# Link Reaction

> Biology · CIE A-Level 9700
> Source: https://www.owlsprep.com/study/cie-9700-u12-link-reaction/

The link reaction is the second stage of aerobic respiration, connecting glycolysis to the Krebs cycle. This guide covers its location, chemical steps, inputs, outputs and role in energy production for CIE A-Level Biology.

**Prerequisites:** [Glycolysis](https://www.owlsprep.com/study/cie-9700-u12-glycolysis/); [Mitochondria structure](https://www.owlsprep.com/study/cie-9700-u04-mitochondria-structure/); [Respiration coenzymes](https://www.owlsprep.com/study/cie-9700-u12-respiration-coenzymes/)

## Learning objectives

- Describe the location and key steps of the link reaction
- Calculate the total inputs and outputs per glucose molecule
- Explain the role of coenzymes in the link reaction
- Distinguish the link reaction from other stages of aerobic respiration

## Overview and Location

**Link Reaction** — A stage of aerobic respiration that converts pyruvate from glycolysis into acetyl CoA, connecting glycolysis to the Krebs cycle

*Example:* It only proceeds when oxygen is available for aerobic respiration

In eukaryotes, the enzymes that catalyse the link reaction are located in the **mitochondrial matrix**. Pyruvate produced by glycolysis in the cytoplasm diffuses across the mitochondrial inner membrane to enter the matrix for the reaction.

**Worked example:** Calculate the total net outputs of the link reaction from one starting molecule of glucose

1. Recall that 1 glucose molecule produces 2 pyruvate molecules at the end of glycolysis, so 2 rounds of the link reaction occur.
2. The overall reaction for 1 round (1 pyruvate) is:
3. $$\text{Pyruvate} + \text{CoA} + \text{NAD} \rightarrow \text{Acetyl CoA} + \text{CO}_2 + \text{reduced NAD}$$
4. Multiply all outputs by 2 to get the total yield from 1 glucose: 2 acetyl CoA, 2 CO₂, 2 reduced NAD. No ATP is produced directly.

> **Exam tip:** Always remember the link reaction produces no ATP — this is a common multiple choice trick.

## Key Steps of the Reaction

1. **Decarboxylation**: The 3-carbon pyruvate loses one carbon as carbon dioxide, leaving a 2-carbon acetyl group.
2. **Oxidation**: The acetyl group is oxidised, losing hydrogen that is transferred to the coenzyme NAD to form reduced NAD.
3. **Acetylation**: The acetyl group combines with coenzyme A (CoA) to form acetyl CoA, which carries the acetyl group to the Krebs cycle.

**Worked example:** Explain why the link reaction is described as both oxidative decarboxylation

1. First, confirm the decarboxylation part: Decarboxylation is the removal of carbon dioxide from a molecule. Pyruvate (3C) loses one carbon as CO₂, so decarboxylation occurs.
2. Next, confirm oxidation: In biological respiration, oxidation is defined as the loss of hydrogen (or electrons). The acetyl group loses hydrogen to NAD, so oxidation occurs.
3. Conclusion: The reaction involves both processes, so it is correctly called oxidative decarboxylation.

## Regulation and Biological Role

The link reaction requires a constant supply of oxidised NAD to proceed. Oxidised NAD is regenerated when reduced NAD drops off hydrogen at the electron transport chain, which requires oxygen. This means the link reaction only occurs in aerobic conditions.

Beyond being the gateway to the Krebs cycle, acetyl CoA produced by the link reaction also acts as a precursor for fatty acid synthesis when the cell has excess energy, linking carbohydrate and lipid metabolism.

**Check your understanding**

Test your understanding:

1. Where does the link reaction occur in eukaryotic cells?

   - Cytoplasm
   - Outer mitochondrial membrane
   - Mitochondrial matrix
   - Intermembrane space

   *Why:* All enzymes for the link reaction are located in the mitochondrial matrix, so this is the site of the reaction.

## Common pitfalls

- **Wrong:** Claiming the link reaction produces ATP directly
  - Why it fails: Students often confuse the link reaction with later ATP-producing stages of respiration
  - Correct: State that no ATP is produced by the link reaction; ATP is only produced in glycolysis, Krebs cycle and oxidative phosphorylation
- **Wrong:** Counting outputs of the link reaction as 1 per product for one glucose
  - Why it fails: Students forget one glucose produces two pyruvate, so two rounds of the reaction occur
  - Correct: Always multiply per-pyruvate outputs by 2 to get the total yield from one starting glucose molecule
- **Wrong:** Saying coenzyme A accepts hydrogen in the link reaction
  - Why it fails: Students mix up the distinct roles of NAD and coenzyme A
  - Correct: NAD accepts hydrogen to form reduced NAD; coenzyme A only carries the acetyl group to the Krebs cycle
- **Wrong:** Claiming the link reaction occurs in anaerobic respiration
  - Why it fails: Students assume all respiration includes the link reaction, regardless of oxygen availability
  - Correct: The link reaction stops in anaerobic conditions because oxidised NAD cannot be regenerated without oxygen

## Cheatsheet

| Component | Per Pyruvate | Per Glucose |
| --- | --- | --- |
| Pyruvate input | 1 | 2 |
| Oxidised NAD input | 1 | 2 |
| Coenzyme A input | 1 | 2 |
| Acetyl CoA output | 1 | 2 |
| Carbon dioxide output | 1 | 2 |
| Reduced NAD output | 1 | 2 |
| ATP produced | 0 | 0 |
| Location | Mitochondrial matrix | Mitochondrial matrix |

## What's next

Now that you understand the link reaction, you are ready to learn the next stage of aerobic respiration: the Krebs cycle, where acetyl CoA is further oxidised to release additional reduced NAD and carbon dioxide. Mastering the inputs and outputs of the link reaction is critical for calculating the total ATP yield from aerobic respiration, a common long answer question that links all stages together. The link reaction also connects carbohydrate metabolism to lipid metabolism, which is often tested in extended response questions about how different nutrients are used for energy.

- [Energy and Respiration (A2 Portion)](https://www.owlsprep.com/study/cie-9700-u13-overview/)
- [Krebs cycle](https://www.owlsprep.com/study/cie-9700-u13-krebs-cycle/)
- [Oxidative phosphorylation](https://www.owlsprep.com/study/cie-9700-u13-oxidative-phosphorylation/)

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