Respiration

You are running late for your morning assembly at school — heart pounding, legs burning, gasping for air. Where does the energy for all that movement come from? The answer is happening right now in every single cell of your body, and it has been running non-stop since you were born. It is called respiration, and the PSLE marker loves it.

Parents: ask your child to predict whether they are still respiring while sleeping before they read on. The interactive experiment in this note lets them test the idea and see the correct equation for themselves — let them tap through it and read the result aloud.

By the end you will be able to: name what respiration needs and what it produces, explain why it happens even in the dark, and tell a PSLE marker exactly how it is different from breathing and from photosynthesis. The four ideas to master are: respiration releases energy, it happens in all cells all the time, glucose + oxygen → energy + carbon dioxide + water, and respiration is not the same as breathing.

Respiration releases energy from food

Your school lunch gives you energy — but a plate of rice cannot power your legs directly. First, your body has to break the food down into a simple sugar called . Then every living cell uses that glucose to release the energy locked inside it. This process is called .

Think of glucose as a tiny battery. Respiration is the process that opens the battery and lets the energy out. Without respiration, cells cannot move, grow, repair themselves, or do anything at all. Your muscle cells need energy to contract, your brain cells need energy to think, and even your taste-bud cells need energy to sense that durian at the void deck.

Respiration happens in all living cells, all the time

Here is the fact that surprises many students: respiration never stops. It does not take a break at night, it does not switch off when you sit still, and it does not only happen in animals. Every living cell — in your body, in a mango tree, in the mushroom on your toast — is respiring right now. Even when you are fast asleep, your heart cells, brain cells, and liver cells keep on respiring to stay alive.

🤔 Predict first: Is a potted plant in your bedroom respiring at midnight, with all the lights off?

Glucose + oxygen → energy + carbon dioxide + water

Respiration is not magic — it is a chemical process with an exact recipe. The cell takes in two things, breaks them down, and releases three things:

Needs: +
Releases: + +

In words: glucose + oxygen → energy + carbon dioxide + water

Try the experiment below. Give a cell its ingredients and watch what comes out — then compare a resting cell with one that is exercising hard.

Feed the cell: does respiration release energy?

Predict first: Do living things respire at night, even when they are asleep?

Notice: when the cell is exercising, it needs more energy, so it carries out more respiration, uses more glucose and oxygen, and produces more carbon dioxide. That is why you breathe faster during exercise — your body is trying to bring in more oxygen and get rid of more carbon dioxide at the same time.

Respiration is not the same as breathing

This is the mix-up that costs the most marks every year. Breathing is the physical act of moving air in and out of your lungs — ribs go up, air rushes in; ribs come down, air rushes out. Respiration is the chemical process inside each cell that uses oxygen and glucose to release energy. They are related (breathing supplies the oxygen respiration needs and removes the carbon dioxide it produces) but they are not the same thing.

Here is the other big contrast — with :

	Respiration	Photosynthesis
Who does it?	All living cells (animals, plants, fungi, bacteria)	Only green plant cells
When?	All the time, day and night	Only when there is light
Needs	Glucose + oxygen	Carbon dioxide + water + light
Produces	Energy + carbon dioxide + water	Glucose + oxygen

Notice they are almost mirror images — what one process uses, the other produces. Plants do both: they photosynthesise during the day (making glucose and oxygen) and they respire around the clock (using glucose and oxygen to release energy).

🤔 Predict first: A plant is kept in complete darkness for three days. Is it still respiring?

Watch out — these are easily mixed up

Note

Respiration vs breathing. Breathing is moving air in and out of the lungs. Respiration is the chemical process in cells that releases energy from glucose. When a PSLE question asks what releases energy, write respiration — not "breathing."

Respiration vs photosynthesis. Photosynthesis makes glucose and needs light. Respiration breaks glucose down and releases energy, and it runs in the dark too. A plant does both — photosynthesis only by day, respiration at all times.

Respiration vs combustion. Burning (combustion) also breaks down fuel using oxygen. But respiration happens inside living cells in a controlled series of steps, releasing energy gradually. Combustion is fast and uncontrolled. The PSLE will not ask you to compare them in detail, but do not write that the cell is "burning" glucose.

Energy vs heat. Respiration releases energy that cells can use for movement, growth, and repair. Some of that energy is lost as heat (which is why you feel warm after exercise), but write "energy" in your answer — not just "heat."

Quick recap

🎯 Mastery check

Answer all 8 — your progress is saved on this device.

What does respiration release that cells need for movement, growth, and repair?
A student says plants only respire during the day. Why is this wrong?
Which two substances does a cell need to carry out respiration?
A person runs 2.4 km during a school cross-country race. Why do they breathe faster during the run?
Which of the following is produced by respiration?
How is respiration different from photosynthesis?
A mushroom is growing on a log in a dark corner of a forest. Is the mushroom respiring?
A student writes that "breathing releases energy." What is wrong with this answer?