Aerobic vs anaerobic respiration

A hard sprint, and what comes after

Step through the story. Each stage is the reason for the next one.

1. 1Oxygen can't keep upIn hard exercise, oxygen sometimes cannot be transported to the muscle cells fast enough for aerobic respiration.
2. 2Muscle cells go anaerobicSo the muscle cells respire anaerobically: glucose is converted to lactic acid,, and energy is transferred more quickly than in aerobic respiration.
3. 3Lactic acid is producedLactic acid is the only product, and it can cause muscle cramps. No carbon dioxide comes from this reaction.
4. 4Recovery: breathing and heart rate stay highAfter exercise, breathing rate and heart rate stay high to take in as much oxygen as possible.
5. 5Oxygen removes the lactic acidThat oxygen removes the lactic acid. The amount of oxygen needed to remove it is called the oxygen debt.

What you need to know

• Respiration isn't breathing. It's an exothermic chemical process in cells that uses glucose as a fuel to transfer energy for life processes.
• Aerobic respiration needs oxygen. Anaerobic respiration happens without oxygen, or when there isn't enough oxygen for aerobic respiration.
• Aerobic respiration: glucose + oxygen → carbon dioxide + water.
• Anaerobic respiration in humans: glucose → lactic acid. Glucose is the only reactant, so no carbon dioxide is produced.
• Have a goA muscle cell is respiring anaerobically. Could carbon dioxide be coming out of it? Settle it using only the equation.

  No. The equation is glucose → lactic acid, and carbon dioxide isn't in it.

  Glucose is the only reactant and lactic acid the only product, so carbon dioxide has nowhere to come from. 'All respiration gives off carbon dioxide' is the trap.

• In plants and microorganisms such as yeast, anaerobic respiration turns glucose into ethanol and carbon dioxide. In microorganisms, that's called fermentation.
• Have a goSam announces, very confidently, 'Anaerobic respiration always makes lactic acid.' Which living thing proves Sam wrong?

  Yeast (or plants). They make ethanol and carbon dioxide.

  Lactic acid is what humans make. In plants and microorganisms such as yeast, glucose is converted to ethanol and carbon dioxide instead.

• Aerobic respiration happens in the mitochondria of eukaryotic cells. Anaerobic respiration happens only in the cytoplasm, with no mitochondria involved.
• Anaerobic respiration transfers less energy than aerobic respiration, but it's a shorter process, so it transfers energy more quickly.
• Have a goA sprinter needs energy transferred right now. Which kind of respiration does that more quickly, and what's the catch?

  Anaerobic respiration, but it transfers less energy than aerobic respiration.

  It's a shorter process, so energy is transferred more quickly. Quicker is not the same as more.

• In hard exercise, oxygen sometimes can't reach muscle cells fast enough. They respire anaerobically, and the lactic acid can cause cramps.
• After exercise, breathing rate and heart rate stay high to take in oxygen, which removes the lactic acid.
• The amount of oxygen needed to remove that lactic acid is called the oxygen debt.

The big picture

Aerobic and anaerobic respiration are both ways of using glucose to transfer energy. Aerobic needs oxygen and makes carbon dioxide and water. Anaerobic works without oxygen: human muscle makes lactic acid only, while plants and yeast make ethanol and carbon dioxide.

⚠ Watch out

Saying human muscle gives off carbon dioxide (or ethanol) when it respires anaerobically, or saying respiration creates energy. Muscle makes lactic acid only, and respiration transfers energy from glucose.

Memory hook

Muscle ends in Lactic acid, Yeast ends in Ethanol + carbon dioxide, and only the oxygen-using kind makes carbon dioxide and water. Look at the products and you can name the process.

Check yourself

A cell turns glucose into lactic acid and nothing else. Is oxygen needed? Is carbon dioxide made? Where does it happen, and how do you know it is not yeast?