1. Chapter 16 Question 30 Presented by Kayako and Leticia Although oxygen does not participate directly in the citric acid cycle, the cycle operates only when O 2 is present. Why?

2. Because… In the presence of O 2, aerobic organisms are able to extract large amounts of additional energy from pyruvate and NADH, that are produced during glycolysis Each pyruvate molecule is then transported across the inner mitochondrial membrane and into the matrix aerobic organisms are able to extract large amounts of additional energy from pyruvate and NADH, that are produced during glycolysis Each pyruvate molecule is then transported across the inner mitochondrial membrane and into the matrix

3. In the matrix, the pyruvate is decarboxylated to form an acetyl group This group is then transferred to coenzyme A to produce acetyl CoA Pyruvate + HS-CoA + NAD + → acetyl CoA + CO 2 + NADH + H + The acetyl CoA then enters the Citric Cycle In the matrix, the pyruvate is decarboxylated to form an acetyl group This group is then transferred to coenzyme A to produce acetyl CoA Pyruvate + HS-CoA + NAD + → acetyl CoA + CO 2 + NADH + H + The acetyl CoA then enters the Citric Cycle

4. In the absence of O 2, The supply of NAD+ is depleted The accumulated NADH allosterically inhibits pyruvate dehydrogenase and α-ketoglutarate dehydrogenase. Pyruvate is then reduced to lactate or another product of fermentation. (does not go into the matrix, stays in cytosol) The supply of NAD+ is depleted The accumulated NADH allosterically inhibits pyruvate dehydrogenase and α-ketoglutarate dehydrogenase. Pyruvate is then reduced to lactate or another product of fermentation. (does not go into the matrix, stays in cytosol)