AQA biology a-level paper 2 questions and answers graded A+.

AQA biology a-level paper 2 questions and answers graded A+. describe and explain the steps in the light dependent reaction of photosynthesis - correct answer.1. photoionisation: light reaches chlorophyll in PSII, which is absorbed by an electron, which becomes excited and moves to a higher energy level. 2. the electron passes to a carrier protein in the thylakoid membrane, and is passed down a series of carrier molecules called an electron transfer chain. 3. as the electron moves down, energy is lost from the electron and is released as ATP. 4. the loss of electron from PSII is 'refilled' by an electron produced by photolysis, which also produces hydrogen and oxygen. 5. the lost electron reaches PSI, which absorbs light energy and boosts another electron to a higher energy level (excitation). 6. this electron also goes down an electron transport chain. 7. this reaches the final electron acceptor which is a proton. they combine to form H and reduce NADP to NADPH. describe and explain the steps in the light independent reaction pf photosynthesis. - correct answer.1. CO2 diffuses into stroma and combines with ribulose bisphosphate (RuBP) using the enzyme rubisco. 2. this forms an unstable 6 carbon molecule, which splits into 2 3 carbon molecules, glyercate-3-phosphate (G3P) . 3. G3P is reduced by NADPH to triose-phosphate (TP), which is aided by ATP for energy. 4. TP can be converted into useful organic substances. 5. TP can also be reformed into RuBP using ATP. describe glycolysis in respiration. give net formation. - correct answer.1. glucose is converted into phosphorylated glucose by 2ATP. this makes it very reactive, so it splits into 2 triose phosphate (TP). 2. 2TP is then oxidised by 2NAD and 4 ATP is formed to form pyruvate. 3. NET: 2ATP, 2Pyruvate, 2NADH, 2H+ describe links reaction in respiration. give net formation. - correct answer.1. pyruvate diffuses into the matrix of mitochondria. 2. pyruvate is oxidised by NAD. CO2 is lost. this forms acetate. 3. acetate and co-enzyme A combine to form acetyl co-enzyme A. 4. NET: CO2, reduced NAD, acetyl co-enzyme A describe krebs cycle in respiration. give net formation. - correct answer.1. acetyl co-enzyme A combines with 4 carbon molecule (oxaloacetate) to form 6 carbon citric acid. 2. CO2 is lost (decarboxylation), molecule is oxidised by NAD and ATP is produce. this forms 5 carbon compound. 3. it is oxidised by 2NADH and FAD, and is decarboxylated. 4. this forms 4 carbon molecule again. describe oxidative phosphorylation in respiration. - correct answer.1. reduced coenzyme passes its H to a carrier protein in the ETC. this splits into a proton and electron. 2. the protons pass through the space between inner and outer mitochondrial membrane. 3. electrons pass through proteins on ETC. 4. protons return back via ATP synthase in the membrane, producing ATP. 5. the protons and electrons recombine to form H, which combines with O to form water. 6. oxygen is the last electron acceptor in the ETC. define biomass - correct total mass of organisms in a given area what is the 'gross primary production' - correct chemical energy stored in a plants biomass what is the 'net primary production' - correct chemical energy stores in a plants biomass after respiratory losses have been considered. this energy is available to consumers. how can you calculate the net primary production? - correct answer.NPP = GPP - R why is converting sunlight energy into biomass in producers inefficient? - correct light isn't the correct wavelength to be absorbed some light doesnt hit chloroplast some light is converted into heat energy some light energy is reflected describe the nitrogen cycle. - correct ion: atmospheric nitrogen can be fixed by rhizbium bacteria. if struck by lightning, it becomes reactive and combines with oxygen to form NO. ammonification: saprobionts feed on organic matter and release ammonia, which then forms ammonium ions in the soil. nitrification: nitrifying bacteria convert ammonium ions into nitrite ions and then to nirate ions. denitrification: anaerobic denitrifying bacteria convert soil nitrates into gaseous nitrogen. homeostasis - correct maintenance of an internal environment within restricted limits in organisms. all cells are in an environment that meets their requirements and allows them to function normally despite external changes. why is homeostasis important? - correct answer.1. the enzymes that control biochemical reactions in cells are sensitive to change e.g. in pH or temperature, which can cause them to denature. homeostasis allows enzyme controlled reactions to take place at a suitable rate. 2. homeostasis allows a constant blood glucose concentration to ensure a constant water potential, so cells don't shrink or burst. 3. homeostasis allows organisms to be more independent of external changes. list the parts of control mechanisms in homeostasis. - correct answer.1. optimum temperature 2. receptor- detects any deviation from the optimum temperature 3. coordinator- info from receptor to effector 4. effector- often a muscle/gland, brings about change to return the system to optimum level 5. feedback mechanism what is negative feedback? - correct is when the change produced by the system leads to a change in the stimulus detected by the receptor, turning the system off. what is positive feedback? - correct is when a deviation from the optimum causes changes that result in an even greater deviation from the norm e.g. in neurones, a stimulus leads to an influx of Na+, which increases the membrane permeability to allow further NA+ to enter. describe the second messenger model. - correct answer.1. Adrenaline binds to transmembrane protein receptor in the cell surface membrane of a liver cell. 2. the binding of adrenaline causes the protein to change shape on the inside of the membrane. 3. the change in tertiary structure activates adenyl cyclase, which converts ATP to cAMP. 4. cAMP binds to kinase, changes structure and activates it. 5. this catalyses the conversion of glycogen to glucose, which moves out of the liver cell and into the blood by facilitated diffusion. glycogenesis - correct rsion of glucose to glycogen. this is when glucose levels are abnormally high. glycogenolysis - correct down of glycogen to glucose. this is when glucose levels are abnormally low. gluconeogenesis - correct ction of glucose from sources other than carbohydrates, such as glycerol or fatty acids. occurs when there's insufficient glycogen. how does insulin and beta cells in the pancreas affect glucose levels? - correct answer.1. the beta cells in the pancreas detect a rise in the blood glucose concentration and respond by secreting insulin into blood. 2. insulin binds to glycoprotein receptors on cells. 3. this causes a change in the tertiary structure of the glucose transport proteins, making them more permeable to glucose and so allowing more in by facilitated diffusion. 4. activates the enzymes that convert glucose to glycogen and fat. how does glucagon and alpha cells in the pancreas affect glucose levels? - correct answer.1. alpha cells detect a fall in blood glucose levels and so secrete glucagon. 2. glucagon attaches to receptors on cell surface membrane of liver cells. 3. this activates enzymes which convert glycogen to glucose. 4. also activates enzymes that convert amino acids to glucose. does insulin increase or decrease glucose levels? - correct ase does glucagon increase or decrease glucose levels? - correct ase does adrenaline increase or decrease glucose levels? - correct ase describe and explain the role of hormones in osmoregulation. - correct eceptors in hypothalamus detect fall in water potential as they begin to shrink, causing hypothalamus to produce ADH. 2. ADH goes to posterior pituitary gland, where it is secreted into capillaries. 3. ADH goes from blood to kidneys, where it binds to receptors on the cells of of distil convoluted tubule and collecting duct. 4. this activates phosphorylase enzyme. 5. this causes vesicles, which contain water channel proteins, to fuse with cell surface membrane. hence, making it more permeable to water. 6. also increases permeability of collecting duct to urea so it passes out and lowers water potential, so more water can pass out by osmosis. describe what is happening to a neurone at resting potential. - correct answer.1. neurone is polarised 2. Na+ actively transported out of axon 3. K+ actively transported in to axon 4. 3 sodium move out for every 2 potassium in hence, the outward movement of Na+ is greater than the inward movement of K+. this creates an electrochemical gradient as the outside is more negative than inside. 5. K+ begins to diffuse back out while Na+ diffuses back in, although most Na+ gates are closed. describe the processes that occur when an action potential is formed. - correct answer.1. the energy of a stimulus causes some sodium voltage-gated channels in the axon membrane to open and so Na+ diffuses into axon, down electrochemical gradient. 2. this triggers a reversal in potential difference across the membrane because Na+ is positively charged. 3. as more Na+ goes in, more channels open and so even more Na+ goes in. 4. when action potential is +40mV the Na+ channels close and the K+ channels open. 5. K+ diffuses out of axon. 6. there is overshoot of electrical gradient- hyperpolarisation. 7. K+ gates shut. resting potential is re-established. this is repolarisation. describe the processes that occur during the passage of an action potential along an unmyelinated axon. - correct answer.1. at rest- the inside of axon is more negative than outside. 2. a stimulus causes influx of Na+ and so the charge of the axon is reversed- depolarised. 3. this causes localised electrical currents to open up the voltage-gated channels further along the axon, so more Na+ enters here and depolarises this area. 4. in the initial area, Na+ gates close and K+ open, so K+ leave the axon down electrochemical gradient. depolarisation moves along membrane. 5. outward movement of K+ and inward movement of Na+ continues until repolarisation; return to resting state. why do action potentials travel faster down a myelinated axon? - correct n sheath prevents action potentials forming. action potentials form at the Nodes of Ranvier, and jump from node to node by saltatory conduction. in an unmyelinated axon, it takes longer as the events of depolarisation take place all the way along an axon. what are factors that affect the speed of an action potential? - correct answer.1. myelin sheath 2. diameter of axon: the greater the diameter, the faster the speed, because there's less leakage of ions from a large axon, so membrane potentials are easier to maintain. 3. temperature: the higher the temperature, the greater the rate of diffusion of ions, and hence the faster the impulse. what is the purpose of the refractory period? - correct answer.1. Ensures action potentials travel in one direction only, since action potentials cannot move to a refractory region. 2. ensures action potentials are separated from one another. 3. limits the number of action potentials. spatial summation - correct answer.a number of different presynaptic neurones collectively release enough neurotransmitters to exceed the threshold value of the postsynaptic neurone, triggering a new action potential. temporal summation - correct answer.a single presynaptic neurone releases neurotransmitters many times over a short period. this exceeds the threshold value, triggering a new action potential. how do drugs affect synaptic transmission and action potentials? - correct answer.1. stimulate the nervous system by mimicking neurotransmitters, stimulating neurotransmitter release or inhibiting enzymes that break down neurotransmitters, and so create more action potentials. 2. inhibit the nervous system by inhibiting release of neurotransmitter or blocking Na+/K+ channels on postsynaptic neurone, hence creating less action potentials. describe the processes that occur during synaptic transmission. - correct answer.1. action potential arrives at end of presynaptic neurone. this stimulates Ca2+ channels to open so Ca2+ enter synaptic knob by facilitated diffusion. 2. this stimulates synaptic vesicles to fuse with presynaptic membrane, releasing acetylcholine into synaptic cleft. 3. acetylcholine binds to receptor sites on Na+ channel proteins on postsynaptic neurone membrane. this causes them to open to Na+ diffuse in. 4. influx of Na+ generates new action potential in postsynaptic neurone. 5. acetylcholinerase hydrolyses acetylcholine into choline and ethanoic acid, which diffuses back across into presynaptic neurone. 6. ATP recombines ethanoic acid and choline, which is stored in synaptic vesicles for later use. describe slow twitch fibres - correct act more slowly and provide less powerful contractions but over a longer period. adapted for aerobic respiration to avoid lactic acid build up. adapted for endurance work. has lots of myoglobin, rich bloody supply and many mitochondria. describe fast twitch fibres - correct act more rapidly and more powerful but over a shorter period of time. adapted for intense exercise have thicker and more numerous myosin filament, have more glycogen, have more enzymes to carry out anaerobic respiration, has phosphocreatine. what is a neuromuscular junction - correct point where motor neurone meets a skeletal muscle fibre. describe the processes that occur during muscle contraction - correct answer.1. action potential travels down T-tubules, which are in contact with the sarcoplasmic reticulum.