Decarboxylation is the step that removes a carboxyl (-COOH) group from an acid and releases it as CO2. In aerobic respiration, CO2 comes out only after glycolysis: 1 CO2 in the link reaction (pyruvate to acetyl CoA) and 2 CO2 in each turn of the Krebs cycle. Memory hook: "Link gives 1, Krebs gives 2, doubled for 2 pyruvate = 6 CO2 per glucose."
CO2 is released only after glycolysis: 1 CO2 in the link reaction and 2 CO2 per Krebs turn. Doubling for two pyruvate gives 6 CO2 per glucose.
Your doubts, answered
How many times does decarboxylation occur in one Krebs (TCA) cycle?
Twice. In one turn there are two decarboxylation steps: isocitrate to alpha-ketoglutaric acid, and alpha-ketoglutaric acid to succinyl-CoA. Each removes one CO2, so 2 CO2 leave per turn. This is a direct NEET 2023 fact.
Is any CO2 released during glycolysis?
No. Glycolysis converts glucose to two molecules of pyruvic acid without releasing any CO2. CO2 release begins only after glycolysis, in the link reaction and Krebs cycle. This is a very common NEET trap.
What is oxidative decarboxylation of pyruvate?
It is the link reaction where pyruvic acid enters the mitochondrial matrix and is converted to acetyl CoA. It removes one CO2 and reduces one NAD+ to NADH. The enzyme is pyruvic (pyruvate) dehydrogenase. It happens twice per glucose because glycolysis makes 2 pyruvate.
How many CO2 molecules are released from one glucose in aerobic respiration?
Six. Per glucose there are 2 pyruvate. Each pyruvate gives 1 CO2 in the link reaction (2 total) and each Krebs turn gives 2 CO2 (2 turns = 4 total). 2 + 4 = 6 CO2, which balances the 6 carbons of glucose.
Does the link reaction count as part of the Krebs cycle?
No. The pyruvate to acetyl CoA step (oxidative decarboxylation) is a separate link reaction. When NEET asks decarboxylations 'in the TCA cycle', the answer is 2, not 3. Count the link reaction only when the question says 'per glucose' or 'total'.
⚠️ The NEET trap ✗ Decarboxylation occurs 3 times in one Krebs cycle (counting the link reaction). ✓ It occurs 2 times inside one Krebs cycle. The link reaction (pyruvate to acetyl CoA) is a separate step and is not part of the TCA cycle. 🧠 Read whether the question asks 'in Krebs cycle' or 'total per glucose'.
Real NEET questions
NEET 2023
How many times decarboxylation occurs during each TCA cycle?
A · Once
B · Twice ✓
C · Thrice
D · Many
Solution: In one turn of the Krebs/TCA cycle there are two decarboxylation steps: isocitrate to alpha-ketoglutaric acid, and alpha-ketoglutaric acid to succinyl-CoA. Each releases one CO2, so decarboxylation occurs twice per turn. The pyruvate to acetyl CoA decarboxylation is the link reaction and is not counted inside the TCA cycle.
NEET 2023
Match List I with List II: A. Oxidative decarboxylation, B. Glycolysis, C. Oxidative phosphorylation, D. Tricarboxylic acid cycle; I. Citrate synthase, II. Pyruvate dehydrogenase, III. Electron transport system, IV. EMP pathway.
A · A-III, B-IV, C-II, D-I
B · A-II, B-IV, C-I, D-III
C · A-III, B-I, C-II, D-IV
D · A-II, B-IV, C-III, D-I ✓
Solution: Oxidative decarboxylation of pyruvate to acetyl CoA is catalysed by pyruvate dehydrogenase (II). Glycolysis is the EMP pathway (IV). Oxidative phosphorylation is driven by the electron transport system (III). The TCA cycle begins with citrate synthase (I). So A-II, B-IV, C-III, D-I.
Solved Respiration in Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Isocitrate to alpha-ketoglutaric acid, and alpha-ketoglutaric acid to succinyl-CoA. Both are oxidative decarboxylations and each releases one CO2.
How many CO2 are released per glucose in total?
Six CO2 per glucose: 2 from the link reaction (1 per pyruvate) and 4 from two Krebs turns (2 per turn).
Why is it called 'oxidative' decarboxylation?
Because CO2 is removed and, at the same time, NAD+ is reduced to NADH (an oxidation of the substrate). Removal of CO2 plus oxidation together make it oxidative decarboxylation.
Does oxygen directly remove the CO2?
No. Decarboxylation itself does not use oxygen directly. Oxygen acts later in the electron transport system as the final electron acceptor. CO2 comes from the carbon skeleton of the acids.
