AQA A LEVEL BIOLOGY Paper 2 Exam 2025

Homeostasis What is negative feedback? - -A response that returns a change in levels, back to normal. What is positive feedback? - -A response that amplifies a change from the normal level. Which factors influence blood glucose concentration? - -Eating food containing carbohydrates, glucose absorbed from intestine to blood. Exercising, more glucose is used in respiration to release energy. What is the effect of insulin? - -Decreases blood glucose concentration. How does insulin work? - -Secreted by beta cells in islets of Langerhans in pancreas, when blood glucose too high. Insulin binds to specific receptors on cell surface membranes of target cells, increases permeability of cell membrane to glucose (increases number of channel proteins in cell membrane), cells take up more glucose by facilitated diffusion, enzymes convert glucose to glycogen and so stored in cytoplasm. Rate of respiration increases. What is the effect of glucagon? - -Increases blood glucose concentration. How does glucagon work? - -Secreted by alpha cells in the islets of Langerhans in pancreas. Binds to specific receptors on cell surface membranes of target cells, activates enzymes to convert glycogen to glucose. Rate of respiration decreases. What is the role of adrenaline? - -Increases blood glucose concentration by activating secretion of glucagon. How does adrenaline work? - -Binds to specific receptors on cell surface membranes of target cells, activates enzymes to convert glycogen to glucose. What is the secondary messenger model? - -Demonstrated by adrenaline and glucagon as they cause glycogenosis to occur inside cell even though they bind to receptors on the outside of the cell. Adrenaline or glucagon bind to specific complimentary receptors on the cell membrane. They activate adenylate cyclase. Convert ATP to cyclic AMP. cAMP activates protein kinase A. Protein kinase A activates a cascade to break down glycogen to glucose. AQA A LEVEL BIOLOGY AQA A LEVEL BIOLOGY What is Type I diabetes? - -Gene mutation. Autoimmune response on beta cells of islets of Langerhans. Body can't produce insulin. What is Type II diabetes? - -Poor diet, lack of exercise, obesity. Glycoprotein loses responsiveness to insulin. Cells don't take up enough glucose. What is osmoregulation? - -The control of water and salt levels in the body, by ADH. How does the body respond to a decrease in water potential? - -Detected by osmoreceptors in hypothalamus. Hypothalamus produces more ADH, posterior pituitary gland secretes more ADH into blood. ADH travels in blood to kidney and attaches to receptors on collecting duct. ADH increases permeability of cell membranes to water so more water is absorbed by osmosis. Less water lost in urine so smaller volume of urine, more concentrated. How does the body respond to an increase in water potential? - -Detected by osmoreceptors in hypothalamus. Hypothalamus produces less ADH, posterior pituitary gland secretes less ADH into blood. Less ADH travels in blood to kidney and attaches to receptors of collecting duct. ADH decreases permeability of cell walls to water so less water absorbed by osmosis. More water lost in urine so large volume of urine, less concentrated. What role does the nephron have in osmoregulation? - -Formation of glomerular filtrate, readsorption of glucose and water by the PCT, maintenance of a gradient of Na+ in the medulla by the loop of Henle, reabsorption of water by the DCT and collecting duct. How is glomerular filtrate formed? - -Diameter of efferent arteriole smaller than afferent arteriole. Buildup of hydrostatic pressure in glomerulus. Water, glucose, mineral ions squeezed out of capillaries into Bowmans capsule to form glomerular filtrate, through pores in capillary endothelium, basement membrane, podocytes. Large proteins aren't pushed out, too big. How are glucose and water reabsorbed by PCT? - -Na+ actively transported out of epithelial cell to capillary, lowers concentration in epithelial cell. Na+ moves via facilitated diffusion from PCT into epithelial cell down conc gradient (co transports glucose/amino acids). Glucose/amino acids move into capillaries via facilitated diffusion down conc gradient, lowers water potential in capillary. Water moves into capillary by osmosis. How is a concentration gradient of Na+ maintained in the medulla by the loop of Henle? - -Na+ actively transported out of ascending limb, ascending limb is impermeable to water so water remains inside, Na+ concentration increased in medulla, lowers water potential. Water moves out of descending limb by osmosis into medulla, water AQA A LEVEL BIOLOGY AQA A LEVEL BIOLOGY reabsorbed by capillaries, filtrate more concentrated as moved down DL. Na+ diffuses into descending limb, recycled in the loop of Henle, reduces water potential further. How is water reabsorbed by the DCT and collecting duct? - -Water moves out of DCT and collecting duct by osmosis, controlled by ADH which changes their permeability. Respiration - - Outline a method for RP9. - -Set up a water bath at 35°C. Add 5 cm³ of the yeast and glucose solution to 3 test tubes, place test tubes in the water bath and leave them for the solution to equilibrate for 10 minutes. Add 2 cm³ of methylene blue to the test tubes and start the timer, shake for 10 seconds and place back in the water bath, record how long it takes for the methylene blue to turn colourless for each test tube. Repeat the experiment using temperatures of 40°C 50°C 60°C and 70°C. Find mean of results for each temperature and calculate the average rate of respiration at each temp. Outline the pathway of respiration. - -Glycolysis, link reaction, Krebs cycle, oxidative phosphorylation. Where does glycolysis happen? - -In the cytoplasm. What happens during glycolysis? - -Phosphorylation - glucose phosphorylated using a phosphate group from molecule of ATP - one molecule of glucose phosphate and one molecule of ADP, ATP used to add another phosphate - hexose bisphosphate - split - 2 triose phosphate. Oxidation - triose phosphate oxidised - 2 pyruvate, NAD collects H+ - NADH. Net gain = 2ATP What happens to pyruvate in anaerobic respiration? - -Converted to ethanol in plants. Converted to lactate in animal cells. (using NADH) What happens during the link reaction? - -Pyruvate decarboxylated (CO2 removed), pyruvate oxidised to form acetate, NAD reduced - NADH, acetate combined with CoenzymeA - acetyl CoA. (No ATP produced) Where does the link reaction take place? - -Mitochondrial matrix. How many times does the link reaction occur for each glucose molecule? - -Twice. What does the Krebs cycle produce? - -ATP and reduced coenzymes. Where does the Krebs cycle take place? - -Mitochondrial matrix. AQA A LEVEL BIOLOGY AQA A LEVEL BIOLOGY What happens during the Krebs cycle? - -Acetyl CoA (2C) combines with oxaloacetate (4C) - citrate (6C). Coenzyme recycled. 6C molecule decarboxylated & dehydrogenated twice - 5C - 4C molecule - 1 FADH & 2 NADH, ATP produced by ADP + Pi (substrate-level phosphorylation). What is the purpose of oxidative phosphorylation? - -Produces a lot of ATP. What is the process of oxidative phosphorylation? - -Hydrogen atoms released from NADH and FADH, H split into protons and electrons. Electrons move down ETC, lose energy at each carrier. Energy pumps protons from mitochondrial matrix to inter-membrane space. Concentration of protons higher in inter-membrane space (electrochemical gradient), protons move down gradient, into mitochondrial matrix via ATP synthase, ATP synthesised (chemiosmosis). Oxygen (final electron acceptor)combines with protons (H+) electrons to form water. 4H+ + 4e- + O2 -- H2O How much ATP is produced from one glucose molecule? - -32 molecules of ATP. Nervous Coordination - - How is a resting potential established? - -Na/K pump actively transports 3 Na+ out and 2 K+ into axon. Electrochemical gradient formed. Membrane more permeable to K+ than Na+, K+ move out of axon by facilitated