Everything is made of them

Peel a thin skin from an onion. Under a microscope, it shows tiny box-shaped cells.

The rule

Structure means what a thing is built from. Function means the job it does.

Living things are one cell, or many cells joined up. Viruses are not cells. The cell is the basic unit of structure and function. Every cell comes from a cell that already existed.

A bacterium is one cell. A banyan tree, a crow and you are made of many cells. Similar cells working together form a tissue. Different tissues working together form an organ, such as the heart.

How big is a cell?

Robert Hooke saw the first cells in 1665. He looked at a slice of cork under a microscope.

Most cells are very small. We need a microscope to see them. We measure cells in micrometres (µm). 1 millimetre (mm) = 1000 µm.

To find a cell's size, first measure the field of view. This is the round area you see through the microscope. Take it as 4 mm, which is 4000 µm. Then count the cells along one line across the field. Take 20 cells.

The 20 cells fill 4000 µm. So one cell = 4000 ÷ 20 = 200 µm.

Magnification tells how many times larger a cell looks. The eyepiece is the lens you look through. The objective is the lens close to the thing you view. A microscope's total magnification is the eyepiece times the objective. A 10× eyepiece and a 40× objective give 400×. Here 10 × 40 = 400.

Try the lab to finish this lesson.