explain that, when oxygen is available, pyruvate enters mitochondria to take part in the link reaction

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – Respiration: The Link Reaction

Respiration – The Link Reaction (Aerobic Pathway)

Learning Objective

Explain that, when oxygen is available, pyruvate enters the mitochondria to take part in the link reaction.

Overview of Aerobic Respiration

Aerobic respiration occurs in three main stages:

  1. Glycolysis (cytoplasm)

  2. The link reaction (mitochondrial matrix)

  3. The Krebs cycle and oxidative phosphorylation (mitochondrial matrix and inner membrane)

Oxygen is the final electron acceptor in the electron transport chain; without it, the link reaction cannot proceed efficiently.

The Link Reaction

Also known as the pyruvate dehydrogenase complex (PDC) reaction, it converts each molecule of pyruvate into acetyl‑CoA, releasing CO₂ and producing NADH.

Key Steps

  • Pyruvate is transported from the cytosol into the mitochondrial matrix via a specific pyruvate carrier.

  • The pyruvate dehydrogenase complex (E1, E2, E3) catalyses the decarboxylation and oxidation of pyruvate.

  • Coenzyme A (CoA‑SH) attaches to the resulting acetyl group, forming acetyl‑CoA.

  • NAD⁺ is reduced to NADH, which will later donate electrons to the electron transport chain.

Chemical Equation

For one molecule of pyruvate:

\$\text{Pyruvate} + \text{CoA‑SH} + \text{NAD}^+ \;\longrightarrow\; \text{Acetyl‑CoA} + \text{CO}_2 + \text{NADH} + \text{H}^+\$

Enzyme Complex Components

ComponentFunction
E1 – Pyruvate dehydrogenase (thiamine pyrophosphate dependent)Decarboxylates pyruvate, forming a hydroxyethyl‑TPP intermediate.
E2 – Dihydrolipoamide acetyltransferaseTransfers the acetyl group to CoA, forming acetyl‑CoA.
E3 – Dihydrolipoamide dehydrogenase (FAD dependent)Regenerates the oxidised lipoamide and reduces NAD⁺ to NADH.

Products per Glucose Molecule

Since each glucose yields two pyruvate molecules, the link reaction occurs twice per glucose:

  • 2 × Acetyl‑CoA (enters the Krebs cycle)

  • 2 CO₂ (released as waste)

  • 2 NADH (later used for ATP synthesis)

Why Oxygen Is Essential

Oxygen itself does not participate directly in the link reaction, but it is required for the re‑oxidation of NADH in the electron transport chain. If oxygen is absent, NADH accumulates, inhibiting the pyruvate dehydrogenase complex and halting the link reaction.

Summary Table

StageLocationMain SubstrateMain ProductsKey Cofactors
GlycolysisCytosolGlucose2 Pyruvate, 2 ATP, 2 NADHADP, NAD⁺
Link ReactionMitochondrial matrixPyruvateAcetyl‑CoA, CO₂, NADHCoA‑SH, NAD⁺, TPP, Lipoic acid, FAD
Krebs CycleMitochondrial matrixAcetyl‑CoA3 NADH, 1 FADH₂, 1 GTP, 2 CO₂H₂O, ADP

Suggested diagram: Flow of pyruvate from cytosol into mitochondrion, showing the pyruvate dehydrogenase complex and the production of acetyl‑CoA, CO₂ and NADH.