Aerobic Respiration

This lesson covers aerobic respiration — the process by which cells release energy from glucose using oxygen — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to recall the word and symbol equations, know where it occurs and understand why organisms need to respire.

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What Is Respiration?

Respiration is a chemical reaction that takes place in every living cell to release energy from glucose. It happens continuously, 24 hours a day, in all living organisms — plants, animals, fungi, bacteria and protists.

Respiration is not the same as breathing:

Term	Meaning
Respiration	A chemical reaction inside cells that releases energy from glucose
Breathing (ventilation)	The mechanical movement of air in and out of the lungs

Exam Tip: Never say "respiration is breathing." Respiration is a cellular process that releases energy. Breathing (ventilation) supplies the oxygen needed for aerobic respiration and removes the CO₂ produced.

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Aerobic Respiration

Aerobic means "with oxygen." Aerobic respiration is the most efficient way for cells to release energy from glucose.

Word Equation

$\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water} + \text{energy (ATP)}$glucose+oxygen→carbon dioxide+water+energy (ATP)

Balanced Symbol Equation

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

Reactant	Formula	Molecules
Glucose	C₆H₁₂O₆	1
Oxygen	O₂	6
Product	Formula	Molecules
Carbon dioxide	CO₂	6
Water	H₂O	6

Exam Tip: Notice that the equation for aerobic respiration is the reverse of photosynthesis. The reactants and products are swapped. This is a common comparison question.

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Aerobic Respiration Is Exothermic

Aerobic respiration is an exothermic reaction — it releases energy to the surroundings. This is the opposite of photosynthesis, which is endothermic.

The energy released is used to make a molecule called ATP (adenosine triphosphate), which is the cell's immediate energy currency. ATP then provides energy for life processes.

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Where Does Aerobic Respiration Occur?

Aerobic respiration takes place mainly in the mitochondria (singular: mitochondrion) of cells.

Stage	Location
Glycolysis (initial breakdown of glucose)	Cytoplasm
Main stages of aerobic respiration	Mitochondria

Cells that need a lot of energy have many mitochondria. Examples:

Cell type	Why it has many mitochondria
Muscle cells	Need large amounts of energy for contraction
Sperm cells	Need energy to swim towards the egg
Nerve cells	Need energy to transmit electrical impulses
Active transport cells (e.g. root hair cells, gut epithelial cells)	Need energy to move substances against the concentration gradient

graph TD
    A["Glucose + Oxygen"] -->|"Enter the mitochondria"| B["Aerobic respiration"]
    B --> C["CO₂ released"]
    B --> D["H₂O produced"]
    B --> E["Energy released (ATP)"]
    E --> F["Muscle contraction"]
    E --> G["Active transport"]
    E --> H["Building molecules (protein synthesis)"]
    E --> I["Maintaining body temperature"]
    E --> J["Nerve impulses"]

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What Is the Energy Used For?

The energy released by respiration is essential for all life processes:

Life process	How energy is used
Muscle contraction	Movement of the body and internal organs
Active transport	Moving substances against their concentration gradient (e.g. mineral ions into root hair cells)
Synthesis of new molecules	Building proteins from amino acids; building cellulose, starch, lipids
Maintaining body temperature	Keeping enzymes at their optimum temperature (in mammals and birds)
Cell division (mitosis)	DNA replication and formation of new cells
Transmitting nerve impulses	Electrical signals along neurons
Growth and repair	Producing new cells and tissues

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Comparing Photosynthesis and Aerobic Respiration

Feature	Photosynthesis	Aerobic respiration
Equation	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Reactants	CO₂ and H₂O	Glucose and O₂
Products	Glucose and O₂	CO₂ and H₂O
Energy	Endothermic (absorbs light energy)	Exothermic (releases energy)
Where	Chloroplasts	Mitochondria
When	Only in the light	All the time (24/7)
Organisms	Plants, algae	All living organisms

graph LR
    A["Photosynthesis"] -->|"Produces glucose and O₂"| B["Aerobic Respiration"]
    B -->|"Produces CO₂ and H₂O"| A
    A -->|"Endothermic"| C["Stores energy in glucose"]
    B -->|"Exothermic"| D["Releases energy from glucose"]

Exam Tip: Plants carry out both photosynthesis and respiration. During the day, the rate of photosynthesis exceeds the rate of respiration, so there is a net uptake of CO₂. At night, only respiration occurs, so the plant releases CO₂.

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The Importance of Oxygen Supply

Aerobic respiration requires a continuous supply of oxygen. The circulatory and respiratory systems work together to deliver oxygen to cells:

1. Ventilation brings fresh air (containing O₂) into the alveoli.
2. Oxygen diffuses from the alveoli into the blood.
3. Red blood cells (haemoglobin) carry oxygen to body tissues.
4. Oxygen diffuses from the blood into cells.
5. Inside the cell, oxygen enters the mitochondria for aerobic respiration.

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Summary

• Respiration is a chemical reaction in cells that releases energy; it is not breathing.
• Aerobic respiration uses oxygen: glucose + oxygen → carbon dioxide + water + energy.
• Balanced equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
• It is an exothermic reaction, releasing energy stored as ATP.
• Aerobic respiration occurs mainly in the mitochondria.
• Energy is used for muscle contraction, active transport, synthesis, maintaining temperature, nerve impulses and cell division.
• Cells with high energy demands (muscle, sperm, nerve cells) have many mitochondria.
• Photosynthesis and aerobic respiration are essentially reverse reactions.

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Worked Example — Balancing the Aerobic Respiration Equation

The equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O must balance for C, H and O: