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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.