Critical notes on Heterocyst and its functions, Flagella and pill

Heterocyst and its function :

Heterocysts are specialized nitrogen-fixing cells formed by some filamentous cyanobacteria, such as Nostoc punctiforme, Cylindrospermum stagnate and Anabaena sphaerica, during nitrogen starvation. They fix nitrogen from dinitrogen (N₂) in the air using the enzyme nitrogenase, in order to provide the cells in the filament with nitrogen for biosynthesis. Nitrogenase is inactivated by oxygen, so the heterocyst must create a microanaerobic environment. The heterocysts’ unique structure and physiology require a global change in gene expression. For example, heterocysts:

i. produce three additional cell walls, including one of glycolipid that forms a hydrophobic barrier to oxygen

ii. produce nitrogenase and other proteins involved in nitrogen fixation

iii. degrade photosystem II, which produces oxygen

iv. up-regulate glycolytic enzymes

v. produce proteins that scavenge any remaining oxygen

vi. contain polar plugs composed of cyanophycin which slows down cell-to-cell diffusion

Heterocyst formation is marked by several changes to the cell, including the formation of three additional cell walls. Heterocysts also begin to produce certain proteins that are involved in nitrogen fixation, and degrade proteins that produce oxygen. Finally, the cell produces more glycolytic enzymes, which enables to cell to produce energy without oxygen. Finally, in order to ensure that no oxygen is present in the cell, the heterocyst cell produces proteins which scavenge any remaining oxygen.

(ii) Flagella and pill:

Flagella are long, thread-like appendages which provide some live single cells with the ability to move, motility. Bacteria which have flagella are either rod or spiral-shaped and are known as bacilli and spirochetes, respectively. Cocci, .or round bacteria, are almost all nonmotile. Animal sperm cells also have flagella. However, prokaryotic cells (such as bacteria) have flagella made up of the protein flagellin. Whereas, eukaryotic cells (such as sperm) which have a nucleus have flagella composed of tubulin proteins.

A pilus is a hairlike appendage found on the surface of many bacteria. All pili are primarily composed of oligomeric pilin proteins. Pili connect a bacterium to another of its species, or to another bacterium of a different species, and build a bridge between the interior of the cells. This enables the transfer of plasmids between the bacteria. An exchanged plasmid can code for new functions, e.g., antibiotic resistance. The pilus is made out of the protein pilin.