The big idea: Cells release energy from glucose to make ATP. They can do this in two ways.
Aerobic respiration uses oxygen, breaks glucose down fully, and releases a lot of ATP.
Anaerobic respiration uses no oxygen, breaks glucose down only partly, and releases only a little ATP.
Which one a cell uses depends mainly on whether oxygen is available.
Glucose branches two ways: with oxygen → carbon dioxide + water + a lot of ATP (in the mitochondria); without oxygen → lactate in animals, or ethanol + carbon dioxide in yeast, with only a little ATP (in the cytoplasm).
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- Cell respiration
- The release of energy from glucose (or other carbon compounds) to make ATP, which powers the cell's processes.
- Aerobic respiration
- Respiration that uses oxygen. Glucose is broken down fully into carbon dioxide and water, releasing a lot of ATP.
- Anaerobic respiration
- Respiration that takes place without oxygen. Glucose is broken down only partly, releasing only a little ATP.
- ATP
- The cell's usable energy currency — the molecule respiration makes to power work in the cell.
- Lactate
- The product of anaerobic respiration in animals, including human muscle cells.
- Ethanol
- A product of anaerobic respiration in yeast (along with carbon dioxide); the basis of bioethanol fuel and brewing.
What the names tell you: Aerobic = with air (oxygen). An-aerobic = without air (no oxygen) — the 'an-' means 'without'.
So the single difference the names point to is simply: is oxygen used, or not?
The two types of respiration differ in five things you should be able to compare: oxygen use, where in the cell it happens, how fully glucose is broken down, the ATP yield, and the products.
Work through them one at a time — the cause of every difference traces back to the same thing: whether oxygen is there to finish the job.
Why aerobic releases so much more ATP: With oxygen, glucose is broken down all the way to carbon dioxide and water, so almost all of its stored energy is released — a lot of ATP.
Without oxygen, glucose is only partly broken down (into lactate, or ethanol + CO₂), so most of its energy is still locked inside that product — only a little ATP comes out.
Cause → effect: no oxygen → incomplete breakdown → far less ATP.
The products depend on the organism: Anaerobic respiration makes different products in different organisms:
In animals (including human muscle), the product is lactate.
In yeast (and plant cells), the products are ethanol + carbon dioxide — this is how bread, alcohol and bioethanol fuel are made.
Aerobic respiration, by contrast, gives the same products in all of them: carbon dioxide + water.
| Feature | Aerobic respiration | Anaerobic respiration |
|---|---|---|
| Oxygen needed? | Yes — uses oxygen | No — oxygen is absent |
| Where in the cell | Mitochondria (after the cytoplasm) | Cytoplasm only |
| How fully glucose is broken down | Fully broken down | Only partly broken down |
| ATP yield per glucose | A lot of ATP | Only a little ATP |
| Products in animals (incl. humans) | Carbon dioxide + water | Lactate |
| Products in yeast (and plants) | Carbon dioxide + water | Ethanol + carbon dioxide |
Aerobic
- Uses oxygen
- Happens in the mitochondria
- Glucose broken down fully
- Releases a lot of ATP
- Products: carbon dioxide + water
Anaerobic
- Uses no oxygen
- Happens in the cytoplasm
- Glucose broken down only partly
- Releases only a little ATP
- Products: lactate (animals) / ethanol + CO₂ (yeast)
A memory hook: Oxygen = more ATP. With oxygen the cell finishes the job and gets a big energy payout; without oxygen it stops early and gets a small one.
And for the products: Animals make lactate; yeast makes ethanol (think 'yeast → used in brewing').
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How this is tested: On Paper 1A a one-mark question often asks for the product and location of anaerobic respiration in human cells — the answer is lactate, made in the cytoplasm.
Another favourite is the contrast itself: state the waste products of aerobic versus anaerobic respiration, or identify what distinguishes the two (aerobic uses oxygen and yields far more ATP).
Look out for the applied versions too: how a muscle cell responds during intense exercise (oxygen runs low → anaerobic → lactate), and how bioethanol is made (yeast respiring anaerobically → ethanol + CO₂).
IB-style question — compare oxygen use, products and ATP yield
Compare aerobic and anaerobic respiration in human cells, referring to the use of oxygen, the products formed, and the amount of ATP released. [4]
How to score all four marks
- Oxygen. Aerobic respiration uses oxygen, whereas anaerobic respiration takes place without oxygen.
- Products in human cells. Aerobic respiration produces carbon dioxide and water, whereas anaerobic respiration produces lactate.
- ATP yield. Aerobic respiration releases a lot of ATP per glucose, whereas anaerobic respiration releases only a little.
- Why. This is because with oxygen the glucose is fully broken down, releasing most of its energy. (Award 1 mark per correct contrasted point, up to 4.)
Final answer
Aerobic uses oxygen, makes carbon dioxide + water and releases a lot of ATP; anaerobic uses no oxygen, makes lactate and releases only a little ATP — because without oxygen glucose is only partly broken down.
✓ Why this scores full marks: A 'compare' answer needs each point set against its opposite — aerobic does X whereas anaerobic does Y.
Simply listing facts about one type (for example, only describing aerobic) loses the comparison marks even if every fact is true.
| Organism | With oxygen (aerobic) | Without oxygen (anaerobic) |
|---|---|---|
| Human / animal muscle | carbon dioxide + water | lactate |
| Yeast (and plant cells) | carbon dioxide + water | ethanol + carbon dioxide |