Respiration, exercise and metabolism – GCSE Combined Science Biology

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Every cell in your body needs energy to work. Respiration is how cells get it. It happens in you, in plants and in every other living thing, all the time.

This matters because it explains why you breathe faster when you run. It also explains why your muscles ache after a hard sprint and why bread dough rises.

By the end you will be able to write the equations for aerobic and anaerobic respiration. You will describe how your body responds to exercise. You will also say what metabolism means.

Aerobic respiration

Respiration is a chemical reaction that releases energy from glucose. It is not the same as breathing. Breathing moves air in and out of your lungs.

Aerobic respiration uses oxygen. Aerobic means with oxygen. Word equation: glucose + oxygen → carbon dioxide + water. Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.

Most of it happens in the mitochondria, which are tiny parts of the cell. Respiration releases energy, so it is exothermic. The energy is used for movement, for keeping warm and for building molecules. Think of it as burning fuel in an engine.

The body and exercise

During exercise your muscles need more energy. So they need more oxygen and glucose, and they make more carbon dioxide.

The heart beats faster, so blood reaches the muscles sooner. Think of a delivery driver working harder at lunchtime. Breathing gets faster and deeper, so more oxygen is taken in and more carbon dioxide is removed.

Faster breathing and a faster heart rate keep aerobic respiration going. If the oxygen still is not enough, the muscles use another route.

Anaerobic respiration in muscles

When there is not enough oxygen, muscle cells also respire without it. This is anaerobic respiration. Anaerobic means without oxygen. It happens in hard exercise, such as a sprint, when the blood cannot bring oxygen fast enough.

Word equation: glucose → lactic acid. It releases much less energy than aerobic respiration, because the glucose is only partly broken down.

Think of it as grabbing a quick snack instead of eating a proper meal. It is fast, but you get much less from it. The lactic acid builds up in the muscles. It can cause cramp and tiredness.

Oxygen debt

After hard exercise you keep breathing fast for a while. The lactic acid in your muscles has to be broken down. This needs extra oxygen.

The extra oxygen needed to break down lactic acid is called the oxygen debt. Think of it as borrowing money. You get the energy now, and you pay back the oxygen later.

The blood carries the lactic acid to the liver. There it is broken down into carbon dioxide and water. The fitter you are, the faster you repay your oxygen debt.

Anaerobic respiration in yeast

Yeast is a tiny fungus. Yeast cells and plant cells can also respire without oxygen. They make ethanol and carbon dioxide. Ethanol is a type of alcohol. Notice that this is different from what happens in muscles.

Word equation: glucose → ethanol + carbon dioxide. This is called fermentation. It also releases less energy than aerobic respiration.

Bakers add yeast to dough. The yeast uses sugar in the dough, and the carbon dioxide gas makes the dough rise. Brewers use fermentation to make alcoholic drinks.

Metabolism

Metabolism is the sum of all the chemical reactions in a cell or the body. Think of it as everything a factory does in a day.

Many reactions build large molecules from small ones. Glucose is made into starch, glycogen and cellulose. Glycerol and fatty acids are joined to make lipids. Glucose and nitrate make amino acids, which are joined into proteins.

Other reactions break molecules down. Respiration breaks down glucose. Excess amino acids are broken down into urea, which the body gets rid of. Many of these reactions use energy from respiration.

Recap

Aerobic: glucose + oxygen → carbon dioxide + water.

Anaerobic in muscles: glucose → lactic acid. Anaerobic in yeast: glucose → ethanol + carbon dioxide.

Exercise raises heart rate and breathing rate. Lactic acid causes an oxygen debt.

Metabolism is the sum of all the reactions in the body.