Connect pyruvate to mitochondrial respiration
Explain that pyruvate enters mitochondria when oxygen is available and becomes a substrate for the link reaction.
A Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; practical investigations need supervised work and human subject-teacher review remains pending.
Before you begin
- ATP transfers energy for cellular work, and enzymes control metabolic reactions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where does pyruvate go for the link reaction in aerobic respiration?
What you will learn
- Relate oxygen availability to pyruvate entering mitochondria
- Connect glycolysis with the link reaction
Build the explanation
When oxygen is available for aerobic respiration, pyruvate produced in the cytoplasm enters mitochondria and reaches the matrix, where the link reaction occurs. Pyruvate is a three-carbon molecule; in the link reaction one carbon is removed as carbon dioxide and the remaining two-carbon acetyl group is transferred to coenzyme A. Oxygen does not react directly with pyruvate in this step, but its availability supports continued aerobic pathways, including oxidative phosphorylation.
Apply it carefully
The phrase 'when oxygen is available' describes the route into aerobic respiration, not oxygen being a reactant in the link reaction. Keep the carbon count explicit: 3C pyruvate loses 1C as CO₂, leaving a 2C acetyl group. Coenzyme A carries the acetyl group into the Krebs cycle.
Pause and explain
How many carbons are in the acetyl group transferred to coenzyme A?
Worked example
What happens to the carbon number of pyruvate in the link reaction?
Show the worked solution
- One carbon leaves as carbon dioxide.
- The remaining acetyl group contains two carbons.
Answer 3C pyruvate loses CO₂ (1C) to form a 2C acetyl group.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how pyruvate from glycolysis reaches the link reaction and what happens to its carbon skeleton.
Give me a hint
Name the location, carbon loss and carrier.
Compare my reasoning
- When oxygen is available, pyruvate enters the mitochondrion and reaches the matrix.
- It loses CO₂ and its remaining 2C acetyl group is transferred to coenzyme A.
When oxygen is available for aerobic respiration, pyruvate enters mitochondria and reaches the matrix. During the link reaction it loses one carbon as CO₂; the remaining 2C acetyl group is transferred to coenzyme A.
Look for these in your work
- Mitochondrial location correct
- Carbon loss stated
- Acetyl-CoA transfer stated
A student writes that oxygen combines directly with pyruvate in the link reaction. Correct this while preserving the condition for aerobic entry.
Give me a hint
Distinguish a condition for aerobic respiration from a reactant in this stage.
Compare my reasoning
- Pyruvate enters the mitochondrion when oxygen is available.
- The link reaction decarboxylates pyruvate and transfers a 2C acetyl group to coenzyme A; oxygen is not directly used in that reaction.
When oxygen is available, pyruvate can enter the mitochondrion for aerobic respiration. In the link reaction, pyruvate loses CO₂ and its 2C acetyl group binds to coenzyme A; oxygen is not a direct reactant in this step.
Look for these in your work
- Condition retained
- Link reaction described
- Oxygen role corrected
A labelled molecule has three carbons before the link reaction and two carbons in acetyl-CoA afterward. What carbon-containing product accounts for the difference?
Give me a hint
Conserve carbon atoms across the step.
Compare my reasoning
- One carbon has been removed.
- It leaves as carbon dioxide during decarboxylation.
One carbon is removed from pyruvate as carbon dioxide during decarboxylation, leaving a 2C acetyl group that combines with coenzyme A.
Look for these in your work
- One-carbon difference identified
- CO₂ named
- Remaining group explained
Common mistakes
- Saying oxygen is used directly in the link reaction
- Calling acetyl-CoA a three-carbon molecule
- Omitting carbon dioxide removal
What can you explain now?
Where does pyruvate go for the link reaction in aerobic respiration?
Compare with the explanation
Into the mitochondrion, to the matrix
The link reaction occurs in the mitochondrial matrix.
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Make it stick
How does 3C pyruvate become acetyl-CoA in the mitochondrial matrix?
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