# Anaerobic respiration

> CIE A-Level Biology · 9700 A2 Energy and Respiration
> Source: https://www.owlsprep.com/study/cie-9700-u13-anaerobic-respiration/

This sub-topic covers anaerobic respiration pathways in eukaryotes, including lactate fermentation in active mammalian muscle and ethanol fermentation in yeast. We explain ATP production without oxygen and compare energy yields to aerobic respiration.

**Prerequisites:** [Glycolysis](https://www.owlsprep.com/study/cie-9700-u13-glycolysis/); [Aerobic respiration](https://www.owlsprep.com/study/cie-9700-u13-aerobic-respiration/)

## Learning objectives

- Distinguish between anaerobic respiration in yeast and mammalian muscle
- Explain the role of fermentation in regenerating NAD+ for ongoing ATP production
- Compare the energy yield of anaerobic and aerobic respiration
- Describe the physiological significance of anaerobic respiration in active muscle

## 1. Overview and Cellular Location

Anaerobic respiration occurs when oxygen is not available to act as the final electron acceptor for the electron transport chain (ETC). It allows continued production of a small amount of ATP by reoxidising the reduced NAD produced during glycolysis.

**Anaerobic respiration** — The breakdown of glucose to produce ATP in the absence of oxygen, involving glycolysis and a fermentation pathway to regenerate oxidised NAD+

*Example:* Occurs in mammalian muscle during vigorous exercise, and in yeast grown in oxygen-free environments

**Worked example:** Explain why regeneration of NAD+ is essential for anaerobic respiration to continue.

1. 1. Glycolysis requires a continuous supply of oxidised NAD+ to accept hydrogen atoms during oxidation of triose phosphate.
2. 2. When oxygen is absent, the ETC cannot oxidise NADH back to NAD+.
3. 3. Without new NAD+, glycolysis would stop as no free NAD+ is available to accept hydrogen.
4. 4. Fermentation reduces pyruvate to oxidise NADH to NAD+, which is reused for glycolysis.

> **Exam tip:** CIE examiners regularly test why NAD regeneration is essential, so always link this to ongoing glycolysis.

## 2. Lactate vs Ethanol Fermentation

There are two main fermentation pathways in eukaryotes that regenerate NAD+, differing in their end products. Both start with pyruvate produced from glycolysis.

| Feature | Lactate fermentation (muscle) | Ethanol fermentation (yeast/plants) |
| --- | --- | --- |
| Location | Cytoplasm | Cytoplasm |
| End products | Lactate | Ethanol + carbon dioxide |
| Reversible in host? | Yes: lactate converted back to pyruvate in liver | No: ethanol is a waste product |
| Decarboxylation occurs? | No | Yes |

**Worked example:** Compare the fate of pyruvate in anaerobic respiration in human muscle and yeast.

1. 1. In human muscle during vigorous exercise, oxygen supply is limited:
2. Pyruvate is directly reduced by NADH to form lactate. NADH is oxidised to NAD+ for reuse. No carbon dioxide is released.
3. 2. In yeast under anaerobic conditions:
4. Pyruvate is first decarboxylated to form ethanal, releasing carbon dioxide. Ethanal is then reduced by NADH to ethanol, oxidising NADH to NAD+.
5. 3. Both pathways achieve the same core goal: regenerate NAD+ to allow glycolysis to continue producing ATP.

> **tip**
>
> No ATP is produced during the fermentation step itself. All 2 net ATP from anaerobic respiration comes from glycolysis.

## 3. Energy Yield and Physiological Role

Anaerobic respiration produces a much lower yield of ATP than aerobic respiration. This is because only glycolysis occurs, and the energy stored in the end products (lactate or ethanol) is not released for ATP synthesis.

$$\text{Anaerobic yield: } 2 \text{ ATP per glucose} \\ \text{Aerobic yield: } 36-38 \text{ ATP per glucose}$$

**Oxygen debt** — The extra volume of oxygen required after exercise to process lactate produced by anaerobic respiration and restore resting metabolic conditions

*Example:* Explains why you breathe heavily after finishing intense exercise: you are repaying the oxygen debt

**Worked example:** Explain why anaerobic respiration cannot be sustained in mammals for long periods.

1. 1. Anaerobic respiration produces only 2 ATP per glucose, so glucose is consumed much faster than in aerobic respiration.
2. 2. Lactate is acidic, so a build-up lowers intracellular pH, inhibiting enzyme activity and causing muscle fatigue.
3. 3. Lactate must be transported to the liver to be converted back to pyruvate, which requires oxygen.
4. 4. Anaerobic respiration only supplements aerobic respiration for short bursts of activity when oxygen is limited.

## Common pitfalls

- **Wrong:** Stating that fermentation produces ATP
  - Why it fails: No ATP is synthesised during the fermentation step itself. All ATP from anaerobic respiration comes from glycolysis.
  - Correct: State that fermentation regenerates NAD+ required for glycolysis to continue producing ATP.
- **Wrong:** Claiming anaerobic respiration occurs in mitochondria
  - Why it fails: Only glycolysis occurs in anaerobic respiration, which takes place in the cytoplasm. The ETC and Krebs cycle do not run without oxygen.
  - Correct: State that all reactions of anaerobic respiration occur in the cytoplasm.
- **Wrong:** Thinking decarboxylation occurs in lactate fermentation
  - Why it fails: Decarboxylation of pyruvate only happens in ethanol fermentation. Pyruvate is not decarboxylated in lactate fermentation.
  - Correct: Remember that carbon dioxide is only produced in ethanol fermentation, not in mammalian lactate fermentation.
- **Wrong:** Claiming lactate is excreted as a waste product
  - Why it fails: Lactate is not immediately excreted. It is transported to the liver for processing after exercise.
  - Correct: State that lactate is oxidised back to pyruvate in the liver to repay oxygen debt.

## Cheatsheet

| Pathway | Organisms | End Products | Net ATP Yield | Site |
| --- | --- | --- | --- | --- |
| Lactate fermentation | Mammalian muscle | Lactate | 2 ATP (glycolysis) | Cytoplasm |
| Ethanol fermentation | Yeast, plants | Ethanol + CO₂ | 2 ATP (glycolysis) | Cytoplasm |
| Aerobic respiration | Aerobic eukaryotes | CO₂ + H₂O | 36-38 ATP | Cytoplasm + mitochondria |

## What's next

Understanding anaerobic respiration builds on your knowledge of glycolysis and aerobic respiration, and connects to broader topics like metabolic regulation and exercise physiology in CIE A-Level Biology. This sub-topic is frequently examined in both multiple choice and extended response questions, often asking to compare energy yields or differences between the two fermentation pathways. Mastering the role of NAD+ regeneration is critical for full marks. Next, you can explore related topics in Unit 13 that build on this foundation to complete your understanding of cellular energy transfer.

- [Respiratory substrates](https://www.owlsprep.com/study/cie-9700-u13-respiratory-substrates/)
- [Measurement of respiration rate](https://www.owlsprep.com/study/cie-9700-u13-measurement-of-respiration-rate/)
- [Photosynthesis](https://www.owlsprep.com/study/cie-9700-u14-overview/)

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