Write a note on heterocyst.

Hint: Heterocysts are colorless cells found in the cyanobacteria that act as the site for nitrogen fixation. Plasmodesmata connections connect these cells with the surrounding cells and help them in getting nutrients from them.

Complete answer:
The orders Nostocales and Stignometales are the orders of the cyanobacteria that form heterocyst. The general characters of the heterocyst are:
It has three cell walls and one of them is formed of glycolipid that forms a barrier to the oxygen.
It does not have photosystem II that produces oxygen so these cells do not perform photosynthesis. But the other vegetative cells of the bacteria perform photosynthesis and form sucrose.
It produces proteins that scavenge any remaining oxygen.
It has enzyme nitrogenase and other proteins that are required for nitrogen fixation.
Heterocysts are developed after every 9-11 cells that give the bacteria a filament like structure. These cells are formed from vegetative cells and once a vegetative cell changes into heterocyst, it cannot revert back.
The heterocyst formation from vegetative cells occurs in a step by step manner. The cell enlarges and its granular inclusion decreases. Then the photosynthetic lamellae reorient and it develops a three-layered wall. The middle wall layer is homogenous and the inner wall is laminated.
The old heterocyst cells undergo vacuolation and breaks from the filament causing fragmentation which is a method of asexual reproduction. These fragments are called hormogonia.

Note: Heterocyst requires a micro anaerobic environment for the enzyme in them to help in nitrogen fixation. These are formed when the filamentous cyanobacteria like Nostoc punctiforme, Anabaena sphaerica, etc. are nitrogen starved.