All living things, from microscopic bacteria to gigantic trees, are made of tiny “units” called cells. They are the building blocks of any living thing, in the same way that bricks fit together to make buildings. Most living things contain millions, sometimes even billions of cells. Most cells are too small to see without a microscope.

Inside a cell

Every cell is surrounded by a skin, the cell membrane. Inside, there is a mass of jelly with smaller parts, called organelles (“little organs”), floating around in it. Plant cells have a tough cell wall outside the cell membrane. Both plant and animal cells have a nucleus, a dark lump that lies at the cell’s centre. It acts like a computer programme that controls the workings of the cell.