Structure of Flagella”Biological Classification” Botany Notes Chapter 2, for Class 12-CBSE

Structure of Flagella

The entire flagellar apparatus is made up of three distinct region : basal body, hook and filament.

  1. Basal body : it is most complex portion of flagellum and has four rings (L,P, S and M) , only two rings S and M are present in gram +ve bacteria. L and P rings in cell wall constitute distal set, while S and M rings are present in plasma membrane, forming proximal set.
  2. Hook : made up of different protein units.
  3. Filament : bacterial flagella are made up of identical spherical subunits of a protein called flagellin. Longitudinal chains of flagellin molecules run longitudinally around each other to form a wavy helical or rope – like structure. Therefore, a cross section of the flagellum reveal a number of flagellin molecule around a central space.

b.Pill and fimbriae are hollow, non helical, filamentous appendages projecting from the walls of Gram- negative bacteria. These are thinner and shorter and more in number than the flagella. These are made up of specific proteins called pilin.

There are different types of pili which serve different functions. One type, known as type I pili (somatic pili) play a major role in infection by facilitating the attachment of bacterial cell to the host cell, another type, termed sex pili, serve as portals of genetic material from donor to recipient cell during conjugation.

4. Protoplast : Cell wall encloses the protoplast, the living matter. It includes (i)  Cell membrane (ii) Cytoplasm (iii) Nucleold and may have plasmid and episome.

(i) Cell membrane : It lies inner to the cell wall, actually representing the outermost layer of the protoplast. It is living and semipermeable, controlling the movements of various dissolved substances in and out of the cells. Functionally, the cell  membrane of bacteria resembles mitochondria of eukaryotic cells as respiratory ETS enzymes and succinate dehydrogenase (Krebs Cycle) are associated with the membrane.

The cell membrane gets invaginated and folded to form a structure called mesosome (chondroid) in some bacteria, particularly the Gram positive bacteria. These may be central or peripheral in position and they are supposed to play a role in replication of DNA during cell division, as these are often attached to the nuclear body. Besides, these increase the surface area of absorption and help in septa formation during binary fission.

1. Cytoplasm : it is homogenous colloidal mass of carbohydrates, fats, proteins, lipids, nucleic acids, minerals and water. It does not show streaming movements. It lacks sap vacuoles (may be present in some bacteria which live in aquatic condition). Typical membrane bound organelles of eukaryotic cells like endoplasmic reticulum, mitochondria, golgi complex and plastids are absent. The cytoplasm appears granular due to the presence of ribosomes. However, these are 70S type in bacteria as compared to 80S type in eukaryotes.

Ribosomes lie scattered freely in the cytoplasm, but sometimes may form a small chian of 4-6 ribosomes attached to mRNA  constituting polyribosome or polysome. Various non living inclusions like glycogen particles , fat bodies, volutin granules (polymetaphosphate – source of energy and lipid molecules acting as food reserve lie disparesed in the cytoplasm. The cytoplasm is usually colourless, lacking pigments. However, in photosynthetic bacteria, the cytoplasm contains pigments like bacteriochlorophyll and bacterioviridin. The pigments wither lie dispersed in the cytoplasm or present in membrane bound spherical vesicles called chromatophores. These pigments are capable of entrapping solar energy for photosynthesis.

iii. Nucleoid (Prochromosome, Genophore, incipient nucleus) – Bacterial cell lacks a well organized nucleus. It consists of  a long double stranded DNA molecule repeatedly folded with the help of RNA   to form a circular ring. DNA has no free ends and not associated with histone proteins(polyamines present). circular DNA ring, without histones is often termed bacterial chromosome.

Plasmid ((Minichromosome) : term plasmid was given by Lederberg and Hays. These are small, extrachromosomal, non-essential, circular , double stranded, free naked DNA molecules. The genes present on them have no vital role in survival and growth  of bacteria. These perform autonomous replication. If plasmids temporarily integrate with bacterial chromosome, then they are called episomes.

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