Respiration

Anaerobic respiration releases energy from glucose without using oxygen. It is an incomplete breakdown of glucose, so it produces much less ATP per glucose molecule than aerobic respiration. The waste products are also different from the aerobic ones, and they vary between organisms.

Anaerobic respiration in animals (including human muscle)

In a hard sprint, your leg muscles need so much energy that the blood cannot deliver oxygen fast enough to keep up with aerobic respiration. The muscle cells switch to anaerobic respiration to top up their ATP supply.

Word equation for anaerobic respiration in animals:

glucose → lactic acid (+ a little energy)

Notice that no oxygen is needed, and the waste product is lactic acid, not carbon dioxide. Glucose is not fully broken down, so most of the energy that was in the glucose ends up locked inside the lactic acid molecules.

Two consequences for the body:

• Lactic acid builds up in the muscle, lowering its pH. Acidic conditions slow down the muscle's enzymes and cause the familiar burning and aching feeling.
• After the exercise, the body must break down the lactic acid using oxygen, in the liver. This takes time and is why you continue to breathe heavily for several minutes after stopping. Your body is still supplying the extra oxygen needed to clear the lactic acid. This extra oxygen demand is called the oxygen debt (or "excess post-exercise oxygen consumption").

Anaerobic respiration in yeast and plants (fermentation)

In yeast cells and in some plant cells, anaerobic respiration produces different waste products:

glucose → ethanol + carbon dioxide (+ a little energy)

This special case of anaerobic respiration in yeast is called fermentation and is industrially important:

• Brewing: yeast ferments the sugars in fruit or grain into ethanol (alcohol), giving beer, wine and spirits.
• Baking: yeast ferments sugars in bread dough. The carbon dioxide produced makes the dough rise; the small amount of ethanol evaporates during baking.
• Biofuels: ethanol from fermented crops is used as a renewable fuel additive.

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