2. Eukaryotes and Prokaryotes

Prokaryotic Cell


The term prokaryote (plural: prokaryotes) came from the Latin pro, meaning “in favour of” or “on behalf of” and káry(on), meaning “nut“, “kernel”. The term prokaryotic is a derived word and used to refer to a prokaryote. Compare: eukaryote. Variant: procaryote.

Prokaryotic Cell

Prokaryotes are unicellular organisms that lack a well-defined nucleus. They have instead a nucleoid region in their cytoplasm where their genetic material occurs in most instances as a single, circular molecule of DNA. They generally reproduce asexually, which is by binary fission or by budding. Most of them are unicellular, others are capable of forming stable aggregate communities. Conjugation, apparently, is the counterpart of sexual reproduction in eukaryotes where two cells exchange genetic materials via a conjugation tube.

Although prokaryotes lack the true organelles found in a eukaryotic cell, they possess certain cytoplasmic structures. For instance, they have a primitive cytoskeleton. Flagellin is the cytoskeletal protein that confers structural support to bacteria that is essential during chemotaxis. Other cytoplasmic structures are ribosomes, carboxysomes, chlorosomes, magnetosomes, and thylakoid systems. Some references regard them as prokaryotic organelles. However, they are not bounded by lipids; rather, they are proteinaceous. For example, carboxysomes are protein-shell compartments that are involved in carbon fixation in certain bacteria. Chlorosomes are light-harvesting complex in green sulfur bacteria. Magnetosomes are present in magnetotactic bacteria. Thylakoids are present in photosynthetic bacteria, such as cyanobacteria. Some prokaryotes have a cell wall that surrounds the cell membrane. Bacterial cell walls are composed chiefly of peptidoglycan. Its thickness can be used to determine if the bacterial cell is Gram-positive (thicker cell wall) or Gram-negative (thinner cell wall). As for the archaea, their cell wall is made up of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides rather than peptidoglycan (except for a group of methanogens).

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