AQA biology a-level paper 2

describe and explain the steps in the light dependent reaction of photosynthesis - 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. - 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. - 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. - 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. - 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. - 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 - answer-the total mass of organisms in a given area what is the 'gross primary production' - answer-the chemical energy stored in a plants biomass what is the 'net primary production' - answer-the 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? - answer-NPP = GPP - R why is converting sunlight energy into biomass in producers inefficient? - answer-some 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. - answer-fixation: 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 - answer-the 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? - 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. - 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? - answer-this 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? - answer-this 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. - 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 - answer-conversion of glucose to glycogen. this is when glucose levels are abnormally high. glycogenolysis - answer-breakdown of glycogen to glucose. this is when glucose levels are abnormally low. gluconeogenesis - answer-production 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? - 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? - 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? - answer-decrease does glucagon increase or decrease glucose levels? - answer-increase does adrenaline increase or decrease glucose levels? - answer-increase describe and explain the role of hormones in osmoregulation. - eceptors in hypothalamus detect fall in water potential as they begin to shrink, causing