Aerobic Respiration

Respiration is the process by which living organisms release energy from glucose. Aerobic respiration uses oxygen and is the most efficient form of respiration. This lesson covers the equations, the site of the reaction, and its importance for all living organisms — a key topic in the AQA GCSE Combined Science Trilogy specification (8464).

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What Is Respiration?

Respiration is a chemical reaction that occurs continuously in every living cell. It is not the same as breathing (which is ventilation). Respiration is the process of releasing energy from glucose to power life processes.

All organisms respire — plants, animals, fungi, bacteria and protists. Respiration happens all the time, in every cell, day and night.

Exam Tip: Never confuse "respiration" with "breathing." Breathing is the mechanical process of ventilating the lungs. Respiration is a chemical process in cells.

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Aerobic Respiration

Aerobic respiration is respiration that requires oxygen. It is the main source of energy for most organisms under normal conditions.

Word Equation

$$\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$$glucose+oxygen→carbon dioxide+water

Balanced Symbol Equation

$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$$C6​H12​O6​+6O2​→6CO2​+6H2​O

Component	Role
Glucose	Reactant — the fuel that is broken down
Oxygen	Reactant — required for aerobic respiration
Carbon dioxide	Product — waste gas, exhaled or released
Water	Product — waste, used or excreted
Energy	Released — transferred to the surroundings and used for life processes

Exam Tip (AQA 8464): You must know both the word equation and the balanced symbol equation for aerobic respiration. Notice that it is the reverse of the photosynthesis equation.

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Where Does Aerobic Respiration Occur?

Aerobic respiration takes place in the mitochondria of cells. Mitochondria are sometimes called the "powerhouses" of the cell because they are the site where energy is released from glucose.

Feature	Detail
Organelle	Mitochondria
Found in	Almost all eukaryotic cells (plant and animal)
Cells with many mitochondria	Muscle cells, liver cells, sperm cells — any cell with a high energy demand
Structure	Double membrane with inner folds (cristae) to increase surface area for reactions

graph TD
    A["Glucose + Oxygen enter the cell"] --> B["Mitochondria"]
    B --> C["Aerobic respiration occurs"]
    C --> D["Energy released"]
    C --> E["CO₂ produced — diffuses out of cell"]
    C --> F["H₂O produced"]
    D --> G["Used for life processes"]

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Aerobic Respiration Is Exothermic

Aerobic respiration is an exothermic reaction — it releases energy to the surroundings. This energy is used to drive all the processes in a living organism.

Process Powered by Respiration	Example
Muscle contraction	Movement — walking, running, heart beating
Active transport	Absorbing mineral ions in roots; absorbing glucose in the gut
Synthesis of new molecules	Building amino acids into proteins; joining glucose into starch or cellulose
Cell division (mitosis)	Growth and repair
Maintaining body temperature	In mammals and birds (endotherms)
Nerve impulses	Sending signals along neurones

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Comparing Energy Yields

Aerobic respiration releases much more energy per molecule of glucose than anaerobic respiration. This is because the glucose molecule is completely oxidised (fully broken down) in the presence of oxygen.

$$\text{1 molecule of glucose} \xrightarrow{\text{aerobic}} \text{maximum energy release}$$1 molecule of glucoseaerobic​maximum energy release $$\text{1 molecule of glucose} \xrightarrow{\text{anaerobic}} \text{much less energy (incomplete breakdown)}$$1 molecule of glucoseanaerobic​much less energy (incomplete breakdown)

This is why organisms preferentially use aerobic respiration whenever oxygen is available.

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Respiration in Plants

Plants also carry out aerobic respiration — in their mitochondria, just like animals. Plants respire all the time (day and night), but they only photosynthesise during the day.

During the day, the rate of photosynthesis usually exceeds the rate of respiration, so the plant has a net uptake of CO₂ and a net release of O₂.

At night, only respiration occurs, so the plant releases CO₂ and takes in O₂.

Time of Day	Photosynthesis	Respiration	Net Gas Exchange
Daytime (bright light)	Fast	Continuous	Net uptake of CO₂; net release of O₂
Night	None	Continuous	Net release of CO₂; net uptake of O₂

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Common Mistakes

Mistake	Correction
Saying "respiration is breathing"	Respiration is a chemical reaction in cells; breathing is ventilation of the lungs
Saying plants only respire at night	Plants respire all the time — day and night
Saying respiration "makes" energy	Energy is released (transferred), not created — this follows the law of conservation of energy
Forgetting where aerobic respiration occurs	It occurs in the mitochondria
Saying aerobic respiration does not need oxygen	Aerobic means "with oxygen" — that is the defining feature

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Summary

• Respiration is a chemical reaction in all living cells that releases energy from glucose.
• Aerobic respiration requires oxygen and takes place in the mitochondria.
• Word equation: glucose + oxygen → carbon dioxide + water.
• Balanced symbol equation: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$C6​H12​O6​+6O2​→6CO2​+6H2​O.
• It is an exothermic reaction — energy is released.
• Energy from respiration is used for muscle contraction, active transport, synthesis of molecules, cell division and maintaining body temperature.
• Aerobic respiration releases much more energy per glucose molecule than anaerobic respiration.
• Plants respire all the time and photosynthesise only during the day.

Exam Tip (AQA 8464): You must be able to write the aerobic respiration equation from memory and identify that it is the reverse of the photosynthesis equation. A common 4-mark question asks you to describe what aerobic respiration is, where it occurs, and what the energy is used for.

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Deeper Exploration: Energy Yields — Aerobic vs Anaerobic