(a) In the link reaction, a two-carbon acetyl group is produced from pyruvate. The acetyl group is transferred to coenzyme A to form acetyl coenzyme A (acetyl-coA). State the terms used to summarise the two chemical changes that occur in the link reaction to produce an acetyl group from pyruvate. (b) Acetyl-coA combines with oxaloacetate in the Krebs cycle to form citrate. This reaction is catalysed by the enzyme citrate synthase. Acetyl-coA has a similar shape to succinyl-coA, one of the compounds made in a later part of the Krebs cycle. An experiment was carried out to investigate the effect of increasing the concentration of acetyl-coA on the activity of citrate synthase. The experiment was repeated, this time adding a solution of succinyl-coA. [Figure 8.1] shows the results of these experiments. With reference to Fig. 8.1, explain how succinyl-coA could help to regulate the Krebs cycle. (c) The coenzyme NAD plays an important role in respiration. Describe the role of NAD in the stages of aerobic respiration that occur in a mitochondrion.

1. (a)State the terms used to summarise the two chemical changes that occur in the link reaction to produce an acetyl group from pyruvate.[2]
2. (b)With reference to Fig. 8.1, explain how succinyl-coA could help to regulate the Krebs cycle.[4]
3. (c)Describe the role of NAD in the stages of aerobic respiration that occur in a mitochondrion.[4]

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