General Structure of Bacterium

The structural difference between the Prokaryotes and the Eukaryotes are so significant that some biologist think that these two groups merit the status of Super Kingdoms. The main distinguishing feature between the Prokaryotes and Eukaryotes is the lack of a true nucleus in the former.

The general size of a prokaryotic cell is about 1-2 um. Note the absence of membrane bound organelles There is no true nucleus with a nuclear membrane The ribosome's are smaller than eukaryotic cells The slime capsule is used as a means of attachment to a surface Only flagellate bacteria have the flagellum Plasmids are very small circular pieces of DNA that maybe transferred from one bacteria to another.

1.2.2 Function of Prokaryotic cell structures

Structure

Function

Cell Wall

Made of murein (not cellulose), which is a glycoprotein or peptidoglycan (i.e. a protein/carbohydrate complex). There are two kinds of bacterial cell wall, which are identified by the Gram Stain technique when observed under the microscope. Gram positive bacteria stain purple, while Gram negative bacteria stain pink. The technique is still used today to identify and classify bacteria. We now know that the different staining is due to two types of cell wall

Plasma membrane

Controls the entry and exit of substances, pumping some of them in by active transport .

Mesosome

A tightly-folded region of the cell membrane containing all the membrane-bound proteins required for respiration and photosynthesis. Can also be associated with the nucleoid. This is now thought to be an artifact of the electron microscope and not a real structure.

Cytoplasm

Contains all the enzymes needed for all metabolic reactions, since there are no organelles

Ribosome's

The smaller (70 S) type are all free in the cytoplasm, not attached to membranes (like RER). They are used in protein synthesis which is part of gene expression.

Naked DNA

Nucleoid is the region of the cytoplasm that contains DNA. It is not surrounded by a nuclear membrane. DNA is always circular (i.e. a closed loop), and not associated with any proteins to form chromatin. Sometimes confusingly referred to as the bacterial chromosome

Slime Capsule

A thick polysaccharide layer outside of the cell wall, like the glycocalyx of eukaryotes. Used for sticking cells together, as a food reserve, as protection against desiccation and chemicals, and as protection against phagocytosis. In some species the capsules of many cells in a colony fuse together forming a mass of sticky cells called a biofilm. Dental plaque is an example of a biofilm.