Aerobic Respiration

Respiration is the process by which living organisms release energy from glucose. It occurs in every living cell, all the time. Aerobic respiration is the type that uses oxygen and is the most efficient way of releasing energy from glucose. This lesson covers the equation, location, purpose and importance of aerobic respiration for AQA GCSE Biology.

What Is Respiration?

Respiration is a chemical reaction that takes place in every living cell to release energy from glucose. It is NOT breathing — breathing (ventilation) is the mechanical process of moving air in and out of the lungs. Respiration is a cellular process.

Respiration is an exothermic reaction — it transfers energy to the environment. This energy is used by the organism for all life processes.

Common Misconception	Correct Understanding
Respiration is breathing	Respiration is a chemical reaction in cells; breathing is gas exchange
Only animals respire	All living organisms respire — plants, animals, fungi, bacteria
Plants photosynthesise instead of respiring	Plants do both — they photosynthesise in the light AND respire all the time
Respiration only happens when you exercise	Respiration is continuous — it occurs 24 hours a day in every living cell

Exam Tip: Never confuse respiration with breathing. If the question asks about respiration, talk about the chemical reaction in cells. If the question asks about breathing or ventilation, talk about inhaling and exhaling. Examiners specifically look for this distinction.

The Equation for Aerobic Respiration

Aerobic respiration is respiration that uses oxygen. The word equation is:

glucose + oxygen ---> carbon dioxide + water (+ energy transferred)

The balanced symbol equation is:

C6H12O6 + 6O2 ---> 6CO2 + 6H2O

Component	Role
Glucose (C6H12O6)	Fuel — the molecule broken down to release energy
Oxygen (O2)	Reactant — needed to fully break down glucose
Carbon dioxide (CO2)	Waste product — exhaled or diffuses out of the organism
Water (H2O)	Waste product — excreted or used in the body
Energy	Released and used for life processes

graph LR
    A[Glucose + Oxygen] --> B[Aerobic Respiration in Mitochondria]
    B --> C[Carbon Dioxide]
    B --> D[Water]
    B --> E[Energy Released]
    E --> F[Movement, growth, warmth, active transport, synthesis]

Exam Tip: You must be able to write both the word equation and the balanced symbol equation for aerobic respiration. Notice that the equation is the reverse of photosynthesis — the reactants of one are the products of the other.

Where Does Aerobic Respiration Occur?

Aerobic respiration takes place in the mitochondria of cells. Mitochondria are small organelles found in the cytoplasm of virtually all eukaryotic cells (plant and animal cells).

Structure and Function of Mitochondria

Mitochondria have a double membrane:

The outer membrane is smooth and forms the boundary of the organelle
The inner membrane is folded into structures called cristae, which increase the surface area for the reactions of aerobic respiration

Cells that require a lot of energy have more mitochondria. For example:

Cell Type	Why It Has Many Mitochondria
Muscle cells	Need large amounts of energy for contraction during movement
Sperm cells	Need energy to swim towards the egg cell
Nerve cells	Need energy for transmitting electrical impulses
Active transport cells (e.g. root hair cells, gut epithelial cells)	Need energy to move substances against a concentration gradient
Liver cells	Carry out many metabolic reactions requiring energy

What Is the Energy Used For?

The energy released by aerobic respiration is used for a wide range of life processes:

In All Organisms

Chemical reactions — building larger molecules from smaller ones (e.g. proteins from amino acids, starch from glucose, lipids from glycerol and fatty acids)
Movement — muscle contraction in animals, cytoplasmic streaming in plants
Active transport — moving substances across cell membranes against a concentration gradient
Maintaining body temperature — particularly important in mammals and birds (endotherms), which maintain a constant body temperature

In Animals Specifically

Muscle contraction — for locomotion, breathing, and heartbeat
Nerve impulses — transmitting electrical signals along neurons
Growth and repair — cell division (mitosis) requires energy

In Plants Specifically

Active transport of mineral ions — root hair cells absorb nitrate ions and other minerals from the soil against a concentration gradient
Growth — cell elongation and division at meristems
Synthesis — building cellulose, proteins, lipids, starch and chlorophyll

graph TD
    A[Energy from Aerobic Respiration] --> B[Movement]
    A --> C[Active Transport]
    A --> D[Synthesis of Molecules]
    A --> E[Maintaining Body Temperature]
    A --> F[Cell Division and Growth]
    A --> G[Nerve Impulses]
    B --> B1[Muscle contraction]
    C --> C1[Absorbing minerals in roots]
    D --> D1[Building proteins, starch, lipids]
    E --> E1[Keeping warm in mammals and birds]