AQA A-Level Biology long answer Qs Exam

Glucose is absorbed from the lumen of the small intestine into epithelial cells. Explain how the transport of sodium ions is involved in the absorption of glucose by epithelial cells. (5 marks) [Unit 1, June 16] - ANS 1. Na+ ions leave epithelial cell and enter blood; 2. (Transport out is by) active transport / pump / via carrier protein using ATP; 3. So, Na+ conc. in cell is lower than in lumen (of gut); 4. Sodium/Na+ ions enter by FACILITATED diffusion; 5. Glucose absorbed with Na+ ions against their concentration/diffusion gradient / glucose absorbed down an electrochemical gradient; Oxygen and chloride ions can diffuse across cell-surface membranes. The diffusion of chloride ions involves a membrane protein. The diffusion of oxygen does not involve a membrane protein. Explain why the diffusion of chloride ions involves a membrane protein and the diffusion of oxygen does not. (5 marks) [Unit 1, June 16] - ANS 1. Chloride ions water soluble/charged/polar; 2. Cannot cross (lipid) bilayer (of membrane); 3. Chloride ions transported by facilitated diffusion OR diffusion involving channel/carrier protein; 4. Oxygen not charged/non-polar; 5. (Oxygen) soluble in/can diffuse across (lipid) bilayer; Blood leaving the kidney eventually returns to the kidney. Describe the pattern of blood circulation in a mammal that causes blood to return to the kidney. (6 marks) [Unit 2, June 16] - ANS 1. (blood flows from kidney along) renal vein to vena cava; 2. (along) vena cava to RIGHT atrium/side of heart; 3. (along) pulmonary artery to lungs; 4. (along) capillaries to pulmonary vein; 5. (along) pulmonary vein to RIGHT atrium/side of heart; 6. (along) aorta to renal artery (to kidney); 7. Blood may pass through several complete circuits before returning to kidney; There are nine subspecies of giraffe. These subspecies evolved when populations of giraffe were separated for long time periods. Each subspecies has distinct coloured skin markings. Some biologists have suggested that up to six of these subspecies should be classified as different species. Explain how different subspecies of giraffe may have evolved from a common ancestor. Use information from the passage in your answer. (5 marks) [Unit 4, June 16] - ANS 1. No interbreeding / gene pools are separate / geographic(al) isolation; 2. Mutation linked to (different) markings/colours; 3. Selection/survival linked to (different) markings/colours; 4. Adapted organisms breed / differential reproductive success 5. Change/increase in allele frequency/frequencies; Crops use light energy to produce photosynthetic products. Describe how crop plants use light energy during the light-dependent reaction. (5 marks) [Unit 4, June 16] - ANS 1. Excites electrons / electrons removed (from chlorophyll); 2. Electrons move along carriers/electron transfer chain releasing energy; 3. Energy use to join AFP and Pi to form ATP; 4. Photolysis of water produces protons, electrons and oxygen; 5. NADP reduced by electrons / electrons and protons / hydrogen; Describe the advantages and disadvantages of using chemical pesticides to control pests of crops. (5 marks) [Unit 4, June 16] - ANS (Advantages) 1. Acts quickly; 2. Can apply to a particular area; 3. Kills all/most/wide variety of pests; (Disadvantages) 4. Needs to be re-applied; 5. Not specific; 6. Pests can develop resistance; 7. (Bio)accumulation; After harvesting, the remains of crop plants are often ploughed into the soil. Explain how microorganisms in the soil produce a source of nitrates from these remains. (5 marks) [Unit 4, June 16] - ANS 1. Protein/amino acids/DNA into ammonium compounds / ammonia; 2. By saprobionts; 3. Ammonium/ammonia into nitrite; 4. Nitrite into nitrate; 5. By nitrifiying bacteria/microorganisms; Describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells. (5 marks) [Unit 1, June 2015] - ANS Any FIVE from: 1. Cell homogenisation to break open cells; 2. Filter to remove (large) debris/whole cells; 3. Use isotonic solution to prevent damage to mitochondria/organelles; 4. Keep cold to prevent/reduce damage by enzymes / use buffer to prevent protein/enzyme denaturation; 5. Centrifuge (at lower speed/1000 g) to separate nuclei/cell fragments/heavy organelles; 6. Re-spin (supernatant / after nuclei/pellet removed) at higher speed to get mitochondria in pellet/at bottom; Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure. (5 marks) [Unit 1, June 2015] - ANS Principles: 1. Electrons pass through/enter (thin) specimen; 2. Denser parts absorb more electrons; 3. (So) denser parts appear darker; 4. Electrons have short wavelength so give high resolution; Limitations: 5. Cannot look at living material / Must be in a vacuum; 6. Specimen must be (very) thin; 7. Artefacts present; 8. Complex staining method / complex/long preparation time; 9. Imagine not in 3D / only 2D images produced;