CBSE Class 9 Answered

Chitin is a tough complex sugar. They form the cell wall of fungi.

Heterocysts are specialized nitrogen-fixing cells formed by some filamentous cyanobacteria, such as Nostoc punctiforme and Anabaena sperica 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.. These cells are large and thick-walled . They occurs at intervals along the filaments of these cyanobacteria.

Diatoms belong to kingdom Protista. They are a group of unicellular, microscopic algae that have rigid cell walls composed of silica. These reproduce asexually and may be marine or freshwater.