Anaerobic Respiration

When oxygen is not available — or cannot be supplied fast enough — cells can still release energy from glucose through anaerobic respiration. This process is less efficient than aerobic respiration but is essential for survival in low-oxygen conditions. This lesson covers anaerobic respiration in animals, plants and microorganisms for AQA GCSE Biology.

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What Is Anaerobic Respiration?

Anaerobic respiration is respiration that takes place without oxygen. It releases energy from glucose but produces different products and much less energy compared to aerobic respiration.

The key difference is:

• Aerobic = with oxygen = complete breakdown of glucose = lots of energy
• Anaerobic = without oxygen = incomplete breakdown of glucose = less energy

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Anaerobic Respiration in Animals (Including Humans)

When animal cells do not have enough oxygen (e.g. during intense exercise), they switch to anaerobic respiration.

The Equation

glucose ---> lactic acid (+ energy transferred)

Note: there is no balanced symbol equation required for anaerobic respiration in animals at GCSE.

Feature	Detail
Reactant	Glucose only (no oxygen)
Product	Lactic acid
Energy released	Much less than aerobic respiration
Location	Cytoplasm (not the mitochondria)
When it occurs	During vigorous exercise when oxygen supply cannot meet demand

graph TD
    A[Intense Exercise] --> B[Muscles need more energy]
    B --> C[Heart and lungs cannot supply O2 fast enough]
    C --> D[Cells switch to anaerobic respiration]
    D --> E[Glucose broken down without oxygen]
    E --> F[Lactic acid produced]
    F --> G[Lactic acid builds up in muscles]
    G --> H[Muscle fatigue and pain]

Why Is Less Energy Released?

In anaerobic respiration, glucose is only partially broken down. The lactic acid that is produced still contains a lot of chemical energy — energy that would have been released if oxygen had been available to fully break down the glucose. This is why anaerobic respiration releases much less energy per molecule of glucose than aerobic respiration.

The Problem: Lactic Acid Build-Up

When lactic acid accumulates in muscles, it causes:

• Muscle fatigue — the muscles become tired and cannot contract as efficiently
• Pain and cramping — a burning sensation in the muscles
• A drop in pH — lactic acid is acidic and lowers the pH in muscles, which can inhibit enzyme activity

The lactic acid must be removed after exercise, which requires oxygen (see the lesson on Response to Exercise).

Exam Tip: The product of anaerobic respiration in animals is lactic acid — not carbon dioxide and alcohol. This is a common mistake. Carbon dioxide and ethanol are the products in plants and yeast, not in animals.

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Anaerobic Respiration in Plants and Yeast (Fermentation)

In plants and yeast (a type of fungus), anaerobic respiration produces different products from animals.

The Equation

glucose ---> ethanol + carbon dioxide (+ energy transferred)

Feature	Detail
Reactant	Glucose only (no oxygen)
Products	Ethanol (alcohol) and carbon dioxide
Energy released	Less than aerobic respiration
Location	Cytoplasm
Common name	Fermentation (especially in yeast)

Fermentation in Yeast

Fermentation is the term specifically used for anaerobic respiration in yeast. It has two major applications:

Application	How Fermentation Is Used
Bread making	Yeast ferments sugar in the dough, producing CO2 gas which makes the bread rise. The ethanol evaporates during baking.
Brewing and wine making	Yeast ferments sugar in grapes (wine) or malted barley (beer), producing ethanol (the alcohol in the drink) and CO2 (which makes beer fizzy).
Biofuel production	Yeast ferments plant sugars to produce ethanol, which can be used as a biofuel.

graph TD
    A[Fermentation in Yeast] --> B[Glucose broken down without oxygen]
    B --> C[Ethanol produced]
    B --> D[Carbon dioxide produced]
    C --> C1[Used in brewing and biofuels]
    D --> D1[Makes bread rise and beer fizzy]

Exam Tip: Make sure you know two different equations for anaerobic respiration — one for animals (glucose ---> lactic acid) and one for plants/yeast (glucose ---> ethanol + carbon dioxide). Examiners often ask you to compare them.

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Anaerobic Respiration in Plant Root Cells

Plant root cells can sometimes become waterlogged (e.g. in flooded soil). When this happens, oxygen cannot reach the cells, and they switch to anaerobic respiration, producing ethanol and carbon dioxide.

If waterlogging continues for too long, the ethanol can become toxic and damage the root cells, eventually killing the plant. This is why good drainage is important for most crops.

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Comparing Aerobic and Anaerobic Respiration

Feature	Aerobic Respiration	Anaerobic Respiration (Animals)	Anaerobic Respiration (Plants/Yeast)
Oxygen needed?	Yes	No	No
Equation	glucose + oxygen ---> CO2 + water	glucose ---> lactic acid	glucose ---> ethanol + CO2
Products	CO2 and water	Lactic acid	Ethanol and CO2
Energy released	Large amount	Small amount	Small amount
Where in the cell?	Mitochondria	Cytoplasm	Cytoplasm
Glucose breakdown	Complete	Incomplete	Incomplete
When does it occur?	All the time (if O2 available)	During intense exercise or O2 shortage	When O2 is unavailable

Exam Tip: A classic exam question asks you to compare aerobic and anaerobic respiration. Structure your answer using the table headings above: oxygen requirement, products, energy yield, and location in the cell. Always state that anaerobic respiration releases less energy because glucose is only partially broken down.

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Why Anaerobic Respiration Is Important

Even though anaerobic respiration is less efficient, it is essential because:

1. It allows cells to continue releasing energy when oxygen is unavailable — during a sprint, for example, the muscles need energy faster than the blood can deliver oxygen.
2. It is a survival mechanism — organisms can survive short periods without oxygen thanks to anaerobic respiration.
3. Fermentation has many uses — bread, alcohol, biofuels and yoghurt production all rely on anaerobic respiration.

However, anaerobic respiration cannot continue indefinitely:

• In animals, lactic acid builds up and causes muscle fatigue
• In plants and yeast, ethanol is toxic at high concentrations

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Oxygen Debt [H]