Anaerobic respiration

Where it happens

Anaerobic respiration happens in the cytoplasm of the cell. In humans it occurs in muscle cells during hard exercise when oxygen runs short; in yeast it occurs during fermentation.

Because no oxygen is used, the mitochondria play no part, and the whole process stays in the cytoplasm where glycolysis takes place. In muscle, anaerobic respiration runs alongside aerobic respiration rather than replacing it: the mitochondria keep working with whatever oxygen arrives, and the extra ATP demand is met anaerobically.

Yeast is a single-celled fungus that switches to fermentation when it is sealed away from air, for example in dough or in a brewing vessel, and some bacteria and plant roots in waterlogged soil respire anaerobically in the same way.

Inputs and outputs

• Input in both cases: glucose, from the blood in humans or from sugar in the surroundings for yeast.
• Input: the enzymes of the cytoplasm; no oxygen is needed at all.
• Output in human muscle: lactic acid and a small amount of energy transferred to ATP.
• Output in yeast: ethanol, carbon dioxide and a small amount of energy transferred to ATP.
• Word equation for muscle: glucose → lactic acid + energy.
• Word equation for yeast: glucose → ethanol + carbon dioxide + energy.

The steps

1. Glucose is available but oxygen is in short supply, so the mitochondria cannot complete the breakdown of glucose.
2. In the cytoplasm, glucose is split into two three-carbon molecules by glycolysis, releasing a small amount of ATP.
3. Because oxygen is absent, the three-carbon molecules cannot enter the mitochondria and are converted instead into an end product.
4. In human muscle, the three-carbon molecules become lactic acid; in yeast, they lose carbon dioxide and become ethanol.
5. The small amount of energy released is transferred to ATP, keeping the cell working for a short time.
6. In humans, lactic acid builds up in the muscle and blood, causing fatigue and a burning feeling until it is removed.
7. After exercise, the lactic acid is carried to the liver and broken down using extra oxygen, which is the oxygen debt being repaid.

Why it matters and how it is controlled

Anaerobic respiration lets cells keep releasing some energy when oxygen is scarce, such as in sprinting muscles. In yeast it is used in baking and brewing.

The switch to anaerobic respiration is controlled by oxygen supply. When the heart and lungs cannot deliver oxygen fast enough to match the demand for ATP, muscle cells top up their energy anaerobically.

The build-up of lactic acid limits how long this can continue, because the acid lowers the pH of the muscle and slows the enzymes, so the muscle tires and the person must slow down. Breathing rate and heart rate stay high after exercise until the extra oxygen has been taken in to oxidise the lactic acid.

In yeast, the rate of fermentation depends on temperature, the concentration of sugar, and the concentration of ethanol that has already built up. Ethanol is toxic to yeast above a certain level, which is why fermentation stops on its own.

Bread rises because the carbon dioxide released by yeast forms bubbles in the dough, and the ethanol evaporates during baking.

How it is examined

You may be asked to compare the products in humans and yeast, to write the word equations, or to explain oxygen debt after exercise.

Experiment-based questions use a boiling tube of yeast and glucose solution covered with a layer of oil, connected to lime water or bicarbonate indicator; you must explain the purpose of the oil layer, which keeps oxygen out, and interpret the change in the indicator. Graph questions show breathing rate or lactic acid concentration before, during and after exercise and ask you to explain why the values stay high for a time after the exercise ends.

Comparison tables of aerobic and anaerobic respiration appear in structured questions and as essay prompts.

Common misconceptions

Worked exam-style question

Question. An athlete ran a 400 m race. Her breathing rate was measured before the race, at the end of the race and every minute for ten minutes afterwards.

The breathing rate rose sharply during the race and stayed above the resting value for eight minutes after she stopped running. (a) Name the type of respiration that took place in her leg muscles towards the end of the race, and state its product.

(b) Explain why her breathing rate remained high for eight minutes after the race. (c) State two differences between this type of respiration and the respiration of yeast in a sealed container.

Model answer. (a) Anaerobic respiration; the product is lactic acid. (b) During the race, oxygen could not be delivered fast enough to meet the demand for ATP, so muscle cells respired anaerobically and lactic acid accumulated.

After the race, extra oxygen is needed to oxidise the lactic acid in the liver and muscles; this is the oxygen debt, so breathing stays rapid until it is repaid. (c) Muscle produces lactic acid whereas yeast produces ethanol; muscle releases no carbon dioxide whereas yeast releases carbon dioxide.

Source:SRC-DSKP-EN

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