What is a cell?
A cell is the smallest unit of life. Everything that is alive is made of one or more cells. Most cells are so small that we need a microscope to see them. Some living things, such as bacteria, are made of only one cell. Others, such as humans and trees, are made of billions of cells.
Parts of a cell
Animal and plant cells share some parts:
- Cell membrane — a thin layer that controls what enters and leaves the cell.
- Cytoplasm — a jelly-like fluid where activities happen.
- Nucleus — the control centre that directs the cell.
Plant cells have three extra parts:
- Cell wall — a stiff outer layer that gives shape and support.
- Chloroplast — contains green chlorophyll to make food by photosynthesis.
- Vacuole — a large space filled with cell sap to store water.
Key idea
Animal cells have a cell membrane, cytoplasm and nucleus. Plant cells have these plus a cell wall, chloroplast and large vacuole.
From cell to organism
In many-celled living things, similar cells are organised in levels:
| cell | → | the basic unit |
| tissue | → | a group of similar cells |
| organ | → | several tissues working together |
| system | → | several organs working together |
| organism | → | the complete living thing |
Remember
- The cell is the basic unit of life.
- Nucleus controls the cell; cell membrane controls what goes in and out.
- Only plant cells have a cell wall, chloroplast and large vacuole.