Respiration

Aerobic respiration

Releases energy from glucose using oxygen, in the mitochondria.

glucose + oxygen → carbon dioxide + water   (+ energy)
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

• Releases lots of energy; it is exothermic.

Anaerobic respiration (no oxygen)

Less efficient — releases much less energy:

• In muscles: glucose → lactic acid (builds up during hard exercise → fatigue and oxygen debt).
• In yeast/plants (fermentation): glucose → ethanol + carbon dioxide (used in brewing and bread-making).

Why we respire

Energy is used for: movement, keeping warm, building larger molecules, active transport.

Metabolism

The sum of all chemical reactions in a cell/body — building up (e.g. proteins) and breaking down molecules.

Exercise response

Heart rate and breathing rate increase to supply more oxygen and glucose and remove CO₂. Hard exercise causes anaerobic respiration → lactic acid → repaid as oxygen debt afterwards.

Exam tip

Don't confuse respiration with breathing. Aerobic respiration releases more energy than anaerobic. In muscles the anaerobic product is lactic acid; in yeast it's ethanol + CO₂.