Respiration Explained: KS3 Biology Guide

Respiration is the chemical process that takes place in every living cell to release energy from glucose. It is not the same as breathing. All living organisms respire continuously; without it, cells cannot function and the organism dies. This is core KS3 biology content covered in Year 7 and Year 8.

What is respiration?

Respiration is a chemical reaction that releases energy from glucose so that cells can carry out life processes: movement, growth, reproduction, active transport, and maintaining body temperature. It happens inside cells — specifically in the mitochondria — not in the lungs.

There are two types:

Aerobic respiration — uses oxygen; releases more energy; produces carbon dioxide and water as waste products.

Anaerobic respiration — does not use oxygen; releases less energy; produces lactic acid (in animals and some bacteria) or ethanol and carbon dioxide (in yeast and plants).

Aerobic respiration

Aerobic respiration is the main type used by the human body during everyday activities. The word equation is:

glucose + oxygen → carbon dioxide + water (+ energy)

The balanced chemical symbol equation (for GCSE reference — not required at KS3 but useful to know):

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

The energy released is used for all cellular activities. Aerobic respiration releases approximately 2,900 kJ per mole of glucose — far more than anaerobic respiration.

Where does aerobic respiration happen?

The reactions of aerobic respiration occur in the mitochondria of cells. This is why cells with high energy demands — such as muscle cells, liver cells and sperm cells — contain large numbers of mitochondria. A liver cell may contain over 2,000 mitochondria.

What happens to the products?

Carbon dioxide diffuses out of cells into the blood, is carried to the lungs, and is exhaled.

Water is partly excreted in urine and exhaled as water vapour. It is also used in chemical reactions within the cell.

Anaerobic respiration

When the body cannot get enough oxygen to cells — for example, during intense sprint exercise — cells switch to anaerobic respiration. The word equation in humans and animals is:

glucose → lactic acid (+ energy)

Anaerobic respiration is much less efficient — it releases only about 1/18th as much energy per glucose molecule as aerobic respiration. It is a temporary solution because lactic acid builds up in muscles, causing the burning sensation and fatigue associated with hard exercise. The lactic acid must be broken down later using oxygen (this is why you continue to breathe hard after a sprint — paying back the "oxygen debt").

Anaerobic respiration in yeast

Yeast cells (fungi) carry out a different type of anaerobic respiration called fermentation:

glucose → ethanol + carbon dioxide (+ energy)

Fermentation has been exploited by humans for thousands of years:

Bread making — yeast ferments sugars in dough, releasing CO₂ that makes the bread rise. The ethanol evaporates during baking.

Beer and wine making — yeast ferments sugars in grain or grapes; the ethanol is the alcoholic product.

Respiration vs. breathing: a critical distinction

One of the most common errors at KS3 is confusing respiration with breathing. They are not the same:

Feature | Respiration | Breathing (ventilation)
What it is | A chemical reaction | A physical process (movement of air)
Where it happens | Inside cells (mitochondria) | In the lungs, airways, and chest
What it involves | Glucose + oxygen → energy | Inhaling and exhaling air
Which organisms do it | All living things | Animals (and some other organisms) with lungs/gills

Breathing gets oxygen into the blood and removes carbon dioxide. Respiration uses that oxygen inside cells to release energy from glucose. Both are essential, but they are different processes.

A worked example: exercise and respiration

Scenario: A Year 9 student sprints 100 m in a PE lesson.

At the start, muscles use aerobic respiration: glucose + oxygen → carbon dioxide + water + energy. The cardiovascular system (heart and blood vessels) delivers oxygen fast enough.

As the sprint intensifies, oxygen delivery cannot keep up with demand. Muscle cells switch to anaerobic respiration: glucose → lactic acid + energy.

Lactic acid accumulates; the student's legs feel heavy and painful.

After the sprint, the student breathes hard for several minutes. This extra oxygen oxidises the lactic acid (converting it to carbon dioxide and water) — repaying the "oxygen debt".

Stage | Type of respiration | Oxygen used? | Products
Jogging | Aerobic | Yes | CO₂, water
Sprinting | Mainly anaerobic | No | Lactic acid
Recovery | Aerobic | Yes (extra) | CO₂, water (from lactic acid)

Why do we need energy from respiration?

Cells require energy for:

Active transport — moving substances against a concentration gradient (e.g., absorbing glucose from the gut).

Protein synthesis — building new proteins for growth and repair.

Cell division — mitosis and meiosis for growth and reproduction.

Muscle contraction — movement.

Maintaining body temperature — warm-blooded animals generate heat as a by-product of respiration.

The Department for Education's Science Programmes of Study for Key Stage 3 requires pupils to be taught that respiration in cells releases energy for the body, to know the differences between aerobic and anaerobic respiration, and to understand that respiration is not the same as breathing.

BBC Bitesize KS3 Biology covers aerobic and anaerobic word equations, the role of mitochondria, and fermentation as key content for Year 7 and Year 8.

Frequently asked questions

What is the word equation for aerobic respiration?

The word equation is: glucose + oxygen → carbon dioxide + water. This reaction takes place inside the mitochondria of cells and releases energy that cells use for all life processes. Glucose comes from the digestion of food; oxygen is absorbed from the air in the lungs and carried to cells by red blood cells.

Why does anaerobic respiration produce lactic acid in humans?

During intense exercise, muscle cells need energy faster than the blood can supply oxygen. Cells switch to anaerobic respiration, which breaks down glucose without oxygen, producing lactic acid as a waste product. Lactic acid lowers the pH inside muscle cells, which interferes with enzyme function and causes the burning sensation and fatigue during hard exercise. Once oxygen becomes available again (during recovery), the liver converts lactic acid back into glucose.

How is respiration different from photosynthesis?

Respiration and photosynthesis are opposite processes. Photosynthesis (in plant cells only, in chloroplasts) uses carbon dioxide and water to make glucose, requiring light energy. Respiration (in all living cells, in mitochondria) breaks down glucose using oxygen to release energy. The overall inputs and outputs are the reverse of each other. Plants carry out both processes simultaneously during daylight — their rate of photosynthesis usually exceeds their rate of respiration during the day, so they are net absorbers of CO₂.

What happens to lactic acid after exercise?

After exercise ends and breathing returns to normal, the body uses the extra oxygen being breathed in to break down accumulated lactic acid in the liver and muscles. The liver converts most lactic acid into glucose (via a process called gluconeogenesis), which can then be stored as glycogen or used for energy. The remainder is fully oxidised via aerobic respiration into carbon dioxide and water. This is why the breathing rate remains elevated for some time after exercise has stopped.

Work through respiration questions with a Socratic biology tutor who guides you to the answer rather than giving it — visit aitutors.me.