8.12
Products of the Citric Acid Cycle
The cells of most organisms—including plants and animals—obtain usable energy through aerobic respiration, the oxygen-requiring version of cellular re…
In the presence of oxygen, pyruvate moves into the mitochondria after glycolysis, where oxidation of one molecule of pyruvate produces one acetyl-CoA, one molecule of carbon dioxide, or CO2, and one NADH. Then, acetyl-CoA enters the citric acid cycle and is modified to produce two molecules of CO2, three NADHs, one ATP, and one FADH2 in addition to the earlier products of glycolysis. So the product of a single glucose molecule, or two pyruvate molecules after oxidation of pyruvate and the citric acid cycle is six molecules of CO2, eight NADHs, two ATPs, and two molecules of FADH2.
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Q1: What are the main products generated from one glucose molecule through the citric acid cycle?
One glucose molecule yields six carbon dioxide molecules, eight NADH molecules, two ATP molecules, and two FADH2 molecules after glycolysis, pyruvate oxidation, and the citric acid cycle. These products result from the cycle running twice, since glycolysis produces two pyruvate molecules that are converted into two acetyl-CoA molecules entering the cycle.
Q2: How many products does each acetyl-CoA molecule generate in the citric acid cycle?
Each acetyl-CoA molecule produces two carbon dioxide molecules, three NADH molecules, one FADH2 molecule, and one ATP molecule during the citric acid cycle. Since one glucose generates two acetyl-CoA molecules through pyruvate oxidation, the cycle runs twice per glucose, doubling these individual products.
Q3: Why are NADH and FADH2 considered critical products of the citric acid cycle?
NADH and FADH2 function as electron carriers that donate electrons to the electron transport chain during oxidative phosphorylation. Although the citric acid cycle produces modest ATP directly, the coenzymes it generates are essential for producing most of the ATP generated by cellular respiration.
Q4: What happens to pyruvate after glycolysis in aerobic respiration?
After glycolysis, pyruvate enters the mitochondria where it undergoes oxidation to produce one acetyl-CoA, one carbon dioxide molecule, and one NADH molecule. This pyruvate oxidation step prepares acetyl-CoA to enter the citric acid cycle for further energy extraction and coenzyme generation.
Q5: How does the citric acid cycle regenerate its starting molecule?
The citric acid cycle regenerates oxaloacetate, the molecule that combines with acetyl-CoA at the cycle's beginning to form citric acid. This regeneration allows the cycle to continue operating repeatedly, processing each acetyl-CoA molecule that enters from pyruvate oxidation in the mitochondria.
Q6: Why does the citric acid cycle run twice for each glucose molecule?
Glycolysis produces two pyruvate molecules from one glucose. Each pyruvate is converted into one acetyl-CoA through oxidation, generating two acetyl-CoA molecules total. Since each acetyl-CoA enters the citric acid cycle once, the cycle must run twice to process all products from a single glucose molecule.
Q7: What is the relationship between the citric acid cycle and ATP yield from glucose?
The citric acid cycle directly produces only two ATP molecules per glucose, but generates six NADH and two FADH2 molecules that serve as electron carriers. These coenzymes drive the majority of ATP production through oxidative phosphorylation, making the cycle's coenzyme output far more significant than its direct ATP yield.