Cell Biology & Energy (BIO 101) · Module 4 of 8
Pyruvate Oxidation & the Citric Acid Cycle
How pyruvate is fully dismantled to CO2, harvesting electron carriers for the payoff stage to come.
Pyruvate Oxidation (Link Reaction)
- Before entering the citric acid cycle, each pyruvate is transported into the mitochondrial matrix and converted to acetyl-CoA, releasing one CO2 and producing one NADH per pyruvate.
- Because there are two pyruvates per glucose, this step happens twice per glucose, yielding 2 NADH and releasing 2 CO2 total.
What happens to pyruvate before it can enter the citric acid cycle?
Solution. It is transported into the mitochondrial matrix and converted into acetyl-CoA by pyruvate dehydrogenase, releasing one CO2 and generating one NADH in the process.
Steps & Yield of the Citric Acid Cycle
- Acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to form citrate (6 carbons), which is progressively oxidized and decarboxylated back to oxaloacetate, completing the cycle.
- Each turn of the cycle produces 3 NADH, 1 FADH₂, and 1 ATP (via GTP), and releases 2 CO2.
Per turn of the citric acid cycle, how many NADH, FADH₂, and ATP are produced?
Solution. 3 NADH, 1 FADH₂, and 1 ATP (via GTP) per turn.
Per-Glucose Totals
- Because each glucose yields two acetyl-CoA molecules, the citric acid cycle runs twice per glucose, producing 6 NADH, 2 FADH₂, and 2 ATP total from this stage.
Why does the citric acid cycle run twice for every one glucose molecule that enters glycolysis?
Solution. Glycolysis splits glucose into two pyruvate molecules, each of which is converted into one acetyl-CoA, so two acetyl-CoA molecules enter the citric acid cycle per original glucose.
Where the Carbon and Energy Go
- All six carbons of the original glucose are eventually released as CO2: 2 during pyruvate oxidation and 4 during the citric acid cycle (2 per turn × 2 turns).
- The citric acid cycle itself makes little ATP directly; its main output is the electron carriers (NADH, FADH₂) that will drive oxidative phosphorylation in the next stage.
If the citric acid cycle produces so little ATP directly, why is it still essential for energy production?
Solution. Its main contribution is generating the electron carriers NADH and FADH₂, which carry high-energy electrons to the electron transport chain, where the vast majority of ATP is actually produced.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What molecule does pyruvate become before entering the citric acid cycle?
2. How many NADH are produced per turn of the citric acid cycle?