Energy Equations

Understanding the chemical equations for aerobic and anaerobic energy production is a core requirement of AQA GCSE PE (spec 3.1.1.3). You must be able to state both equations, explain what each component means, and link them to performance in sport. This lesson covers the aerobic and anaerobic equations in detail, explains the differences between them, and provides strategies for remembering them under exam pressure.

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The Aerobic Energy Equation

When the body has a sufficient supply of oxygen, glucose is broken down completely through the aerobic energy system:

glucose + oxygen → energy + carbon dioxide + water

Breaking Down the Equation

Component	Role
Glucose	The fuel — obtained from food (carbohydrates are broken down into glucose during digestion)
Oxygen	Breathed in through the lungs, transported to muscles via red blood cells in the bloodstream
Energy	The purpose of the reaction — used for muscle contraction and all bodily functions
Carbon dioxide (CO2)	A waste product — transported in the blood to the lungs and breathed out
Water (H2O)	A waste product — removed through sweat, urine and breathing

graph LR
    A[Glucose] --> C{Aerobic<br>Respiration}
    B[Oxygen] --> C
    C --> D[Energy]
    C --> E[Carbon Dioxide]
    C --> F[Water]
    style A fill:#f39c12,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#95a5a6,color:#fff
    style F fill:#95a5a6,color:#fff

Key Points About the Aerobic Equation

• The reaction is complete — glucose is fully broken down, releasing a large amount of energy.
• The by-products (CO2 and water) are harmless and easily removed from the body.
• This is the same equation as aerobic respiration in GCSE Biology. The PE exam will use the word equation, not the chemical formula.
• Because more energy is released, the aerobic system is more efficient than the anaerobic system.

Exam Tip: AQA will only ask for the word equation in GCSE PE. You do not need to know the chemical symbols. However, you must be able to state all five components (two reactants on the left, three products on the right).

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The Anaerobic Energy Equation

When the body does not have enough oxygen — during high-intensity exercise — glucose is broken down incompletely through the anaerobic energy system:

glucose → energy + lactic acid

Breaking Down the Equation

Component	Role
Glucose	The fuel — same source as for aerobic respiration
Energy	Released for muscle contraction — but less than in aerobic respiration because the breakdown is incomplete
Lactic acid	A toxic waste product that builds up in the muscles, causing fatigue, pain and a burning sensation

graph LR
    A[Glucose] --> B{Anaerobic<br>Respiration}
    B --> C[Energy<br>- less than aerobic -]
    B --> D[Lactic Acid]
    style A fill:#f39c12,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#e74c3c,color:#fff
    style D fill:#c0392b,color:#fff

Key Points About the Anaerobic Equation

• No oxygen appears in this equation — that is the defining feature of anaerobic respiration.
• The reaction is incomplete — glucose is not fully broken down, so less energy is released.
• The by-product is lactic acid, which is harmful. It causes muscles to fatigue, ache and eventually stop working effectively.
• Lactic acid must be removed after exercise — this is where oxygen debt and EPOC come in (covered in the next lesson).

Exam Tip: A very common mistake is to write "carbon dioxide" or "water" as products of the anaerobic equation. The only by-product is lactic acid. Oxygen is not involved, so CO2 and water cannot be produced.

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Comparing the Two Equations

Feature	Aerobic	Anaerobic
Reactants	Glucose + Oxygen	Glucose only
Products	Energy + CO2 + Water	Energy + Lactic acid
Oxygen needed?	Yes	No
Energy yield	High (complete breakdown)	Low (incomplete breakdown)
Harmful by-products?	No (CO2 and water are harmless)	Yes (lactic acid causes fatigue)
Can be sustained?	Yes, for long periods	No, fatigue builds rapidly

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Why Does the Anaerobic System Produce Less Energy?

The answer lies in the completeness of the reaction: