Locomotory organ: Flagella

Flagella are extremely thin hair like appendages. They originate from basal body and come out through cell wall. The basal bodies are present just beneath the cell membrane in the cytoplasm. The bacterial flagella is made of about 20 proteins, with another 30 proteins required for its regulation and assembly. Flagella are chiefly made up of a protein flagellin. Most of the bacilli and spiral shaped bacteria have flagella. Cocci rarely have flagella. Flagella of prokaryotes and eukaryotes differ from each other. Prokaryotic flagella are one-tenth of the width of eukaryotes. Bacterial flagella are not covered by extension of membrane. Bacteria are classified into different taxonomic groups on the basis of presence of flagella, pattern of attachment of flagella and the number of flagella.

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I. Atrichous: In this case, bacteria are without flagella.


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  1. Monotrichous: In this case, single polar flagellum is present.
  2. Lophotrichous: In this case tuft of flagella is present only at one pole of bacteria
  3. Ampitrichous: In this case, tuft of flagella is present at each of the two poles of bacteria.
  4. Peritrichous: In this case flagella surround the whole cell. Function of flagella

a) The primary function of flagella is locomotion. Flagella are used for swimming through water: bacterial gliding and twitching. These movements also change the buoyancy to allow vertical motion. Swimming bacteria frequently move near 10 body lengths per second and a few as fast as 100. This makes them at least as fast as fish, on a relative scale. In twitching motility, bacterial use their pili as a grappling hook. Bacteria repeatedly extend it, anchor it and then retract it with remarkable force. (a) Bacteria can detect a chemical signals with the help of flagella and move in its response. Such type of behaviour is called Chemotaxis.

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