MoBio Prokaryotic Cells Chapter 1

Figure 1-A-2. Schematic drawing of a prokaryotic cell. The plasma membrane is surrounded by the cell wall, which is wrapped by the capsule.

A prokaryotic cell consists of DNA, cytoplasm, and a surface structure which includes the plasma membrane and some of the following components:

  • Cell wall
  • Capsule
  • Slime
  • Flagella
  • Fimbriae/Pili

Capsule and slime are the hydrophilic gel surrounding the cell wall in most bacteria. The capsule is more closely associated with the cell than the slime. Flagella are long, rigid protein rods, facilitating the movement of motile bacteria. Fimbriae and pili are short hair-like structures used to attach other cells. They are essential for infecting other organisms.

In bacteria, the cell wall contains a unique structure called peptidoglycan. Archaea do not possess peptidoglycan, but some archaea may contain pseudopeptidoglycan, which is composed of N-acetyltalosaminuronic acid, instead of N-acetylmuramic in peptidoglycan. Because of this structural difference, archaea are resistant to many cell wall antibiotics.

Gram Staining


Figure 1-A-3. A Gram stain of mixed Staphylococcus aureus (Gram positive, in purple) and Escherichia coli (Gram negative, in red).

Bacteria may be divided into two groups, on the basis of their cell wall structures and the response to Gram stain: Gram-negative and Gram-positive.

In Gram-negative bacteria, the cell wall is composed of three layers: (1) the periplasmic space which is an open area located outside the plasma membrane, (2) a thin layer of peptidoglycan external to the periplasmic space, and (3) the outer membrane surrounding the peptidoglycan.

The Gram-positive bacteria do not have the periplasmic space and outer membrane, but have a thicker peptidoglycan layer. As a result, they are quite sensitive to lysozyme and penicillin.