Biology · The Very Small

One cell.A whole machine.

Float alongside the smallest unit of life. Inside this hovering capsule is a factory of organelles — each with a job that keeps everything alive.

Enter the cell ↓5 LESSONS · ORGANELLES · MICROSCOPY · SPECIALISATION

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Biology · The Very Small · Topic 01

The building blocks

of life

Every living thing — from a blade of grass to a blue whale — is built from the same tiny units. They were invisible until the microscope revealed them, and inside each one is a miniature factory keeping life running. This is where biology begins: the cell.

5 LESSONS · CELL THEORY · MICROSCOPY · ORGANELLES · SPECIALISATION

In this lesson01Living vs non-living02The microscope03Inside the cell04Plant vs animal05Specialised cells

01Lesson 1 · Cell theory

Living, non-living, and the cell theory

How do we know something is alive? Biologists use seven signs — and every one of them happens inside cells.

It seems obvious that a dog is alive and a rock is not. But scientists need a precise test. The seven characteristics of living things are remembered with MRS NGREN.

Letter	Process	What it means
M	Movement	Changes position or moves parts of itself
R	Respiration	Releases energy from food
S	Sensitivity	Detects and responds to surroundings
N	Nutrition	Takes in food or makes its own
G	Growth	Gets larger and develops
R	Reproduction	Makes more of its own kind
E	Excretion	Removes waste products
N	—	(MRS NGREN is the memory aid)

MRS NGREN — the seven signs every living thing shows.

Key term · Cell

The smallest unit of life. Every living organism is made of one or more cells, and all cells come from existing cells.

That idea took 200 years to assemble. It is now called the cell theory, and it rests on three statements built by many scientists — starting with Robert Hooke, who first saw and named "cells" in a sliver of cork in 1665.

1665

Robert Hooke

Looks at cork through a microscope and names the tiny boxes 'cells'.

1674

Van Leeuwenhoek

Sees living single-celled organisms — 'animalcules' — in pond water.

1838

Schleiden & Schwann

Propose that all plants and all animals are made of cells.

1855

Rudolf Virchow

Adds the final piece: every cell comes from another cell.

The cell theory was built piece by piece over two centuries.

✦

The three rules of cell theory

1 · All living things are made of cells. 2 · The cell is the basic unit of life. 3 · All cells come from pre-existing cells.

Some organisms are a single cell — unicellular, like bacteria. Others are made of trillions working together — multicellular, like you. Cells are astonishingly small: most are between 10 and 100 micrometres, far too small to see without help.

37tn

cells in your body

~30µm

typical animal cell

1665

cells first seen

🧪Practical activity

Living or Non-Living Sort

Sort a set of images into living and non-living, justifying each with MRS NGREN. The tricky ones — a seed, a flame, a virus — spark the best arguments.

02Lesson 2 · Microscopy

The instrument that revealed life

Cells stayed hidden until the light microscope. Learning to drive one — and to measure what you see — is a core scientific skill.

A light microscope shines light through a thin sample and magnifies it through two lenses. Knowing each part, and how to focus safely, lets you observe cells without damaging the instrument or the slide.

The parts of a compound light microscope and how magnification is calculated.

Key term · Magnification

How many times larger an image appears than the real object. Total magnification = eyepiece lens × objective lens.

Eyepiece	Objective	Total magnification
10×	4× (low power)	40×
10×	10× (medium)	100×
10×	40× (high power)	400×

Worked magnification examples — multiply the two lenses together.

Lowest objective

start on 4×

Place & clip slide

specimen over the light

Coarse focus

move stage, watch from the side

Fine focus

sharpen the image

Increase power

switch to higher objective

The safe routine for focusing a slide — always start on the lowest power.

✦

Scientific drawings

Use a sharp pencil, draw clear unbroken lines, no shading, label with straight ruled lines, and always record the magnification. A good drawing communicates what a photo can't.

🧪Practical activity

Observing Real Cells

Prepare slides of onion epidermis and pond water. Find cells on low power, switch to high, and produce a labelled scientific drawing with the magnification recorded.

03Lesson 3 · Organelles

Inside the cell: the organelles

A cell is not an empty bag. It's packed with tiny structures — organelles — each doing a specific job, like rooms in a factory.

Each organelle has a role. Together they keep the cell alive: making energy, building proteins, storing instructions and controlling what comes in and out.

A typical animal cell. Every organelle has a specific function.

Organelle	Function	In plant / animal?
Nucleus	Control centre; holds DNA	Both
Cell membrane	Controls what enters and leaves	Both
Cytoplasm	Jelly where reactions happen	Both
Mitochondria	Release energy (respiration)	Both
Ribosomes	Build proteins	Both
Cell wall	Rigid support	Plant only
Chloroplast	Photosynthesis (captures light)	Plant only
Vacuole	Stores water & keeps cell firm	Large in plants

The major organelles and what each one does.

✦

A useful analogy

If the cell is a factory: the nucleus is the manager's office (instructions), mitochondria are the power generators, ribosomes are the assembly line, and the membrane is security at the gate.

🧪Practical activity

Observing Plant & Animal Cells

Compare prepared slides of cheek cells and onion cells. Which organelles can you actually see at 400×, and which are too small?

04Lesson 4 · Plant vs animal

Two designs, one blueprint

Plant and animal cells share most organelles — but three features set plant cells apart, all tied to how plants live.

Animals move to find food; plants stand still and make their own. That difference shows up directly in their cells.

A plant cell — note the rigid wall, chloroplasts and one large vacuole.

Animal cell

▸No cell wall — flexible shape
▸No chloroplasts
▸Small, scattered vacuoles
▸Often rounded

Plant cell

▸Rigid cell wall for support
▸Chloroplasts for photosynthesis
▸One large central vacuole
▸Fixed, boxy shape

Both still have a nucleus, membrane, cytoplasm, mitochondria and ribosomes — the shared core every cell needs to live. (Fungal cells add a twist: they have a cell wall too, but made of chitin, not cellulose, and no chloroplasts.)

🧪Practical activity

Build a Cell Model

Make a 2D or 3D model of a plant or animal cell from clay or craft materials, then draw a Venn diagram showing which organelles are shared and which are unique.

05Lesson 5 · Specialisation

Structure follows function

In a multicellular body, cells don't all look the same. Each is shaped for its specific job — a principle that runs through all of biology.

Key term · Specialised cell

A cell with a structure adapted to perform one particular function especially well.

Four specialised cells — each shape is a clue to its job.

Cell	Job	Special adaptation
Nerve cell	Carry electrical signals	Long thin fibre to reach far
Red blood cell	Carry oxygen	No nucleus = more room; disc shape
Muscle cell	Produce movement	Packed with fibres that contract
Fat cell	Store energy	Large oil droplet fills the cell

How four specialised cells are adapted to their roles.

Organisation

Cell

basic unit

Tissue

similar cells together

Organ

tissues working as one

System

organs cooperating

Organism

the whole living thing

From a single cell to a whole organism — the levels of organisation.

✦

The big idea

A cell's shape is never random — it always serves its function. Spot the adaptation and you can predict the job, and vice versa.

🧪Practical activity

Design a Specialist

Invent a cell for an extreme job — a 'super-absorber' for the gut (huge surface area), an 'invader-destroyer' (engulfs bacteria), or a desert plant's 'water-saver' (giant vacuole). Justify every feature.

← Year 8 index Next · Atoms & Elements →

Research spotlight · imaging living cells

We can now watch cells work in real time. Fluorescence and cryo-electron microscopy let scientists film organelles moving inside a living cell and map proteins atom-by-atom. What Hooke saw as tiny empty boxes in 1665, we now explore as busy, three-dimensional machines.

Check · Test yourself

Quiz

10 questions to check your understanding.