Aerobic & Anaerobic Respiration

Respiration is the process that releases energy from glucose inside living cells. Cells need this energy for everything — muscle contraction, making new molecules, active transport and keeping warm.

• It happens in every living cell, all the time.
• The energy released is stored in a molecule called ATP, which the cell uses straight away.
• There are two types: aerobic (uses oxygen) and anaerobic (does not use oxygen).

Watch out — respiration is NOT breathing. Breathing (ventilation) moves air in and out of the lungs. Respiration is a chemical reaction inside cells that releases energy. Examiners reject answers that swap the two.

Aerobic respiration uses oxygen to break glucose down completely. Because the glucose is fully broken down, it releases a large amount of energy (a large ATP yield).

Word equation:

glucose + oxygen → carbon dioxide + water

Balanced symbol equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

• The products are carbon dioxide and water — these are removed as waste (CO₂ is breathed out).
• The large energy release is the key advantage of aerobic respiration.
• It happens whenever there is enough oxygen, e.g. in muscle cells at rest or during gentle exercise.

Anaerobic respiration releases energy from glucose without using oxygen. The glucose is only partly broken down, so it releases a small amount of energy (a small ATP yield). The products are different in animals compared with plants and yeast.

In animal cells (e.g. human muscle during hard exercise):

glucose → lactic acid

• Used when muscles work so hard that oxygen cannot be delivered fast enough.
• Produces lactic acid, which builds up and causes muscle fatigue/cramp.

In plant cells and in yeast (a fungus):

glucose → ethanol + carbon dioxide

• This is called fermentation.
• In yeast, it is used to make bread (the CO₂ makes dough rise) and alcoholic drinks (the ethanol).

Spec point 2.36 asks you to compare the two types. Learn this table — comparison questions are very common.

Feature	Aerobic	Anaerobic
Oxygen	Used	Not used
Breakdown of glucose	Complete	Incomplete (partial)
Energy / ATP released	Large amount	Small amount
Products	CO₂ + water	lactic acid (animals); ethanol + CO₂ (plants/yeast)

Why does anaerobic release less energy? Because the glucose is only partly broken down — the products (lactic acid, or ethanol) still contain a lot of locked-up chemical energy that has not been released.

Exam tip. A common 2-mark question is "give two differences". A clean answer pairs the SAME feature on both sides, e.g. "aerobic uses oxygen whereas anaerobic does not; aerobic releases more energy than anaerobic".

During hard exercise, muscle cells respire anaerobically and lactic acid builds up. After exercise, this lactic acid must be broken down, and that needs extra oxygen. The extra oxygen the body takes in after exercise to remove the lactic acid is called the oxygen debt.

This is why, after a sprint, you keep breathing fast and deeply for a while even though you have stopped:

• Hard exercise → oxygen supply can't keep up → muscles respire anaerobically → lactic acid builds up.
• You then "owe" oxygen — you breathe deeply and your heart keeps beating fast.
• The extra oxygen breaks the lactic acid down, repaying the oxygen debt.

Exam tip. Link the chain clearly: not enough oxygen → anaerobic respiration → lactic acid → oxygen debt repaid by continued deep breathing after exercise.