Ch 5 Metabolism and enzymes

Transcription

1 Ch 5 Metabolism and enzymes Think about (Ch 5, p.2) 1. Enzymes are proteins that act as biological catalysts to speed up metabolic reactions. 2. Enzymes catalyse the breakdown of cellulose fibres of the jeans so that the jeans become soft. 3. This reduces the overall cost of stonewashing, such as water, electricity, wages etc. Quick Practice (Ch 5, p.9) A (3) is incorrect. The active site of an enzyme has a specific shape. Only the substrate that fits exactly into the active site can bind to the enzyme. Quick Practice (Ch 5, p.21) 1. D 2. D A is incorrect. Most enzymes are inactive, but not denatured, at 0 o C. B is incorrect. 37 o C is the body temperature of humans but the optimum temperature of most human enzymes is 40 to 45 o C rather than 37 o C. C is incorrect. Enzymes are not living thing. They are denatured, rather than killed, by high temperatures. High temperatures, extreme phs and inhibitors that bind permanently to the enzymes can cause the active sites of enzymes to change shape, so that the active sites no longer fit the substrates. Quick Practice (Ch 5, p.28) A Enzyme P works well in moderate temperatures which do not cause damage to the clothes. It can tolerate in a slightly alkaline medium of the detergent. B is incorrect. Enzyme Q works well only in high temperatures, which can cause damage to the clothes. C is incorrect. Enzyme R works well only in acidic medium, while the washing powder solution is alkaline. D is incorrect. Enzyme S works well only in extremely high temperatures, which can cause damage to the clothes. 1

2 Section exercise (Ch 5, p.31) 1. (a) catabolism 2. T 3. F 4. T 5. F 6. F 7. T 8. F 9. F (b) anabolism (c) respiration (d) photosynthesis 10. (a) The activation energy of the reaction catalysed by enzymes is lower than the uncatalysed reaction. (b) It is because enzymes speed up chemical reactions taking place in an organism. 11. (a) C: enzyme 12. B 13. C 14. D D: product X: enzyme-substrate complex (b) lock-and-key hypothesis (c) Both the active site of the enzyme and the key have a specific shape. In order to open a lock, the key with a specific shape must fit into the lock. Similarly, each enzyme can act only on substrates that fit into its active site. (d) formation of protein or starch The enzyme is denatured upon heating. It loses its activity permanently. 15. (a) protease (b) pectinase (c) cellulose (d) protease and amylase 2

3 Revision exercise (Ch 5, p.33) 1. A 2. B 3. D 4. C 5. B 6. A 7. B 8. D 9. A The graph represents an anabolic process as the higher energy level of product indicates an increased energy level, i.e. energy is absorbed during the reaction. The activation energy of the reaction catalysed by the enzyme is lower than the uncatalysed reaction. The energy levels of products are the same in catalysed and uncatalysed reactions. When the temperature is raised to 80 o C, most enzyme are denatured and the shape of the active site changes. The frequency of collision between enzyme and substrate increases at a higher temperature. The rate of reaction becomes higher, therefore the time for completion of reaction become shorter. The rate of enzymatic reaction beyond 40 o C should also be studied in order to draw a conclusion on the optimum temperature of this enzyme. When the temperature increases beyond the optimum temperature, the shapes of the active sites of the enzyme start to change. The active sites no longer fit the substrates, therefore the enzyme activity decreases. The two curves intercept at ph 5. The rates of reactions catalysed by both enzymes are equal at this point. At ph 4, the rate of reaction catalysed by enzyme A is higher than that of enzyme B. A is incorrect. Enzyme A is more active than enzyme B in acidic medium but there is no evidence on the graph to show that enzyme A is found in gastric juice. B is incorrect. Enzyme B is more active than enzyme A only between ph range between 5 and 7.5. C is incorrect. The optimum ph of enzyme B is 6. Temperature is one of the controlled variables in this study. It must be kept constant when reactions occur so that the variation of the rate of reaction obtained is merely due to the change in ph. 3

4 10. C 11. B 12. B 13. C 14. A 15. B B is incorrect. This statement is true. However, if both enzymes are kept at a low temperature, the result is still reliable. Only the time required to obtain result becomes longer. C is incorrect. Enzyme becomes denatured at high temperatures. D is incorrect. There is no control experiment in this study. The ph value of the saliva in the mouth stays between 6.5 and 7.5. Enzymes work best at their optimum ph. This does not mean that enzymes are active only at a particular ph. They are active at a narrow range of ph. A is incorrect. Enzymes are biological catalysts which speed up metabolic reactions without being used up. C and D are incorrect. The curve levels off at the 3rd minute. It means that there is no more oxygen released, i.e. the reaction stops at the 3rd minute. The rate of enzymatic reaction is faster at 35 o C, i.e. it would take a shorter time to complete the reaction. In both experiments, the accumulative amount of oxygen would be the same. Proteases in fresh papaya extract catalyse the breakdown of milk protein in the milk agar plate, leaving a clear zone on the plate. The enzymes are denatured in extreme ph and by boiling. Hole D is the control of this experiment. According to the data, high concentrations (1 x 10-4 M) of both copper salts and magnesium salts inhibit the activity of the enzyme. 16. Each enzyme molecule has an active site that has a specific shape. (1) Only the substrate that fits exactly into the active site can bind to the enzyme. (1) An enzyme-substrate complex is formed. (1) Once the reaction has completed, the complex breaks up to release the product(s). (1) The enzyme remains unchanged and is reusable. (1) (Total: 5 marks) 4

5 17. (a) amylase (1) (b) The optimum ph of amylase is about 7, which is neutral. (1) The acidic condition of the stomach denatures amylase. (1) (c) temperature / inhibitor (1) 18. HKCEE Biology 2011 Paper 1 Q2 19. HKCEE Biology 2008 Paper 1 Q6 (Total: 4 marks) 20. (a) cells damaged / some cell content released (during blending) (1) (b) Either enzyme A or enzyme B increases the yield of apple juice. (1) A combination of enzymes yields a greater volume of juice than either enzyme on its own. (1) (c) Any four of the following: (4) Disrupt the cell wall. (1) Break down / digest the pectin / cellulose. (1) Resulting in an increase in permeability / juice able to leak out of cells (1) Enzyme A catalyses the breakdown of a different component to enzyme B. (1) Two enzymes together destroy more molecules / cell wall. (1) (Total: 7 marks) 21. (a) Title (1) Correct choice of axes (0.5) Correct labelling of axes with units (1) Correct plotting and joining of points (1.5) (b) The gas was oxygen. (1) It can relight a glowing splint. (1) (c) (i) The one originally kept at 0 ºC. (1) (ii) In the sample kept at 100 ºC, the enzyme was denatured. (1) In the sample kept at 0 ºC, the enzyme was temporarily inactive only (1) and it resumed its function again at room temperature. (1) (d) No (1) Enzyme action is specific. (1) (e) ph / the amount of hydrogen peroxide (or substrate) used (1) (Total: 13 marks) 5

6 22. (a) (i) X = 80 ºC (1) Y = 15 ºC (1) Z = 40 ºC (1) (ii) X: At 80 ºC, the enzyme in the biological washing powder was denatured. The fat stain could not be removed / broken down. (1) Y: At 15 ºC, the enzyme activity was low. Only a small amount of fat was removed. (1) Z: 40 ºC is / near to the optimum temperature of the enzyme. Most fat stain was removed. (1) (b) Any two of the following: (2) size of the stain (1) concentration of the enzyme (1) amount of washing powder (1) (c) lipase (1) (d) If the clothes are not rinsed thoroughly, some enzymes may remain in the fabric. (1) These enzymes may degrade proteins on skin, causing allergic responses / dermatitis. (1) (e) As biological washing powder works effectively even at a relatively lower temperature, hot water is not required, (1) much less electricity is used. (1) (Total: 13 marks) 23. (a) (i) Raw pineapple juice contains proteases. (1) Proteases catalyse the digestion of proteins (collagen, a type of fibrous protein) in the connective tissues and so soften meat. (1) (ii) Proteases in fresh pineapple catalyse the breakdown of proteins in tongue, causing the sensation of tongue pain. (1) High temperature denatures proteases. (1) (b) (i) Lactase catalyses the breakdown of lactose into galactose and glucose. (1) (ii) Lactose-intolerant people are not able to digest lactose. (1) Undigested lactose decreases the water potential in the small intestine. (1) Water enters the small intestine by osmosis, (1) producing a large amount of watery faeces. 24. HKCEE Biology 2010 Paper 1 Q1 (Total: 8 marks) 6

7 25. Temperature Enzymes are inactive at low temperatures. (1) As the temperature increases, enzyme activity increases. (1) Each enzyme works best at its optimum temperature. (1) Enzymes are denatured at high temperatures. (1) ph Different enzymes have different optimum phs. (1) Each enzyme work best at its optimum ph. (1) Most enzymes are denatured at extreme ph values. (1) Inhibitors Inhibitors bind to enzymes and reduce enzyme activity. (1) (Total: 8 marks) 26. Properties (max. 3) Enzymes are biological catalysts / speed up chemical reactions to give a higher product yield. (1) Enzymes are specific in action that would reduce the production of undesirable products. (1) Enzymes are reusable that would be required in small amount only. (1) Examples (max. 3) Meat tenderisers contain proteases, which soften the meat by speeding up the breakdown of the proteins in it. (1) Enzymes catalyse the breakdown of cell walls of plant cells in fruits and release the juice inside. (1) Enzymes clarify fruit juice by speeding up the digestion of residues remaining in it. (1) Sugars in food and drinks are made by breaking down starch into glucose and fructose. The breakdown process is catalysed by enzymes. (1) In making baby food, enzymes are used to help pre-digest food into simpler food molecules, which can be absorbed by the baby directly. (1) Lactose-free milk is suitable for people with lactose intolerance. It is made by adding the enzyme lactase to regular milk. Lactase catalyses the breakdown of lactose into galactose and glucose. (1) Communication (max. 3) (Total: 9 marks) 7

8 Article reading (Ch 5, p.40) 1. Enzymes are specific in action. An enzyme / active site has a specific shape. (1) Only glucose molecules can fit to the active site of glucose oxidase. (1) Therefore, only glucose can form enzyme-substrate complex but not other substances. 2. Enzyme activity is greatly affected by temperature. Change in temperature would give an unreliable result. (1) 3. Any two of the following: Higher specificity: Glucose biosensor only detects glucose whereas Benedict s detects all reducing sugars like maltose and lactose. (1) Quantitative result: Glucose biosensor provides a quantitative reading whereas Benedict s test only shows color change but does not measure concentration (semi-quantitative). (1) Higher sensitivity: Glucose biosensor can detects glucose at a lower concentration than Benedict s test. (1) Others: Color of the blood masks the result in Benedict s test. / Only small amount of blood sample is needed for glucose biosensor. / A glucose biosensor is reusable (1) 8