Process of Respiration

Respiration is the chemical reaction that releases energy from glucose inside living cells. It happens in every living cell, every second, for your whole life — in animals, plants, fungi and bacteria alike.

The energy is needed because cells are busy: they contract, transport substances, build new molecules and grow. None of that is free — it all needs an energy supply.

Key points the examiner wants:

• Respiration releases energy (it does not make or create energy).
• The energy comes from glucose (a sugar).
• It happens in all living cells, all the time — not only in animals and not only during exercise.
• Most aerobic respiration takes place in the mitochondria of the cell.

Exam tip. A very common 1-mark question is "State what is meant by respiration." The mark scheme wants "the release of energy from glucose" (or food). Saying "breathing" scores zero.

Cells do not use the energy from glucose directly. Instead, respiration uses that energy to make a molecule called ATP (adenosine triphosphate).

Think of ATP as the cell's "money":

• Glucose is like a banknote that is too big to spend in a vending machine.
• Respiration breaks that energy down into small, usable amounts — ATP — that the cell can spend exactly where and when it is needed.
• When a cell needs energy, it breaks down ATP to release a small, controlled amount of energy.

So the chain is:

glucose → (respiration releases energy) → ATP made → ATP broken down → energy used for cell processes

Exam tip. When asked "What is ATP for?", write that ATP stores and supplies energy for the cell's reactions — it is the molecule the cell actually uses, made during respiration.

ATP provides the energy for all the work a cell does. The Double Award spec wants you to know these examples of what that energy is used for:

• Muscle contraction — moving the body, the heart beating, the gut squeezing food along.
• Active transport — moving substances against a concentration gradient (e.g. mineral ions into root hair cells; glucose absorbed in the gut/kidney). This needs energy because it goes the "uphill" way.
• Synthesis (building large molecules) — joining small molecules into large ones, e.g. amino acids → proteins, glucose → starch.
• Growth — making new cells and new cell material as the organism gets bigger.

A handy extra (often credited): some energy is also released as heat, which in mammals and birds helps keep body temperature constant.

Exam tip. If a question says "Give two uses of the energy released by respiration", pick from the list above. Active transport and muscle contraction are the safest, clearest marks.

This is the mistake examiners flag year after year. In everyday English "respiration" can mean breathing — but in Biology the two are different things:

	Respiration	Breathing (ventilation)
What it is	A chemical reaction in cells	A physical movement of air
Where	Inside every living cell	In the lungs / chest
Purpose	Releases energy from glucose (makes ATP)	Moves air in and out for gas exchange
Who does it	All living things	Only animals with lungs/gills etc.

A plant, a bacterium and a yeast cell all respire — but they do not breathe. You respire in every cell of your body, even while holding your breath.

Exam tip. If a question gives marks for "explain why respiration is not the same as breathing", you must say respiration is a chemical reaction inside cells that releases energy, while breathing is the movement of air in and out of the lungs. Both ideas are needed.

Most respiration (the aerobic part) takes place in the mitochondria — tiny organelles often called the cell's "powerhouses".

Because different cells do different amounts of work, they need different amounts of ATP:

• Very active cells — such as muscle cells, nerve cells and sperm cells — use a lot of energy, so they contain many mitochondria to make plenty of ATP.
• Less active cells need less ATP, so they have fewer mitochondria.

This links straight to the spec idea that respiration produces ATP: the more ATP a cell needs, the more "factories" (mitochondria) it has to make it.

Exam tip. A classic application question: "Explain why muscle cells contain many mitochondria." The answer: muscle cells contract and need a lot of energy, mitochondria carry out (aerobic) respiration to make ATP, so more mitochondria means more ATP for contraction.