Cells and their sizes

There are two distinct types of cell without any known intermediates: prokaryotic and eukaryotic. Prokaryotic cells are much simpler than eukaryotic cells. They have very few internal components and little genetic information. All prokaryotic cells are single cell bacteria and are thought to have been some of the first organisms to exist. The more complex eukaryotic cells make up all other living organisms.

Prokaryotic cells are the smallest cells, typically a few micro-meters (µm) long. For example, an Escherichia coli (often referred to as E. coli) bacterium is typically a few µm long.

Mammalian red blood cells are among the smallest eukaryotic cells. They are typically about 8µm in diameter and have a distinctive bi-concave shape in humans. It is interesting to note that the shape can be very different. For example the red blood cells of sheep are nearly spherical and those of the camel are elliptical.

The amoeba is a relatively large single celled organism, typically 10 to 100 µm across.

Technically, the yolk of an ostrich egg is one cell though it is rather specially adapted. All the nucleotide material is contained in a small volume at the edge of the yolk. The bulk is a supply of `raw material' that the cells of the growing embryo use to assemble the cells of the ostrich chick.

Single giraffe nerve cells can be the length of the giraffe, from head to toe, several meters! Of course, these cells are still extremely narrow and the diagram below is very schematic.