What is the difference between aerobic and anaerobic respiration, and why do we get muscle cramps after running?

Question

HBSE Class 10, Science chapter Life Processes. I get mixed up between what happens in yeast and what happens in our muscles when there is no oxygen. Where does each step take place and which one gives more energy? Also why do legs pain after a long race?

GradeHunt_Team · Answer

All three pathways in your textbook start the same way. The difference is what happens to pyruvate.

Step 1 (common to all), in the cytoplasm:
Glucose (6-carbon) breaks down into pyruvate (3-carbon) + a small amount of energy.

Step 2 depends on oxygen:

1. Aerobic respiration (oxygen present), in the mitochondria:
Pyruvate breaks down into carbon dioxide + water + energy.
This releases much more energy than the other two.

2. Anaerobic respiration in yeast (no oxygen), in the cytoplasm:
Pyruvate changes to ethanol + carbon dioxide + energy.
This is fermentation. It is why bread dough rises (the CO2) and why yeast is used to make alcohol.

3. Lack of oxygen in our muscle cells, in the cytoplasm:
Pyruvate changes to lactic acid + energy.

So yeast makes ethanol and CO2, our muscles make lactic acid, and only the aerobic pathway gives CO2 and water.

Why cramps after running? During hard exercise your muscles need energy faster than your blood can supply oxygen. Some muscle cells switch to the no-oxygen pathway, and lactic acid builds up. Your textbook says this build-up of lactic acid causes the cramps, and that is the answer to write in the exam. The ache you feel the next day is a different thing. Lactic acid is cleared within about an hour after you stop, so next-day soreness comes mostly from tiny damage to the muscle fibres during the race.

How to write it in the exam: draw the three-branch flowchart (glucose to pyruvate, then three arrows). Label the place (cytoplasm or mitochondria) and the presence or absence of oxygen on each arrow. If the question says differentiate, also give a short table: place (cytoplasm or mitochondria), oxygen (needed or not), end products, and energy released (more or less).

One more point the chapter makes: the energy released during respiration is used to make ATP, which the cell uses as its energy currency for other activities.

What is the difference between aerobic and anaerobic respiration, and why do we get muscle cramps after running?

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