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.
Organization of eukaryotic cells
As said before, eukaryotic cells are delimited by a membrane (animals) or wall (plants) and have a nucleus which is the organelle containing the genome of the individual.
In the eukaryotic cell there are also organelles which are either part of the endo-membrane system, or part of closed organelles (peroxisomes, mitochondria and chloroplasts).
The endo-membrane system corresponds to the set of saccules limited by simple membranes in permanent communication with each other, and with the plasma membrane thanks to vesicles (reticulum-endoplasmic, nuclear envelope, Golgi apparatus, lysosomes and endosomes ). They all use energy.
Closed organelles are the main energy transformers of the cell, they allow the formation of energy.
On the other hand, the cytoskeleton allows the maintenance of cell morphology, the position of organelles in the cell and the transport of different cytoplasmic components. Among them are actin microfilaments, microtubules and intermediate cytokeratin filaments.
“The medium in which most cells in the multicellular eukaryotic organism are bathed is the interstitial portion of the extracellular fluid.
The normal functioning of cells depends on the consistency of this liquid and it is therefore not surprising that in multicellular eukaryotes, multiple regulatory mechanisms have developed to maintain its conditions. Homeostasis describes the various physiological arrangements that restore normal state after a disturbance.
The distinctive characters between prokaryote and eukaryote
Prokaryotic cells do not have nuclei and have circular or linear DNA, located in the cytoplasm and haploid in the vegetative state. In this way the replication, transcription and translation of DNA takes place directly in the cytoplasm.
Prokaryotes have no cytoplasmic partitioning and their membranes do not have sterols but they are lined with a layer of peptidoglycan forming the cell wall (see earlier in the course). The basic substance of the cytoplasm is called the cytosol which is rigid in prokaryotes, with an absence of flow (neither exocytosis nor endocytosis). Prokaryotes have neither organelles nor cytoskeletons.
Eukaryotic cells have a nucleus which is the largest organism and which is bounded by a double membrane called the nuclear envelope. In the nucleus, DNA replication and transcription takes place; translation takes place in the cytoplasm of the cell.
Eukaryotes have cytoplasmic partitions allowing the formation of organelles (endoplasmic reticulum nucleus, golgi apparatus, lysosomes, peroxysomes and vesicles), these organelles swim in the cytosol which in eukaryotes is fluid with the presence of flow thanks to the cytoskeleton.
Plasma membranes are not lined with a wall for animals, but doubled for plants (pectocellulosic wall) and for fungi (polysaccharide wall); in all cases there is absence of peptidoglycan but presence of sterols.