Anaerobic Respiration

During vigorous exercise, such as a sprint, muscles contract faster than the blood can bring them oxygen, so as well as aerobic respiration they respire anaerobically.

glucose \(\rightarrow\) lactic acid, with a small amount of energy released. No carbon dioxide is made.

The lactic acid collects in the muscles and the blood. It causes muscle fatigue, so the muscles ache, tire and stop contracting efficiently.

Anaerobic respiration is useful because it releases energy at once and without oxygen. It cannot be kept up for long, because the lactic acid builds up.

The oxygen debt is the extra oxygen the body needs after exercise to break down the lactic acid that built up in the muscles.

The blood carries the lactic acid from the muscles to the liver. There it is broken down using oxygen, and some is converted back to glucose.

A person keeps breathing deeply and their heart rate stays high for several minutes after stopping. This takes in the extra oxygen and carries it to the muscles and liver, which is why you pant after a sprint.

The time that breathing rate and heart rate take to return to resting levels is the recovery time. A fitter person recovers faster, so the oxygen debt is repaid sooner.

glucose \(\rightarrow\) ethanol + carbon dioxide, with a small amount of energy released. Ethanol is the alcohol in drinks.

Yeast makes ethanol and carbon dioxide, not lactic acid. This is the main difference to learn between anaerobic respiration in yeast and in animal muscle.

Plant cells, such as the roots of a waterlogged plant, and some bacteria can also respire anaerobically to make ethanol and carbon dioxide.

If oxygen is present, yeast respires aerobically instead and makes carbon dioxide and water, so fermentation to ethanol only takes place when oxygen is absent.

Yeast is added to a sugary liquid, such as a barley extract for beer or grape juice for wine, in a closed vessel without oxygen. The ethanol makes the drink alcoholic and the carbon dioxide makes beer fizzy.

Yeast is mixed into bread dough with a little sugar. The carbon dioxide makes bubbles in the dough, which rises and is set by baking into a light, airy loaf. The ethanol evaporates in the hot oven.

Yeast ferments fastest in a warm place, about 30 to 40 degrees Celsius, because its enzymes work best there. Dough is left somewhere warm to rise, and the yeast is killed in a hot oven.

Brewers keep air out of the vessel so that the yeast ferments instead of respiring aerobically, which would make less ethanol.

In aerobic respiration the glucose is broken down completely, so most of its energy is released. In anaerobic respiration it is only partly broken down, and the lactic acid or ethanol that is left still contains a lot of energy.

Aerobic respiration releases many times more energy from each glucose molecule, roughly fifteen times as much usable energy as anaerobic respiration.

A muscle that depends on anaerobic respiration has to use up glucose far faster to release the same amount of energy, which is why it cannot be kept up for long.

It is still useful because it needs no oxygen and releases energy at once, giving a burst of energy in a sprint or in a cell that has no oxygen at all.

During exercise the muscle cells respire faster, so the heart rate and breathing rate increase, and each breath gets deeper. More oxygen arrives and more carbon dioxide is removed.

In gentle exercise the blood can deliver enough oxygen, so the muscles respire aerobically. In vigorous exercise it cannot, so they also respire anaerobically and lactic acid builds up.

Heart rate and breathing rate stay high until the oxygen debt has been repaid and the lactic acid has been removed. They then fall back to resting levels.

You can measure the pulse in beats per minute and the breathing rate in breaths per minute at rest, straight after exercise and then every minute during recovery.