Cambridge O Level Biology · Syllabus 5090 · Respiration
Lactic Acid
What is Lactic Acid?
Lactic acid is the product of anaerobic respiration in human muscle cells. It forms when the rate at which the blood delivers oxygen to a muscle is not sufficient for the rate of aerobic respiration required, so some glucose is broken down anaerobically instead. Because the glucose is only partly broken down, the lactic acid still contains chemical energy. It accumulates in the muscles and in the blood during vigorous exercise and is transported in the blood to the liver, where it is broken down using oxygen during recovery.
This definition is part of the Respiration chapter in Cambridge O Level Biology.
Common mistakes with Lactic Acid
- 3. Swapping the anaerobic products between humans and yeast Human muscle gives lactic acid. Yeast gives ethanol and carbon dioxide. A yeast answer containing lactic acid, or a human answer containing ethanol, scores zero for that mark however good the rest of the sentence is. Repair Memorise them as a pair, together, so that recalling one always drags the other into view.
- 4. Defining oxygen debt as lactic acid, or as breathlessness EPOC is a quantity of oxygen — the extra oxygen consumed after exercise, above the resting rate. Lactic acid is a substance. Breathlessness is a sensation. Repair “The extra oxygen taken in after exercise, used in the liver in the breakdown of lactic acid.”
- Careful with fatigue. It is safe to write that lactic acid accumulates in muscle and blood during vigorous exercise and that it must be removed afterwards. It is not safe to write that lactic acid is the proven cause of every ache, cramp or feeling of tiredness — the evidence for that is far less settled than older textbooks suggest. Say what accumulates and what happens to it.
- 11. “Humans produce ethanol when they respire anaerobically.” Ethanol is the yeast product. Human muscle produces lactic acid. Write glucose → lactic acid (human muscle).
- 12. “Yeast produces lactic acid.” The mirror image of the previous error, and equally fatal. Write glucose → ethanol + carbon dioxide (yeast).
- 14. “Oxygen debt is the lactic acid in the muscles.” Lactic acid is a substance. EPOC is a quantity of oxygen. Write EPOC is the extra oxygen consumed after exercise, above the resting requirement, used in the liver in the breakdown of lactic acid.
Examiner tips on Lactic Acid
- Why order matters. A question worth four marks is marked on the links, not the vocabulary. “Heart rate stays high” earns nothing on its own; “heart rate stays high so that the blood can carry lactic acid from the muscles to the liver” earns the mark. Attach a purpose to every step.
Questions students ask about Lactic Acid
What are the products of anaerobic respiration in humans and in yeast?
In human muscle, glucose → lactic acid. In yeast, glucose → ethanol + carbon dioxide. The two must never be swapped: yeast does not make lactic acid, and humans do not make ethanol.
Why does anaerobic respiration release less energy?
Because the glucose is only partly broken down. The products — lactic acid, or ethanol — still contain a great deal of chemical energy, so less of the energy in the original glucose molecule is released to the cell.
What is oxygen debt, or EPOC?
EPOC stands for excess post-exercise oxygen consumption, often called oxygen debt. It is the extra oxygen consumed after exercise, above the amount that would have been used at rest. That oxygen is used in the liver in the breakdown of the lactic acid that accumulated during the exercise.
Why do you keep breathing hard after you stop running?
Lactic acid has accumulated in the muscles and blood. Breathing stays deeper and faster so that extra oxygen enters the blood, and heart rate stays high so that the blood carries the lactic acid to the liver, where oxygen is used in breaking it down. Both return gradually to resting values once the lactic acid has been removed.
Does lactic acid cause muscle soreness the next day?
For O Level purposes, say that lactic acid accumulates in muscle and blood during vigorous exercise and is removed afterwards using oxygen in the liver. The claim that it causes soreness a day or two later is not well supported by evidence, so avoid asserting it; describe what accumulates and what happens to it.