Key Stage 4 science · Biology – Cell biology

GCSE enzymes and respiration: a guide for parents

Every living cell runs on chemical reactions, and enzymes are the biological catalysts that make those reactions fast enough to keep us alive. At GCSE, students learn how enzymes work, why temperature and pH change their speed, and how cells use respiration to release energy from glucose, with oxygen (aerobic) or without it (anaerobic). The topic links directly to digestion, exercise and fermentation.

Enzymes and respiration, explained

Enzymes are proteins that speed up chemical reactions without being used up. Each enzyme has a specially shaped region called the active site, and only the substrate whose shape fits that site can bind to it. Teachers often call this the lock and key model. The substrate is changed into products, the products leave, and the enzyme is free to work again. Digestive enzymes such as amylase (starch to sugars), protease (proteins to amino acids) and lipase (fats to fatty acids and glycerol) are the examples students meet first.

The rate of an enzyme-controlled reaction depends on conditions. As the temperature rises, particles move faster and collide with the active site more often, so the reaction speeds up. Beyond the enzyme’s optimum, the bonds holding the protein in shape break, the active site changes shape and the substrate no longer fits. The enzyme is then denatured, and this is permanent. A change in pH can denature an enzyme in the same way, and different enzymes have different optimum values: pepsin in the stomach works best in acid, while amylase in the mouth works best close to neutral. Concentrations of enzyme and substrate also matter.

Respiration is the process that releases energy from food in every living cell, all the time, in plants as well as animals. It is not the same as breathing, which is just how oxygen reaches the cells. Aerobic respiration uses oxygen: glucose plus oxygen gives carbon dioxide plus water, and energy is transferred to the cell. Most of it takes place in the mitochondria, and the energy is used for muscle contraction, building new molecules, active transport and, in mammals, keeping warm.

When muscles work hard and oxygen cannot be delivered fast enough, cells also respire anaerobically. In humans glucose is broken down to lactic acid, which releases much less energy than aerobic respiration. Lactic acid builds up in the muscles, which then tire. After exercise the body needs extra oxygen to react with the lactic acid and remove it, which is why breathing stays fast and deep for a while. This extra oxygen is called the oxygen debt. Plant cells and yeast respire anaerobically in a different way, making ethanol and carbon dioxide. In yeast this is called fermentation, and it is used in bread-making and brewing. Students also learn that respiration releases energy to the surroundings, so it is an exothermic reaction.

Words your child will hear

Catalyst: A substance that speeds up a reaction without being used up. Enzymes are biological catalysts.
Substrate: The molecule an enzyme acts on, for example starch for amylase.
Active site: The part of an enzyme whose shape fits the substrate, where the reaction takes place.
Denatured: Describes an enzyme whose shape has been permanently changed, usually by heat or extreme pH, so it no longer works.
Optimum: The temperature or pH at which an enzyme works fastest.
Aerobic respiration: Releasing energy from glucose using oxygen, producing carbon dioxide and water.
Anaerobic respiration: Releasing energy from glucose without oxygen, giving lactic acid in muscles, and ethanol and carbon dioxide in plant cells and yeast.
Oxygen debt: The extra oxygen the body needs after hard exercise to react with the lactic acid that has built up.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about enzymes
Students should be taught about factors affecting the rate of enzymatic reactions
Students should be taught about the importance of cellular respiration; the processes of aerobic and anaerobic respiration

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A model exam answer

The enzyme amylase breaks down starch. In an investigation its rate of reaction rose from 10 °C to 37 °C, then fell sharply, and at 60 °C no reaction took place. Explain these results.

From 10 °C to 37 °C: as the temperature rises, the enzyme and starch particles have more kinetic energy, so they move faster and collide more often. More collisions with the active site mean more successful reactions each second, so the rate increases.
Around 37 °C: this is close to the optimum, where the rate is highest. The shape of the active site fits the starch substrate best here.
Above the optimum: the extra energy starts to break the bonds that hold the protein in its three-dimensional shape. The active site begins to change shape, so fewer starch molecules fit and the rate falls.
At 60 °C: the active site has lost its shape completely, so starch can no longer bind. The enzyme is denatured, which is not reversible, so cooling the mixture again would not restore the reaction.

Rate rises with temperature up to the optimum because of more frequent collisions, then falls as the enzyme’s active site changes shape. At 60 °C amylase is denatured, so the substrate no longer fits and no reaction occurs.

Common mistakes and how to help

“High temperature kills the enzyme”

The word “killed” is used loosely, but enzymes are molecules, not living things. They have never been alive, so they cannot be killed.

What helps: Use “denatured” and describe what happens: the shape of the active site changes. Examiners expect the precise wording, so it is worth practising.

Respiration and breathing are the same thing

In everyday speech people say “respiratory” for breathing, and the two do happen together. Students then write that respiration “takes in oxygen”.

What helps: Breathing moves gases in and out of the lungs. Respiration is a chemical reaction inside cells that releases energy. Ask: “Which one is happening in a potato?”

Plants photosynthesise by day and respire by night

Plants are often taught as the opposite of animals, so students assume plants don’t need to release energy from glucose in the light.

What helps: Plants respire all the time in every living cell. In the light they photosynthesise as well, and usually make more glucose than they use.

Respiration “makes” or “produces” energy

Everyday language talks about producing or making energy, and the phrase is common in films, news and adverts.

What helps: Energy is never created. Respiration transfers energy from the chemical store in glucose so that the cell can use it, and some goes to the surroundings as heat. “Releases” or “transfers” is the safe word.

How to help at home

Taste an enzyme at work. Chew a plain cracker or piece of bread for a minute without swallowing. It gradually tastes sweeter as amylase in saliva breaks starch into sugars. Ask your teenager to explain why chewing helps as well.
Watch yeast respire. Mix a pinch of dried yeast and a spoon of sugar into a bottle of warm (not hot) tap water and stretch a balloon over the top. The balloon inflates with carbon dioxide. Repeat with cold water and compare. Wash hands and don’t drink it.
Compare breathing and heart rate. Take a pulse and count breaths at rest, then after a minute of star jumps. Ask what is happening in the muscles and why the body needs extra oxygen afterwards, then listen for the words aerobic, anaerobic and lactic acid.
Ask for a short explanation. Pick one graph from their notes, such as rate against temperature, and ask for a 30-second explanation in their own words. Being able to say “denatured” and “active site” precisely is what earns the marks.