AQA A level Biology Revision

AQA A level Biology Revision . [2] - Answer- Active site / enzyme not complementary; Active site changes (shape) / is flexible; (Change in enzyme allows) substrate to fit / E-S complex to form; Describe one way that the lock and key model is different from the induced fit model. [1] - Answer- Active site does not change (shape) / is fixed (shape) / is rigid / does not wrap around substrate / (already) fits the substrate / is complementary (before binding); Explain why the rate of reaction of an enzyme is low away from its optimum pH. [3] - Answer- (change in pH) leads to breaking of bonds holding tertiary structure/ changes charge on amino acids; enzyme/protein/active site loses shape/denatured; substrate will not bind with/fit active site; fewer/no ES complexes formed; A protein molecule contains 150 amino acids. What is the total number of peptide bonds in this molecule? [1] - Answer- 149 How do you calculate magnification? [1] - Answer- M= I (Image)/ A (actual size) Describe the ways in which prokaryotic cells and eukaryotic cells differ. [3] - Answer- Prokaryotic cells do not have a nucleus / have genetic material in cytoplasm; DNA in loop / ring; Not associated with proteins / do not have chromosomes /chromatin / do not divide by mitosis; Smaller ribosomes; No membrane-bound organelles; Such as mitochondria / lysosomes / endoplasmic reticulum / Golgi / chloroplasts; Prokaryotic cells may have mesosomes; Prokaryotic cells smaller; May be enclosed by capsule; What is the function of: a)Ribosome b)Smooth ER c)Golgi apparatus d) Chloroplast e)Mitochondria [5] - Answer- a)Protein synthesis b)Lipid synthesis and transport c)Processing and packaging proteins for secretion d)Photosynthesis e)Aerobic respiration to produce ATP Explain the advantages and limitations of using a transmission electron microscope [5] - Answer- Advantages: Small objects can be seen; TEM has high resolution as wavelength of electrons shorter; Limitations: Cannot look at living cells as cells must be in a vacuum; must cut section / thin specimen; Preparation may create artefact; Does not produce 3D or colored image; Why an electron microscope can be used to produce images [2] - Answer- EM gives high resolution due to short wavelength of electrons; Explain how viruses cause damage to cells. [3] - Answer- uses / breaks up / digests host nuclear / genetic material (allow references made to DNA /RNA instead of nuclear /genetic); virus DNA / genetic material inserted into hosts DNA / chromosome / genetic material; host cells amino acids are used to synthesize viral proteins; cell lysis; by enzyme (produced by expressing a virus gene); toxin production; Describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells. [5] - Answer- Cell homogenisation to break open cells; 1. Accept suitable method of breaking open cells. Filter to remove (large) debris / whole cells; 2. Reject removes cell walls. Use isotonic solution to prevent damage to mitochondria / organelles; Keep cold to prevent / reduce damage by enzymes / use buffer to prevent protein / enzyme denaturation; Centrifuge (at lower speed / 1000 g) to separate nuclei / cell fragments / heavy organelles; Re-spin (supernatant / after nuclei / pellet removed) at higher speed to get mitochondria in pellet / at bottom. Name two structures present in eukaryotic cells that are not present in the cells of prokaryotes. [2] - Answer- Nucleus; mitochondria; chloroplast, Golgi, RER; SER; centrioles Describe the fluid-mosaic structure of a cell surface membrane.(5/8 marks) - Answer- Phospholipids and proteins; Phospholipid bilayer; Arrangement of phospholipid molecules 'Tails to tails';'Floating'(protein) molecules / molecules can move in membrane;Intrinsic proteins extend through bilayer; Extrinsic proteins in outer layer only;(Ref. to intrinsic and extrinsic, unqualified, gains 1 mark);Detail of channel proteins / protein shapes / glycoproteins;Presence of cholesterol between phospholipids. Describe the part played by cell surface membranes in regulating the movement of substances into and out of cells.(6/10 marks) - Answer- 1. Non-polar/lipid soluble molecules move through phospholipid layer/bilayer;2. Small molecules/water/gases move through phospholipid layer/bilayer;3. Ions/water soluble substances move through channels in proteins;4. Some proteins are gated;5. Reference to diffusion;6.Carriers identified as proteins;7. Carriers associated with facilitated diffusion;8. Carriers associated with active transport/transport with ATP/pumps;9. Different cells have different proteins;10. Correct reference to cytosis; Describe the role of the enzymes of the digestive system in the complete breakdown of starch. (6 marks) - Answer- 1. Amylase;2. (Starch) to maltose:3. Maltase;4. Maltose to glucose;5. Hydrolysis;6. (Of) glycosidic bond; Explain how the small intestine is adapted to its function in the absorption of the products of digestion. (6/9 marks) 1. - Answer- Large surface area provided by villi / microvilli;2. long / folds increase surface area / time for absorption;3. thin epithelium;4. short diffusion pathway;5. capillary network absorbs amino acids / sugars;6. lacteal for absorption of digested fats;7. Maintains a steep concentration gradient;8. mitochondria supply ATP / energy for active transport;9. carrier proteins (in membranes); Describe and explain the role of diffusion in the absorption of digested food by the ileum. (4 marks) - Answer- 1. movement along / down concentration gradient;2. monoglycerides / micelles/fatty a