Question

Primary function of heterocyst in Nostoc is

• A. Food storage
• B. Nitrogen fixation
• C. Reproduction
• D. Protection

Answer 1

The correct option is B Nitrogen fixation

Heterocysts are specialized, pale-yellow, thick-walled cells with the function of nitrogen-fixing formed during nitrogen starvation by some filamentous cyanobacteria, such as Nostoc punctiforme, Cylindrospermum stagnale, and Anabaena sphaerica. They fix nitrogen from di-nitrogen (N${}_2$) 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 micro anaerobic environment. Hence, their main function is nitrogen fixation. Thus, the correct answer is option B.