AQA biology a-level paper 2, Exams of Biology

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AQA biology a-level paper 2 describe and explain the steps in the light dependent reaction of photosynthesis - 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. - 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. - 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. - 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. - 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. - 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 - the total mass of organisms in a given area what is the 'gross primary production' - the chemical energy stored in a plants biomass what is the 'net primary production' - the chemical energy stores in a plants biomass after respiratory losses have been considered. this energy is available to consumers. 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 - conversion of glucose to glycogen. this is when glucose levels are abnormally high. glycogenolysis - breakdown of glycogen to glucose. this is when glucose levels are abnormally low. gluconeogenesis - 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? - 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? - 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? - decrease does glucagon increase or decrease glucose levels? - increase does adrenaline increase or decrease glucose levels? - increase describe and explain the role of hormones in osmoregulation. - 1.osmoreceptors 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. - 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. - 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. - 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? - myelin 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? - 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. mechanical pressure when at rest, what occurs at a pacinian corpuscle? - the sodium ion channels on the membrane around the neurone are narrow and so don't allow Na+ to pass along them. it has resting potential. what happens when pressure is applied to pacinian corpuscle? - 1. it deforms, so membrane around neurone stretches. 2. this widens Na+ channels and so Na+ diffuses into neurone. 3. this changes the potential of the membrane, depolarising it. a generator potential is formed. 4. generator potential forms an action potential. what kind of summation occurs in rod cells? - spatial summation what kind of summation occurs in cone cells? - temporal summation where are rod cells absent at? - the fovea where are cone cells concentrated at? - the fovea where are rod cells more highly distributed at? - periphery of the retina do rod cells give good or poor visual acuity? - poor visual acuity do cone cells give good or poor visual acuity? - good visual acuity how many types of rod and cone cells are there? - rod- 1 cone- 3, all responding to different wavelengths why do rod cells give poor visual acuity? - many rod cells link to the same bipolar cells (spatial summation), so when light stimulates rod cells which share the same neurone, only 1 impulse will travel to the brain. so, the brain can't distinguish between separate sources of light that stimulated them. resolution is poor, hence low visual acuity. why do cone cells give high visual acuity? - each cell is connected to a separate bipolar cell. if 2 separate rod cells are stimulated by light, then 2 separate impulses will be sent to the brain. so, the brain can distinguish between two sources of light that stimulate two different rod cells, hence better resolution, hence better visual acuity. what is a taxis? - when the direction of a stimulus determines a simple response. motile organisms will move towards it if favourable (positive), or away if unfavourable (negative). what is a kinesis? - a form of response whereby an organism changes the speed in which it moves and changes direction. this occurs when an organism reaches an area of unfavourable stimuli, so that it may return to a favourable environment. example of kinesis - woodlice increase rate of movement/turning when they reach a dry area. this will allow them to reach a favourable damp area, where they don't turn as much. example of taxis - earthworms have negative phototaxis and so stay deeper in soil to aid their chances of survival, so they are more likely to find food, avoid predators, and conserve water. what is a tropism? - the growth of part of a plant in response to a directional stimulus. there can be positive and negative responses. describe the control of phototropism by IAA. - 1. cells in shoot tip produce IAA which is transported evenly throughout all regions and then down the shoot. 2. light causes IAA to accumulate on the shaded side of the shoot, so much that there is a greater build up of IAA on shaded than unshaded side. 3. IAA causes the shaded side of the shoot to elongate further than the non shaded side. this causes the shoot to eventually grow and bend towards the light. describe the control of gravitropism by IAA. - 1. root tip cells produce IAA, which is distributed evenly and down the root. 2. gravity causes IAA to accumulate on the LOWER side of the shoot than upper side, so there is a higher concentration on lower than upper. 3. IAA inhibits elongation in root cells, so inhibits elongation in lower side than upper side. so, the upper side of the shoot elongates further and bends towards gravity. what effect does IAA have on roots? - inhibits growth what effect does IAA have on shoots? - stimulates growth describe the components of a reflex arc and give an example. - 1. stimulus- heat 2. receptor- heat receptors to sensory neurone codominance - occurs where heterozygote has a phenotype that is different from both homozygotes. neither allele is dominant over the other; they both contribute equally to the phenotype. sex linkage - alleles carried on the X chromosome. why are sex linked diseases more common in males than females? - because males only have one X chromosome, and so if there is a recessive allele there, then there will be no dominant allele on Y chromosome to "hide" it. multiple alleles - this is where there are several different alleles of a gene e.g. blood type: IA, IB (codominant), and IO (recessive). dihybrid inheritance - involves 2 genes at different loci. epistasis - when one gene locus interacts with another gene at a different locus. linked genes - genes on the same chromosome Hardy-Weinberg principle - p2 + 2pq + q2 p2 = homozygous dominant q2 = homozygous recessive 2pq = heterozygous In any hardy-weinberg problem, start with homozygous recessive individuals. what assumptions need to be in place to use the hardy weinberg principle? - 1. No mutations 2. Population is isolated 3. No selection 4. Large population 5. Mating is random how can a population be separated and form different species? - 1. Populations become separated. physical barriers may come between two groups. 2. Therefore they stop interbreeding. 3. Populations adapt to new environment. Selection pressures will be different in different areas. 4. Allele frequencies will change in the different populations. 5. Over time they become so different that they can no longer interbreed. formula to calculate the mean density of individuals from quadrats - total no of individuals counted ------------------------------------ no of quadrats x area of quadrat allopatric speciation - When populations of a species become geographically isolated. Gene flow between them ceases (reproductive isolation). the new environment will trigger a change in the gene pool due to natural selection imposed on them. If the populations are relatively small, they may experience a founder effect. Selection and genetic drift will act differently on these two different genetic backgrounds, creating genetic differences between the two new species. sympatric speciation - become reproductively isolated from each other even though they occupy the same geographic range. Factors that could lead to them becoming reproductively isolated from each other are things like changes in courtship behavior, changes in feeding behavior, changes in coloration. The most common way this occurs is polyploidy. Rapid genetic changes can alter morphology, behavior, and habitat preferences. totipotent cells - cells that can mature into any kind of specialized body cell. can divide to form a whole organism. they are found in very early mammalian embryos. after this stage, some of the genes become switched off and so are not translated into RNA, hence specialized. pluripotent cells - can become any kind of specialized body cell, but cannot divide to form a whole organism. induced pluripotent stem cells - produced from unipotent cells. genetically altered in labs (transcription factors) to make them have the characteristics of embryonic stem cells. they turn on the genes that were otherwise turned off. multipotent cells - can divide into some, but not all, specialized cells. unipotent cells - divide to form just one type of cell. transcription factors - what is the role of the loop of henle? - reabsorbs water from collecting duct, so it can concentrate urine so that it has a lower water potential than blood. how does the loop of henle concentrate urine? - 1. filtrate enters descending limb. water passes out by osmosis and into the interstitial space as the walls are permeable. Na+ actively transported in. 2. this lowers water potential. lowest water potential is at the bottom of the hairpin. 3. at the ascending limb, the walls are not permeable to water and so it cant leave. Na+ is actively transported out into the interstitial space. this creates a higher water potential. 4. interstitial space between collecting duct and ascending limb has a wp gradient, from high to low, and so any water left passes out. explain how the loop of henle acts as a counter current multiplier. - water in the loop of henle meets water in the interstitial space which is of a lower water potential, and so water can pass out by osmosis for the whole length of the descending tube and collecting duct. how does the distil convoluted tubule reabsorb material from the filtrate? - the cells lining the DCT have microvilli and mitochondria. it selects which ions to reabsorb. this controls the pH of the filtrate. list the steps in osmoregulation - 1. Ultrafiltration in Bowmans capsule 2. reabsorption in the proximal convoluted tubule 3. concentration in the loop of henle distil convoluted tubule explain the role of hormones in osmoregulation - 1. osmoreceptors in the hypothalamus detect fall in water potential. this causes water to be lost from osmoreceptors. 2. this makes them produce ADH, which goes to pituitary gland to pass into blood. 3. ADH goes to kidneys and binds to surface of DCT ad collecting duct, activating phosphorylase. this causes vesicles to fuse with membrane and form aquaporins. 4. ADH increases permeability to urea, which passes out and decreases WP in interstitial space. this causes water to leave by osmosis, so more water is reabsorbed. effect of oestrogen on transcription - 1. diffuses into cytoplasm of cell. Binds with complimentary TF. 2. alters tertiary shape of the binding site of TF, so it is activated and can now bind to genes. 3. TF enters nuclear pore and binds to gene. 4. This stimulates transcription.