Introduction

Look around you and it is easy to sort objects into two groups: things that are alive and things that are not. But what actually draws that line? What does a living organism have that an inanimate object — a stone, a chair, a drop of water — simply lacks?

The answer lies in one shared feature: every living organism, without exception, is built from cells. The cell is the basic unit of life — the smallest package of living material an organism can be made of, and it is this cellular organisation that separates the living from the non-living.

One cell, or many

Not every organism needs many cells to be alive. Based on how many cells make up their body, organisms fall into two broad groups:

• Unicellular organisms — the entire body is a single cell. That one cell does everything the organism needs to survive.
• Multicellular organisms — the body is built from many cells working together. Human beings, along with most animals and plants we see around us, belong to this group.

Whether an organism's body is a single cell or a vast cooperating community of them, the cell remains the starting point for understanding how life is organised — which is exactly what the rest of this chapter explores.