Lesson 2 of 5 / Respiration and ATP production
The link reaction and Krebs cycle
Where do glucose carbons leave as CO2?
I can outline mitochondrial oxidation, dehydrogenation and decarboxylation.
I can recognise chloroplast and mitochondrial components in drawings and micrographs.Syllabus
Syllabus 9477, Core 3(a). Recognise chloroplast and mitochondrial components in drawings and micrographs.
I can outline the link reaction and Krebs cycle, including dehydrogenation and decarboxylation.Syllabus
Syllabus 9477, Core 3(g). Outline the link reaction and Krebs cycle, including dehydrogenation and decarboxylation.
About 8 min
Make a guess. You are not marked.
Where do the carbon atoms of glucose leave as carbon dioxide during aerobic respiration?
Show the answer
In the link reaction and the Krebs cycle.
Decarboxylation of pyruvate and of Krebs cycle intermediates releases all six carbons as CO2.
The key idea
The 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.
Watch out for this
A student says: "The four-carbon acceptor is used up once and must be supplied anew for every turn." What is wrong with this?
Show the answer
It is regenerated as the cycle proceeds, allowing repeated acetyl-group entry.
Check your understanding
How many Krebs turns are associated with one glucose?
Show the answer
Two
Each of the two acetyl CoA enters a turn.
Exam-style questions
Check your understanding
One glucose molecule is fully respired aerobically. How many molecules of CO2 are released in the Krebs cycle itself?
Show the answer
4
Each turn releases 2 CO2 and one glucose gives two acetyl CoA, so 2 turns x 2 = 4. The other 2 come from the link reaction.
Quick recall
Where do glucose carbons leave as CO2?