The two main types of cellular organism, prokaryotic and eukaryotic, have a common unicellular ancestor called protocell or progenote which is a prokaryotic organism.
Prokaryotes are identified with bacteria: most live as single-celled organisms, but some bacteria associate in a chain. Prokaryotes have their DNA in the cell’s cytoplasm.
Eukaryotes (or “true-nucleus”) have a nucleus, a compartment separate from the rest of the cellular content, which contains DNA.
Beware viruses, or acaryotes, are elements (and not cells) that have neither nuclei nor cytoplasm and can reproduce only by parasitizing a host cell by hijacking cellular machinery.
Prokaryotic cells are divided into two cell types:
Archaeobacteria which take into account methanogenic cells, halophilic cells and thermoacidophilic cells. Archaeobacteria are the first to colonize bare rocks because they survive with the minimum of resources.
The eubacteria (or “real bacteria”) are the closest to today’s bacteria. They take into account contemporary bacteria, mycoplasmas and cyanobacteria. The classic prokaryote is Escherichia-coli (or E-coli), which is a bacteria living in the human intestinal flora thanks to a rigid cell wall.
Bacteria are distinguished by their cell walls highlighted by Gram staining. There are “gram +” bacteria and “gram -” bacteria:
Gram + bacteria retain the dye, purple coloration. Their walls have a single layer of peptidoglycan which rests on the plasma membrane, the two constitute the cell wall. We could take as an example staphylococci.
Gram – bacteria are much more permeable to dye, pink coloring. Their walls are made up of a thin layer of peptidoglycans which rests on the plasma membrane surrounded by an outer membrane: there are therefore three layers. The most relevant example will be Escherichia-coli.
Prokaryotic cells contain a single compartment, the cytoplasm, containing a single chromosome or DNA molecule that is most often circular and called the nucleoid.
The bacteria replicate quickly by cell division or scissiparity. They can be pathogenic or non-pathogenic.
Eukaryotes correspond to multicellular organisms (animals, plants, fungi) as well as to some unicellular eukaryotes. The eukaryotic model is a worm called Caenorhabditis Elegans which has the same molecular and biochemical mechanisms as all multicellular organisms while being easily studied because it has a limited number of cells (131 cells).
Monocellular eukaryotes correspond to protists which are of two types: animal protozoa and plant protophytes. The protist model is yeast or Saccharomyces Cerevisae which is a fungus with a rigid cell wall which absorbs sugars to secrete alcohol and CO2.
Plant cells are the pinnacle of plant evolution: they are capable of synthesizing all organic substances from inorganic matter and light. They contain chloroplasts with voluminous vacuoles limited by a double membrane which correspond to saccules stacked one on the other called thylakoid, where photosynthesis is carried out and therefore which contain chlorophyll. Chloroplasts, like mitochondria, can reproduce and have their own DNA.