Diagram 3.1: A variety of animal cells
The cell is the basic building block of living organisms. Bacteria and the parasite that causes malaria consist of single cells, while plants and animals are made up of trillions of cells. Most cells are spherical or cube shaped but some are a range of different shapes (see diagram 3.1).
Most cells are so small that a microscope is needed to see them, although a few cells, e.g. the ostrich’s egg, are so large that they could make a meal for several people.
A normal cell is about 0.02 of a millimetre (0.02mm) in diameter. (Small distances like this are normally expressed in micrometres or microns (μm). Note there are 1000 μms in every mm).
Diagram 3.2: An animal cell
When you look at a typical animal cell with a light microscope it seems quite simple with only a few structures visible (see diagram 3.2).
Three main parts can be seen:
- an outer cell membrane (plasma membrane),
- an inner region called the cytoplasm and
- the nucleus
Diagram 3.3: An animal cell as seen with an electron microscope
However, when you use an electron microscope to increase the magnification many thousands of times you see that these seemingly simple structures are incredibly complex, each with its own specialized function. For example the plasma membrane is seen to be a double layer and the cytoplasm contains many special structures called organelles (meaning little organs) which are described below. A drawing of the cell as seen with an electron microscope is shown in diagram 3.3.