Summarizing Structure of Prokaryotes

Summary

Prokaryotes (domains Archaea and Bacteria) are single-celled organisms lacking a nucleus. They have a single piece of circular DNA in the nucleoid area of the cell. Most prokaryotes have a cell wall that lies outside the boundary of the plasma membrane. Some prokaryotes may have additional structures such as a capsule, flagella, and pili. Bacteria and Archaea differ in the lipid composition of their cell membranes and the characteristics of the cell wall. In archaeal membranes, phytanyl units, rather than fatty acids, are linked to glycerol. Some archaeal membranes are lipid monolayers instead of bilayers.

The cell wall is located outside the cell membrane and prevents osmotic lysis. The chemical composition of cell walls varies between species. Bacterial cell walls contain peptidoglycan. Archaean cell walls do not have peptidoglycan, but they may have pseudopeptidoglycan, polysaccharides, glycoproteins, or protein-based cell walls. Bacteria can be divided into two major groups: Gram positive and Gram negative, based on the Gram stain reaction. Gram-positive organisms have a thick cell wall, together with teichoic acids. Gram-negative organisms have a thin cell wall and an outer envelope containing lipopolysaccharides and lipoproteins.

Glossary

capsule

external structure that enables a prokaryote to attach to surfaces and protects it from dehydration

conjugation

process by which prokaryotes move DNA from one individual to another using a pilus

Gram negative

bacterium whose cell wall contains little peptidoglycan but has an outer membrane

Gram positive

bacterium that contains mainly peptidoglycan in its cell walls

peptidoglycan

material composed of polysaccharide chains cross-linked to unusual peptides

pilus

surface appendage of some prokaryotes used for attachment to surfaces including other prokaryotes

pseudopeptidoglycan

component of archaea cell walls that is similar to peptidoglycan in morphology but contains different sugars

S-layer

surface-layer protein present on the outside of cell walls of archaea and bacteria

teichoic acid

polymer associated with the cell wall of Gram-positive bacteria

transduction

process by which a bacteriophage moves DNA from one prokaryote to another

transformation

process by which a prokaryote takes in DNA found in its environment that is shed by other prokaryotes