Lesson 4 of 6 / Photosynthesis and respiration
Release carbon dioxide and reduce carriers
Where does pyruvate carbon go under aerobic conditions?
In this lesson: Identify initial reactants, final products and mitochondrial locations of the link reaction and Krebs cycle.
About 6 min
The key ideaThe 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.
Is this true? "The Krebs cycle is where glucose first splits directly into pyruvate."
Glucose-to-pyruvate conversion occurs in glycolysis in the cytoplasm. The Krebs cycle receives acetyl groups via acetyl-CoA in the mitochondrion.