Link Reaction: Pyruvate to Acetyl CoA (Pyruvate Oxidation)
Biology · Respiration in Plants · NEET
The link reaction is the step that joins glycolysis to the Krebs cycle. Each pyruvic acid (3-carbon) enters the mitochondrial matrix and is changed into acetyl CoA (2-carbon) by the enzyme pyruvate dehydrogenase. This step removes one CO2 and one pair of hydrogens, so per pyruvate you get 1 CO2 and 1 NADH. Memory hook: "3C pyruvate loses 1 CO2 to become 2C acetyl, hooks onto CoA, and drops off NADH."
The link reaction: pyruvic acid (3C) enters the mitochondrial matrix and pyruvate dehydrogenase removes one CO2 and one NADH, forming acetyl CoA (2C), which then enters the Krebs cycle. Per glucose this happens twice.
Your doubts, answered
Is the link reaction part of glycolysis or the Krebs cycle?
It is neither. Glycolysis ends when glucose becomes 2 pyruvate in the cytoplasm. The Krebs cycle starts when acetyl CoA joins oxaloacetic acid. The link reaction (pyruvate to acetyl CoA) sits between them, which is why it is called the 'link' reaction. NEET often traps students who count its CO2 as part of the Krebs cycle. Remember: the 2 decarboxylations counted 'in the TCA cycle' do NOT include this one.
Where in the cell does pyruvate become acetyl CoA?
In the mitochondrial matrix. Pyruvate is made in the cytoplasm during glycolysis, then it is transported into the mitochondrion. NCERT clearly says pyruvate enters the mitochondrial matrix and undergoes oxidative decarboxylation there. The electron transport system, in contrast, is on the inner mitochondrial membrane, not in the matrix.
How many carbons are lost when pyruvate becomes acetyl CoA?
One carbon is lost as CO2. Pyruvic acid has 3 carbons; acetyl CoA has a 2-carbon acetyl group. So exactly one carbon leaves as carbon dioxide. Because one glucose gives 2 pyruvate, the link reaction releases 2 CO2 and 2 NADH per glucose.
Which enzyme catalyses the link reaction?
Pyruvate dehydrogenase (NCERT calls it pyruvic dehydrogenase). It is actually a complex of enzymes. The reaction needs coenzymes including NAD+ and Coenzyme A. In the NEET matching PYQ, 'oxidative decarboxylation' is matched with pyruvate dehydrogenase, while the TCA cycle is matched with citrate synthase.
Why is the step called oxidative decarboxylation?
'Decarboxylation' means a carboxyl carbon is removed as CO2. 'Oxidative' means hydrogens/electrons are removed and given to NAD+, making NADH. Both happen together in one step, so pyruvate is both decarboxylated and oxidised, giving acetyl CoA plus CO2 plus NADH.
⚠️ The NEET trap ✗ The link reaction is one of the decarboxylation steps of the Krebs cycle, so the cycle has 3 decarboxylations. ✓ The Krebs cycle itself has only 2 decarboxylations (isocitrate to alpha-ketoglutarate, and alpha-ketoglutarate to succinyl CoA). The pyruvate to acetyl CoA decarboxylation is the link reaction and is NOT counted inside the TCA cycle. 🧠 Count carbon: pyruvate loses its CO2 BEFORE the cycle starts. The cycle begins only when acetyl CoA meets OAA.
Real NEET questions
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 (A-II). Glycolysis is the EMP pathway (B-IV). Oxidative phosphorylation is driven by the electron transport system (C-III). The TCA cycle begins with citrate synthase (D-I). So the answer is A-II, B-IV, C-III, D-I.
NEET 2016
Which of the following biomolecules is common to respiration-mediated breakdown of fats, carbohydrates and proteins?
A · Glucose-6-phosphate
B · Fructose 1,6-bisphosphate
C · Pyruvic acid
D · Acetyl CoA ✓
Solution: Acetyl CoA is the common meeting point. Carbohydrates give pyruvate which becomes acetyl CoA (the link reaction), fatty acids are broken directly to acetyl CoA, and many amino acids also enter as acetyl CoA. Glucose-6-phosphate and fructose 1,6-bisphosphate are only in carbohydrate glycolysis, and fats bypass pyruvic acid. So acetyl CoA is the shared molecule.
Solved Respiration in Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What are the products of the link reaction from one glucose?
From one glucose you get 2 pyruvate, so the link reaction gives 2 acetyl CoA, 2 CO2 and 2 NADH.
Does the link reaction make ATP directly?
No. It makes no ATP directly. Its energy is stored in NADH, which later makes ATP in the electron transport system.
What is the role of Coenzyme A here?
Coenzyme A picks up the 2-carbon acetyl group to form acetyl CoA, which then carries the acetyl group into the Krebs cycle.
Is oxygen directly used in the link reaction?
No, oxygen is not a direct reactant. But the reaction happens only in aerobic respiration because NAD+ must be regenerated later by the ETS, which needs oxygen.
