Respiration

What respiration is

Respiration is the process that releases energy from glucose in every living cell. It is an exothermic reaction and happens continuously — it is not the same as breathing (that's ventilation). The energy released is used for movement, keeping warm, growth and building larger molecules.

There are two types: aerobic (with oxygen) and anaerobic (without oxygen).

Aerobic respiration

Uses oxygen and releases a large amount of energy. It happens mainly in the mitochondria.

Word equation:

glucose + oxygen → carbon dioxide + water

Symbol equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Anaerobic respiration

Happens when there is not enough oxygen (e.g. hard exercise). It releases much less energy because glucose is only partly broken down.

In muscle cells:

glucose → lactic acid

In plants and yeast (fermentation):

glucose → ethanol + carbon dioxide

Fermentation in yeast is used to make bread (CO₂) and alcohol (ethanol).

Aerobic vs anaerobic

	Aerobic	Anaerobic
Oxygen?	Yes	No
Energy released	A lot	A little
Glucose broken down	Completely	Partially
Products (muscle)	CO₂ + water	Lactic acid

Oxygen debt

During vigorous exercise, muscles respire anaerobically, building up lactic acid. Afterwards you keep breathing hard to take in extra oxygen — the oxygen debt — to break down the lactic acid. This is why you pant after a sprint.

Metabolism

Metabolism is the sum of all the chemical reactions in a cell or body — including respiration, and reactions that build larger molecules (like proteins from amino acids) and break them down. Respiration provides the energy for these reactions.

Worked example

Why does a sprinter breathe heavily for a few minutes after finishing a race?

• During the sprint, muscles respired anaerobically, producing lactic acid and creating an oxygen debt. The extra breathing supplies oxygen to break down the lactic acid. ✓

Common mistakes

• Confusing respiration (releasing energy in cells) with breathing (moving air in/out).
• Saying respiration "makes" energy — energy is released/transferred, not created.
• Giving ethanol as the anaerobic product in muscles — that's plants/yeast; muscles make lactic acid.

Exam tips

• Learn both respiration equations, and the two different anaerobic products (lactic acid vs ethanol + CO₂).
• Be able to explain oxygen debt clearly.
• State that aerobic respiration releases more energy than anaerobic.

Key facts to remember

• Respiration releases energy from glucose in all cells; exothermic; not the same as breathing.
• Aerobic: glucose + oxygen → CO₂ + water (lots of energy, in mitochondria). Anaerobic (muscle): glucose → lactic acid; (yeast/plants): glucose → ethanol + CO₂.
• Hard exercise builds lactic acid and an oxygen debt; metabolism = all the body's chemical reactions.