Respiration (Years 7–11) — Science

How every cell releases energy from glucose — aerobic and anaerobic respiration.

Right now, in every one of your trillions of cells, tiny power stations are quietly burning sugar to keep you alive. That process is respiration — and it never, ever stops, not even while you sleep.

Why this matters: Every movement, thought, heartbeat and repair your body makes is paid for with energy released by respiration. Photosynthesis stores energy in glucose; respiration is how living things spend it.

Respiration releases energy inside the mitochondria

Respiration is not breathing

This trips everyone up. Breathing is moving air in and out of your lungs. Respiration is a chemical reaction inside cells that releases energy from glucose. Breathing merely supplies the oxygen that respiration uses. The reaction happens in tiny structures called mitochondria — think of them as the cell's power stations, quietly burning fuel around the clock.

With oxygen: aerobic respiration

When plenty of oxygen is available, cells use the most efficient method, aerobic respiration:

glucose + oxygen → carbon dioxide + water (+ lots of energy)

Aerobic respiration: glucose burns with oxygen, giving CO₂, water and plenty of energy.

The carbon dioxide is a waste product you breathe out; the water is reabsorbed or lost. This releases a large amount of energy from each glucose molecule.

Without enough oxygen: anaerobic respiration

During hard exercise your muscles demand energy faster than your blood can deliver oxygen. Cells switch to anaerobic respiration — respiration without oxygen. In muscles this produces lactic acid, which builds up and causes that burning, tired feeling. It releases less energy per glucose than the aerobic route.

In yeast, anaerobic respiration is different: it makes ethanol + carbon dioxide. Humans have used this for thousands of years — the carbon dioxide makes bread rise, and the ethanol is the alcohol in brewing.

Yeast respires without oxygen to make ethanol + carbon dioxide — used in baking and brewing.

Feature	Aerobic	Anaerobic (muscle)
Needs oxygen?	✓ Yes	✗ No
Energy released	Lots	Less
Products	CO₂ + water	Lactic acid

Worked reasoning. A sprinter's legs ache after a 200 m dash. Ask: "was oxygen keeping up?" — no, the muscles worked too fast. "So which respiration ran?" — anaerobic. "What did it produce?" — lactic acid, and that build-up is the ache. After the race, deep breathing brings extra oxygen to break the lactic acid down.

Sprinting outpaces oxygen supply, so muscles respire anaerobically and lactic acid builds up.

Common mix-up: saying respiration happens in the lungs or that it is breathing. Respiration is a chemical reaction inside cells' mitochondria; breathing is just the pump that delivers the oxygen.

Memory hook: Aerobic needs air (oxygen) and gives you a lot of energy; anaerobic means no air and gives you a nibble. Glucose is always the fuel; mitochondria are always the furnace.