Lesson 2 of 5 / Respiration and ATP production
The link reaction and Krebs cycle
Where do glucose carbons leave as CO2?
In this lesson: Outline mitochondrial oxidation, dehydrogenation and decarboxylation.
About 8 min
The key ideaThe link reaction forms acetyl coenzyme A; the Krebs cycle releases its carbon as CO2 while reducing electron carriers.
Explore the idea
Count per pyruvate, then per glucose
| Stage | CO2 | NADH | FADH2 | ATP equivalent |
|---|---|---|---|---|
| Link | 1 | 1 | 0 | 0 |
| Krebs | 2 | 3 | 1 | 1 |
One pyruvate gives one acetyl group and one Krebs-cycle turn. The four-carbon acceptor is regenerated rather than used up at every turn.
The diagram follows carbon counts, not the identity of individual carbon atoms: the first CO2 released need not contain the newly added acetyl carbons. NADH and FADH2 are reduced carriers. The cycle's ATP equivalent is produced by substrate-level phosphorylation, not a fixed estimate of later oxidative ATP yield.
Explanation
Pyruvate enters the mitochondrial matrix. In the link reaction, each three-carbon pyruvate is decarboxylated to a two-carbon acetyl group, releasing CO2, and dehydrogenated, reducing NAD. The acetyl group joins coenzyme A to form acetyl CoA. Per glucose this gives two acetyl CoA, two CO2 and two reduced NAD.
In the Krebs cycle, a two-carbon acetyl group combines with a four-carbon acceptor to form a six-carbon compound. Subsequent reactions regenerate the four-carbon acceptor. Decarboxylations release two CO2 per turn, while dehydrogenations transfer reducing power to NAD and FAD.
Per acetyl CoA, the cycle produces three reduced NAD, one reduced FAD and one ATP equivalent through substrate-level phosphorylation. Since one glucose yields two acetyl CoA, double these values: six reduced NAD, two reduced FAD, two ATP equivalents and four CO2.
The cycle itself does not directly consume oxygen, but depends on oxidised carrier regeneration by aerobic electron transport. If that regeneration fails, reduced carriers accumulate and oxidation slows. The syllabus requires locations, inputs, outputs and dehydrogenation/decarboxylation logic, not a memorised list of every intermediate structure or enzyme.
Step by step
- 1
Set the denominator
Per pyruvate, acetyl CoA or glucose?
- 2
Balance carbons
Track CO2 release and acceptor regeneration.
- 3
Count reduced carriers
These carry energy to oxidative phosphorylation.
Worked example
Work through the evidence
What is the total CO2 from link reaction plus Krebs cycle per glucose?
One way to explain it
Two from the link reactions plus four from two Krebs turns gives six CO2.
Why this answer works
- Glycolysis releases no CO2 in this standard pathway.
- The six glucose carbons are accounted for across the later oxidation steps.
Is this true? "The four-carbon acceptor is used up once and must be supplied anew for every turn."
It is regenerated as the cycle proceeds, allowing repeated acetyl-group entry.