10.3 Anaerobic Respiration

2026 📋 Syllabus Objectives

By the end of this section, you should be able to:

  1. Describe anaerobic respiration as the release of a relatively small amount of energy by the breakdown of glucose without using oxygen
  2. State the word equation for anaerobic respiration in humans
  3. State the word equation for anaerobic respiration in yeast
  4. Explain why lactic acid builds up in muscles and blood during vigorous exercise, causing Excess Post-exercise Oxygen Consumption (EPOC), also called an 'oxygen debt'
  5. Outline how the oxygen debt is removed after exercise, including the role of a fast heart rate and deeper/faster breathing

📖 Main Content

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What is Anaerobic Respiration?

You already know that cells need energy to do everything — move, grow, repair, and more. Cells get this energy from a process called respiration, which breaks down glucose (a type of sugar) to release energy.

There are two types of respiration:

  • Aerobic respiration — uses oxygen, releases a large amount of energy
  • Anaerobic respiration — does not use oxygen, releases a relatively small amount of energy

💡 "Anaerobic" simply means "without air" (or more specifically, without oxygen). Think of it as the body's emergency backup system for making energy when oxygen runs out.

Key definition:

Anaerobic respiration is the breakdown of glucose to release energy without using oxygen. It releases only a relatively small amount of energy compared to aerobic respiration.

Because no oxygen is needed, anaerobic respiration can happen very quickly — this is why your muscles can keep working for a short time even when your body hasn't had time to deliver enough oxygen to them.

Word Equation for Anaerobic Respiration in Humans

In humans (and other animals), when oxygen is in short supply — such as during very intense exercise — muscle cells switch to anaerobic respiration. Glucose is broken down, but instead of being fully broken down (as in aerobic respiration), it is only partially broken down, producing a substance called lactic acid.

Word equation:

glucose→lactic acid (+ energy)\text{glucose} \rightarrow \text{lactic acid} \text{ (+ energy)}
  • Glucose is the sugar that gets broken down.
  • Lactic acid is the waste product produced. It is an acid that builds up in the muscles.
  • A small amount of energy is released and used by the cells.

⚠️ Notice there is no oxygen on the left side and no carbon dioxide or water on the right side — this is what makes it different from aerobic respiration.

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