Per glucose molecule, glycolysis produces two 3-carbon pyruvates. How many carbons from these pyruvates ultimately enter the Krebs cycle as acetyl groups?
A6 — all carbons are preserved as acetyl-CoA for the Krebs cycle
B4 — one carbon per pyruvate is lost as CO₂, leaving two 2-carbon acetyl groups
C2 — only one pyruvate is processed at a time, contributing one acetyl group
D3 — one full pyruvate enters intact while the other becomes CO₂
Pyruvate oxidation releases one CO₂ per pyruvate (oxidative decarboxylation), reducing each 3-carbon pyruvate to a 2-carbon acetyl group. Two pyruvates per glucose means 2 CO₂ released and 4 carbons remaining as two acetyl-CoA molecules. Students who answer 6 are forgetting the decarboxylation step entirely — the whole point of this bridge step is that one carbon per pyruvate is removed before the Krebs cycle.
Question 2 Multiple Choice
The pyruvate dehydrogenase complex is inhibited by high levels of acetyl-CoA and NADH, and activated when CoA and NAD⁺ are abundant. What does this regulation accomplish?
AIt ensures pyruvate is converted to acetyl-CoA at a steady, constant rate regardless of energy status
BIt commits carbon to energy extraction only when the cell is actually energy-depleted, not when products are already abundant
CIt keeps acetyl-CoA and NADH at precisely equal concentrations for Krebs cycle efficiency
DIt accelerates pyruvate oxidation when ATP is high, building a reserve of acetyl-CoA for biosynthesis
High NADH and acetyl-CoA signal that the cell already has abundant downstream energy products; further pyruvate oxidation would wastefully consume carbon that could serve biosynthetic purposes. Product inhibition slows the complex under energy-replete conditions. When NAD⁺ and CoA are high (energy-depleted state), the complex is activated to replenish the NADH pool. This is feedback regulation applied to an irreversible commitment-point reaction — the logic of not running an irreversible process when its products are already in excess.
Question 3 True / False
The CO₂ released during pyruvate oxidation represents carbons that will not enter the Krebs cycle.
TTrue
FFalse
Answer: True
True. Oxidative decarboxylation removes one carbon from each pyruvate as CO₂, which exits the cell. Only the remaining 2-carbon acetyl group is loaded onto CoA and delivered to the Krebs cycle. This is why the step is called 'oxidative decarboxylation' — 'decarboxylation' literally means carbon removal. The CO₂ released here is distinct from the CO₂ produced in the Krebs cycle, where the remaining four carbons are ultimately released.
Question 4 True / False
Because acetyl-CoA can be converted back to pyruvate, the cell can use fatty acids (which are degraded to acetyl-CoA) to synthesize glucose via gluconeogenesis.
TTrue
FFalse
Answer: False
False. Pyruvate oxidation is irreversible — the pyruvate dehydrogenase complex cannot run in reverse, and the CO₂ released by decarboxylation cannot be recaptured. This means acetyl-CoA cannot be converted back to pyruvate. Since fatty acids are degraded to acetyl-CoA via beta-oxidation, and acetyl-CoA cannot enter gluconeogenesis, fats cannot yield net glucose in animals. This is a major metabolic asymmetry: carbohydrates can be converted to fat, but fat cannot be converted back to net carbohydrate.
Question 5 Short Answer
Why is pyruvate oxidation described as a 'metabolic commitment point,' and what metabolic consequence follows from this irreversibility for organisms burning fat during starvation?
Think about your answer, then reveal below.
Model answer: Pyruvate oxidation is irreversible because the decarboxylation step releases carbon as CO₂ — a gas that cannot be recaptured. Once pyruvate becomes acetyl-CoA, that carbon is committed to the Krebs cycle and cannot re-enter gluconeogenesis. During starvation, fatty acids are degraded to acetyl-CoA (beta-oxidation), but because acetyl-CoA cannot be converted back to pyruvate, fat cannot serve as a net source of glucose. The organism must rely on amino acids or glycerol for gluconeogenesis rather than on its fat stores.
This irreversibility explains why prolonged fasting or uncontrolled diabetes leads to ketosis — acetyl-CoA from fat oxidation accumulates faster than the Krebs cycle can consume it (which requires oxaloacetate, a gluconeogenic intermediate that becomes depleted when carbohydrates are scarce). Understanding the commitment point reveals that the cell's carbohydrate and fat pathways are not simply reversible — the one-way door at pyruvate oxidation has profound consequences for whole-body energy metabolism.