5.2.2(d)-(e) - Link reaction and Krebs cycle
Glycolysis leaves the cell with pyruvate, but most of the chemical energy remains in the pyruvate molecules. In this lesson you will follow what happens to pyruvate in the mitochondrial matrix: the link reaction makes acetyl CoA, and the Krebs cycle releases carbon dioxide, reduces coenzymes and makes a small direct yield of ATP. Separate carbon loss, hydrogen transfer and direct ATP formation to see how these stages support the later electron transport chain.
Matrix And Carbon Flow
Glycolysis happens in the cytoplasm and produces two pyruvate molecules from one glucose molecule. This lesson starts after that point. Pyruvate enters the mitochondrion, and the link reaction and Krebs cycle both take place in the mitochondrial matrix.
Mitochondrial Matrix
The mitochondrial matrix is the fluid-filled inner region of a mitochondrion, enclosed by the inner mitochondrial membrane.
The site matters because several respiration stages are close together but not identical. The mitochondrial matrix is the correct site for the link reaction and the Krebs cycle. The inner mitochondrial membrane and cristae become important in oxidative phosphorylation, which is the next pathway rather than the focus of this lesson.
The easiest way to keep the pathway clear is to track carbon atoms.
| Point in pathway | Carbon atoms in the molecule | Meaning |
|---|---|---|
| Pyruvate | 3 | Product of glycolysis that enters the mitochondrion |
| Acetyl group in acetyl CoA | 2 | Formed from pyruvate in the link reaction |
| Oxaloacetate | 4 | Accepts the acetyl group at the start of the Krebs cycle |
| Citrate | 6 | Forms when the 2-carbon acetyl group combines with 4-carbon oxaloacetate |
| Oxaloacetate again | 4 | Regenerated so the cycle can continue |
Because one glucose molecule makes two pyruvate molecules, the link reaction and the Krebs cycle happen twice for each glucose molecule that completes aerobic respiration.
The Link Reaction
The link reaction links glycolysis to the Krebs cycle. It converts pyruvate into acetyl CoA. The reaction is short, but it uses three important ideas.
Decarboxylation
Decarboxylation is the removal of carbon dioxide from a molecule.
Pyruvate has three carbon atoms. During the link reaction, one carbon atom is removed as carbon dioxide, so the remaining acetyl group has two carbon atoms.
Dehydrogenation
Dehydrogenation is the removal of hydrogen atoms from a molecule.
When hydrogen is removed from pyruvate, NAD accepts the hydrogen and is reduced to reduced NAD. In this wording, NAD is the molecule being reduced. Do not write that NAD is oxidised in the link reaction.
The two-carbon acetyl group then combines with coenzyme A to form acetyl CoA. Acetyl CoA carries the acetyl group into the Krebs cycle. No ATP is made directly in the link reaction.
For one pyruvate molecule, the link reaction produces:
| Product | Amount per pyruvate | Why it forms |
|---|---|---|
| Acetyl CoA | 1 | The 2-carbon acetyl group attaches to coenzyme A |
| Carbon dioxide | 1 | One carbon is removed by decarboxylation |
| Reduced NAD | 1 | NAD is reduced when hydrogen is removed |
Coenzyme A is a carrier, not a two-carbon molecule: only its attached acetyl group contains the two carbons being tracked here. The rest of the coenzyme also contains carbon, but is recycled rather than oxidised as part of this fuel molecule. Decarboxylation removes carbon as CO₂; dehydrogenation transfers hydrogen to NAD. Neither term means adding oxygen directly to pyruvate.
The Krebs Cycle
The Krebs cycle is a cycle because one of its starting molecules, oxaloacetate, is regenerated at the end. This allows another acetyl group to enter and the sequence to continue.
Acetyl CoA delivers a two-carbon acetyl group to oxaloacetate. The acetyl group combines with four-carbon oxaloacetate to form six-carbon citrate. Coenzyme A is released and can be reused.
Continue this lesson.
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