Structures External To The Cell Wall | Medical Microbio

Cells Structure of Prokaryotes

Structure / September 26, 2017

Prokaryotes, found in both Domain Archaea and Bacteria, are unicellular organisms that lack membrane-bound organelles and a defined nucleus.

Learning Objective

  • Describe the basic structure of a typical prokaryote

Key Points

  • Prokaryotic cells lack a defined nucleus, but have a region in the cell, termed the nucleoid, in which a single chromosomal, circular, double-stranded DNA molecule is located.
  • Gram-positive organisms typically lack the outer membrane found in gram-negative organisms and contain a large amount of peptidoglycan in the cell wall, roughly 90%.
  • Gram-negative bacteria have a relatively thin cell wall composed of a few layers of peptidoglycan.

Terms

  • osmotic pressure

    the hydrostatic pressure exerted by a solution across a semipermeable membrane from a pure solvent

  • plasmid

    a circle of double-stranded DNA that is separate from the chromosomes, which is found in bacteria and protozoa

  • nucleoid

    the irregularly-shaped region within a prokaryote cell where the genetic material is localized

The Prokaryotic Cell

Prokaryotes are unicellular organisms that lack organelles or other internal membrane-bound structures . Therefore, they do not have a nucleus, but, instead, generally have a single chromosome: a piece of circular, double-stranded DNA located in an area of the cell called the nucleoid. Most prokaryotes have a cell wall outside the plasma membrane.

Prokaryotic cell structure

The features of a typical prokaryotic cell are shown.

The composition of the cell wall differs significantly between the domains Bacteria and Archaea, the two domains of life into which prokaryotes are divided . The composition of their cell walls also differs from the eukaryotic cell walls found in plants (cellulose) or fungi and insects (chitin). The cell wall functions as a protective layer and is responsible for the organism's shape. Some bacteria have a capsule outside the cell wall. Other structures are present in some prokaryotic species, but not in others. For example, the capsule found in some species enables the organism to attach to surfaces, protects it from dehydration and attack by phagocytic cells, and increases its resistance to our immune responses. Some species also have flagella used for locomotion and pili used for attachment to surfaces. Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea.

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