All living organisms on Earth are made up of living cells. All cells fall into one of the two major categories:
- Prokaryotes – single-celled organisms such as bacteria
- Eukaryotes – complex cells that make up organisms such as plants and animals.
What is a cell?
A cell is the smallest basic unit that you are made up of. One cell contains all the genetic information necessary to make a human being. When you look at yourself in the mirror, you’re seeing about 10 trillion cells (1 trillion is 1,000,000,000,000 or a million millions).
The main purpose of a cell is to organise. Each cell has a different set of functions. There are about 200 different cell types. For example, our muscles are made of muscle cells and our livers are made of liver cells. The cells in our bodies work together to help us function as a human being. Everything that happens in our bodies – such as repair, growth and reproduction – happens at a cellular level.
If you were only made of one cell, you’d only be able to grow to a certain size. Think of a bacterium, which is a single-celled organism. It can only grow as big as one cell. You won’t find a single-cell organism that’s as large as a horse, for example.
All our cells group together into tissue that comes together to form organs. The organs form systems and the systems form the organism – you! If you were only one cell, you wouldn’t have a blood system or an immune system. There’d be no muscles for movement, no nerves for feeling, no brain cells for thinking and definitely no playing of computer games. The trillions of cells in your body make your life possible.
Parts of a cell
Nucleus:A large round structure that functions as the cell centre, a bit like the brain. It contains the cell’s DNA – the genetic code that gives the cell instructions on how to function.
Organelles: These are all the parts inside the cell that work together to carry out the operations of the cell.
Cell division
Our cells divide to reproduce themselves. This is how we grow. After growth, there is continual construction and repair of the body. In one lifetime, our bodies will have experienced about 10,000 trillion cell divisions.
Find out more about cell division.
The immune system
The immune system is a group of cells (within the bloodstream) with specific functions that cause them to work together to eliminate pathogenic cells that might enter your body.
These amazing cells can change shape so they can work their way from the bloodstream and squeeze through tissue to reach the site of an infection.
Bacteria cells
Bacteria cells
Bacteria cells are single-celled organisms – they can live by themselves.
From left to right: Campylobacter bacteria; the Bacillus subtilis bacterium found in soil and the Helicobacter bacteria that is known to cause stomach ulcers.
Helicobacter bacteria courtesy of David Gregory & Debbie Marshall, Wellcome Images
Find out more about bacteria.
Activity ideas
Introduction to cells – In this activity, students are introduced to cells. They learn that we are made up of cells. Students also learn how to use a microscope to observe onion cells.
Inside a cell – In this activity, students learn about the contents of a cell. They explore some of the main organelles within a cell using the analogy of a school, an online game and/or by making something edible.
Useful link
Read this tutorial about Cell Structure and learn about the Fluid Mosaic Model, cell junctions and organelles on the Biology Online website.
