Cambridge International A Level Biology Revision Guide

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Cambridge International A Level Biology 282 Summary ■■ ■■ ■■ ■■ ■■ Organisms must do work to stay alive. The energy input necessary for this work is either light, for photosynthesis, or the chemical potential energy of organic molecules. Work includes anabolic reactions, active transport and movement. Some organisms, such as mammals and birds, use thermal energy released from metabolic reactions to maintain their body temperature. Reactions that release energy must be harnessed to energy-requiring reactions. This involves an intermediary molecule, ATP. ATP can be synthesised from ADP and phosphate using energy, and hydrolysed to ADP and phosphate to release energy. ATP therefore acts as an energy currency in all living organisms. Respiration is the sequence of enzyme-controlled steps by which an organic molecule, usually glucose, is broken down so that its chemical potential energy can be used to make the energy currency, ATP. In aerobic respiration, the sequence involves four main stages: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. In glycolysis, glucose is first phosphorylated and then split into two triose phosphate molecules. These are further oxidised to pyruvate, giving a small yield of ATP and reduced NAD. Glycolysis occurs in the cell cytoplasm. When oxygen is available (aerobic respiration), the pyruvate passes to the matrix of a mitochondrion. In a mitochondrion, in the link reaction, pyruvate is decarboxylated and dehydrogenated and the remaining 2C acetyl unit combined with coenzyme A to give acetyl coenzyme A. The acetyl coenzyme A enters the Krebs cycle in the mitochondrial matrix and donates the acetyl unit to oxaloacetate (4C) to make citrate (6C). The Krebs cycle decarboxylates and dehydrogenates citrate to oxaloacetate in a series of small steps. The oxaloacetate can then react with another acetyl coenzyme A from the link reaction. ■■ ■■ ■■ ■■ Dehydrogenation provides hydrogen atoms, which are accepted by the carriers NAD and FAD. These pass to the inner membrane of the mitochondrial envelope, where they are split into protons and electrons. In the process of oxidative phosphorylation, the electrons are passed along a series of carriers. Some of the energy released in oxidative phosphorylation is used to move protons from the mitochondrial matrix to the intermembrane space. The movement of electrons sets up a gradient of protons across the inner membrane of the mitochondrial envelope. Protons pass back into the matrix, moving down their concentration gradient through protein channels in the inner membrane. An enzyme, ATP synthase, is associated with each of the proton channels. ATP synthase uses the electrical potential energy of the proton gradient to phosphorylate ADP to ATP. At the end of the carrier chain, electrons and protons are recombined and reduce oxygen to water. In the absence of oxygen as a hydrogen acceptor (in alcoholic and lactic fermentations), a small yield of ATP is made through glycolysis, then dumping hydrogen into other pathways in the cytoplasm which produce ethanol or lactate. The lactate pathway can be reversed in mammals when oxygen becomes available. The oxygen needed to remove the lactate produced during lactic fermentation is called the oxygen debt. The energy values of respiratory substrates depend on the number of hydrogen atoms per molecule. Lipids have a higher energy density than carbohydrates or proteins. The respiratory quotient (RQ) is the ratio of the volume of oxygen absorbed and the volume of carbon dioxide given off in respiration. The RQ reveals the nature of the substrate being respired. Carbohydrate has an RQ of 1.0, lipid 0.7 and protein 0.9. Oxygen uptake, and hence RQ, can be measured using a respirometer.
Chapter 12: Energy and respiration End-of-chapter questions 1 What does not occur in the conversion of glucose to two molecules of pyruvate? A hydrolysis of ATP B phosphorylation of ATP C phosphorylation of triose (3C) sugar D reduction of NAD [1] 2 Where does each stage of aerobic respiration occur in a eukaryotic cell? Link reaction Krebs cycle Oxidative phosphorylation A cytoplasm mitochondrial matrix mitochondrial cristae B mitochondrial cristae cytoplasm mitochondrial matrix C cytoplasm mitochondrial cristae mitochondrial matrix D mitochondrial matrix mitochondrial matrix mitochondrial cristae 3 The diagram summarises how glucose can be used to produce ATP, without the use of oxygen. glucose X Y in mammals [1] Z in yeast Which compounds are represented by the letters X, Y and Z ? X Y Z A ethanol pyruvate lactate B lactate ethanol pyruvate C pyruvate ethanol lactate D pyruvate lactate ethanol 4 Distinguish between: a an energy currency molecule and an energy storage molecule [2] b decarboxylation and dehydrogenation. [2] [Total: 4] 5 State the roles in respiration of: a NAD [1] b coenzyme A [1] c oxygen. [1] [Total: 3] 6 Copy and complete the table to show how much ATP is used and produced for each molecule of glucose respired in the various stages of respiration. [1] 283 glycolysis link reaction Krebs cycle oxidative phosphorylation Total ATP used ATP produced Net gain in ATP [5]
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Mary Jones, Richard Fosbery, Jennif
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Index gluconeogenesis, 317 glucose,
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Acknowledgements Meiosis” in Tuat
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Cambridge International A Level Biology Revision Guide

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