Anaerobic Respiration

This lesson covers anaerobic respiration — respiration without oxygen — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to know the word equations for anaerobic respiration in animals and in yeast/plants, understand the concept of oxygen debt and compare aerobic with anaerobic respiration.

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What Is Anaerobic Respiration?

Anaerobic means "without oxygen." Anaerobic respiration is the release of energy from glucose without using oxygen. It occurs when cells cannot get enough oxygen to meet their energy demands — for example, during vigorous exercise.

Anaerobic respiration releases much less energy per molecule of glucose than aerobic respiration because glucose is only partially broken down.

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Anaerobic Respiration in Animals

In animal cells (including humans), the word equation is:

$\text{glucose} \rightarrow \text{lactic acid} + \text{energy}$glucose→lactic acid+energy

Key points:

• The glucose molecule is not fully broken down — lactic acid still contains a lot of chemical energy.
• No carbon dioxide or water is produced (unlike aerobic respiration).
• The process takes place in the cytoplasm, not the mitochondria.
• It releases much less energy than aerobic respiration.

Feature	Aerobic respiration	Anaerobic respiration (animals)
Oxygen required?	Yes	No
Products	CO₂ + H₂O	Lactic acid
Energy released	Large amount (~38 ATP per glucose)	Small amount (~2 ATP per glucose)
Location	Mitochondria	Cytoplasm
Glucose	Fully broken down	Partially broken down

Exam Tip: In anaerobic respiration in animals, the only product is lactic acid (plus a small amount of energy). Do not write CO₂ or water — these are products of aerobic respiration only.

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Lactic Acid and Its Effects

During vigorous exercise:

1. Muscles contract rapidly and need more energy.
2. The heart and lungs cannot deliver oxygen fast enough.
3. Muscle cells switch to anaerobic respiration to supplement their energy supply.
4. Lactic acid builds up in the muscle cells.

The effects of lactic acid build-up:

Effect	Explanation
Muscle fatigue	Muscles become tired and may cramp
Muscle pain	Lactic acid irritates nerve endings
Reduced performance	The muscles cannot contract as effectively

The lactic acid must be removed after exercise — this is where oxygen debt comes in.

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Oxygen Debt (Excess Post-Exercise Oxygen Consumption)

After vigorous exercise, you continue to breathe heavily for a period of time. This is because you have built up an oxygen debt — the extra oxygen your body needs to:

1. Break down lactic acid — lactic acid is transported in the blood to the liver, where it is converted back into glucose using oxygen.
2. Replenish oxygen stores in the blood and muscles.
3. Restore ATP and creatine phosphate reserves in the muscles.

graph TD
    A["Vigorous exercise"] --> B["O₂ demand > O₂ supply"]
    B --> C["Anaerobic respiration in muscles"]
    C --> D["Lactic acid builds up"]
    D --> E["Muscle fatigue and pain"]
    A --> F["Exercise stops"]
    F --> G["Heavy breathing continues"]
    G --> H["Extra O₂ taken in (oxygen debt repaid)"]
    H --> I["Lactic acid → glucose (in the liver)"]
    H --> J["O₂ stores replenished"]

Exam Tip: The term "oxygen debt" refers to the extra oxygen consumed after exercise above what would normally be needed at rest. If asked to explain why breathing rate remains high after exercise, always mention: to repay the oxygen debt by breaking down lactic acid in the liver.

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Anaerobic Respiration in Yeast and Plants

In yeast cells and plant cells, anaerobic respiration produces different products:

$\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide} + \text{energy}$glucose→ethanol+carbon dioxide+energy

This process is called fermentation (specifically, alcoholic fermentation).

Feature	In animals	In yeast / plants
Products	Lactic acid	Ethanol + CO₂
Reversible?	Yes (lactic acid → glucose in the liver)	No (ethanol is toxic; cannot be converted back to glucose)
Location	Cytoplasm	Cytoplasm
CO₂ produced?	No	Yes

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Uses of Fermentation

Fermentation by yeast has important applications:

Application	How fermentation is used
Bread making	Yeast ferments sugars in the dough; CO₂ gas causes the dough to rise; ethanol evaporates during baking
Brewing (beer and wine)	Yeast ferments sugars in grain (beer) or grapes (wine); ethanol is the desired product; CO₂ causes fizz
Bioethanol production	Yeast ferments plant sugars (e.g. sugar cane, corn) to produce ethanol as a biofuel

graph TD
    A["Glucose (from sugar or starch)"]
    A -->|"Yeast added"| B["Anaerobic conditions"]
    B --> C["Fermentation"]
    C --> D["Ethanol"]
    C --> E["Carbon dioxide"]
    D -->|"Beer / wine / biofuel"| F["Useful product"]
    E -->|"Bread rises / fizzy drinks"| G["Useful product"]

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Comparing Aerobic and Anaerobic Respiration

Feature	Aerobic	Anaerobic (animals)	Anaerobic (yeast/plants)
Oxygen	Required	Not required	Not required
Equation	Glucose + O₂ → CO₂ + H₂O	Glucose → lactic acid	Glucose → ethanol + CO₂
Energy released	Large amount	Small amount	Small amount
Where	Mitochondria	Cytoplasm	Cytoplasm
Glucose breakdown	Complete	Incomplete	Incomplete

Exam Tip: A 6-mark question might ask you to compare aerobic and anaerobic respiration. Make sure you include: whether oxygen is used, the products, the amount of energy released, where in the cell it occurs and whether glucose is fully or partially broken down.

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The Response to Exercise

During exercise, the body makes several changes to meet the increased demand for energy:

Change	Reason
Heart rate increases	Pumps blood (and therefore oxygen and glucose) to muscles faster
Breathing rate and depth increase	Takes in more oxygen and removes more CO₂
Blood vessels to muscles dilate (vasodilation)	Increases blood flow to active muscles
Glycogen in muscles is broken down to glucose	Provides more fuel for respiration

If the demand for energy still exceeds what aerobic respiration can supply, anaerobic respiration makes up the difference — but lactic acid accumulates and oxygen debt builds.

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Summary

• Anaerobic respiration releases energy from glucose without oxygen.
• In animals: glucose → lactic acid (+ small amount of energy). Takes place in the cytoplasm.
• In yeast/plants: glucose → ethanol + carbon dioxide (+ small amount of energy). This is fermentation.
• Anaerobic respiration releases much less energy than aerobic because glucose is only partially broken down.
• Lactic acid causes muscle fatigue; it is broken down in the liver using oxygen after exercise.
• Oxygen debt is the extra oxygen needed after exercise to break down lactic acid and restore normal conditions.
• Fermentation by yeast is used in bread making, brewing and biofuel production.

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Worked Example — Comparing ATP Yield