AQA biology a-level paper 2 Games and activities

AQA biology a-level paper 2 Questions And Actual Answers acetylation of DNA - acetyl groups added to histones, so DNA is less tightly wound round it, making it easier for RNA polymerase and transcriptional factors to bind, so transcription can occur. 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. 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. define biomass - the total mass of organisms in a given area define community. - all the populations of different species living and interacting in a particular place at the same time. define niche. - the role and position a species has in its environment; how it meets its needs for food and shelter, how it survives, and how it reproduces. A species' niche includes all of its interactions with the biotic and abiotic factors of its environment. define population. - a group of individuals of one species that occupy the same habitat at the same time and are potentially able to interbreed. describe and explain the role of hormones in osmoregulation. - 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 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