In the biological hierarchy of living species, the cell is the basic unit of each organism's structure and function.
Together, an organism's cellular activities enable its survival and reproduction.
There are two major cell groups:
- Prokaryotic cells which compose bacteria and cyanobacteria.
- Eukaryotic cells, specific to the four major classes of living organisms: protists, plants, fungi and animals.
For pluricellular organisms (animals and plants) specialized cells form tissues and organs.
It is estimated that the human body is made up of 50 000 billion cells and about 220 specialized cell types.
We will only discuss animal eukaryotic cells from here on.
