Structure and Function

Think of a brick wall. It looks like one solid thing, but up close it is built from thousands of small bricks. Living things work the same way: they are built from tiny units called cells.

Cells are far too small to see with just your eyes — you need a microscope. A full stop on this page could sit on top of dozens of them.

Two words will come up again and again in biology:

• Structure — what something is made of and what it looks like (its parts and shape).
• Function — the job that something does.

Some living things are made of just one cell (we call them unicellular) — like bacteria. Most living things you can see are made of many cells (multicellular) — like a tree, a dog, or you.

The big idea to carry through this whole topic: a part's structure is suited to its function. Once you spot that pattern, biology starts to make a lot more sense.

Every animal cell has the same four main parts, each with its own job.

• Cell membrane — a thin skin around the cell. It controls what goes in and out, like a security gate.
• Cytoplasm — a jelly-like filling where most of the cell's chemical reactions happen.
• Nucleus — the control centre. It holds the instructions (DNA) that tell the cell what to do.
• Mitochondria — tiny "power stations" that release energy from food so the cell can work.

A plant cell has the same four parts as an animal cell — and three extra ones.

The three extra parts are:

• Cell wall — a tough layer made of cellulose, outside the membrane. It gives the cell strength and a fixed shape.
• Chloroplasts — green parts that contain chlorophyll. They trap light energy to make food in photosynthesis.
• Large vacuole — a big space filled with cell sap. When full, it pushes outwards and keeps the cell firm.

Not all cells look the same. Many are specialised — their structure is built to suit one job. Here is the big idea in action.

• A red blood cell is a flat disc with no nucleus. That leaves more room inside to carry oxygen around the body.
• A root hair cell has a long, thin projection. This gives it a large surface area to absorb water from the soil.
• A nerve cell is very long and thin, so it can carry signals from one part of the body to another.

Whenever you meet a new cell, ask: what shape is it, and how does that shape help its job?

In a multicellular living thing, cells do not work alone — they are organised into bigger and bigger teams.

• A tissue is a group of similar cells that work together on one job — for example, muscle tissue.
• An organ is made of several tissues working together — like the heart, the stomach, or a leaf.
• An organ system is a group of organs that team up — like the digestive system.
• All the organ systems together make a complete organism — a whole living thing.

Understanding cells now makes the rest of biology much easier:

• Body systems — the circulatory, digestive and breathing systems are all built from organs, tissues and cells.
• Photosynthesis and respiration — these happen inside chloroplasts and mitochondria.
• Microscopes — you will use the skill of observing cells throughout school science.
• Health and disease — understanding healthy cells helps you understand what happens when things go wrong.

Whenever a biology topic feels tricky later, it often helps to go back to the cell. Take your time — really understanding this topic is worth far more than rushing through it.