Learn · Science · Class 8

Cell Structure & Function

The organelles, what each does, and why cells specialise.

10 parts
Scroll to begin ↓
A plant cell and what each part doesCell wallrigid supportCell membranecontrols what entersNucleusholds the instructionsVacuolesap keeps it firmChloroplastphotosynthesisMitochondriarelease energy
A plant cell, with what each part actually does.
🧩 The partsThe organellesPart 2 of 10
🔍 In detailWall and membrane are differentPart 3 of 10
✏️ Worked exampleSpecialised cells and their shapesPart 4 of 10
🔍 In detailDiffusion and osmosisPart 5 of 10
✏️ Worked exampleWhy cells cannot grow largePart 6 of 10
🌍 In the real worldWhy cell biology mattersPart 7 of 10
⚠️ Common mistakesWhere it usually goes wrongPart 8 of 10
✅ SummaryWhat to rememberPart 9 of 10
✏️ Worked exampleWatch one worked throughPart 10 of 10
1📘 DefinitionPart 1 / 10

The unit of life

The cell theory states three things:

  • All living things are made of cells.
  • The cell is the basic unit of structure and function.
  • All cells come from existing cells.

That third point mattered enormously. Before it was established, people believed living things could arise spontaneously from non-living matter. Rejecting that changed medicine. It is why we understand infection.

2🧩 The partsPart 2 / 10

The organelles

  • Cell membrane: selectively permeable; controls what enters and leaves.
  • Cytoplasm: jelly where most reactions occur.
  • Nucleus: contains DNA, the instructions; controls the cell.
  • Mitochondria: release energy by respiration. Cells needing lots of energy have many.
  • Ribosomes: build proteins.
  • Vacuole: storage; large and permanent in plant cells.

Plant cells additionally have: a rigid cell wall of cellulose, and chloroplasts for photosynthesis.

3🔍 In detailPart 3 / 10

Wall and membrane are different

A common confusion, and the difference is functional.

Cell membrane: present in all cells. Thin, flexible, and selectively permeable: it decides what passes.

Cell wall, plants, fungi and bacteria only. Thick, rigid, and fully permeable: it lets everything through and provides support, not control.

So a plant cell has both, with the wall outside the membrane.

Why plants need a wall: they have no skeleton. The wall plus a full vacuole pressing outwards is what keeps a plant upright, which is why a plant wilts when short of water.

4✏️ Worked examplePart 4 / 10

Specialised cells and their shapes

Cells become specialised, and their structure always matches the job.

  • Red blood cell: biconcave disc, no nucleus. Losing the nucleus makes room for more haemoglobin, so it carries more oxygen. The shape increases surface area for absorbing it.
  • Nerve cell: very long and thin, to carry signals over distance. Branches connect to many other cells.
  • Root hair cell: a long projection, hugely increasing surface area for absorbing water.
  • Sperm cell: a tail for movement and many mitochondria to power it.
  • Muscle cell: packed with mitochondria, because contracting needs continuous energy.

The reasoning to practise: given a cell's job, predict its features. Given its features, deduce its job. That is what exam questions actually test.

5🔍 In detailPart 5 / 10

Diffusion and osmosis

Diffusion: particles spread from high concentration to low, until evenly distributed. No energy required; it happens because particles move randomly.

Oxygen entering a cell and carbon dioxide leaving both work this way.

Osmosis: the movement of water across a partially permeable membrane, from where water is more concentrated to where it is less.

What this means practically:

  • A cell in pure water gains water. A plant cell becomes firm; an animal cell may burst, having no wall.
  • A cell in concentrated salt solution loses water and shrinks.

This is why salt preserves food. It draws water out of bacteria, and they cannot survive it.

6✏️ Worked examplePart 6 / 10

Why cells cannot grow large

The question. Why is a large organism made of trillions of tiny cells rather than a few big ones?

The answer is the surface-area-to-volume ratio.

Everything a cell needs enters through its surface. Everything it must supply is throughout its volume.

Double a cell's width: surface area increases ×4, but volume increases ×8. Demand doubles relative to supply.

So beyond a certain size, diffusion cannot deliver materials to the centre fast enough, and the cell dies.

The solution life uses: stay small, and make more cells. Many small cells have far greater total surface area than one large one.

The same principle appears repeatedly: alveoli in lungs, villi in intestines, root hairs in plants, all structures that exist purely to increase surface area.

7🌍 In the real worldPart 7 / 10

Why cell biology matters

Medicine. Cancer is uncontrolled cell division. Many treatments target rapidly dividing cells, which is also why they damage hair and gut cells, causing side effects.

Antibiotics often work by attacking the bacterial cell wall. Human cells have no wall, which is why the drug harms the bacterium and not you.

Agriculture. Understanding osmosis explains why over-fertilised soil damages crops, high salt concentration draws water out of the roots.

Food preservation. Salting, sugaring and drying all remove water from microbes by osmosis.

8⚠️ Common mistakesPart 8 / 10

Where it usually goes wrong

  • "Plant cells have a wall instead of a membrane." They have both.
  • "Osmosis and diffusion are the same." Osmosis is water across a membrane.
  • "All cells look alike." They are specialised.
  • "Red blood cells have a nucleus." Mature ones do not.
  • "Diffusion needs energy." It does not.
9✅ SummaryPart 9 / 10

What to remember

  • All cells: membrane, cytoplasm, nucleus, mitochondria, ribosomes.
  • Membrane controls; wall supports, plants have both.
  • Structure always matches function.
  • Cells stay small because volume outgrows surface area.
10✏️ Worked examplePart 10 / 10

Watch one worked through

Here is one done slowly, with the reason for every move. Then one for you.

Watch one first

A muscle cell has far more mitochondria than a skin cell. Explain what that tells you.

  1. Recall what a mitochondrion does.
    mitochondrion = releases energy (aerobic respiration)
    Start from the job of the structure. Structure and function always match, and that is the idea being tested.
  2. Ask what a muscle cell spends its time doing.
    muscle = contracts = needs energy
    Contraction is active work, and active work costs energy. Skin cells mostly sit there as a barrier.
  3. More of a structure means more of that job being done.
    more mitochondria → more energy released
    This reasoning works for any organelle: more ribosomes means more protein, more chloroplasts means more photosynthesis.
  4. State the conclusion in terms of need.
    high energy demand → many mitochondria
    Answer the question that was asked. It wants what the observation tells you, not a list of what mitochondria are.

Answer: Muscle cells need a lot of energy to contract, so they need more mitochondria to release it

Now you try

Which organelle makes proteins? One word.

Show the answer

ribosome

Practise Cell Structure & Function

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🧠 11 quizzes on Cell Structure & Function (8 easy, 2 medium, 1 hard)

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🖨️ Printable worksheet

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