Aerobic respiration uses oxygen to break glucose down fully and releases a large amount of energy. Anaerobic respiration works without oxygen and releases less energy, leaving lactic acid or ethanol.
This lesson opens the SPM Biology cellular respiration chapter. The next lesson looks at substrates, products and energy in more detail.
What are the word equations?
Aerobic respiration: glucose + oxygen → carbon dioxide + water + energy.
Anaerobic respiration in muscle: glucose → lactic acid + energy.
Anaerobic respiration in yeast: glucose → ethanol + carbon dioxide + energy.
Worked example: a sprint and a loaf of bread
Two situations use the same idea. In the first, a student sprints 100 metres. In the second, yeast is mixed into dough.
| Aerobic (jogging muscle) | Anaerobic (sprinting muscle) | Anaerobic (yeast in dough) | |
|---|---|---|---|
| Oxygen needed | Yes | No | No |
| Substrate | Glucose | Glucose | Glucose |
| Products | Carbon dioxide and water | Lactic acid | Ethanol and carbon dioxide |
| Energy released | Large amount | Small amount | Small amount |
| Where it happens | Cytoplasm, then mitochondria | Cytoplasm | Cytoplasm |
The sprinting muscle builds up lactic acid, which is why the legs feel tired. The yeast releases carbon dioxide, which makes the dough rise.
The mistake that costs marks
The common slip is to write that anaerobic respiration releases no energy. It does release energy, only a smaller amount, because the glucose is broken down only partly.
| Wrong | Right | |
|---|---|---|
| Energy without oxygen | None | Less, because glucose is partly broken down |
| Muscle product | Ethanol | Lactic acid |
| Yeast product | Lactic acid | Ethanol and carbon dioxide |
Students also mix up the two anaerobic routes. Link each product to its organism before the exam: muscle with lactic acid, yeast with ethanol.
How should you write a comparison answer?
Give one point for each side in the same sentence. For example: “Aerobic respiration needs oxygen, but anaerobic respiration does not.”
A comparison with only one side, such as “aerobic respiration needs oxygen”, earns less than one that names both.
Check yourself
A baker notices that a dough with yeast rises slowly in a cold room. State the products of the yeast’s respiration and say which one makes the dough rise.
Answer
Yeast respires anaerobically on the sugars in the dough. The products are ethanol and carbon dioxide, and energy is released.
The carbon dioxide forms gas bubbles that make the dough rise.
What to study next
Continue with relating substrates, products and energy. The biological data interpretation trainer helps you practise reading results from investigations, and the mistake log and paper-error review records slips.
If you would like a teacher to check your comparison sentences, see online one-to-one Biology tuition.