Student book anSwerS Chapter 9 Inside cells

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Question set 9.11 Cellular metabolism is a term that describes all of the chemical processes occurring in a living cell.

2 Biochemical pathways are ordered, enzyme-regulated reaction sequences.

3 Intracellular enzymes occur inside cells, speeding up metabolic reactions. Extracellular enzymes are produced by cells, but achieve their effects outside the cell.

4 a This stepwise progression allows for the regulation of each reaction and the management of the energy requirements (or energy release) of the reaction.

b As all reactions are reversible, any build-up of product would reverse the reaction or produce a feedback effect in which the forward reaction would be slowed down.

5 In anabolic reactions, such as photosynthesis, simple molecules are combined into more complex molecules, whereas in catabolic reactions, such as cellular respiration, complex molecules are broken down into simpler molecules.

6 Exergonic reactions release energy and endergonic reactions use energy. The breakdown of organic compounds to simpler compounds such as carbon dioxide and water is an exergonic reaction. The synthesis of complex organic compounds such as starch from simpler organic compounds such as sugar is an endergonic reaction.

7 a Anabolic reactions always use energy so they are always endergonic.

b Catabolic reactions always release energy so they are always exergonic.

8 A mitochondrion increases its internal surface area by the folding and stacking of internal membranes. These are studded with enzymes. This increases the number of enzymes available for chemical reactions.

9 Endergonic reactions require an energy input and are dependent upon the net energy released from exergonic reactions. Exergonic reactions release energy when complex molecules break down into simpler molecules and are dependent on endergonic reactions to build up complex molecules.

10 a The reaction is releasing energy; therefore, it is exergonic. In this reaction, heat energy is released causing the temperature to increase.

b Enzymes (protein)

c Molecules would be larger before the reaction than they would be after the reaction.

Student book anSwerS

Chapter 9 Inside cells

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experiment 9.1: the effect of temperature on enzyme activityDiscussion

1 This will depend on students’ results.

2 The colour of the iodine indicates whether starch is still present. The more active the enzyme, the less starch is present. In some test tubes, the colour of the iodine changes throughout the testing time. This is because not all of the starch breaks down at the same time. If not enough enzyme is present, the enzyme can be reused; hence, the conversion of all starch to sugar takes time.

3 These tests were used to make sure that iodine did not react with the amalyse or the distilled water. Because neither of these substances caused a reaction, it is safe to assume that the colour changes noted in the experiments were due to the presence of the starch.

4 Three identical test tubes for each condition were used, to increase the accuracy of results.

5 Tubes 1–3 were set up as controls so that any difference between them and test tubes A–C would indicate that it was caused by the variable – the presence of amylase.



Question set 9.21 Enzymes are proteins with a specific shape. Substrates bind to an enzyme’s active site. As a result of these

stresses on the substrate, the activation energy required to create a reaction is lowered. Enzyme activity is affected by temperature and pH.

2 As enzymes are not destroyed or altered by the reactions they catalyse, they can be used again.

3 Enzymes act on substrates by lowering the activation energy required for a substrate to react.

4 The conformation of the active site, and hence the activity of an enzyme, is sensitive to changes in pH and temperature.

5 • Theyactveryrapidly.Thelock-and-keymechanismexplainsthisbecausetheenzyme-substratecomplex causes the substrate to change, producing the end product.

• Theyarenotdestroyedoralteredbythereactionstheycatalyse;theycanbeusedagain.Thelock- and-key mechanism explains this because the substrate fits into the enzyme active site, then leaves it. The active site remains intact after the substrate leaves.

• Theycanworkineitherdirection.Thelock-and-keymechanismexplainsthisbecausethesubstratemolecule attaches to the enzyme.

• Theyareaffectedbytemperatureandhaveanoptimalrangeinwhichtheyoperate.Thelock-and-keymechanism explains this because the active site shape changes when the temperature changes.

• TheyaresensitivetopH.Thelock-and-keymechanismexplainsthisbecausetheactivesiteshapechanges when the pH changes.

• Theyareusuallyspecifictoparticularreactions.Thelock-and-keymechanismexplainsthisbecauseeach enzyme has a specific shape that fits closely with the substrate.

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6 When enzymes are denatured, the active site loses its functional shape and is no longer able to accommodate the substrate.

7 A build-up of product within the cell will act as an inhibitor, binding to a site on the enzyme and slowing down the reaction. If the product is removed, the reaction will speed up again.

8 A non-competitive inhibitor, such as the build-up of product, may inhibit the activity of the enzyme, but not permanently. Other inhibitors that compete with the substrate are called competitive inhibitors. Their effect can be long lasting.

9 Without enzymes, reactions in cells would occur so slowly, they would hardly proceed at all. This would be incompatible with the maintenance of life.

10 There are thousands of different reactions occurring in the human body. Each reaction requires a different enzyme, hence the need for the thousands of different types of enzymes.

11 In the induced-fit model of enzyme action, the enzyme’s active site can actually change shape to ensure that the substrate is accommodated by the active site. In the lock-and-key model, the active site is a static structure into which the substrate fits.

12 If our body temperature increases, enzymes change shape. If they change to such an extent that they are no longer able to function, important reactions in our body may not proceed at a rate fast enough to maintain life.

13 a This is a reversible reaction.

b The reaction would go from left to right, that is glucose 6-phosphate to fructose 6-phosphate.

c If there is more fructose 6-phosphate than glucose 6-phosphate

d These types of reactions are advantageous when equilibrium is required. There will be a constant amount of substrate and product.

Question set 9.31 ATP molecules and hydrogen ions are the products of light-dependent reactions used as inputs in light-

independent reactions.

2 Glucose

3 6CO2 1 12H2O → C6H12O6 1 6O2 1 6H2O

4 Only living plant cells that contain chloroplasts carry out photosynthesis. Root cells, for instance, do not.

5 In the light-dependent reaction, solar energy is absordbed by pigments within the thylakoid membranes (grana) of the chloroplast. This energy is used to split water molecules into hydrogen ions and oxygen gas. In this process, molecules of ATP are produced. The light-independent reaction occurs in the stroma of the chloroplast. In this reaction, carbon dioxide and hydrogen ions join to produce glucose.

6 ADP is composed of an adenosine molecule with two phosphate groups attached. When another phosphate group is attached to ADP it produces ATP. ATPase is the enzyme that speeds up the reaction of ATP breaking down to ADP 1 P.

7 A battery stores chemical energy that can be released as electrical energy to power various appliances. In the same way, ATP stores chemical energy in the high-energy chemical bonds attaching the last phosphate group to ATP. This stored energy is released when the bonds are broken and a phosphate group is removed. This energy is now available to fuel a cellular reaction.

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experiment 9.2: the effect of light on photosynthesisDiscussion

1 The only difference between the tubes was the presence of the foil. All other factors were the same. If the tubes had been put in the cupboard to keep them away from the light, other factors could vary (e.g. temperature, humidity).

2 Tube A was the control for tube B, and tube C was the control for tube D. Tube C was also a control for tube A, and tube D was a control for tube B. Variables being tested were the presence of light and the presence of the plant.

3 a Photosynthesis: 12H2O 1 6CO2 → 6O2 1 C6H12O6 1 6H2O

b Cellular respiration: 6O2 1 C6H12O6 → 6H2O 1 6CO2 1 36ATP

4 Plants photosynthesise when all the requirements for photosynthesis are present (i.e. water, carbon dioxide, light, chlorophyll). Plants carry out cellular respiration at all times.

5 The experiment is testing the production and use of carbon dioxide in photosynthesis and respiration. If atmospheric carbon dioxide is allowed to enter and leave the test tubes, it makes it extremely difficult to draw the conclusion that the processes being investigated are changing the carbon dioxide levels. Any carbon dioxide produced by the experiment would be lost to the atmosphere.

6 Tubes C and D acted as controls to show that the colour of the indicator, and hence the pH, did not change because of any factor other than the presence of the plant.

7 Oxygen could also be used to measure the rate of photosynthesis and respiration; however, the concentration of carbon dioxide is easier to measure using an indicator because it dissolves in water to form carbonic acid. Oxygen would not change the pH of the water.

8 Water plants are adapted to an aquatic environment and can more easily utilise dissolved carbon dioxide or oxygen. Land plants depend on gases entering through stomata.

9 Students will discuss this from their experiences during the experiment.

Conclusion

1 a The rate of photosynthesis is less than the rate of cellular respiration.

b The rate of photosynthesis is more than the rate of cellular respiration.

c The rate of photosynthesis is equal to the rate of cellular respiration.

2 Light allows photosynthesis to take place. Without light, the process will not occur. Light has no direct effect on the rate of cellular respiration; however, in order for cellular respiration to occur within the closed system, as established in this exercise, the amount of glucose present will have an effect on the rate of cellular respiration.

Question set 9.41 C6H12O6 1 6O2 → 6CO2 1 6H2O 1 ATP

2 The initial substrate in the glycolysis pathway is glucose and the final product is the three-carbon compound called pyruvate.

3 Glycolysis takes place in the cytosol.

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4 Aerobic respiration takes place in the mitochondria, whereas fermentation takes place in the cytosol. The final products and the amount of net ATP produced are different in both pathways.

5 In animal cells and plant cells, the final products of aerobic respiration are carbon dioxide and water. Products vary in anaerobic respiration; in animals, the final product is lactic acid and in plants, carbon dioxide and ethanol.

6 In aerobic cellular respiration, carbon dioxide is the by-product of a reaction that involves pyruvate and oxygen.

7 Fermentation begins to occur when oxygen is absent or not always available. Many plants (or parts of plants) can respire anaerobically for a short time, such as germinating seeds and roots living in water-logged soil, where there is little oxygen.

8 The fermentation process is used to make cakes and breads rise. A product of alcohol fermentation is carbon dioxide gas. It is the carbon dioxide bubbles that pass through the dough, causing it to rise. In lactic fermentation, there is no liberation of carbon dioxide; therefore, the desired result cannot be obtained.

9 Lactic acid fermentation results in the release of a small amount of energy; therefore, muscle cells can contract for a short while without oxygen.

Chapter review questions1 The active site is a place on the enzyme’s surface. Its precise shape allows substrate molecules to become

attached; it is this close contact that causes a reaction.

2 Temperature, pH, substrate concentration, amount of enzyme, competitive and non-competitive inhibitors

3 Suitable temperature, water, light and oxygen

4 Anabolic reactions produce more complex molecules: a, c and d are anabolic. Catabolic reactions produce more simple molecules; b and e are catabolic.

5 Mitochondria and chloroplasts are both double membrane-bound structures with internal membranes, which increase the surface area of reactive surfaces. The inner membrane of a mitochondrion is tightly folded, whereas the membrane sacs in chloroplasts are stacked into piles called grana.

6 Alcoholic fermentation Lactic acid fermentation Aerobic respirationStarting materials Glucose Glucose Glucose, oxygenEnd products Ethanol, carbon dioxide Lactic acid Carbon dioxide, waterEnergy produced for each molecule of glucose

2 ATP molecules 2 ATP molecules 36 ATP molecules

7 Although the reactants, products and energy transformations of the two reactions appear to be opposite, they are better described as complementary. Each is a unique biological pathway, with specific intermediate compounds controlled by specific enzymes and cofactors.

8 Enzymes are highly specific because of the shape of their active site. Only specific substrates of one enzyme will bind to the active site. Each enzyme generally catalyses one specific chemical reaction.

9 When the active site permanently loses its functional shape, the substrate can no longer bind with it. This change in the active site shape denatures the enzyme.

10 Oxygen is produced by the breakdown of water.

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11 ATP is made up of adenosine, bound to a chain of three phosphate groups. When a cell requires energy to drive an endergonic reaction, the high-energy chemical bonds attaching the last phosphate group to ATP are broken, thus releasing stored energy.

12 Body fluids are normally buffered to maintain the optimum pH for enzyme activity.

13 a 20 minutes

b The plant was in put in the dark; the carbon dioxide produced in cellular respiration is not being used in photosynthesis.

c The plant was put in light conditions.

d The rate of photosynthesis was at its maximum capacity. Limiting factors could be the amount of water, the amount of light, the temperature and the amount of oxygen (if the plant was in a contained environment).

e Enzymes would denature. Cellular metabolism would slow down to the stage where life could not be sustained.

f Carbon dioxide is continually produced in cellular respiration but is used in photosynthesis. The lower the carbon dioxide concentration, the greater the rate of photosynthesis.

14 Responses will vary.


16 When money is earned, it can be deposited into a bank. When energy is gained, it is used to add a phosphate group to an ADP molecule. The energy is then stored in the chemical bond. When money is withdrawn from the bank, it can be spent on a range of goods. When the phosphate is removed from an ATP molecule, the energy released can be ‘spent’ on other chemical reactions.

17 At the compensation point, all produced glucose is consumed and there is no incorporation of material into the plant. Plant growth is likely to cease.

18 Food is produced by plants in the form of glucose. Glucose can’t be directly used for cellular metabolism: it has to be converted into simpler compounds. The energy released builds up ATP molecules, which are then converted to ADP 1 P. The energy released can then be used for cellular activities. This process of building ATP is cellular respiration, which depends on mitochondria.

19 If mitochondria failed to work, cellular respiration would not proceed and the cell would die.

20 During a heart attack, the heart is deprived of oxygen, causing muscle cells to switch to an anaerobic pathway to maintain a supply of energy. Lactate is the end product of this anaerobic pathway.

21 Penicillin is a drug that inhibits enzymes necessary for the synthesis of peptidoglycans, a constituent of the bacterial cell wall. With the inhibition, the bacterial population stops growing because there is no new cell wall formation.

22 Alcohol fermentation is happening. To prevent this happening again, a readily available supply of oxygen is needed.

23 a At a lower temperature, all chemical reactions proceed at a lower rate.

b At a higher temperature, the enzyme will become denatured.

c A lower temperature does not permanently affect the function of the enzyme’s active site. At a higher temperature, the active site will not be preserved.

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24 Arsenic is a chronic poison that competes with phosphate as a substrate. Once it occupies an active site, it binds permanently with the enzyme so the substrate is permanently excluded and energy-producing reactions cannot occur.

25 An increase in temperature would adversely affect the enzymes involved in sperm production.

26 Lemon juice is acidic. The change in pH will adversely affect the activity of the enzyme polyphenol oxidase and the brown colouration will not be produced.

27 Plants in the dense tropical rainforest would have more thylakoid membranes in their chloroplasts. When less light is available, it is advantageous to have more thylakoid membranes, in order to maximise light absorption. More thylakoid membranes would provide more area for absorption of what light there is.

28 Tube A would contain cells only capable of performing aerobic respiration. This is predicted because aerobic respiration releases much more energy than anaerobic respiration. The experimental design will depend on students’ response.

29 The molecular degradation during the decomposition of organs and tissues is catalysed by enzymes. Cooling organs and tissues destined for transplantation reduces that enzyme activity and thus lessens the natural decomposition process. The cooling also reduces the metabolic work of cells and prevents degradation of their own structures to obtain energy.



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Student book anSwerS Chapter 9 Inside cells