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A Level Biology

The ability of a lens to change its curvature, so that it is able to focus on both distant and nearby objects.

A Level Biology

A temporary, unstable, high-energy combination of atoms that are a transition between reactants and products.

A Level Biology

The process by which energy is transferred from organic molecules to ATP to drive metabolic reactions and processes. The energy that is "lost as heat" is actually necessary so that endotherms can keep a constant body temperature.


Role of oxygen: Is present, and therefore required, for the link reaction, Krebs cycle and oxidative phosphorylation (makes up chemiosmosis together with the electron transport chain (ETC)), but is only used in oxidative phosphorylation.
1) Glycolysis (in the cytosol): The process by which a glucose molecule is converted to 2 pyruvate molecules, by:
a) Phosphorilation:
1 ATP is used to convert glucose to glucose phosphate.
0 ATP is used to convert glucose phosphate to its isomer fructose phosphate.
1 ATP is used to convert fructose phosphate to fructose biphosphate.
b) Lysis: The phosphorylated glucose molecule (a hexose) splits to form two pyruvate molecules (trioses).

d) Substrate-level phosphorylation: 4 ATP's are produced. 1 ATP is produced two times, every time that a glucose molecule is converted to pyruvate.
c) Oxidation (dehydrogenation): NAD, and the enzyme for this purpose, dehydrogenase, work together to remove excess hydrogen from the pyruvate, because NAD is a molecule that can accept hydrogen.

2) Link reaction (in the matrix of the mitochondria): The process by which a pyruvate molecule is converted to acetyl coenzyme A, by:
Dehydrogenation: Transferring a hydrogen atom to NAD, after which NAD becomes reduced, but then passes on the hydrogen to other receptor molecules.
Decarboxylation: Removal of a carbon dioxide molecule.

3) Kreb's cycle (in the matrix of the mitochondria twice for every glucose molecule): The process by which more reduced NAD and carbon dioxide is made, by:

Acetyl coenzyme A + Oxyloacetate (an oxidised four-carbon sugar) = Coenzme A + Citrate (a six-carbon sugar).
Citrate + NAD or FAD = A five-carbon sugar + carbon dioxide + reduced NAD or reduced FAD.
5C + NAD or FAD = A four-carbon sugar + carbon dioxide + reduced NAD or reduced FAD.
4C + NAD + FAD + adenosine diphosphate + phosphate = oxyloacetate + reduced NAD + reduced FAD + adenosine triphosphate.


It is also called the citric acid cycle, because all the acetyl coenzyme A is ultimately converted to oxyloacetate, that is used to ultimately convert the next acetyl coenzyme A molecule to oxyloacetate. So oxyloacetate is being synthesised and used up all the time. However, there are molecules that break free from this cycle, namely reduced NAD and reduced FAD with electrons to be used in chemiosmosis (electron transport chain and oxidative phosphorylation), and carbon dioxide to leave the cell, likely to be used in photosynthesis at some point.



Tip: There is an important table is on page 245 of your textbook.


Throwback tip: Difference between glycolysis and the link reaction in terms of output per molecule of glucose at the end of Kreb's cycle: Both reduce 2 NAD's each but no FAD's. Glycolysis makes 2 ATP's and no CO2, and link reaction makes two CO2's but no ATP.


Outputs of the Kreb's cycle:
4 CO2's (2 from each molecule that is or came from acetyl coenzyme A).
2 ATPs (1 from each molecule that is or came from acetyl coenzyme A).
6 reduced NAD's (3 from each molecule that is or came from acetyl coenzyme A).
2 reduced FAD (1 from each molecule that is or came from acetyl coenzyme A).

4) Oxidative phosphorylation (in the mitochondria): The hydrogen released from therefore-oxidised NAD splits to protons and electrons, that travel along the electron transport chain (E.T.C.), releasing energy along the way, that is used to pump protons from the matrix into the intermembrane space, then then diffuse by facilitated diffused through the special conjugated proteins in the walls of the mitochondria (ATP synthase) to join up with oxygen (terminal receptors) to form water. Most of the ATP is synthesised during the electron movement.

Outputs of oxidative phosphorylation:
ATP yield: Theoretically, about 34, but practically about 28, because some of the ATP is used to transport ADP and Pi into the matrix, and to transport ATP out of the matrix. This definition, description and explanation applies to the aerobic respiration of glucose, but the aerobic respiration of fats and proteins produce more reduced NAD and FAD because they have a greater concentration of hydrogen, therefore producing more net ATP than glucose per quantity.
Average Theoretical yield of:
Reduced NAD: 3 ATPs.
Reduced FAD: 2 ATP.
Average Realistic yield of:
Reduced NAD: 2.5 ATPs.
Reduced FAD: 1.5 APTs.
ATP use: None.
No carbon dioxide or reduced NAP are synthesised.


Net outputs of the aerobic respiration of a glucose molecule:
ATP:
Theoretical: 38.
Practical: 32.
C.O.2: 6.
reduced NAD: 10.
reduced FAD: 2.

A Level Biology

Explanation for the fact that respiration takes place only for a short time according to the Z-notes:
Why only glycolysis takes place under anaerobic respiration:

Glycolysis, the link reaction, and the Krebs cycle do not require the oxygen molecule or atom to participate in the reaction as a reactant or as an enzyme in order for the reaction to take place. However, the link reaction and the Krebs cycle require the electron transport chain to be able to transport electrons that are released from NAD. So, if there is no oxygen in the matrix to terminally recieve the electrons, forming metabolic water, the electrons cannot move through the chain. Therefore, the chain stops, and the link reaction and Krebs cycle also stop, because all the NAD and FAD available becomes reduced, leaving none to receive hydrogen from the four-carbon sugar, five-carbon sugar or citrate in the Krebs cycle (Krebs cycle cannot continue without the oxidised four-carbon sugar), and the pyruvate in the link reaction to form acetyl coenzyme A (without which the Krebs cycle cannot take place and the link reaction cannot be completed). Also, the pyruvate itself cannot be formed, so glycolysis eventually also stops.


How anaerobic respiration can be maintained (only the reduced NAD in the cytoplasm can be oxidised again, therefore only enabling substrate-level phosphorylation):

Lactate fermentation: Takes place in animals.

In the oxidative phosphorilation stage, oxygen would have been the receptor of hydrogen to indirectly oxidise reduced NAD and form water, but, since there is no oxygen available, pyruvate oxidises reduced NAD without the electron transport chain in the presence of lactate hydrogenate to lactic acid and oxidised NAD, making NAD available for reuse so that anaerobic respiration, although always inefficient compared to aerobic respiration, can continue.

Alcoholic fermentation: Takes place in yeasts and plants (same category).
In the oxidative phosphorilation stage, reduced NAD would have sent its hydrogen down the electron transport chain, but, since electrons cannot move along the chain because there is no oxygen available to accept them in the matrix, pyruvate releases a carbon dioxide molecule, then the remaining molecule oxidises reduced NAD in the presence of ethanal to give ethanol and oxidised NAD.
However, according to the Z-notes, the reaction is as ff:
Pyruvate = Carbon dioxide + Ethanal.
Ethanal + Hydrogen = Ethanol.
Is ethanal an enzyme, or a product?
Alcoholic fermentation: Fermentation with the end products being alcohol and carbon dioxide. Undergone by certain flowering plants (eg. waterlogged root cells of rice) AND Many species of yeasts (Saccharomyces) (some even respire anaerobically in the presence of oxygen). Yeast fermentation has been exploited by humans for the production of beer and wine for thousands of years. The buildup of alcohol can be harmful to the organism, but some yeasts are able to break down alcohol to sugars to use as a source of energy, while some species are not (which are useful in the production of alcoholic beverages).

Lactate fermentation: Occurs in animals.
Fermentation with the end product being lactic acid. Undergone by tissues in the absence of oxygen, eg. muscles whose current energy needs exceed the amount of energy provided by aerobic respiration. The buildup of lactic acid can be harmful to the organism, but, in animals, the lactic acid diffuses from the muscles to the blood, and is pumped with the blood to the liver where it is broken down aerobically to provide sugars to be used as a source of energy. The oxygen required to break down lactic acid is called the oxygen debt, that causes the pulse and breathing rate of the organism to remain high for awhile after exercise.

A Level Biology

A unique nucleotide that occurs in all cells, that is the main source of energy for metabolism (eg. anabolic reactions, movement AND active transport across the cell surface membrane) and heating where heating is required, but is not a means of long-term storage.

Concentration: 0.5-2.5mg/cm^3
Energy: 10-14 kJ (a small enough amount to be more than enough to drive individual reactions).
Synthesis: ADP + Pi = ATP during respiration.
ATP produced during 24 hours of “rest”: 40kg
ATP produced during 30 seconds of very strenuous activity: 1.5kg
Use (takes place very fast after synthesis): ATP = ADP + Pi during life processes, mostly as a phosphorilated intermediate between energy providing and energy requiring reactions by causing the Pi to bind with the molecule, causing it to react, and then by being released for reuse, but sometimes works in other ways, eg. in muscular contractions.
Transport between cells: Facilitated diffusion (easily).

Synthesis:

Substrate phosphorilation (produces a trivial amount of ATP): Occurs in the cytosol, twice in the step of respiration called glycolysis, and once during the Krebs cycle.

Steps of glycolysis: See Aerobic Respiration.

Where the bulk of ATP synthesis takes place: In the membrane bound organelles, mitochondria and chloroplasts, along the pathways of electron transfer.

Reversable reactions:

ATP + Water = ADP PLUS 1 phosphate.

ADP + Water = AMP PLUS 1 phosphate.

AMP + Water = Adenosine PLUS 1 phosphate.

A Level Biology

How green plants manufacture their proteins, lipids and all other requirements from sugars from photosynthesis and mineral ions from the soil.

A Level Biology: Plant Coordination

IAA diffuses into the leaf through passive auxin channels.
IAA diffuses to the leaf, where it, or another auxin, functions as a transcription promoter or transcription inhibitor (for protein synthesis).
IAA diffuses to the cell walls on the side of the cell, separates the cross-linking polysaccharides (may be hemicelluloses) from the cellulose of the cell walls, then breaks them down. Now that the inhibition is destroyed, the turgidity of the cells maintained by their central vacuole causes the cell walls to stretch.

A Level Biology: Homeostasis

A thin membrane separating epithelial cells from underlying tissues.

A Level Biology: Plant Coordination

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A Level Biology: Homeostasis

Glucagon:
Step 1: Hormone binds to the receptor embedded in the liver cell membrane, that activates the G-protein embedded in the cell membrane, that activates the enzyme adenylyl cyclase.
Step 2: The enzyme catalyses the transformation of ATP to AMP (the second messanger).
Step 3: AMP catalyses a cascade of reactions where specific enzymes are activated by reaction with ATP that amplify the signal produced by a single hormone to catalyse the production of glucose from glycogen to be released into the bloodstream.

A Level Biology: Homeostasis

Step 1: Hormone binds to the receptor embedded in the liver cell membrane, that activates the G-protein embedded in the cell membrane, that activates the enzyme adenylyl cyclase.
Step 2: The enzyme catalyses the transformation of ATP to AMP (the second messanger), and AMP activates and binds to kinase (an enzyme that transfers phosphate from ATP to the receptor, in this case, phosphorilase), forming 10 to the power of 10 activated AMP molecules.
Step 3: The effect of adrenaline is amplified when AMP kinase catalyses a cascade of enzymes where phosphate is added to phosphorilase to form 10 to the power of 6 activated phosphorilase molecules to catalyse the production of 10 to the power of 9 glucose 1 phosphate molecules from glycogen to be released into the bloodstream and increase the blood glucose concentration. Phosphorilase can catalyse glycogenesis of carbohydrate containing compounds other than glycogen to form glucose 1 phosphate. Eventually, cyclic AMP is deactivated.

A Level Biology: Homeostasis

The glucose concentration in the blood is a direct determiner of the rate of respiration, since glucose is the main substrate in respiration. The set concentration of blood glucose is 4.0 mM/L, but it fluctuates between 3.6 mM/L and 5.8 mM/L in a healthy person.
Increase in blood glucose concentration:
After a meal, glucose is absorbed from the villi into the hepatic artery, where its first destination is the liver. The liver converts some of the glucose to glycogen to be stored within it, but it cannot remove all of the excess glucose after a meal high in carbohydrates, so the blood glucose concentration throughout the body is increased. The Islets of Langerhans (an endocrine gland in an exocrine gland called the pancreas composed of a-cells and b-cells with no duct to transport the hormones away, so the hormones produced go directly into the blood in the rich capillary network that surrounds the glans. The gland is surrounded by groups of cells that synthesise and secrete pancreatic juice with ducts for transportation into the duodenum) detect the change, and b-cells secrete insulin. Insulin enters the blood, instructing the liver, most tissues of the body, muscles and brain to absorb glucose from the blood and synthesise it into glycogen (glycogenesis), lipids and fatty acids, and regulates fat distribution within the body as lipids and fatty acids are synthesised from glucose in the liver.
Hyperthermia: An extreme case of high blood glucose concentrations when the blood glucose concentrations are so high that water travels into the blood from the tissue fluid and cells via osmosis, causing the kidneys to excrete more water to maintain the homeostatic amount of water in the blood. The result is dehydration and the inability of the body to maintain the blood pressure.
Decrease in blood glucose concentration:
After prolonged, intense physical activity or a period of starvation, blood glucose concentration decreases, decreasing the rate of respiration. When this change is detected by the Islets of Langerhans, a-cells secrete glucagon (that also decreases rate of respiration) that is transported throughout the body in the blood, that stimulates the liver and most tissues to break down glycogen and amino acids (gluconeogenesis, by activating the enzymes that break down glycogen) in order to release glucose into the blood.

A Level Biology: Homeostasis

Hypothermia: An extreme case of low blood glucose concentration where the blood glucose concentration is below 2 mM/L, that can result in feinting, and eventual convulsions and coma if the glucose is not replenished.

Therefore, it is very important to maintain blood glucose concentrations at homeostatic levels.

A Level Biology

Plants in tropical or sub-tropical regions or tropical grasses, eg sugar cane, maize (whose male flowers are at the top and whose female flowers become the cob after fertilisation) and sorghum, with enzymes with a high optimum temperature so that the plant can metabolise under high temperatures and light intensities, with the photorespiration pathway, whose product is a two-carbon-sugar called phosphoglycolate that wastes ATP and reducing power to recycle, possibly because its decomposition produces little to no useful substances, and one molecule of glycerate-3-phosphate instead of two.
How C4 cells avoid this inefficiency:
1) C4 photosynthetic cells in C4 plants called bundle sheath cells that surround the vascular bundles are surrounded by special mesophyll cells that fix carbon dioxide into a four-carbon sugar called malate by the following process:
Carbon dioxide (diffused into the cell) + a 3 carbon molecule called PEP IN THE PRESENCE OF PEP CARBOXYLASE = Oxyloacetate = Malate, transported to the bundle sheath cells directly after it is formed, and broken down to give carbon dioxide when the plant closes its stomata to reduce water loss, so that carbon dioxide can out-compete oxygen for the active site of rubisco, and pyruvate.
Philosophical note: You can close the stomata to retain a resource, therefore blocking the entry of another, knowing that you have enough of that resource stored within you until it is safe enough to open the door again. However, as a plant, it is impossible to block out the light.
2) Smaller spaces between mesophyll cells so that less to no oxygen can diffuse past the cells to the bundle sheath cells, but the bundle sheath cells obtain their carbon dioxide from malate from the surrounding mesophyll cells.
3) Higher optimum temperatures for enzymes.

A Level Command Words

Work out from given facts, figures or information.

A Level Biology

An instrument used to measure heat production, that allows the calculation of the energy value of substance.

How it works:
Burn a specific volume of a substance in oxygen in the crucible at the bottom of the calirometer. The energy value is determined by measuring the temperature of the water surrounding the crucible in a given time.

You will find the energy values in the following descending order:
Lipids (highest concentration of hydrogen).
Proteins (second highest concentration of hydrogen).
Carbohydrates (lowest concentration of hydrogen and highest concentration of oxygen).

A Level Biology

Tropical grasses or tropical or subtropical plants (for example sorghum, maize (whose male flowers are at the top of the plant and whose female flowers become the cob after fertilisation) and sugar cane,

A Level Biology

History of the theory:

First hypothesised by Peter Mitchell in 1961, after which it was eventually recognised, and Peter Mitchell was awarded the Nobel prize for his discovery, since there is enough evidence to support the hypothesis that it is now classified as a theory.

Process:
In the matrix:
Hydrogen ions and electrons are released by the oxidation of nicotinamine adenine dinucleotide (NAD) and flavine adenine dicleotide (FAD), and water is synthesised from (half an O2 molecule, a proton and an electron) X2.
Protons are transported across the inner membrane that is impermeable to electrons and ions by special carrier protiens, causing protons to accumulate in the intermembrane space, causing the pH to drop, creating a potential difference, more spcifically a proton gradient, across the inner membrane.
Protons passively diffuse back into the matrix through ATP synthase, providing energy for the synthesis of ATP, catalysed by the ATP synthase.

A Level Biology

Extraction:

1) Remove a healthy green leaf from a green plant.

2) Cut the leaf up into small pieces.

3) Grind the small pieces up with a mortar and sand as an abrasive.

4) Dissolve the pieces in an organic solvent (eg. nine parts petroleum ether to one part 90% aqueous propanone, the solvents with which chlorophyll a. has an Rf. value of 0.65 and chlorophyll b. has an Rf. value of 0.45).

5) Put the solution through a centrifuge to get rid of the large insoluble particles.

6) Store in foil in a dark place, because chlorophyll disassociated from the grana is unstable and will bleach in the light energy that it otherwise would utilise to drive most food chains of the earth.
Philosophical note: “I know that pictures don’t change. Just the people inside of them do. The only difference between combustion and respiration is the how well the transfer of energy is controlled.”

Composition:

Photosynthetic pigments: Chlorophyll A. with CH3 as part of the head.
Chlorophyll B. with an aldehyde group (-CHO) as part of the head.
Accessory pigments:
Pigments that transfer the light energy they have absorbed to the photosynthetic pigments.
eg.
Carotene (a yellow pigment as an important source of vitamin A)
Xanthophyll.

Position:

Sandwiched between the protein and lipid membranes of the grana, and occur with the enzymes and protein carriers that make photosynthesis possible.

A Level Biology

1) Absorption spectra:

The amount of each wavelength of light absorbed by chlorophyll, discovered to be mainly blue and red light, that can be determined by measuring how much of each wavelength in turn is absorbed by chlorophyll.

Representation:

On a graph.
X axis: Wavelength/ frequency.
Y axis: Amount of light absorbed.

OR

On a similar graph combined with the action spectrum.

2) Action spectrum:

The amount of each spectrum of light used by chlorophyll for photosynthesis (rate of photosynthesis at each wavelength), discovered to be mainly blue and red light (very similar to the absorption spectrum), that can be determined by exposing aquatic green pondweed to each distinct wavelength of visible light in turn and for a unit of time, and measuring the rate of photosynthesis by oxygen evolution at each wavelength.

Representation:
In a graph.
X axis: Wavelength/ frequency.
Y axis: Rate of photosynthesis.

OR

On a similar graph combined with the absorption spectrum.


When the absorption and action spectra are very different: Extracted chlorophyll can absorb light energy, but cannot convert it to potential energy in sugars (photosynthesise), because it is separated from its membranes and enzymes that are essential for photosynthesis.


Why the absorption and action spectra will never be 100% analogous: Waves with lower (shorter) wavelengths (high frequency) have more energy than those with higher (longer) wavelengths (low frequency). Therefore, even if the amount of wave absorbed for two difference wavelengths is the same, the rate of photosynthesis at the higher frequency will be greater.

A Level Biology

Porphyrin Head: Hydrophyllic and associated with proteins.

Composed of nitrogen and magnesium, and:

Chlorophyll A: CH3
Chlorophyll B: -CHO

Hydrophobic Tail: Hydrophobic and associated with lipids; folded.

A Level Biology

Composition: Two chlorophylls and a non-photosynthetic substance.

Structure:

Porphyrin Head: Hydrophyllic and associated with proteins.

Composed of nitrogen and magnesium, and:

Chlorophyll A: CH3
Chlorophyll B: -CHO

Hydrophobic Tail: Hydrophobic and associated with lipids; folded.

A Level Biology

Function: All stages of photosynthesis.
Size: 4-10 microns long and 2-3 microns wide (one of the largest organelles), that can be observed in outline by light microscopy, but whose structure can only be seen via electron microscopy.
Structure (relating to function):
Double membrane that is fully permeable to the inputs and first outputs of photosynthesis and ATP, and whose inner membrane consists of thylakoid membranes that make up the grana and stroma, and whose outer membrane is called the chloroplast envelope.
Stroma: Loosely-arranged tubular thylakoid membranes in a watery medium, that contains enzymes for carbon dioxide fixation (rubisco that is ribulose biphosphate carboxylase), reduction by NADP and regeneration of ribulose biphosphate, as well as the synthesis of many product molecules.

Grana.
Intergranum lamella: Thylakoid membranes that join grana to each other.Photosystems on thylakoid membranes: Funnel-shaped systems of photosynthetic and accessory pigments directing light to a chlorophyll molecule called the reaction centre, that carry out the light-dependent stages of photosynthesis.
Chloroplasts sandwiched in the grana with their hydrocarbon tails in lipids and their hydrophillic heads in proteins: Within the photosystems, catalyse photosynthesis.
Starch grains, lipids and ribosomes: Storage of the products of photosynthesis, and protein synthesis.

A Level Biology

The process used to separate components of a mixture, that works well when only a small sample is available, ideal for separating biologically active molecules because biochemists often only have access to small samples.

Stationary phase: The support medium up which the moving phase moves, through the loads, eg. absorbent paper in paper chromatography, a thin film of dried solid in thin-layer chromatography, and powdered solid in column chromatography.

Process:

1) Dissolve the mixture to be separated in solvent A, evaporate the solution for concentration, then place drops of the solution on a line near but not on the edge of the stationary phase, and allow them to dry.

For paper and thin-layer chromatography:

2) Suspend the bottom part of the stationary phase, below the load, in a solvent B (moving phase) inside a closed system (eg. boiling tube or small glass tank). The solvent will move up the stationary phase by capillary action, through the load, the different components of which will move up at different rates, because they have different solubilities in the solvent, and they relate differently to the stationary phase.

3) Before the solvent front reached the end of the stationary phase, stop the process, mark the position of the solvent front, and note the positions of the coloured pigments.

Differences with thin-layer chromatography:
Stationary phase: Thin-level chromatography plate: A layer of plastic coated with silica gel.
Environment: Flat-bottomed tank with a close-fitting bung with a slit for the stationary phase, with a small level of water.

A Level Biology: Plant Coordination

A sheath protecting a young shoot.

A Level Command Words

Give an informed opinion.

A Level Command Word

Identify or comment on similarities and/or differences.

A Level Biology: Coordination

The process beginning with fertilisation and ending with implantation, resulting in pregnancy.

A Level Biology: Coordination

What it is:


Composed of two hormones that are chemically very similar to oestrogen and progesterone.


How it prevents conception:


Decreases the production of FSH and LH by the pituitary gland, therefore decreasing oestrogen levels, therefore decreasing the amount of secondary follicles produced and preventing the production of the Graafian follicle, therefore preventing the production and release of the secondary oocyte, therefore preventing the production of the corpus luteum, therefore preventing the release of progesterone, therefore preventing the maintenance of the endometrium, therefore decreasing the volume and duration of endometrium lost during menstruation (you can't lose what isn't there) and preventing ovulation and decreasing the likelihoods of implantation.
Thickens the mucus plug of the cervix, making it harder for sperm cells to enter the uterus, with the added benefit of making it harder for bacteria to enter the uterus, preventing infection.


How it must be taken:
Fertile, sexually active biological women who have vaginal sex with biological men who wish to decrease their likelihoods of pregnancy and who are prepared for the potential negative side effects of the pill should take one pill at the same time every day for 21 days, and then should take a placebo containing iron supplement (a lot is lost during menstruation) or no pill at all for the next 7 days, during which a lighter and shorter period of menstruation will take place.


Possible negative side effects: The pill may increase the likelihoods of:
Nausea, vomiting, headaches and mood swings.
Breast cancer (slight increase in likelihood).
Rise in blood pressure, increasing the risk of thrombosis (blood clots are life-threatening).


Possible positive side effects: The pill may decrease the likelihoods of:
Ovarian cysts, and ovarian and uterine cancer.
Almost guaranteed benefits of the cell besides contraception:
Lighter, shorter periods, and reduced pre-menstrual stress.
Decreased likelihood of bacteria infection beyond the cervical plug.

A Level Biology: Coordination

All systems in multicellular organisms need some level of coordination. The mechanisms of coordination in mammals are the nervous and endocrine systems.

Sensitivity, one of the seven life processes, is essential for life.

A Level Biology

Use: When carbon dioxide concentrations in a cell are low.
When carbon dioxide is a limiting factor, the Calvin cycle cannot take place, and reduced NAD cannot be oxidised again, so it builds up, using up more and more NAD so that NAD concentrations fall, and the photosystems 1 and 2 of the light-dependent stage can no longer work together as they would otherwise have under non-cyclic photophosphorilation. Therefore, the photosystems revert to cyclic photophosphorilation so that ATP can still be synthesised while organic molecules cannot be synthesised.

Process:

The reaction centres in photosystems 1 and 2 release two specific electrons each, excited by light energy, onto two electron transport chains.

Ground electrons in photosystem 1 are replaced by regrounded electrons from photosystem 2 whose energy has been used for the active transport of protons into the thylakoid space.

Ground electrons in photosystem 2 are replaced by regrounded electrons from photosystem 1 whose energy has been used for the actice trabsport of protons into the thylakoid space.

Protons travel down their concentration gradient through the gap in ATPase, providing energy to tranfer to ATP on the stroma side of the thylakoid membrane, possibly because it will be used to transfer energy to reacting molecules in the stroma during the Calvin cycle.

A Level Command Words

Give precise meaning.

A Level Command Words

State the points of a topic

OR

Give characteristics and main features.

A Level Command Words

Establish an answer using the information available.

A Level Biology: Homeostasis

A group of diseases in which the body fails to recognise blood glucose levels.

Glucose levels in mammals with diabetes:
1) Glucose levels in urine.
Principle:
Glucose PLUS Oxygen IN THE PRESENCE OF GLUCOSE OXIDASE EQUALS Gluconic acid PLUS Hydrogen peroxidase (H2O2).
Hydrogen peroxidase (H2O2) PLUS Reduced chromagen (D.H2) IN THE PRESENCE OF PEROXIDASE EQUALS Water PLUS Chromagen.
The colour of the chromagen is determined by the concentration of glucose in the urine (glucose is completely reabsorbed by the blood in the kidneys of a non-diabetic person).
Philosophical note: If something is to much where it is supposed to be (in the blood), some of it spils over into where it is not supposed to be (in the urine).
Dip the probe into the urine sample, and compare its colour with the scale on the box.

2. Glucose levels in blood.
Glucose biodetector: A device that uses biological compounds, or in some cases cells, to detect the level of glucose, that consists of:
Pin used to prick the tips of the fingers to obtain a blood sample.
Processing unit.
Probe consisting of:
Platinum electrode.
Inner membrane consisting of cellulose acetate.
Immobilised glucose oxidase.
Outer membrane consisting of polycarbonate.
Process:
Outer layer comes into contact with blood sample.
Glucose in the blood is broken down with oxygen by immobilised glucose oxidase to give gluconic acid and hydrogen peroxide as soon as it comes in contact with the enzyme.
Decrease in oxygen levels is detected by platinum electrode, creating an electrical signal. This electrical signal is directly proportional to blood glucose concentration. Blood glucose concentration is displayed on a digital readout.

If blood glucose concentrations are too high, type 1 diabetics need to take insulin.

A Level Biology

Carriers that transport electrons along a redox chain (referring to the carriers, not the electrons themselves), involved in the synthesis of ATP in the process.

A Level Biology

Metabolic reactions that require energy.

A Level Biology: Homeostasis

Intersection between the two systems:

Hypothalamus monitors the hormone levels in the blood, and controls them by negative feedback.
Hypothalamus releases hormones to regulate the activities of the pituatary gland.
The pituatory gland (the master gland of the endocrine system) regulates activities of the nervous and endocrine systems.

Differences between endocrine and nervous systems:
1) Transmission:
Endocrine: In the blood.
Nervous: Impulses (electrochemical action potentials) travelling along neurones.
2) Where messanger can be detected:
Endocrine: Throughout the body, but only responded to by the target tissues and/or organs.
Nervous: Only target cells.
3) Effect of message:
Endocrine: Change in metabolism.
Nervous: Contraction of muscle or secretion by gland.
4) Time between transmission and effect of message:
Endocrine: Minutes or hours.
Nervous: A matter of milliseconds.
5) Length of effect:
Endocrine: Long lasting.
Nervous: Short-lived and reversible.

A Level Biology: Coordination

Hormones area released from endocrine glands at a distance from the target cells in target organs that have special receptor molecules to receive, process and respond to the signal from the hormone, usually in the form of a change in metabolism. The liver is continuously deactivating hormones by breaking them down so that they can be excreted by the kidneys, so, in order for the effect of a hormone to be long-lasting, it must be secreted in a continuous process, which many hormones are.

A Level Biology

1) Life processes (nutrition, respiration, excretion, movement, growth, reproduction and sensitivity to the environment).
For example:

Respiration:
Synthesis and release of enzymes.

Movement:
Movement within cells of organelles, in and out of cells across the cell membrane (active transport), AND of cells and prostrusions from cells eg. muscular contractions and the movement of cilia and flagella.

Growth:
1) Anabolism (synthesis of large molecules from smaller ones).
2) Synthesis of all cell organelles.
3) Cell division.

Sensitivity to the internal and external environment:
1) Synthesis and release of hormones.
2) Nerve signals (setting up and maintaining action and rest potentials).
3) Homeostasis.

Reproduction:
1) DNA and protein synthesis.
2) Bioluminescence.

A Level Biology

Metabolic reactions that release energy.

A Level Biology

A gland that makes substances such as sweat, tears, saliva, milk or digestive juices, and releases them through a duct or opening to the body surface.

A Level Command Word

Set out purposes or reasons.

OR

Make the relationship between things evident.

OR

Provide why or how, giving relevant evidence.

A Level Biology

Diffusion across a membrane facilitated by molecules in the membrane, not requiring metabolic energy.

A Level Biology

When a molecule has been broken down, the part of the chemical potential energy that is available for useful work.

Philosophical note: See the Law of the Conservation of Value, Law of the Conservation of Effort and Law of the Conservation of Experiences in mental notes.

A Level Biology: Plant Coordination

General roles:
Activate enzymes that promote germination.
Promote additional mitosis (for example, to increase internodal length of rice plants) that promotes the elongation of stems.
Delays abscission of leaves.
Inhibits the initiation of lateral roots.

How it causes the stimulation of a barley seed:
Dormant seed responds to its DNA by absorbing water.
Water activates gibberelinic acid.
Gibberelinic acid activates the enzymes that break down the colyledon(s) to provide energy for the growth of the shoot.

How it relates to dwarfism in pea plants:
Both tall and short varieties of pea plants contain gibberelinic acid, but only tall varieties contain the enzymes that activate gibberelins, coded for by the dominant allelle for the enzyme. Short varieties do not contain the enzymes, because they contain the two recessive allelles that do not code for the enzymes.

A Level Command Words

Produce an answer from a relevant source.

OR

Recall an answer from.

A Level Biology: Homeostasis

1) Glucose levels in urine.
Principle:
Glucose PLUS Oxygen IN THE PRESENCE OF GLUCOSE OXIDASE EQUALS Gluconic acid PLUS Hydrogen peroxidase (H2O2).
Hydrogen peroxidase (H2O2) PLUS Reduced chromagen (D.H2) IN THE PRESENCE OF PEROXIDASE EQUALS Water PLUS Chromagen.
The colour of the chromagen is determined by the concentration of glucose in the urine (glucose is completely reabsorbed by the blood in the kidneys of a non-diabetic person).
Philosophical note: If something is to much where it is supposed to be (in the blood), some of it spils over into where it is not supposed to be (in the urine).
Dip the probe into the urine sample, and compare its colour with the scale on the box.

2. Glucose levels in blood.
Glucose biodetector: A device that uses biological compounds, or in some cases cells, to detect the level of glucose, that consists of:
Pin used to prick the tips of the fingers to obtain a blood sample.
Processing unit.
Probe consisting of:
Platinum electrode.
Inner membrane consisting of cellulose acetate.
Immobilised glucose oxidase.
Outer membrane consisting of polycarbonate.
Process:
Outer layer comes into contact with blood sample.
Glucose in the blood is broken down with oxygen by immobilised glucose oxidase to give gluconic acid and hydrogen peroxide as soon as it comes in contact with the enzyme.
Decrease in oxygen levels is detected by platinum electrode, creating an electrical signal. This electrical signal is directly proportional to blood glucose concentration. Blood glucose concentration is displayed on a digital readout.

If blood glucose concentrations are too high, type 1 diabetics need to take insulin.

A Level Biology

A demonstration of the light-dependent stages of photosynthesis, as long as the chloroplasts are suspended in an isotonic buffer to prevent osmotic damage, and either the natural electron-carrier and hydrogen-acceptor molecules, or a suitable alternative, are present in the solution. It can be used to investigate the splitting of water molecules, for example, because the splitting of water molecules results in the evolution of oxygen.

In a research lab, the amount of oxygen evolved is detected using oxygen electrodes. In your lab, a hydrogen-acceptor dye that changes colour when reduced (addition of hydrogen from the splitting of water molecules) that does not cause damage when added to the chloroplast suspension, eg. DCPIP, a blue liquid that turns colourless when reduced, can be used.

2 DCPIP PLUS 2 Water EQUALS 2 DCPIPH2 PLUS Oxygen.

(DCPIP stands for dichlorophenolindophenol.)

How the chloroplasts are isolated:
1) Homogenise (blend) chilled de-veined leaf tissue in a chilled homogenising medium.
2) Filter the homogenate through 8 layers of muslin (cheesecloth) through a funnel into a centrifuge tube in a filtered homogenate ice bucket to remove the larger debris (part ground cells).
3) Centrifuge the liquid at a low speed in a cold centrifuge head.
4) Decant the supernatant and discard the solid that collects at the bottom.
5) Centrifuge the liquid again at a high speed in a cold centrifuge head.
6) Dispose of the supernatant and re-suspend the pellets composed of lower-mass organelles (chloroplasts) in a cold re-suspending medium, with a pipette, stirring the mixture with a glass rod.

A Level Biology: Homeostasis

An organism maintaining its internet environment within certain limits.


Regulators: Organisms that are able to maintain their internal environment within narrow limits, eg. mammals and birds, with the following advantages:
Temperature is maintained at the optimum temperature for the majority of enzymes that drive metabolism.Muscles can contract and the nervous system can function properly at any temperature.
Animal can move to catch prey or escape from a predator, which may provide an advantage over non-regulators.
Animal is able to live in a wide variety of habitats, from the equator to the poles.

Mammals as regulators: Although they are a recent evolutionary development, they have been able to colonise every habitat in the world as a result of homeostasis by negative feedback. Internal conditions maintained at a relatively constant level as a result of negative feedback:
Of blood:
Concentration of respiratory substances, glucose and water.
pH.
Pressure in arteries.
Of body:
Temperature.

Heart rate.
Concentration of essential ions.

A Level Biology: Homeostasis

Endotherms: Animals that undergo thermoregulation (regulation of body temperature). Mammals are endotherms that maintain a high and fairly constant internal environment, by heat production within the body by respiration, and by carefully controlling heat exchange through the skin. The internal body temperature of humans varies between 35.5 degrees and 37 degrees in a 24-hour period when we are in good health.



Where heat is produced in mammals:
At rest, 70% from other abdominal organs, including heart and kidneys, but also from the lungs and brain. During physical activity, skeletal muscles generate a great deal of heat as products of both respiration and contraction. Heat is then distributed to the rest of the body by the blood circulation.

How heat is transferred between mammals and the external environment:

To and from mammals:
Conduction
Convection
Radiation

From mammals only:
Evaporation

A Level Biology

Why temperature affects rate of respiration:

Respiration is a series of enzyme-catalysed reactions, and enzymes have an optimum temperature beyond which they begin to denature, although the optimum temperature of an different types of enzyme vary greatly (eg. some enzymes live in hot springs at or above boiling conditions).

General rule for chemical reactions:
Up till just below optimum temperature, rate of reaction at least doubles for every 10 degree rise in temperature.

With methyline blue (test for reducing agents):

Dependent variable: Time taken for the redox indicator to change colour.

Independent variable: Temperature of water in test tube.

Control variables: Amount and concentration of redox indicator, yeast and oil.

Apparatus:
X amount of test tubes.
Yeast suspension.
Water bath.
Methylene blue (a redox indicator that is blue when in an oxidised state but colourless if exposed to a reducing agent).
Oil (prevents the dissolving of oxygen in the yeast suspension, so that anaerobic respiration can take place, and prevents oxygen from the air to cause oxidation of methylene blue so that it would turn blue again).
Stopwatch.

Steps:
1. Place volume V. of yeast suspension in a test tube at temperature a., with y. drops of methyline blue.
2. Place volume v. of oil on top of the yeast suspension.
3. Start the stopwatch, and stop it as soon as the yeast suspension has changed colour. Record your result in a table.
4. Repeat steps 1-3 for temperatures b., c., d., e., etc. at regular intervals. Represent your results in a conprehensive manner.

With a differential respirometer:

Change the temperature of the water in the water bath.

A Level Biology: Homeostasis

The control centre for thermoregulation in a region of the forebrain, called the thermoregulator, that consists of receptor cells and two regions (heat production centre and heat retention centre), and connected to the rest of the body via the nervous system.
How it works:
Neurones detect changes in the temperature of the blood flowing through the brain (internal environment) and the brain receives impulses from neurones in the skin and other organs that detect changes in the temperature of the blood flowing outside of the brain (external environment).
Philosophical note: If you want to be able to influence the outside world, you will not be able to do so effectively, if at all, if you sever yourself from it. You need to have connections to many parts of the outside world in order to influence it.
If the temperature is too high: The heat loss centre inhibits the heat gain centre.
The hypothalamus sends nerve signals to the rest of the body, to:
Stop the following mechanisms that make the body warmer:
Vasoconstriction that results in constriction of capillaries and dilation of bypass vessel between the arterioles and venules.
Contraction of skeletal muscles to increase their heat production by respiration (shivering), to add to the heat released from other organs by respiration (eg. stomach organs, heart and brain) and transported by the blood.
Standing up of body hairs to trap humid air so that it blows away slower, decreasing evaporation.
Start the following mechanisms to make the body cooler:
Vasodilation that results in the dilation of the capillaries and the constriction of the bypass vessel that channels blood directly from the arteriole to the venule.
Relaxation of body hairs so that they do not trap humid air close to the body, increasing evaporation.
Sweating, to increase evaporation (the process that transfers heat from the body to the external environment, not vice versa), AND Decrease metabolic rate to decrease heat production.
When the body temperature becomes too low, the opposite applies.

A Level Biology: Homeostasis

Change in position, activity and (for humans) adjustment of clothing.

A Level Biology: Homeostasis

The insulin-rich hormone called thyroxin secreted by the thyroid gland that stimulates oxygen consumption and basal metabolic rate in the body, that is released as a result of impulses from the heat gain centre of the hypothalamus to increase rate of respiration and therefore increase heat production.

Shivering.

Enhanced “brown fat” respiration.

A Level Biology: Homeostasis

Skin that is especially exposed (eg. nose, ears and extremities) has an extensive capillary network, whose arterioles can be dilated or constricted.

A Level Command Words

Recognise OR Name OR Select.

A Level Biology

A molecule formed as part of a metabolic pathway that is neither the first or the last in a metabolic pathway.

A Level Biology

Located or occuring within a cell or cells.

A Level Biology

The action or process of being turned inside out or folded back on itself to form a cavity or pouch.

A Level Command Words

Support a case with evidence or argument.

A Level Biology: Homeostasis

Structure: Very similar to that of the proximal convoluted tubule, but performs different functions.

Functions:
1) pH regulation.
Slight changes in the pH of the blood are regulated by the blood proteins that function as a pH buffer, but when the blood pH varies more than the buffer can prevent, above a pH of 7.4, the distal convoluted tubule secretes hydrogen ions into and absorbs hydrogencarbonate from the blood, to maintain the blood pH between 7.35 and 7.45, although the pH of the urine varies greatly, between pH 4.5 and 8.2.

2) Regulation of mineral ions:
Potassium ions are transferred from the blood to the filtrate as necessary, and sodium ions are reabsorbed back into the blood as necessary.

A Level Biology: Homeostasis

Role: Makes water conservation in the kidneys possible.

Explanation:
A significant issue in the removal of the waste products of metabolism from the body: It is impossible to avoid some water loss, since urea and mineral ions pass out of the body in solution. To reduce this, a water potential gradient from high water potential to low water potential between the cortex and the medulla must be maintained, to make sure that the urine is, where necessary, more concentrated than the blood, and, in humans, up to five times as concentrated as the blood.

Mechanism: Counter-current multiplier that facilitates counter-current exchange, using the principle of exchange between two fluids travelling in opposite directions in two systems, a process that occurs throughout the regions of the loop. At every level of the loop, the contents of the ascending loop are slightly less concentrated than the contents at the corresponding position in the descending loop.
The descending loop is fully permeable to water and most mineral ions. Because of the large concentration of mineral ions in the interstitual fluid (fluid between the cells of the medula), water travels out of the loop into the interstitual fluid by osmosis, and mineral ions, such as sodium and chloride, diffuse into the loop.
The vasa recta supplies the metabolically active cells with oxygen and rids them of CO2, and its descending part is saltier than its ascending part, since the blood travels in the opposite direction to the fluid in the loop. Therefore, the cells of the loop are serviced without the removal of the buildup of salts in the medulla.
At the hairpin bend, in the pyramid region of the medulla consisting largely of collecting ducts, the concentration of mineral ions in the loop has increased to the point at which mineral ions diffuse out of the loop into the medulla, because of ions diffusing in and water diffusing out. Therefore, in the medulla, the concentration of salts is the greatest in and around the hairpin bend, as sodium and chloride ions diffuse out, so that water can enter by osmosis from the water collecting ducts as necessary.
The ascending loop is permeable to mineral ions, but, unusually, not to water. Salts may continue to diffuse out until the solute potential between the loop and the fluid between the cells of the medulla is equal. Then mineral ions are removed from the loop by active transport.

A Level Biology: Homeostasis

Role in excretion: Ultrafiltration.

Anatomy and mechanisms:
Afferent arterioles (wide) supply the glomurulus, and efferent arterioles (narrow) carry the blood away from the glomurulus. Therefore, a high hydrostatic (blood) pressure potential is created within the glomurulus, that is higher than its water potential (the adherence of water molecules toward each other, decreased by a greater solute potential. Philosophical note: By this you by your sheer numbers will turn the world upside down, if you have love for one another. In life, adherence, in effect, creates more.) Therefore, many of the water molecules, glucose molecules, amino acids and urea molecules, and some of the smaller soluble proteins (but none of the larger blood proteins because they are too big) are forced out through the pores in the endothelium of the capillaries, through the basement membrane (that inhibits most proteins trying to pass through it, as well as (NB!) white blood cells and red blood cells), and past the extensions of the podocytes (epithelial cells of the renal capsule with foot-like extensions that adhere close to the capilaries, forming a network) through the spaces between the podocytes into the lumen of the renal capsule.

How the fluid in the renal capsule differs from tissue fluid:
Proteins are largely (yet not completely) absent, and white blood cells are absent.

A Level Biology: Homeostasis

Role in excretion: Ultrafiltration.

Anatomy and mechanisms:
Afferent arterioles (wide) supply the glomurulus, and efferent arterioles (narrow) carry the blood away from the glomurulus. Therefore, a high hydrostatic (blood) pressure potential is created within the glomurulus, that is higher than its water potential (the adherence of water molecules toward each other, decreased by a greater solute potential. Philosophical note: By this you by your sheer numbers will turn the world upside down, if you have love for one another. In life, adherence, in effect, creates more.) Therefore, many of the water molecules, glucose molecules, amino acids and urea molecules, and some of the smaller soluble proteins (but none of the larger blood proteins because they are too big) are forced out through the pores in the endothelium of the capillaries, through the basement membrane (that inhibits most proteins, leucocytes and erythrocytes trying to pass through it), and past the extensions of the podocytes (epithelial cells of the renal capsule with foot-like extensions that adhere close to the capilaries, forming a network) through the spaces between the podocytes into the lumen of the renal capsule.

How the fluid in the renal capsule differs from tissue fluid:
Proteins are largely (yet not completely) absent, as well as blood cells.

A Level Biology: Homeostasis

Role: Selective reabsorption.

Anatomy:
The longest section of the nephron, one cell thick, whose cells are packed full of mitohondria for active transport. The cell surface membranes in contact with the filtrate are covered with a “brush border” of microvilli, increasing the surface area for reabsorption by simple diffusion, facillitated diffusion, osmosis, active transport and bulk transport.

Mechanisms:
Co-transport: A kind of active transport, where a proton pump pumps protons out of the cell to which the molecules must be pumped, creating a potential difference in the form of a proton gradient. Protons therefore diffuse down their concentration gradient back into the cell through the co-transporter pump, taking sugars, amino acids or mineral ions with them.

Facilitated diffusion:
Mineral ions.

Simple diffusion:

Glucose (all of it in a healthy person).
Amino acids (all of it in a healthy person).
Urea (in response, harmful substances are pumped back into the nephron via active transport).

Osmosis: Water (as a result of the decrease in water potential of the blood as opposed to the proximal convoluted tubule cell (PCT cell).

Pinocytosis (uptake of a droplet of liquid into a cell involving the invagination of the plasma membrane).
The few proteins that made it into the nephron.

Ion exchange, for example exchange of a sodium ion for a proton:
Mineral ions (sodium ions exit the PTC cell, and potassium ions enter it, by ion exchange).

A Level Biology: Homeostasis

Anatomy that I didn’t take note of in IGCSE:

Voluntary sphincter that opens or closes the urethra from the external environment.
Dorsal artery that supplies the renal artery.
Vena cava that drains the renal vein.
Convoluted nephrons are found in the cortex and the medulla.
Loops of Henlé and collecting ducts are found largely in the medulla.
Malpighian body: The first part of the nephron that forms the glomurulus inside of the renal capsule, that occurs in the cortex.
Note: The nephron is the unit of kidney function, and each part of the nephron has a function in homeostasis.
Summary of the functions of:
Malpighian body: Ultrafiltration (powered by a pressure potential that is greater than the water potential as a result of the input capillaries being wider than the capillaries in the glomurulus, causing most of the water, glucose, amino acids and urea to be forced out of the glomurulus through the endothelium, basement membrane and epithelium of the renal capsule into the renal capsule).
Porpheral convoluted nephrons: Selective filtration.
Loop of Henlé (largely in medulla): Water conservation.
Distal convoluted nephrons: Filtration of water and ions.
Pelvis: Large area that functions as the collecting part of the nephron.

Functions:

1) Excretion: A characteristic of living things where the waste products of metabolism are removed from the organism so that they do not accumulate to have a toxic effect on the organism.
How urea is excreted:
In the liver, amino acids that are not needed for protein synthesis are broken down by the following process:
Deamination: Removal of the amino group to form pyruvate (respired by the link reaction, Krebs cycle and oxidative phosphorilation) and ammonia (a very soluble and toxic compound that is combined immediately with carbon dioxide to form the harmless compound urea.
Urea travels in the bloodstream to the kidneys where it is forced into the nephrons and not rebsorbed by the blood so it can exit the body via the urine.
The body produces about 1-1.5 litres of urine a day, about 40 to 50 grams of which are extretory products, composed of about 20 grams of urea and a maximum of about 15 grams of sodium chloride. Nephrons produce urine in a continuous process that we conveniently divide into different steps.
Therefore, the kidneys control excretion by constantly changing the composition of the blood.

2) Osmoregulation (usually of sodium and chlorode ions)
The kidneys regulate the concentrations of water and inorganic organs in the body.

A Level Biology: Homeostasis

Voluntary sphincter that opens or closes the urethra from the external environment.
Dorsal artery that supplies the renal artery.
Vena cava that drains the renal vein.
Convoluted nephrons are found in the cortex and the medulla.
Loops of Henlé and collecting ducts are found largely in the medulla.
Malpighian body: The first part of the nephron that forms the glomurulus inside of the renal capsule, that occurs in the cortex.
Note: Each part of the nephron has a function in homeostasis.
Summary of the functions of:
Malpighian body: Ultrafiltration (powered by a pressure potential that is greater than the water potential as a result of the input capillaries being wider than the capillaries in the glomurulus, causing most of the water, glucose, amino acids and urea to be forced out of the glomurulus through the endothelium, basement membrane and epithelium of the renal capsule into the renal capsule).
Porpheral convoluted nephrons: Selective filtration.
Loop of Henlé (largely in medulla): Water conservation.
Distal convoluted nephrons: Filtration of water and ions.
Pelvis: Large area that functions as the collecting part of the nephron.

Functions:

1) Excretion: A characteristic of living things where the waste products of metabolism are removed from the organism so that they do not accumulate to have a toxic effect on the organism.
How urea is excreted:
In the liver, amino acids that are not needed for protein synthesis are broken down by the following process:
Deamination: Removal of the amino group to form pyruvate (respired by the link reaction, Krebs cycle and oxidative phosphorilation) and ammonia (a very soluble and toxic compound that is combined immediately with carbon dioxide to form the harmless compound urea.
Urea travels in the bloodstream to the kidneys where it is forced into the nephrons and not rebsorbed by the blood so it can exit the body via the urine.
The body produces about 1-1.5 litres of urine a day, about 40 to 50 grams of which are extretory products, composed of about 20 grams of urea and a maximum of about 15 grams of sodium chloride. Nephrons produce urine in a continuous process that we conveniently divide into different steps.
Therefore, the kidneys control excretion by constantly changing the composition of the blood.

2) Osmoregulation (usually of sodium and chlorode ions)
The kidneys regulate the concentrations of water and inorganic organs in the body.

A Level Biology: Homeostasis

Function: Blood Water Concentration Regulation.
1) Hypothalamus (front region of the brain that regulates many functions of negative feedback, such as thermoregulation, osmoregulation and parts of the endocrine system) secretes anti-diuretic hormone (ADH) into the back of the pituatary gland (called the Master Gland because of all the different hormones it secretes) that is below it, yet connected to it, where it is stored in vesicles at the ends of their neurosecretory cells.
2) If you drink too little water, consume too much salt, or sweat profusely, thirst receptors will be triggered, likely to cause you to drink more fluids, that, if sufficient to return blood water potential to set conditions, will be offset by the hypothalamus before the below becomes necessary. Otherwise, the sensor cells in the hypothalamus or the sensor cells in specific organs joined by nerve cells in the hypothalamus will detect the decrease in the water potential of the blood, and trigger the pituatary gland to release ADH into the blood.
3) The ADH enters the kidney, and causes the protein carrier molecules in the collecting duct to open, so water diffuses out into the medulla, and is reabsorbed by the blood vessels to take the blood water potential back to set conditions. The urine is concentrated. Less water diffuses from the medulla into the collecting ducts because of the water potential gradient between the collecting ducts and the medulla, set up by the Loop of Henlé.
4) ADH is absorbed and deactivated by the liver as it passes through it, and excreted by the kidneys in the urine.
5) The hypothalamus detects that the blood water potential is now slightly too high, because it is possible to avoid some overshoot in negative feedback, so the thirst receptors are not stimulated, likely causing you to drink less fluids, and the ADH is not released from the pituatary gland, so the protein carrier molecules in the collecting duct remain closed, and the urine is dilute.
The above process here by number 5 occurs at any time that the blood water potential is too high, due to the over-consumption of fluids, too little minerals in the blood, etc.

A Level Biology: Homeostasis

1) Glucose.
Detected in the urine by Clinistix, or as a concentration that is too high in the blood by a glucose biosensor.
May indicate diabetes.
2) Protein.
Detected in the urine by a dipstick test for albumin.
Indicates a proteinura, that might be an indicater of a disease caused by some sort of damage to the glomurula membranes, or severe hypertension.
3) Ketones.
Detected in the urine by a dipstick test called Ketostix.
Indicates type 1 diabetes, an untreated or advanced case of type 2 diabetes, or an eating disorder, because the body metabolises lipids, producing ketones in high enough concentration to be detectable in the urine, when levels of glucose are very low.

A Level Biology: Homeostasis

Structure: Very similar to that of the proximal convoluted tubule, but performs different functions.

Functions:
1) pH regulation.
Slight changes in the pH of the blood are regulated by the blood proteins that function as a pH buffer, but when the blood pH varies more than the buffer can prevent, above a pH of 7.4, the distal convoluted tubule secretes hydrogen ions into and absorbs hydrogencarbonate from the blood, to maintain the blood pH between 7.35 and 7.45, although the pH of the urine varies greatly, between pH 4.5 and 8.2.

2) Regulation of mineral ions:
Potassium ions are transferred from the blood to the filtrate as necessary, and sodium ions are reabsorbed back into the blood as necessary.

A Level Biology: Homeostasis

Role: Makes water conservation in the kidneys possible.

Explanation:
A significant issue in the removal of the waste products of metabolism from the body: It is impossible to avoid some water loss, since urea and mineral ions pass out of the body in solution. To reduce this, a water potential gradient from high water potential to low water potential between the cortex and the medulla must be maintained, to make sure that the urine is, where necessary, more concentrated than the blood, and, in humans, up to five times as concentrated as the blood.

Mechanism: Counter-current multiplier that facilitates counter-current exchange, using the principle of exchange between two fluids travelling in opposite directions in two systems, a process that occurs throughout the regions of the loop. At every level of the loop, the contents of the ascending loop are slightly less concentrated than the contents at the corresponding position in the descending loop.
The descending loop is fully permeable to water and most mineral ions. Because of the large concentration of mineral ions in the interstitual fluid (fluid between the cells of the medula), water travels out of the loop into the interstitual fluid by osmosis, and mineral ions, such as sodium and chloride, diffuse into the loop.
The vasa recta supplies the metabolically active cells with oxygen and rids them of CO2, and its descending part is saltier than its ascending part, since the blood travels in the opposite direction to the fluid in the loop. Therefore, the cells of the loop are serviced without the removal of the buildup of salts in the medulla.
At the hairpin bend, in the pyramid region of the medulla consisting largely of collecting ducts, the concentration of mineral ions in the loop has increased to the point at which mineral ions diffuse out of the loop into the medulla, because of ions diffusing in and water diffusing out. Therefore, in the medulla, the concentration of salts is the greatest in and around the hairpin bend, as sodium and chloride ions diffuse out, so that water can enter by osmosis from the water collecting ducts as necessary.
The ascending loop is permeable to mineral ions, but, unusually, not to water. Salts may continue to diffuse out until the solute potential between the loop and the fluid between the cells of the medulla is equal. Then mineral ions are removed from the loop by active transport.

A Level Biology: Homeostasis

Role in excretion: Ultrafiltration.

Anatomy and mechanisms:
Afferent arterioles (wide) supply the glomurulus, and efferent arterioles (narrow) carry the blood away from the glomurulus. Therefore, a high hydrostatic (blood) pressure potential is created within the glomurulus, that is higher than its water potential (the adherence of water molecules toward each other, decreased by a greater solute potential. Philosophical note: By this you by your sheer numbers will turn the world upside down, if you have love for one another. In life, adherence, in effect, creates more.) Therefore, many of the water molecules, glucose molecules, amino acids and urea molecules, and some of the smaller soluble proteins (but none of the larger blood proteins because they are too big) are forced out through the pores in the endothelium of the capillaries, through the basement membrane (that inhibits most proteins trying to pass through it), and past the extensions of the podocytes (epithelial cells of the renal capsule with foot-like extensions that adhere close to the capilaries, forming a network) through the spaces between the podocytes into the lumen of the renal capsule.

How the fluid in the renal capsule differs from tissue fluid:
Proteins are largely (yet not completely) absent.

A Level Biology: Homeostasis

Role in excretion: Ultrafiltration.

Anatomy and mechanisms:
Afferent arterioles (wide) supply the glomurulus, and efferent arterioles (narrow) carry the blood away from the glomurulus. Therefore, a high hydrostatic (blood) pressure potential is created within the glomurulus, that is higher than its water potential (the adherence of water molecules toward each other, decreased by a greater solute potential. Philosophical note: By this you by your sheer numbers will turn the world upside down, if you have love for one another. In life, adherence, in effect, creates more.) Therefore, many of the water molecules, glucose molecules, amino acids and urea molecules, and some of the smaller soluble proteins (but none of the larger blood proteins because they are too big) are forced out through the pores in the endothelium of the capillaries, through the basement membrane (that inhibits most proteins trying to pass through it), and past the extensions of the podocytes (epithelial cells of the renal capsule with foot-like extensions that adhere close to the capilaries, forming a network) through the spaces between the podocytes into the lumen of the renal capsule.

How the fluid in the renal capsule differs from tissue fluid:
Proteins are largely (yet not completely) absent.

A Level Biology: Homeostasis

Role: Selective reabsorption.

Anatomy:
The longest section of the nephron, one cell thick, whose cells are packed full of mitohondria for active transport. The cell surface membranes in contact with the filtrate are covered with a “brush border” of microvilli, increasing the surface area for reabsorption by simple diffusion, facillitated diffusion, osmosis, active transport and bulk transport.

Mechanisms:
Co-transport: A kind of active transport, where a proton pump pumps protons out of the cell to which the molecules must be pumped, creating a potential difference in the form of a proton gradient. Protons therefore diffuse down their concentration gradient back into the cell through the co-transporter pump, taking sugars, amino acids or mineral ions with them.

Facilitated diffusion:
Mineral ions.

Simple diffusion:
Urea.
In response, harmful substances are pumped back into the nephron via active transport.

Osmosis: Water.

Pinocytosis (uptake of a droplet of liquid into a cell involving the invagination of the plasma membrane).
The few proteins that made it into the nephron.

Ion exchange, for example exchange of a sodium ion for a proton:
Mineral ions.

A Level Biology

A single factor that slows down an entire process, since a process is limited by its slowest step. The factor slows down the entire process because, although the process is determined by more factors than just this one, the factor that limits the process has the lowest values.

Limiting factors are taken into consideration in intensive agriculture in the following ways, to increase the rate of photosynthesis, and therefore increase crop yields, usually in greenhouses where conditions can be more easily controlled and monitored than in the natural environment:


Sensors in greenhouses, linked to computers, detect humidity, light intensity, and carbon dioxide concentration (not temperature), and control optimum conditions.


Light intensity: Banks of lamps radiating photosynthetic wavelengths are used.
Temperature: Ambient temperatures are moderately increased.

Carbon dioxide concentration: In most greenhouses, carbon dioxide concentrations are increased from 340 to 1000 parts per million. This needs to be monitored with infra-red detectors linked to computers to maintain pre-set, minimum and maximum levels of carbon dioxide concentration. Therefore, the extensive benefits are offset to some extent by the costs involved, so the crops need to be marketed at a premium price in order to be profitable.


The effect of limiting factors can be measured using an aquatic plant such as Elodea or Cabomba in photosynthometer.

A Level Biology

Whenever the stomata are open, the proportions of oxygen and carbon dioxide that diffuse into the stomata are always proportional to the proportions of these gases in the atmosphere, so the gases that diffuse into the open spaces between mesophyll cells is a control variable in photosynthesis.

When there is no light, photosynthesis cannot take place at all, but respiration occurs, using oxygen (importing oxygen, therefore decreasing oxygen concentrations within the leaf) to produce carbon dioxide (exporting carbon dioxide, therefore increasing carbon dioxide concentrations within the leaf).

When light begins to shine on green plants, for example at dawn, under low levels of light, the oxygen evolved is directly propotional to the levels of light. Therefore, as the earth turns, oxygen levels increase to the point at which all the carbon dioxide produced in respiration is used in photosynthesis, and all the oxygen produced in photosynthesis is used in respiration. The leaf is neither importing (using a net amount of) or exporting (evolving a net amount of) oxygen or carbon dioxide, so it is in compensation mode.

As light intensity increases even more, photosynthesis takes place more than respiration, so that more oxygen is evolved by photosynthesis than is used in respiration (oxygen is exported, so oxygen levels rise). At some point, as light intensity increases, photosynthesis can no longer increase, due to other limiting factors such as water (for the light-dependent stage), carbon dioxide concentrations (fixed in the light-independent stage to give glucose and oxygen), and number of chloroplasts that contain chlorophyll that reduces the activation energy required for photosynthesis.

A Level Biology

At low light intensities, temperature has little to no effect on the rate of photosynthesis.

At high light intensities, temperature is directly proportional to the rate of photosynthesis.

These facts were used to conclude that photosynthesis has two distinct phases, later discovered by biochemical analysis, eg. of chloroplasts, that the light-dependent stage, just like all other photochemical reactions, is unaffected by temperature and the light-independent stage, just like all other biochemical enzyme-catalysed reactions, is directly influenced by temperature.

A Level Biology: Coordination

A process composed of two cycles, which take about 28 days combined:
The ovarian cycle, concerned with the production and release of a mature egg.
The uterine cycle, concerned with the buildup, thickening, maintenance by continued buildup, and breakdown, of the endometrium.


Day 1-5: Menstruation

As a result of the decrease of progesterone and oestrogen, the endometrium is broken down and removed from the body (uterine cycle).
As a result of the decrease of progesterone, the production of FSH (follicle-stimulating hormone) from the pituitary gland is stimulated.
FSH stimulates the development of secondary follicles and the Graafian follicle in the ovaries (ovarian cycle).
Immature eggs begin to grow in the secondary follicles (ovarian cycle).


Day 6-11: Oestrogen
One of the immature eggs matures in the Graafian follicle to form a secondary oocyte.
Follicles release oestrogen, that causes negative feedback by inhibiting the release of FSH by the pituitary gland, and causes the thickening of the endometrium.


Day 12-18: Ovulation
Oestrogen levels in the blood rise to the point at which they stimulate a surge in the production of LH, and a slight surge in the production of FSH (positive feedback). LH causes ovulation (ovarian cycle).


Day 18-24: Corpus luteum
LH causes the Graafian follicle, now empty, to change into a temporary gland called a corpus luteum, that secretes progesterone that inhibits the production of LH (negative feedback), and small amounts of oestrogen. Progesterone maintains the endometrium by continuing to build it up (uterine cycle).


Day 25-28: Breakdown of corpus luteum
Now that the levels of LH and FSH are low, there is not enough to maintain the corpus luteum, so it breaks down, subsequently decreasing the concentrations of progesterone and oestrogen in the blood (ovarian cycle).

A Level Biology

Interconnected network of chemical reactions that occur in living cells, mostly catalysed by enzymes.

A Level Biology

Double outer membrane: Permeable to pyruvate, oxygen, carbon dioxide, reduced NAD and oxidised NAD.

Inner membrane:
Has cristae (folds into the matrix) for a greater surface area, so that more ATP synthesis in ATP synthase can take place within the membrane.

A Level Biology: Plant Coordination

Since most plants are firmly anchored into the ground and cannot move from place to place, even under unfavourable conditions, their responses may not be as evident as those of animals, but they are no less important to the plant. Most responses occur as growth responses, for example:


Response of a plant to light:
Under dark conditions: Bolted, long stem, small leaves.
Under bright conditions: Dark green, short and stury.


Abscission of a leaf:
The auxin I.A.A. is synthesised at the tips of the stem and leaves, and are accessed by the leaf at its vertex and from the junction between the leaf and the stem, that prevent abscission. When I.A.A. synthesis slows down and stops, ethanol in synthesised in the leaf, counteracting the effect of I.A.A. and creating an abscission layer. Abscission slows down and stops.


Response of the plant Mimosa pudica to touch (Tip: Mimosa pudding hits the spot.)


Rapid closure of the leaves of the carnivorous plant Drosera roduntiflora. (Tip: Some people think that the leaves of the flora are redundant because they don't photosynthesise, but that is most likely not true.)

A Level Biology: Homeostasis

The system by which regulators maintain homeostasis by counteracting changes in the internal environment.


Process:
Input: There may or may not be a change to the system.Detector cells measure the level of a variable, and communicate the level of the variable to the control centre via the nerves.
The control centre measures the variable against the set variable. If there is a difference between the measured variable and the set variable, the control centre communicates with an effector organ via the nerves to counteract the change in an attempt to move the variable back to set conditions.

When the receptor cells detect that the change has been reversed, the control centre tells the effector to switch off its response. The variable will have been changed slightly in the opposite direction, so then the negative feedback response will be in the opposite direction. This process continues, maintaining internal conditions within narrow limits instead of being completely constant, because, as efficient as the system of negative feedback is, it is impossible to avoid some level of overshoot (overcompensation) when trying to bring a variable back to set conditions.


Anatomy of negative feedback in mammals:
Receptor: Specialised cells in organs such as the brain and pancreas.
Control centre: Brain.
Effector: Organs such as the skin, kidneys and liver.

A Level Biology: Coordination

Due to the deporalisation of an excitable cell (for example a nerve cell, used in this definition), nerve impulse travels along an axon, not like a current flowing, but due to the rapid reversal of electric potential of its cells by the rapid exchange of potassium and sodium ions, unlike the rest potential that is defined by the electric potential remaining the same (about negative 70 millivolts).

A Level Biology: Coordination

During the rest potential, the voltage gated carriers are closed.
1) Electrical energy is transferred from a receptor cell or a sensitive nerve ending to the body of a neurone, where it is used to open sodium voltage gated channels (globular carrier-proteins that span the width of the cell surface membrane), allowing sodium ions to rapidly diffuse into the cytoplasm, progressively increasing its potential from negative 70 millivolts to positive 40 millivolts. The potential has been reversed.
2) The moment the sodium voltage channels close, the potassium voltage gated channels open, allowing the potassium ions that have diffused in to rapidly diffuse out, rapidly and progressively decreasing the potential of the cytoplasm, but not all the way back to negative 70 millivolts, because the potassium ion gradient between the inside and outside of the nerve cell is not that great, since some of the potassium ions, during the resting potential, diffused out of the cell because of how permeable the cell membrane is to potassium ions.
3) The action potential will be transmitted along the length of the nerve cell.
This is an example of positive feedback.
4) The potassium-sodium pump immediately begins to simultaneously pump sodium ions out and potassium ions into the cell, bringing the nerve cell back to its rest potential.

A Level Biology: Coordination

Action potentials have a standard amplitude, but their frequency is determined by the intensity of the input energy (light, pressure, temperature or chemicals) and detected by the brain. For example, the brain detects the difference between a light touch on your hand, and your hand being crushed to splinters in a motor, even though both are mechanical pressure. Pain, therefore, is necessary, because it is an indication when the frequency of impulses is too much (nearly or at the maximum possible frequency, constricted because of the refractory period in which the neurone needs to be taken back to a resting potential so that sodium ions will be able to diffuse in again, because diffusion of potassium ions out will not take it back to a rest potential of negative 70 millivolts).


Sometimes, the impulse is so small that an action potential will not be created. In that case, sodium ions diffuse in, potassium ions diffuse out, and rest potential is set up again, without the impulse being transmitted along the dendrites and axon. It is relatively insignificant impulses that do not make it across the synapse to the next neurone.

A Level Biology: Coordination

The parts of the axon and dendrites within the myelin sheath (usually found in the peripheral nervous system) are insulated from changes in electrical potential. At the nodes (1 to 2 millimetres apart), however, entrance of sodium ions through sodium channels and exit of potassium ions through potassium channels occur alternatively (when the sodium channel at one node closes, the potassium channel at the next opens). Therefore, the electrical potential travels along a neurone by jumping from one node to the next, which is an advantage, because this systems allows for faster transmission of the impulse than is possible with non-myelinated neurones. Most neurones in vertebrates are myelinated.

A Level Biology: Coordination

Action potential flows steadily all along the non-myelinated dendrites and axon of the neurone. Rate of flow of action potential can be greatly increased by increasing the width of the axon and dendrites, such as in invertebrates (animals in which non-myelinated neurones occur) such as squid and earthworms, in which the passage of action potential was first studied, but rate of flow of action potential will still not be as great as in myelinated neurones.

A Level Biology: Coordination

The property of a sense cell is to transfer the energy of a stimulus (mechanical, chemical, thermal or photic energy) into the electrochemical energy of an impulse.

A capsule found in joints, composed of layers of fibres and flattened cells surrounding nerve endings, sensitive to pressure.

How it works:
When the neuron is at rest and localised pressure is exerted on the Pacinian corpuscle:
The collagen forming the capsule is deformed.
Membrane around nerve endings becomes more permeable (temporarily), allowing sodium ions to diffuse into the neuron, depolarising it and positivising its voltage. The greater the pressure, the greater the effect. Once the potential difference reaches a certain threshold level, an action potential is created in the axon, transmitting the nerve signal along to the next neuron.

A Level Biology: Coordination

Organised paths among the neurones that coordinate reflex responses of muscles or glands. Mechanism:

1) Receptor detects some form of mechanical, chemical, thermal or light energy being applied to it, creating an action potential in the receptor.
2) Impulse travels along sensory neuron, to interneuron(s), to motor neuron.
3) Impulse from motor neuron stimulates a response in the effector, that may be a muscle or a gland.

A Level Biology: Coordination

The only time that an organism is completely at rest is when it is dead. Even when an animal is asleep, its internal organs are still working, like the sea, never stopping, and its brain is consolidating all the memories taken in during the waking hours. Similarly, when the sensory neurones are not actively transmitting signals, their state is called “at rest”, but they are not truly resting, because they are actively setting up and maintaining a potential difference between the interior and the exterior of the cell, by the following mechanisms:

1) Active transport via sodium-potassium pumps:
Potassium ions are actively transported into the cell.
Sodium ions are actively transported out of the cell, gradually causing the neuron and its immediate surroundings to be equally positively charged.
2) Facilitated diffusion:
The membrane is way more permeable to potassium ions than to sodium ions. Therefore, potassium ions diffuse out of the cell, and much fewer sodium ions diffuse back in, causing a difference in potential between the nerve cell (more negative) and its immediate surroundings (more positive).

The rest potential was discovered by inserting microelectrodes into the inside and outside of an isolated neuron, amplifying the signal and displaying it on a cathode-ray-oscilloscope.

A Level Biology: Coordination

Structure of any nerve cell (neuron):
Cell body consisting of the nucleus and the majority of the cytoplasm.
Cytoplasmic processes that are often very long, that are often protected by a myelin sheath composed of glia cells such as Schwann cells wrapped around the axon many times, therefore consisting mainly of lipid, and some protein.
The neurons are adapted for the rapid and specific coordination of muscles and glands, with impulses travelling at speeds of 30 to 120 metres per second.

Motor neuron:
Dendrites (long progressions that recieve signals from the central nervous system (brain or spinal cord) and transmit them to the cell body).
Cell body in the central nervous system.
Axon (often myelinated, that transmits impulses away from the cell body).

Interneuron:
Consists of many short fibres that are extensions of the nerve cell, and the cell body.
Role: Transmit impulses between nerve cells. Does not have dendrites, myelinated dendrons or an axon.

Sensory neuron:
Dendrites transmit signals from the sensory cell to the often-myelinated dendron or dendrons.
Dendron transmits signals to the cell body.
Cell body.
Axon transmits signals away from the cell body.

A Level Biology: Coordination

Structure (what):

Region on pre-synaptic neurone called the synaptic knob, containing a Golgi body that synthesises the neurotransmitter substance (common ones include acetylcholine used by cholinergic neurones, and noradrenaline released by adrenergic neurones, and common ones in brain include dopamine and glutamic) and vesicles in which the neurotransmitter substance is temporarily stored.
Synaptic cleft (about 20 nanometers).
Region of dendrite or cell body on post-synaptic neurone, containing receptor molecules for the specific neurotransmitter substance that form sodium channels for entrance of sodium ions, potassium channels for subsequent exit of potassium ions, and other ion channels.


Rationale (why):
An action potential cannot be directly transmitted across a synapse, so it must be transmitted via a neurotransmitter substance that triggers an action potential in the post-synaptic neurone. The fact that neurones do not touch, but that there is a space between them, slightly slows down the passage of the impulse, but this disadvantage is offset by the following advantages of synapses:
Factors out small impulses of little importance.
Prevention of synaptic fatigue: If the continuous passage of impulses takes place faster than the regeneration of the neurotransmitter substance, no impulses will be able to travel for a time. Synapses prevent this from happening.
Innovation: Allows for variation in the response of the central nervous system, particularly of the brain.
Consolidation: Sometimes impulses from different neurones, and even from different types of neurones such as both exciting and inhibiting neurones, enter a single synapse, so the synapse consolidates the impulses to ultimately create the appropriate response.


Philosophical note: Synapses are like doubt. They break traction, and run the risk of you changing your path, and hence are regarded as dangerous by those who regard themselves on the absolute, irrefutably right path, but they have the advantage of helping one to confirm what information is really necessary, allow for a moment of silence and reflection to prevent burnout, helps you to think up ways to be more innovative in your journey, and gives you the opportunity to consolidate what you know so that you can assess whether the action you are taking can be improved. Too much doubt, however, will slow down your action too much. No doubt, however, can allow you to run out of control.


Mechanism (how) of cholinergic synapses: Tip: First part of neurotransmitter PLUS Last part of allergic neurone.
1) Action potential in pre-synaptic neurone causes calcium channels to open, and calcium ions diffuse from the synaptic cleft into the synaptic knot.
2) Calcium ions in synaptic knot cause vesicles that contain neurotransmitter substance to fuse with the cell surface membrane, releasing their contents in the synaptic cleft.
3) Neurotransmitter diffuses quickly across the synaptic cleft into the receptor molecules for that neurotransmitter specifically, immediately triggering the opening of the sodium channels. Sodium ions diffuse into the post-synaptic neurone, progressively but quickly setting up an action potential (called facilitation). In a myelinated neurone, sodium channels then close, simultaneously with the opening of the potassium channels in the next node for the diffusion of potassium ions out of the neurone.
4) Neurotransmitter is broken down by an enzyme, after which it leaves the receptor molecule and diffuses back into the synaptic knob, where it is regenerated into the neurotransmitter using energy from ATP.
5) The moment the neurotransmitter leaves the receptor molecules, the sodium channels close, and the post-synaptic neurone will begin setting up a rest potential by the active transport of sodium ions out of the cell, and of potassium ions out of the cell, and the subsequent diffusion of potassium ions out of a cell, to set up and maintain a rest potential of negative 70 millivolts.

A Level Biology: Coordination

The level below which a stimulus will not create an action potential in a neurone.

A Level Biology

1) Building blocks of nucleaic acids.

2) Metabolic reactions, for example:

A) Adenoside triphosphate.

B) NAD (Nicotinamide adenine dinucleotide)

Structure: Two nucleotides joined by two phosphate molecules. In one of the nucleotides, the organic base is replaced by nicotinamide, a molecule that comes from nicotinic acid derived from a vitamin of the B complex, whose significance lies in the fact that it can accept both hydrogen ions (protons and electrons) to become reduced, but can give up electrons easily to oxidising agents. This flow of electrons is important in certain metabolic reactions.

Mode of function (summary):
Functions as a coenzyme, working with different kinds of enzymes, as well as fulfilling other roles in cells. Works in redox reactions.
NAD+ (part of molecule that is recognised by molecules with which NAD is a coenzyme) + 2H+ + 2e-
= NADH + H+

C) FAD (Flavine adenine dinucleotide):

Structure: Derived from vitamin B2. Is a hydrogen carrying molecule involved in oxidation and reduction reactions, including the Krebs cycle. Occurs tightly associated with a protein, forming a complex known as flavoprotein.

Function:
Functions as a coenzyme.

D) Coenzyme A (CoA):



Structure: A vitamin + Nucleoside (adenine + ribose)


Tip to remember the composition of a nucleoside: Replace the T with an S, add a pyrine base (adenine) and the RNA 5-carbon sugar (ribose).

Function: Assists in the removal of acetyl units from glucose and lipids, for example during aerobic respiration, and transporting the acetyl units to be used in the Krebs cycle.

A Level Biology

Any substance used or required by an organism as food.

A Level Biology: Coordination

Hypothalamus secretes a hormone that instructs the pituitary gland to secrete follicle-stimulating hormone (FSH) and luitenizing hormone (LH) for the first time (so named because they were first studied in biological females, but occur in both biological males and biological females. These hormones stimulate the ovaries and/or testes to produce oestrogen and testosterone, that stimulate target organs to develop primary and secondary sexual characteristics, as the body is prepared to take part in sexual reproduction.

A Level Biology: Z notes

An amount of oxygen that needs to be taken in, above the amount that is needed under resting conditions, just after exercise, to repay the oxygen deficit, that is needed for the following processes:

Lactic acid is converted to:
1) Pyruvate in the liver (making anaerobic respiration in animals a reversable process).
Tip: Lactic acid takes one step back.
2) Glycogen in the liver.
Tip: Lactic acid takes so many steps back that it may or may not ever have been there in the first place.
3) Carbon dioxide and water by oxidation.
Tip: Lactic acid can get the destination without most of the journey. Do you still call that freaken inefficient?
Yes, because it never produces 32 ATPs.

A Level Biology: Z notes

Turgid central vacuole pushes cytoplasm tightly against cell wall (also decreases the distance to diffuse through).
Large cells are arranged in a way for efficient interception and absorption of light.
Chloroplasts can move closer to the surface of the cell to absorb more light, or further away to avoid damage.

A Level Biology: Z notes

Short distance to diffuse through:
Thin cell walls.
Large vacuole helps push chloroplasts towards edge of cell.


Large distance to diffuse across:
Large surface area.


Unknown benefit:
Moist cell surfaces (possibly so that gases can still diffuse in solution for some reason).

A Level Biology

Photosynthesis evolved when the ratio of carbon dioxide to oxygen in the air was much greater than it is now, and is the reason why oxygen levels increased to make up 21% of the atmosphere.
Regardless, rubisco still functions normally, especially in temperate regions. In tropical regions where light intensity and temperatures are higher than in other parts of the world, light is no longer a limiting factor, but carbon dioxide is, to the point at which oxygen becomes a competitive inhibitor of rubisco. When that happens, ribulose biphosphate (a 5-carbon sugar) is broken down to a 2-carbon sugar with little use and a single molecule of glycerate biphosphate. This process, called photorespiration, reduces the amount of sugars made by photosynthesis.
Tropical plants, therefore, have adopted a measure to significantly reduce the levels of photorespiration, by transporting a higher concentration of carbon dioxide to the chloroplasts.

A Level Biology

Processes of photolysis, oxygen evolution, ATP synthesis and NADP reduction occur in photosystems 2 and 1 in which processes occur simultaneously, steps of which may repeat for as long as conditions remain favourable (a level of light intensity, and supply of water, as well internal conditions such as pigments, enzymes, electron carrier proteins and NADP):


1) Photoactivation (remember this word!):
Light energy is transferred to the special chlorophyll molecule that is the reaction centre in photosystems 2 and 1.

2) Two specific ground electrons in photosystems 2 and 1 become excited, and subsequently are transferred to electron carrier molecules, eg. proteins.

3) Photolysis and oxygen evolution:
Photosystem 2:
Water molecules are split by enzymes into two ground electrons to fill the space left by the two electrons that entered the electron carriers, two protons (possibly to be pumped into the thylakoid space later) and an oxygen atom, that bonds with another oxygen atom from an oxygen molecule to be transferred into the environment.
Photosystem 1:
Two ground electrons from photosystem 2 fill the space left by the two electrons.

Oxygen diffuses out of the grana out of the chloroplast out of the plant cell out of the air spaces between the cells out of the leaf or stem into the environment.

Protons build up, in preparation for the next step.

5) Photosystem 2:
Electrons move along the electron transport chain, transferring their energy for non-cyclic photophosphorilation that occurs by chemiosmosis (energy is used to pump protons into the thylakoid space, creating a proton gradient across the thylakoid membrane, after which protons will diffuse down their proton gradient out of the thylakoid space through ATPase, providing energy for the synthesis of ATP), progressively falling back down to the ground state.

Photosystem 1:
Electrons move along their electron transport chain, combining with protons to form hydrogen atoms, and reducing NADP to NADPH.

A Level Biology

Summary: Carbon dioxide is fixed to form sugars, starch, lipids, waxes, hormones, growth factors, vitamins, chlorophyll and other wall-building molecules, and proteins such as enzymes and cytoplasm.

Disclaimer:
Depends on heat and the products of the light-dependent stage (ATP, also from respiration), and the enzymes are activated by light. It is independent from light, however, regarding the source of energy, that is ATP generated with energy from light, rather than light energy directly.

Step 1: Carbon dioxide is combined with ribulose biphosphate (RuBH) (a five-carbon sugar) IN THE PRESENCE OF RIBULOSE BIPHOSPHATE CARBOXYLASE (RUBISCO) to give an unstable 6-carbon sugar.

The unstable 6-carbon sugar is split into two molecules of glycerate-3-phosphate (GP, sometimes called PGA).

Source of reactants:Carbon dioxide diffuses readily into the chloroplasts.

Rubisco is abundant in the stroma, is the most abundant protein in green plants, and is the most abundant enzyme in the living world.
Ribulose biphosphate is synthesised from trioses (5 out of the each 6 trioses makes two molecules of ribulose biphosphate).
Step 2: Glycerate-3-phosphate PLUS Reduced nicotinamide adenine dinucleotide phosphate EQUALS Triose phosphate PLUS Nicotinamide adenine dinucleotide phosphate.

Energy: ATP synthesised by electrical potential energy from light energy.

Glycolase-3-phosphate is an organic acid, because it is not at the energy level of a sugar, but triose phosphate is a high-energy sugar.

Step 3 (Don't forget the ATP): For every six triose phosphate molecules synthesised, five are converted into 3 acceptor molecules (ribulase phosphate) using energy from ATP, and one is used to synthesise sugars, starch, lipids, waxes, amino acids for proteins such as enzymes and cytoplasm (with ions), vitamins, hormones and growth factors using energy from ATP. Both acceptor regeneration and the synthesis of organic molecules require energy from ATP. Sugars (sometimes later converted to starch for storage), lipids and amino acids are translocated from source to sink.

A Level Biology

Suspend the freshly cut shoot of aquatic green pondweed upside down in a very dilute solution of sodium hydrogencarbonate that provides carbon dioxide in solution in the form of hydrogencarbonate, in the microburette/photosynthometer.

The plant will produce a vigorous stream of bubbles from the plant base, showing that photosynthesis is taking place. The bubbles are collected in the tube, and their length is measured in the capillary tube to find the volume of oxygen evolved in unit time.

This apparatus has been used to measure the effect of limiting factors such as light intensity, temperature and carbon dioxide on the rate of photosynthesis.

This experiment can be simulated on a computer.

Philosophical note: When is a dilute input enough to produce a vigorous output? Maybe it is dilute in comparison with, not the output evolution, but what it could have been ...

A Level Biology

The two processes that occur simultaneously:

1) Conversion of inorganic carbon (carbon-containing molecules that do not contain hydrogen, mainly carbon dioxide and carbonates) to organic compounds (carbon-containing molecules that contain carbon and hydrogen, of which there are at least 2.5 known, more than the total number of known compounds made from all other elements, the diversity of which makes possible the diversity of life on earth).
Carbon dioxide results in glucose.

2) Light energy trapped by chlorophyll during the light-dependent stage is converted to chemical energy during the light-independent stage.
BECAUSE
Photosynthesis is a redox reaction, most clearly in terms of hydrogen transfer that works similar to electron transfer:
Carbon dioxide is reduced to glucose.
Water is oxidised to oxygen.
Where reduction occurs, energy is absorbed.

The two interconnected stages discovered by investigations by biochemists:

1) Light-dependent stage: Everything in preparation for the light-independent stage.

A photochemical reaction, so, like all photochemical reactions, it is largely independent of temperature, but it is dependent on light.
Process:
Water molecules are split (photolysis)
Reduced hydrogen receptor (reduced NADP, a molecule found in plant cells) for reducing power is formed by the removal of hydrogen from nicotinamide adenine dinucleotide phosphate (as in NAD, the nicotinamide molecule that replaces the second sugar is the action centre where oxidation, reduction and electron transport takes place).
Simultaneously, Energy is transferred to 2 ATPs.
Oxygen is released as a waste product.

2) Light independent stage: The whole point of photosynthesis with immediate results for the plant in the first place.

A biochemical reaction, so it is catalysed by enzymes and dependent on temperature, reducing power and ATP, but is it independent of light.

Results: Using the energy from the 2 ATPs synthesised during the light-independent stage, carbon dioxide is fixed to form trioses that are used to form hexoses.

A Level Biology

Several hundreds of photosynthetic and accessory pigments arranged in the thylakoid membranes of the grana, that all absorb light of different wavelengths and channel the energy to a single molecule of chlorophyll called the reaction centre. There are two different types of photosystems with different functions that occur grouped together in the thylakoid membranes along with proteins that often function as electron carriers.

Photosystem 1 (P700): Reaction centre absorbs light of wavelength 700 nanometres only.
Photosystem 2 (P680): Reaction centre absorbs light of wavelength 680 nanometres only.

A Level Biology: Plant Coordination

The five groups of hormones that are the main regulatory substances in plants, with the following properties:

Synthesis: Synthesised in a variety of tissues rather than in discreet endocrine glands.
Transport: Transported from their region of synthesis (not all are) by diffusion or active transport from cell to cell, or in the phloem.
Concentration: Are at very low concentrations, so it is often difficult to determine their precise role.
Interactions: Some hormones enhance each others' effects (synergonism), but other hormones counteract each others' effects (antagonism, for example I.A.A. and ethanol).
Effect: Effect can differ, sometimes vastly, depending on concentration, which tissue it is found is, and the developmental stage of the tissue it is found in.

Examples:

1) Auxins, mainly I.I.A.

Discovery: First by Charles Darwin, while investigating the curvature of coleoptiles towards a unilateral light source, and later by others, including the devising of a biological assay to determine its concentration (because its concentrations are so low).

Roles:
Promotes extension growth of stems and roots.
Prevents abscission of leaves.
Promotes fruit growth.
Dominance of plant buds.

Synthesis:
At the tips of stems and roots, and in green leaves.

Gibberelins (group of different types of gibberelinic acid):

Roles:
Promotes extension growth of stems (not roots).
Inhibits lateral root production from the pericycle.
Delays (not prevents) abscission of leaves.
Promotes germination of seeds.

Synthesis: In germinating seeds.


Abscissic acid (a stress hormone):


Roles:
Closure of stomata when water is scarce.
Abscission of leaves.

A Level Command Words

State what might happen based on available information.

A Level Biology: Coordination

1) The neurotransmitter molecules from the motor neuron quickly diffuse across the synapse and binds to the receptor molecules in the sarcoplasmic membrane of the muscle fibre.
2) The signal triggers the release of calcium ions, not necessarily from the synapse into the post-synaptic cell, but from the sarcoplasmic reticulum into the cytosol.
3) Calcium ions react with the protein troponin, that, now activated, removes the blocking protein tropomyosin from the binding sites in the actin filament.
4) The charged bulbous heads (containing ADP and inorganic phosphate) of the myosin filaments fits into the binding site in the actin filaments.
5) The inorganic phosphate molecule leaves the bulbous head, after which the ADP does.
6) Power stroke: The myosin head moves into an angle of about 45 degrees as it moves the myosin and actin filaments in opposite directions. The myofibril is contracting as the myosin and actin filaments slide over each other.
7) A molecule of ATP from respiration attaches to the bulbous head, containing ATP-ase that breaks down ATP to form ADP and inorganic phosphate, causing the bulbous head to become charged again.
8) The charged bulbous head detaches from the binding site of the actin filament.
9) If the signals for contraction continue to come, steps 4 to 9 repeat. If no more signals come, calcium moves back into the sarcoplasmic reticulum, tropomyosin blocks the binding sites of actin again, and the sarcomere relaxes and returns back to its original length.

A Level Biology

Synthesis of large molecules from smaller ones.
Movement.
Active transport.
Etc.

A Level Biology

Breaking down of a larger molecule into smaller ones.

A Level Biology

Measured in joules by a calorimeter:

Carbs: 1600-1760 kilojoules per 100g.
Lipids: 3700-4000 kilojoules per 100g.
Proteins: 1700-1720 kilojoules per 100g.

Why lipids and proteins have a high relative energy value compared to carbohydrates:
Most ATP is released during oxidative phosphorilation, when reduced NAD is oxidised and water molecules are synthesised. Therefore, the more reduced NAD there is, the more ATP will be generated. Therefore, the more hydrogen formed part of the initial organic molecule, the more ATP will be generated. (The primary function of proteins is as a building block in organisms, but they can also be deaminated for safe respiration when carbs and fats are scarce).

A Level Biology

Proportion of carbon dioxide produced to oxygen taken in, by a given organism, in a given time.
RQ = Carbon dioxide produced DIVIDED BY Oxygen taken in.
The RQ of a substance can be used to determine what kind of substance it is.
RQ of carbohydrates: 1.
RQ of lipids: 0.7.
RQ of proteins and amino acids: 0.9.
The RQ of a substance can also be used to determine whether or not it is respiring anaerobically. Under anaerobic respiration that takes place in plants, the amount of carbon dioxide produced will be greater than the amount of oxygen taken in, so the RQ will be greater than 1. However, in muscles that are respiring anaerobically, the RQ will be undefined or 0, because no carbon dioxide is produced when muscle cells are respiring anaerobically. (Respiration is the production of ATP, and contraction is movement. Do not confuse the two, but remember that they are different yet interconnected life processes.)
Sometimes, the RQ may not be a precise number, possibly causes by different substances being respired at different rates (substance is not pure).

A Level Biology

Simple respirometer:

A sealed test tube connected to a manometer with a drop of coloured water, containing respiring organisms at the bottom, and soda lime in a perforated metal cage that absorbs carbon dioxide before its presence can be detected by an increase in pressure. Therefore, only the absorbing of oxygen can cause a change (drop) in pressure, detected by the movement of the drop of water. Change in volume of the apparatus can be estimated from measurements of the movement in the manometric fluid, using a stopwatch/stopclock to calculate time taken.

Problems with the simple respirameter: Change in temperature and atmospheric pressure can also cause the drop of water to move.

Differential respirometer:

The test tube, same as above, is joined by the manometer with a much longer columb of water, to a control tube, with identical contents (except that in the control tube, the respiring organisms are replaced with beads of equal size and mass), and connected to a calibrated syringe (showing volume of gas absorbed by unit time) to measure rate of respiration. Any pressure changes on both sides of the manometer will cancel each other out. When the absorbing of oxygen causes a drop of pressure in the test tube, the level of water on the side of the test tube moves now. Adjust the syringe until the water levels are equal again, then note the reading on the syringe.

Independant variable: Any 1 factor that influences the rate of respiration, for example, temperature.

Dependent variable: Oxygen absorbed by the organism.

According to the Z-notes, it seems like pressure is a control, and that the differential respirometer functions according to the direct proportion between mass and volume. However, how is the pressure kept the same? If the inverse proportion between pressure and volume is ised, that required mass to remain the same, but in many respirometers, carbon dioxide evolution leaves the system, so it is disregarded. In other respirometer, depending on the energy source being respired (its RQ), the change in mass could be due to how much more oxygen is absorbed than carbon dooxide is evolved. So, if mass is kept constant, the respironeter is redundant.

A Level Biology

Type of plant: Semi-aquatic Grass.

Type of crop: Likely cultivated for human use before wheat (first cultivated in south Asia thousands of years ago), making it most likely to be the crop that has been used by humans the most. Now, is the staple of 60% of the world’s population.

Type of soil: Waterlogged an anaerobic, with all the oxygen used up in the respiration of saprophytes.

Why this is strange: Plant roots need oxygen to undergo aerobic respiration to produce ATP for the active transport of ions from the soil into the root hairs, and for the synthesis and transport of amino acids around the plant.

Adaptations that enable rice to survive and thrive with its roots in anaerobic conditions:
1) Aerenchyma: Parenchyma (ground tissues) with large spaces between them through which oxygen diffuses, that enables the rice plants to undergo normal aerobic respiration.
2) Able to undergo anaerobic respiration when needed.
3) Contains enzymes that are able to respire ethanol, and has roots that are more tolerant to ethanol that other roots.
4) Submerged leaves have hydrophobic lower corrugated surfaces, creating pockets of air containing oxygen that can diffuse into the leaves.
5) When water levels rise, so do the levels of ethanol, that activate gibberelins that increase the internodal length, causing the plant to grow taller, lifting alot of its aerial tissues out of the water.

Rice makes sense. It knows how to work efficiently even under inefficient conditions, and, when it inevitably needs to be either inefficient or ineffective, it knows is some is better than nothing, so, not only does it produce something, but it also uses the results of its inefficiencies to enable it to continue.

Even the most inspiration plant on the earth has aspects that are afraid of its conditions. But not only does it channel its fear to its advantage, but it refuses to grow where it is planted. Plants are known to be stationary organisms, but rice, inspired by by its inefficiencies, rises above its circumstances.

A Level Biology: Homeostasis

Compounds such as cyclic AMP that are formed intracellularly, and, after the initial molecule has been stimulated by the stimulus external to the cell, activate specific cell responses.

A Level Command Word

Provide structured evidence leading to a given result.

Tip: The difference between justification and showing could be the difference between the calculated and spontaneous approaches to life - working with the facts to draw a conclusion in a structured, lengthy way, or stating the conclusion, and briefly stating the evidence afterwards.

A Level Command Words

Make a simple freehand drawing showing the key features.

A Level Biology

A plant that grows well in hot, semi-arid environments with high light-intensities where the typical annual rainfall is only 400 to 600 millimetres, way too dry for maize. It has become the fourth most important grain globally, after wheat, rice, maize and barley.


Properties:


Height: 0.5 to 5 metres according to variety
Morphology from top to bottom:
Cluster of many flowers.
Hinge cells in leaves enable the leaf to roll up when water is scarce, reducing evaporation.
Stem surrounded by circular leaf bases.
Leaf blade: Epidermis covered by layer of wax (xeromorphic feature).
Prop roots are strengthened by additional fibres.
Exceptionally extensive adventitious roots spread out just below the soil surface, tapping the soil to a depth of 1.8 metres and spreading out 1.5 metres wide (xeromorphic feature).
Immediately around the root tips, free-living nitrogen-fixing bacteria raise the nitrate and ammonia concentrations in the soil, that are taken up by the sorghum.

A Level Command Words

Express something in clear terms.

A Level Biology: Homeostasis

Two guard cells, connected to normal epidermal cells of the leaves or stem (concentrated at the underside of the leaf in broad leaves, but equally distributed across the leaf in pin-shaped leaves, for example in grasses) whose cell walls facing the external environment and cytosol are thick to prevent movement, but whose cell walls facing each other are thin to allow for movement, control the stoma, that is a pore in the epidermal cells of a plant that allow for contact between the air outside of the plant and the interconnected air spaces inside the plant.
How the stomata (plural for stoma) open:
Before sunrise, or maybe soon after in xeromorphic plants, potassium ions from the surrounding epidermal cells are transported into the central vacuoles of the guard cells (anions) and starch stored in the cytosol of the guard cells in broken down into organic acids (cations) and transported into the central vacuole, increasing the solute potential of the guard cells, thus causing water to enter the guard cells. The guard cells become turgid, therefore pushing against the surrounding epidermal cells due to their thick cell walls, and pulling away from each other due to their cell walls, creating a space between them.
How the stomata close:
1) At night:
Philosophical note to answer the crooked letter called Y.:
They have nothing to gain but everything to lose by remaining open, so they close. They only take risks when there is gain to make the loss worth it. But one thing that the guard cells cannot block out is light.
Mechanism (reversal of the mechanism to open stomata):
Potassium ions are transported out of the central vacuoles of the guard cells into the surrounding epidermal cells, and organic acids are transported out of the central vacuole into the cytoplasm, where they are converted to starch.

A Level Biology: Homeostasis

2. During slight wilting (water stress)
a. Stress-hormone ABA is released, that triggers the mass secretion of calcium ions into the guard cells, that directly triggers:
b. Exit of potassium ions and sodium ions out of the guard cells across the cell surface membrane, and the disallowing of potassium ions diffusing back into the guard cells, that directly triggers:
c. Water potential in the guard cells that is higher than that of the epidermal cells, that directly triggers:
d. Exit of water out of the guard cells by osmosis, resulting in their flaccidity, that directly triggers:
e. Closing of the stomata.

3. During severe wilting:
Cells contain a significantly lower amount of water, resulting in automatic flaccidity, and therefore automatic closing of the stomata.

Experimental evidence that opening and closing of stomata is directly determined by the turgidity of the guard cells:
When a guard cell in the position for open stomata was poked with a drainage tube, removing some of the cell sap in the central vacuole and therefore reducing the turgidity, half of the stomata immediately disappeared as the poked guard cell changed its position.

A Level Biology: Coordination

Structure (what):
From sarcomere to muscle (small to big):


Alternating thin filaments made of actin protein, light coloured, (about 7 nanometers in diameter), held in place with transverse Z. lines, and thick filaments made of myosin protein with bulbous heads, (about 15 nanometers in diameter), dark coloured, held in place by transverse M. lines, form a sarcomere. Many sarcomeres joined end-to-end form a myofibril. (Although the sarcomere is a unit of the myofibril, it is technically only a subdivision, as a myofibril is more like an unbroken unit of a light section, a dark section, and another light section (sarcomere), repeating.) A bunch of myofibrils are called a sarcolemma, found in muscle fibres with a sarcoplasm membrane enclosing the other components of a muscle fibre such as the:
Sarcoplasmic reticulum: Specialised ER (a network of membranous channels) formed by the infolding of the sarcoplasmic membrane, with a tubular shape, often called T-tubes, containing calcium ions, and forming a network on the myofibrils though which calcium ions pass into the sarcoplasm.
Sarcoplasm: Specialised cytoplasm.
Mitochondria sandwiched between the myofibrils.
Multiple nuclei.

These muscle fibres are arranged in bundles of thousands of alternating light and dark muscle fibres. The bundles form the striated muscles, joined together by connecting tissues and held in place to bones or other organs by tendons.


Mechanism of contraction (how):
The feature of myofibrils that enables the muscles to contract is that they can shorten to half or even a third of their relaxed length, because the heads of the thick lines and the thin lines can fit into each other at complementary points when the blocking protein is removed by calcium ions released from the specialised ER into the cytosol through the sarcoplasmic reticulum as a result of acetylcholine binding to receptor molecules in the sarcoplasmic membrane from a pre-synaptic motor neurone in the neuromuscular junction.
Steps and further explanations to follow.

A Level Biology

The interior of the chloroplast where the thylakoid membranes are more loosely arranged than in the grana, between the grana, the side of which ATP is synthesised, and the site where the light-independent phase of photosynthesis (Calvin cycle, the enzymes of which are activated by light) takes place.

The stroma recieve a constant supply of ATP and reduced NADP throughout the daylight hours.

A Level Biology

The molecule that takes part in the first step of a chemical reaction, forming a complex with an enzyme.

A Level Biology

Compounds:

1) with the general formula Cx(H2O)y where x. is approximately equal to y.

2) that contain an aldehyde or ketone group.

A Level Command Words

Apply knowledge and understanding to situations in order to make proposals, where there are a range of valid possibilities.

A Level Biology: Plant Coordination

Why:
In subtropical, wet regions, for example on the east coast of the US, decomposition is often slow, and decomposing organisms and organic matter are often washed away before their essential mineral ions can be taken up by the roots of plants. Therefore, plants such as the Venus flytrap, that obtain their essential mineral ions from small animals, thrive over plants that don’t in such environment.

How:
When a fly bends one of the side teeth of the leaves to a certain angle, the leaves snap shut, fast enough to trap the fly. The leaves then secrete enzymes that digest the fly, absorb the needed ions, and open up again to let the unneeded parts blow away. This is the fasted response known in Kingdom Animalia.

Hypotheses of mechanism:
A sudden change in osmotic concentrations.
Active transport.