Prokaryotic Cell: What it is and Its Parts

The prokaryotic cell is the most basic cell on the tree of life. It consists of a plasma membrane that surrounds a gelatinous medium called cytoplasm.

The distinctive feature of the prokaryotic cell is the absence of nucleusa membrane structure that encloses deoxyribonucleic acid (DNA).

The word “prokaryote” derives from the Greek pro– which means “before” and karyon which means “core”. From prokaryotic cells, eukaryotic cells evolved, with the appearance of the nucleus and other organelles.

Prokaryotic cells are currently classified into two groups:

  • bacteria: we can mention the Escherichia coli (which lives in the intestine of animals) and the Staphylococcus aureus (found in the skin).
  • archaea: among these are known the Halobacterium salinarum (which grows in the salt flats) and the Thermoproteus neutrophilus (found in hot springs).

Prokaryotic cells are widely distributed on Earth: in the oceans, the soil, on the skin of animals, on the leaves of plants, and even in the intestines of animals.

Parts of the prokaryotic cell

prokaryotic cell structure

Image of the structure of a prokaryotic cell

The prokaryotic cell has the following parts:

  • Membrane cell phone:also known as the plasma membrane, is a semipermeable phospholipid bilayer that maintains the integrity of the cell.
  • Cytoplasm: is the gelatinous substance inside the prokaryotic cell, where biological reactions take place. It contains proteins, carbohydrates, electrolytes and nucleic acids.
  • nucleoid: region in the cytoplasm where DNA is concentrated. DNA forms a circular chromosome.
  • ribosomes: protein and RNA complexes that have the function of synthesizing proteins.
  • Cellular wall: It is a structure external to the cell, outside the cell membrane, which protects and shapes it. In bacteria, the cell wall is made of peptidoglycan (carbohydrates and small proteins); in archaea, the cell wall is constructed of a pseudopeptidoglycan. Some prokaryotic cells, such as mycoplasmas, do not have a cell wall.
  • Capsule: some prokaryotic beings, such as the pneumococcus Streptococcus pneumoniae, have an additional outer layer of carbohydrates, with a slimy appearance adherent to the surfaces of their environment. This capsule gives you additional protection.
  • flagella: It is a long whip-shaped structure that allows the cell to swim.
  • Cilia: they are small hairs that come out of the membrane, which give mobility to the prokaryotic cell.
  • fimbria: It is a small tube that comes out of the membrane.
  • magnetosome: consists of a lipid membrane that surrounds a crystal of magnetite (Fe3EITHER4) or greigite (Fand3S4). Its function is to guide the bacteria in a magnetic field. It is found in magnetotactic bacteria such as magnetospirillum magneticum.
  • photosynthetic compartments: some prokaryotes present in the cytoplasm compartments formed by membrane where the machinery for photosynthesis is concentrated. For example, purple bacteria (such as Rhodopseudomonas palustris) have chromatophores, cyanobacteria have a thylakoid membrane, and green bacteria (such as Chlorobium tepidum) have chlorosomes.
  • Carboxysomes: found in cyanobacteria and chemoautolithotrophic bacteria. Its function is to concentrate the reactions for the fixation of carbon dioxide.
  • gas vesicles: are found in non-motile bacteria and archaea, such as Halobacterium halobium. Its function is to allow them to float in the liquid medium.
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prokaryotic and eukaryotic cell

The prokaryotic cell is considered the ancestor of the eukaryote. Both have a plasma membrane, cytoplasm, DNA, RNA, and ribosomes.

The eukaryotic cell differs from the prokaryotic cell by having a nucleus, where the DNA is found. Prokaryotic DNA forms a single circular chromosome, while eukaryotic DNA occurs in several chromosomes.

The ribosomes of prokaryotic cells are smaller than those of eukaryotic cells, although they fulfill the same function of protein synthesis.

The cell wall of prokaryotic cells is made up of peptidoglycan (bacteria) or pseudopeptidoglycan (archaea). The cell wall of the eukaryotic cell can be made of cellulose (plant cell) or chitosan (fungal cell).

Prokaryotic cells form unicellular organisms, while eukaryotic cells can form multicellular beings, such as animals and plants.

The Swiss-French biologist Édouard Chatton (1883-1947) named the cell without a nucleus a prokaryotic cell, to differentiate it from the eukaryotic cell with a nucleus.

See also Eukaryotic cell and Cell types.

References

Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., Walter, P. (2008) Molecular Biology of The Cell 5th ed. Garland Science. NY.

Hardin, J., Bertoni, G., Kleinsmith, LJ (2012) Becker’s World of the cell. Pearson Education.

Murat, D., Byrne, M., Komeili, A. (2010) Cell biology of prokaryotic organelles. Cold Spring Harbor Perspectives in Biology. 2:a000422. doi: 10.1101/cshperspect.a000422