Structure of Bacteria

There are many different types of bacteria. The following description applies to a generalized bacterial cell.

1. Flagellum

The bacterial flagellum can be divided into three parts, a basal granule, a hook and a main filament.
a) Basal granule. In Gram-negative bacteria the basal granule consists of two sets of rings, a proximal set and a distal set.
b) The book penetrates the cell wall connects the main filament to the basal granule.

2. Fimbriae (Pili)

Many Gram-Negative bacteria have hair-like appendages called fimbriae or pili given off from their surface. These appendages are shorter and straighter than flagella. Electron microscope studies suggest that each F-pilus is made up of two parallel proteins with a groove between them. Each rod is made up of F-pilin monomers.

3. Capsule

Many bacterial cells are surrounded by a capsule. The capsule is secreted by the cell, and is usually composed of polysaccharides or disaccharides, but is sometimes made up of polypeptides.

4. Cell Wall

The bacterial cell is surrounded by a rigid cell wall, which can withstand pressures of up to 20 atmospheres. The cell wall gives the bacteria their characteristic shape, and also protects the cell from the high osmotic pressure difference.
The matrix substances in the walls of Gram positive bacteria may be polysaccharides or teichoci acids. The latter are very widespread, but have been found only in Gram positive bacteria. Teichoic acids are of two main types, ribitol teichoic acids and glycerol teichoci acids. Of the two types glycerol teichoci acids are move widespread. These acids are polymers of ribitol phosphate and glycerol phosphate, respectively.

5. Plasma Membrane

Electron microscopy has revealed the presence of a plasma membrane just beneath the cell wall. The membrane appears to be similar to that of higher plants and animals. Recently an alternative structure called the fluid mosaic model has been proposed, and is receiving much support.

Functions of the bacterial plasma membrane

1. The bacterial cell membrane provides an osmotic barrier which retians cell metabolites and prevents unrequired outside substances from entering the cell.

2. The electron transport system of bacteria is located in the plasma membrane, which is, therefore, functionally equivalent to the mitochondrion.

3. The bacterial plasma membrane contains many specific transport systems for compounds like sugars, amino acids, etc., and mineral ions.

6. Mesosome

In many bacteria, especially Gram-Positive bacteria, the cell membrane becomes inavaginated and folded to form structures called mesosomes. The mesosomes were formerly thought to be equivalent to mitochondria of higher cells.

7. Internal membrane system

There is no definite endoplasmic reticulum is bacteria. Electron micrographs, however, show the presence of an internal membrane system which may be concealed because of densely packed ribosomes.

8. Ribosome

In contrast to most eukaryote ribosomes the bacterial ribosomes do not appear to be attached to the endoplasmic reticulum, but are found free in the cytoplasm.

9. Storage Granules

In many bacteria the cytoplasm contains large granules of storage materials. These serve as stores of energy of organic compounds.

a) Polymetaphoshpate or volutin granules stain reddish violet with methylene blue or toludine blue blue.

b) Polyglucan granules, which stain blue, reddish blue or brown with iodine, are found in many bacteria. Polyglucans are barached molecules which resemble in structure glycogen of mammalian liver.

10. Photosynthetic Apparatus

The basic unit of the photosynthetic apparatus is a membranous sac called the thylakoid. Primitive photosynthetic bacteria contain small vesicles called chromatophores.
Photosynthetic bacteria are of two types, green bacteria and purple bacteria. Bacteria photosynthesis differs in certain respects from photosynthesis in green plants.

11. Nucleoid (Nucleus)

In eukaryotes the nucleus is bounded by a nuclear envelope. The DNA of bacteria is, however, not enclosed within a nuclear envelope and is hence known as the nucleoid or chromatin body.


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