2.6.1Structure and Functions in Living Organisms: Part 1

Process of Respiration — Edexcel International GCSE Science Double Award (Modular) - Biology Unit 1 Revision Notes & Topical Questions

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Part 1 of 4What respiration is

A chemical reaction in every cell

A chemical reaction in every cell Respiration is the chemical process that releases energy from glucose.
- It happens continuously in all living cells, in plants as well as animals.
It is not the same thing as breathing.

Aerobic and anaerobic

Aerobic and anaerobic Aerobic respiration uses oxygen, whereas anaerobic respiration releases energy without oxygen.
Aerobic respiration releases much more energy per glucose molecule.

Energy is transferred, not made

Energy is transferred, not made Energy cannot be created.
- Respiration transfers the chemical energy stored in glucose into a form cells can use, and some is lost as heat.

Glucose is broken down in the cell, and the energy released is used to build molecules of ATP.

ATP molecules move to where energy is needed inside the cell.

When an ATP molecule is broken down, it releases a small packet of energy for a cell process.

The products are then recycled and rebuilt into ATP using energy from further respiration.

Energy from ATP

Building molecules

Building molecules Small molecules are joined together to make larger ones, for example amino acids into proteins.

Muscle contraction

Muscle contraction Muscle fibres need ATP to shorten, allowing movement, breathing and the beating of the heart.

Keeping warm

Keeping warm Mammals and birds release heat from respiration to maintain a steady body temperature at which enzymes work well.

Growth and cell division

Growth and cell division Making new cells and tissues requires a continuous energy supply.

	Gas exchange	Respiration
Where	Lungs (alveoli), leaf surfaces, gills	Inside every living cell
What happens	Oxygen moves in and carbon dioxide moves out by diffusion	Glucose is broken down to release energy as ATP
Type of process	Physical	Chemical
Link	Supplies the oxygen needed	Produces the carbon dioxide and water that must be removed

Term	Meaning
Respiration	The chemical reactions in cells that release energy from glucose.
ATP	The molecule that transfers energy from respiration to processes that need it.
Aerobic respiration	Respiration that uses oxygen to break down glucose.
Anaerobic respiration	Respiration that releases energy from glucose without using oxygen.
Gas exchange	The diffusion of oxygen into, and carbon dioxide out of, an organism.

Confusing respiration with breathing or gas exchange. Respiration is the chemical release of energy inside cells.

Saying that energy is 'made' or 'produced'. It is transferred from glucose to ATP and then to cell processes.

Thinking only animals respire. Plants respire all the time, including during the day.

Writing that ATP is food or an energy store like fat. ATP is a short-term carrier of energy that is constantly broken down and remade.

Last updated: 20 October 2026