Prokaryotes

Article

July 1, 2022

Prokaryotic cells (Greek: pro — "before", Greek: karyon — "sail"; "before the sail") are small, simple in structure, surrounded by a cell wall and membrane, and do not have a nucleus or cell organelles, except for ribosomes. Prokaryotic organisms include bacteria, archaea and blue-green algae (cyanophytes), while all other organisms, whether unicellular or multicellular, are eukaryotes.

Cytoplasm structure

Genetic material (DNA) is found free, in a part of the cytoplasm called the nucleoid ("like a nucleus", because it is not separated by a membrane from the rest of the cytoplasm). DNA is in the form of a ring (circular DNA), and most often one prokaryotic cell has one molecule of DNA (one chromosome). The DNA molecule is bound to proteins, but that bond is not as strong and stable as in eukaryotes. The nucleoid is a very dynamic structure, which is probably related to the fact that prokaryotic cells divide every 15-20 minutes. They reproduce by simple cell division (amitosis). Mitosis and meiosis do not exist in them. In addition to not having a true, differentiated nucleus, a prokaryotic cell also contains no cell organelles, except for ribosomes in which proteins are synthesized. The ribosomes of prokaryotes are smaller than those of eukaryotes and are designated as 70S (S is the sedimentation coefficient, i.e. deposition, which determines the size and shape of a molecule; it is also called Svedberg's constant).

Extracellular structures

The cell wall is unique in composition because it contains peptidoglycans, polymers unique to prokaryotes. Unlike eukaryotic plants, the prokaryotic cell wall never contains cellulose. In addition to the cell wall, some bacteria are also surrounded by a capsule made of polysaccharides or polypeptides. Some bacteria form a huge number of protein filaments around the cell, the so-called. saw, which are used to fasten it to a suitable substrate. Motile bacteria form long flagella (flagellum) made of flagellin protein.

Functions

Although it is without cell organelles, this cell performs similar functions as the eukaryotic one in certain parts of the cell. Cellular respiration is carried out by means of a series of enzymes (respiratory enzyme chain) which are bound to the cell membrane. In addition, DNA is attached to the membrane, at least in one place, and s