Cells, but bitesize.

Part 1The limits of your eyes

Look at a hair on your hand. You can see it — but you can't see what it's made of. Look at a grain of salt. You can see it's there, but you can't see the tiny crystals it's built from. Some things in the world are too small to see with just your eyes. We call this "with the naked eye" — without any lenses or anything to make it look bigger.

One of those things is a cell. Cells are the tiny building blocks of every living thing — every plant, every animal, you. They are far too small to see with the naked eye. That's why scientists invented the microscope.

Part 2Magnification + focus

A microscope does two things, not one. This is a common point students get wrong.

1. Magnification. The microscope makes the image of the object look bigger. The actual object hasn't grown — the image of it has been enlarged.

2. Focus. When an image is magnified it can look fuzzy and blurred. The microscope also focuses the image — making it sharp and clear enough to actually see the details.

A magnified image that isn't focused is useless. A focused image that isn't magnified just shows you what you'd see anyway. You need both.

Try the interactive microscope below. Move the slide, change the magnification, and adjust the focus — you'll see exactly what each one does.

Part 1The four key parts

Part 2How to use a microscope

Using a microscope properly is a skill you'll have practised in class — maybe using onion cells stained with iodine, or cheek cells stained with methylene blue. Here's the order, which is what examiners look for:

1. Carry it carefully. Two hands — one under the base, one on the arm.

2. Clip your slide onto the stage. Make sure it's positioned so the specimen is over the hole in the stage where the light comes through.

3. Start at the LOWEST magnification. Always. The lowest magnification has the widest field of view, so it's easier to find your cells.

4. Focus using the focus knob. Slowly turn the wheel until the image is sharp.

5. Once you've found the cells, increase the magnification. Switch to a stronger objective lens. You may need to focus again.

Part 1Organisms — one cell or many

An organism is a living thing. There are two main kinds, depending on how many cells they're made of:

Unicellular organisms are made of just one cell. A bacterium is unicellular. So is yeast. The whole organism IS a single cell — it has to do everything itself to survive.

Multicellular organisms are made of many cells working together. A plant. A fish. You. We have billions of cells, all specialised to do different jobs.

Cells need certain things to survive: they need food (for energy and to grow), they need oxygen (for respiration), and they need to remove waste like carbon dioxide. To do all that, every cell has a set of structures inside it.

Part 2The four parts of every cell

Every cell — whether it's from a plant, an animal, or a bacterium — has these four sub-cellular structures:

1. Cell membrane. A thin layer that surrounds the cell and controls what goes in and out. Oxygen and food can come in; waste like carbon dioxide can go out.

2. Cytoplasm. A gel-like substance that fills the cell. This is where most of the chemical reactions of the cell happen.

3. Genetic material. The instructions the cell uses to make the proteins it needs. (You'll meet DNA later — for now, "genetic material" is enough.)

4. Ribosomes. Tiny structures where proteins are actually made, using the instructions from the genetic material.

Try the cell-zoom interactive below to see these parts inside a real cell, and zoom from the whole cell right down to its smallest features.

Part 1The nucleus and mitochondria

Multicellular organisms — animals and plants — have cells with two extra structures that simple cells (like bacteria) don't have:

Nucleus. The nucleus is a structure that contains the genetic material of the cell. It controls the cell's activities. Every cell in your body has a nucleus (except mature red blood cells — you'll meet them later).

Mitochondria. These are tiny structures where respiration takes place. Respiration is how the cell releases energy from food. Cells that need a lot of energy (like muscle cells) have lots of mitochondria.

Part 2What plant cells have that animal cells don't

Plant cells have everything an animal cell has — cell membrane, cytoplasm, ribosomes, nucleus, mitochondria — plus three extras:

Chloroplast. Where photosynthesis happens. Photosynthesis is how plants make their own food using sunlight, water, and carbon dioxide. Chloroplasts are green because they contain a pigment called chlorophyll.

Cell wall. A rigid layer outside the cell membrane, made of cellulose. It gives the plant cell a fixed shape and stops it bursting. (Animal cells don't have one — they rely on their cell membrane and the bones/skin around them.)

Permanent vacuole. A large fluid-filled space in the middle of the cell, filled with cell sap. It keeps the cell rigid (helps the plant stand up) and stores nutrients.

Part 1What magnification really means

You already know magnification means "making the image bigger". Now we're going to put a number on it.

If a cell is 1 mm across in real life, and through a microscope its image looks 10 mm across, the microscope has magnified it by 10 times. We write this as ×10.

The magnification tells you how many times bigger the image is than the actual object.

Part 2The magnification equation

This is the equation you need to know:

image size = actual size × magnification

Part 1Living as one cell

A unicellular organism is a complete living thing made of just one cell. It has to do everything alone — find food, get rid of waste, move around, defend itself. There's no other cell to help.

The most well-known unicellular organisms are bacteria. You probably think of bacteria as germs that make you ill — and some do. But many bacteria are completely harmless, and some are useful. There are bacteria in your gut that help you digest food. There are bacteria in soil that help plants grow. There are bacteria used to make yoghurt and cheese.

Part 2What a bacterial cell looks like

A bacterial cell has all four basic sub-cellular structures you already know: cell membrane, cytoplasm, genetic material, ribosomes. It also has a cell wall (like plants do — but made of different stuff). Note: bacteria do not have a nucleus — their genetic material just floats free in the cytoplasm.

Bacterial cells also have a couple of structures that animal and plant cells don't have, because they need to survive on their own:

Flagellum. A long whip-like tail that lets the bacterium move around — to find food or escape danger. (Plural: flagella.)

Slime capsule. A protective layer outside the cell wall that helps the bacterium stick to surfaces and protects it from the body's defences.

Part 1Concentration: lots in one place

Before diffusion, one word: concentration. Concentration means how packed-in something is in a space.

If a box has lots of particles squashed into it, we say the box has a high concentration of those particles. If a box has only a few particles spread out, we say it has a low concentration.

Part 2Diffusion

Particles don't stay still — they're always moving around. If you put a high concentration of particles next to a low concentration, the particles will spread out until both areas have the same concentration.

This process — particles moving from a high concentration to a low concentration — is called diffusion.

DIFFUSION: high concentration → low concentration

You can see diffusion at home. Spray perfume in one corner of a room and walk to the opposite corner. After a minute or two, you can smell it. The perfume particles have diffused from where they were highly concentrated (next to the spray) to where they were less concentrated (the rest of the room).

Part 3Diffusion in and out of cells

Cells use diffusion to get the things they need and to get rid of waste.

Oxygen IN. There's lots of oxygen outside the cell (in the blood, in the lungs, in water). There's not much oxygen inside the cell, because the cell uses it up for respiration. So oxygen diffuses INTO the cell — from high concentration (outside) to low (inside).

Carbon dioxide OUT. Respiration produces carbon dioxide as a waste product, so there's lots of CO₂ inside the cell and not much outside. CO₂ diffuses OUT of the cell — from high (inside) to low (outside).

Both substances pass through the cell membrane, which is designed to let small particles diffuse through it.

Part 1One organism, many jobs

If you're unicellular (like a bacterium), one cell has to do everything. But if you're multicellular (like a plant or an animal), different cells can do different jobs. Some cells are good at carrying oxygen. Some are good at contracting to make you move. Some are good at sending signals.

Cells that have a specific job, with a specific structure to do that job well, are called specialised cells. The way the cell is shaped to do its job is called an adaptation.

Part 2Four specialised cells you need to know

1. Red blood cell — carries oxygen.

· No nucleus, so there's more room inside for a substance called haemoglobin that holds the oxygen.
· Biconcave shape (like a flattened disc) — gives a large surface area for oxygen to diffuse across.
· Flexible, so it can squeeze through narrow blood vessels.

2. Muscle cell — contracts to make movement.

· Long and thin, so it can shorten when it contracts.
· Lots of mitochondria — because contracting needs a lot of energy from respiration.
· Often in bundles with other muscle cells, so they can pull together with more force.

3. Nerve cell — carries electrical signals.

· Very long — some are over a metre, so a single nerve cell can carry a signal a long way through the body.
· Branched ends (called dendrites), so it can connect to many other cells.
· An insulating sheath around the long bit (axon) so signals travel fast and don't leak out.

4. Root hair cell — absorbs water from soil.

· Long extension (the "hair") that sticks out into the soil, giving a much bigger surface area to absorb water.
· Thin walls, so water can pass through easily.
· No chloroplasts — root hair cells are underground, so there's no light to photosynthesise with anyway.