All living things are made of cells, and cells are the smallest units that are capable of independent life. Living organisms are classified into five kingdoms, each with their own characteristic types of cell. However the biggest division is between prokaryote cells (the bacteria) and the eukaryotic cells of the other four kingdoms (animals, plants, fungi and protoctista). This division is shown in the chart below.
Viruses are included here for information purposes, but they are not one of the five kingdoms. Viruses and Prions are biological, but are not constructed from cells, and there is considerable debate as to whether they represent organisms or not, as they are incapable of independent life.
The basics structure of cells has been known for hundreds of years, but it has only been since the invention of the electron microscope that the detailed structure of a vast array of sub-cellular components has been discovered. The term for a structure within a cell is organelle. Biological membranes surround many organelles though some have no membrane. The numbers, and range of organelles varies between cells. The next two sections will look at the organelles present in eukaryote and prokaryote cells.