Lesson 4 of 6 / Photosynthesis and respiration
Release carbon dioxide and reduce carriers
Where does pyruvate carbon go under aerobic conditions?
I can identify initial reactants, final products and mitochondrial locations of the link reaction and Krebs cycle.
I can identify inputs and outputs of glycolysis, link reaction, Krebs cycle and oxidative phosphorylation.Syllabus
Syllabus 8876, Core 3(d). Identify inputs and outputs of glycolysis, link reaction, Krebs cycle and oxidative phosphorylation.
I can locate glycolysis in cytoplasm and other aerobic stages in mitochondria.Syllabus
Syllabus 8876, Core 3(e). Locate glycolysis in cytoplasm and other aerobic stages in mitochondria.
About 6 min
Make a guess. You are not marked.
You breathe out carbon dioxide. Which stages of respiration produce it?
Show the answer
The link reaction and the Krebs cycle.
Carbon from pyruvate and acetyl groups is released as CO2 in these mitochondrial stages.
The key idea
The link reaction converts pyruvate to acetyl-CoA, releasing carbon dioxide and reducing NAD. The Krebs cycle oxidises acetyl groups, releasing more carbon dioxide and reduced carriers.
Explore the idea
Prepare acetyl groups, then reduce carriers
One carbon leaves each pyruvate as CO2. The remaining acetyl group joins coenzyme A; NAD is reduced.
Original models of required stage inputs and outputs. Shapes are not full chemical formulae; exact pathway ATP yields are not required here.
Explanation
The link reaction occurs in the mitochondrial matrix. A three-carbon pyruvate loses carbon dioxide and is oxidised as NAD becomes reduced. The remaining two-carbon acetyl group joins coenzyme A, forming acetyl-CoA.
The Krebs cycle also occurs in the matrix. Acetyl-CoA supplies an acetyl group to a repeating reaction cycle, and coenzyme A is released for reuse. The cycle releases carbon dioxide and produces reduced NAD, reduced FAD and ATP or an equivalent transferable energy product.
NAD and FAD are electron/hydrogen carriers. Their reduced forms pass reducing equivalents to the later oxidative-phosphorylation stage, where much of the aerobic ATP production occurs. They are not themselves carbon dioxide or the final waste product.
A cycle must regenerate its starting acceptor rather than use it up permanently. H1 requires the initial reactants and final products, not the intermediate names or enzymes. At this level, account for carbon dioxide release, carrier reduction and the connection to the next stage rather than memorising every internal step.
Step by step
- 1
Link pyruvate to acetyl-CoA
Carbon dioxide is released and NAD is reduced as the acetyl group is prepared.
- 2
Run the carbon-oxidation cycle
Acetyl groups enter the Krebs cycle and carbon dioxide is released.
- 3
Deliver reduced carriers
Reduced NAD and FAD connect these stages to oxidative phosphorylation.
Worked example
Identify the one-carbon loss
A three-carbon pyruvate becomes a two-carbon acetyl group in the link reaction. What accounts for the missing carbon?
One way to explain it
It is released as carbon dioxide. At the same time NAD is reduced, and the acetyl group combines with coenzyme A to form acetyl-CoA.
Why this answer works
- Conserve carbon across the stage.
- Name both the carbon product and the carrier change.
Watch out for this
A student says: "The Krebs cycle is where glucose first splits directly into pyruvate." What is wrong with this?
Show the answer
Glucose-to-pyruvate conversion occurs in glycolysis in the cytoplasm. The Krebs cycle receives acetyl groups via acetyl-CoA in the mitochondrion.
You may meet this in school papers
Not required by the 2027 syllabus, but school papers still ask it. Learn it after the main idea.
Counting carbons in the Krebs cycle
School papers sometimes ask for carbon counts. The two-carbon acetyl group joins a four-carbon compound to make a six-carbon compound. Two decarboxylations then release two CO2, giving back the four-carbon compound. One glucose gives two pyruvate, so there are two link reactions (2 CO2) and two turns of the cycle (4 CO2): all six carbons leave as CO2. H1 does not require these counts.
Check your understanding
Which pair carries reducing equivalents towards oxidative phosphorylation?
Show the answer
Reduced NAD and reduced FAD.
They deliver electrons to the later energy-transfer system.
Exam-style questions
Check your understanding
Which happen in the mitochondrial matrix? 1: Glycolysis. 2: The link reaction. 3: The Krebs cycle. 4: Release of CO2.
Show the answer
2, 3 and 4 only
The link reaction and Krebs cycle take place in the matrix and both release CO2. Glycolysis is in the cytoplasm.
Quick recall
Where does pyruvate carbon go under aerobic conditions?