Key Stage 3 science · Biology – Structure and function of living organisms

KS3 cells: plant and animal cells, microscopes and organisation

Cells are the building blocks of all living things, and Key Stage 3 is where pupils first study them properly. They use a light microscope, learn what each part of a plant or animal cell does, see how diffusion moves materials in and out of cells, meet some single-celled organisms, and learn how cells are organised into tissues, organs and whole organisms. It is the foundation for nearly all later biology.

Cells and organisation, explained

Pupils begin with the light microscope. They learn to prepare a slide, focus from the low-power lens upwards, and make a clear, labelled drawing in pencil with ruled label lines and no shading. Total magnification is the eyepiece lens multiplied by the objective lens, so a ×10 eyepiece with a ×40 objective gives ×400. Many schools also teach the formula for working out real size from a drawing: actual size = image size ÷ magnification. Schools choose the specimens; onion skin is a common first slide, and many schools also look at cheek cells, leaf cells or pond water.

Both plant and animal cells have a nucleus, which contains the genetic material and controls the cell’s activities; cytoplasm, where many chemical reactions happen; a cell membrane, which controls what enters and leaves; and mitochondria, which release energy from food in respiration. Plant cells have three more features: a cell wall made of cellulose that supports the cell and gives it a fixed shape, chloroplasts, which contain the green substance chlorophyll and absorb light for photosynthesis, and a large permanent vacuole filled with cell sap that helps keep the cell firm. Animal cells may have small vacuoles, but not the large permanent one.

Substances do not have to be pushed into a cell: many exchanges happen by diffusion. This is the net movement of particles from where there are many to where there are fewer, caused by their random motion, and it needs no energy from the cell. Oxygen diffuses into a cell that is using it up, and carbon dioxide diffuses out. Pupils also study unicellular organisms, in which one cell does everything. Schools choose the examples, often amoeba, euglena or paramecium, and the focus is how the cell’s structure suits the way it lives, for instance the chloroplasts and light-sensitive eyespot of euglena, the tiny hairs (cilia) that move a paramecium along, or the way an amoeba changes shape to surround its food.

In larger organisms cells are specialised and organised in levels. A group of similar cells doing the same job is a tissue, such as muscle tissue. Different tissues working together make an organ, such as the stomach, and organs working together make an organ system, such as the digestive system. The whole set of systems makes up the organism. Being able to move up and down this hierarchy, with an example at each level, is a standard KS3 question.

Words your child will hear

Nucleus: The part of the cell that holds its genetic material and controls what the cell does.
Mitochondria: Tiny structures where respiration releases energy for the cell. One is a mitochondrion.
Chloroplast: A green structure in plant cells where photosynthesis happens.
Cell wall: A rigid layer outside the membrane in plant cells that supports and protects them.
Diffusion: The net movement of particles from a place where there are lots to a place where there are fewer.
Unicellular: Made of just one cell, as an amoeba is.
Tissue, organ, system: Levels of organisation: a group of similar cells, a group of tissues, and a group of organs working together.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about cells as the fundamental unit of living organisms, including how to observe, interpret and record cell structure using a light microscope
Pupils should be taught about the functions of the cell wall, cell membrane, cytoplasm, nucleus, vacuole, mitochondria and chloroplasts
Pupils should be taught about the similarities and differences between plant and animal cells
Pupils should be taught about the role of diffusion in the movement of materials in and between cells
Pupils should be taught about the structural adaptations of some unicellular organisms
Pupils should be taught about the hierarchical organisation of multicellular organisms: from cells to tissues to organs to systems to organisms

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

Try it: working out how big a cell really is

A pupil looks at a cheek cell through a ×10 eyepiece and a ×40 objective lens and draws it. The drawing is 24 mm wide. How wide is the real cell, in millimetres and in micrometres?

Work out the total magnification by multiplying the two lenses: 10 × 40 = ×400.
Check the drawing. It should be in pencil, use most of the space available, have clear lines with no shading, label lines ruled and not crossing, and a title saying what it is and the magnification.
Use the formula: actual size = image size ÷ magnification. The image (drawing) is 24 mm wide.
```
actual = image ÷ magnification
       = 24 mm ÷ 400
       = 0.06 mm
```
Convert to micrometres. There are 1,000 micrometres (µm) in 1 millimetre, so 0.06 mm × 1,000 = 60 µm.
Check it makes sense: 0.06 mm is far smaller than a printed full stop, and tens of micrometres across is a typical size for a cheek cell. If the answer had been 6 mm, you would be able to see it with your eyes, so you would suspect a mistake.

The real cell is 0.06 mm (60 µm) wide.

Common mistakes and how to help

Thinking plants do not have mitochondria because they photosynthesise

Photosynthesis is well known, so pupils think of it as plants’ equivalent of respiration.

What helps: Plants respire all the time, in every living cell. Chloroplasts make the food, and mitochondria release energy from it. Both are needed, which is why plant cells have both.

Confusing the cell wall with the cell membrane

Both are described as “the outer layer”, and the words sound alike.

What helps: The wall is rigid, made of cellulose and only in plants (and some other organisms). The membrane is thin and flexible, in every cell, and controls what passes through. Plant cells have both.

Believing diffusion is something the cell does using energy, or that it stops when the concentration is equal

Words such as “move” suggest particles are being sent somewhere on purpose. When the concentration evens out, it looks as though everything has stopped.

What helps: Particles keep moving randomly all the time. Diffusion is the net movement from high to low concentration. At equal concentration, there is just no net change.

Mixing up a tissue, an organ and an organ system

The terms are used loosely, so a heart can be called a “muscle” or a “tissue” in conversation.

What helps: Build up an example together: a muscle cell, muscle tissue, the stomach (an organ with several tissues), and the digestive system, in which the stomach is one part.

How to help at home

Look at cells with a simple magnifier. If you have a pocket microscope or can borrow one, ask an adult to peel the thin, almost see-through skin from the inside of an onion layer, then lay it flat in a drop of water on a slide and look. A hand lens will show the surface of leaves and fruit.
Watch diffusion. Add one drop of food colouring to a glass of still, cold water and watch it spread without stirring. Ask your child to describe it in terms of particles moving from high to low concentration.
Build a model cell. Make a plant cell on a baking tray or plate from household items, such as a clear food bag of jelly for the membrane and cytoplasm, a grape for the nucleus, green peas for chloroplasts, a small balloon for the vacuole and a shoebox lid for the cell wall. Then ask what is missing for an animal cell.
Sort the levels of organisation. Write cell, tissue, organ, organ system and organism on separate pieces of paper. Ask your child for an example of each in a human or a plant, such as muscle cell, muscle tissue, stomach, digestive system and human, and explain how each level fits inside the next.

Builds on

Year 6: Classifying living things

More on biology – Structure and function of living organisms

Key Stage 3: Breathing, gas exchange and drugs
Key Stage 3: Diet and digestion
Key Stage 3: Reproduction in humans and plants
Key Stage 3: Skeleton and muscles

Everything in Key Stage 3 science

Updated 20 October 2026.