In the filamentous cyanobacterium Nostoc, label A represents a heterocyst, which is a specialized, thick-walled cell. Its primary function is nitrogen fixation, converting atmospheric nitrogen into ammonia using the enzyme nitrogenase. The thick cell wall prevents oxygen from entering, maintaining the anaerobic conditions necessary for the enzyme to function. These cells are typically larger and paler than the surrounding vegetative cells, which handle photosynthesis.

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