What is the name of the specialized nitrogen-fixing cell formed by some filamentous cyanobacteria?

The specialized nitrogen-fixing cell formed by some filamentous cyanobacteria is called a heterocyst. These cells are crucial for the process of nitrogen fixation, which is the conversion of atmospheric nitrogen (N2) into a form that is usable by living organisms, primarily ammonia. Heterocysts provide an anaerobic environment necessary for the nitrogen-fixing bacteria to function, as the enzyme responsible for nitrogen fixation, nitrogenase, is sensitive to oxygen.

In filamentous cyanobacteria, heterocysts are interspersed between regular vegetative cells along the filament. This organization allows for the efficient exchange of nutrients and metabolites. While the vegetative cells are involved in photosynthesis, heterocysts focus on nitrogen fixation, thereby promoting the growth of the cyanobacterial colony in nutrient-poor environments.

The other options refer to different structures or functions in plant cells and cyanobacteria. Chloroplasts are the organelles where photosynthesis occurs in plant cells, the stroma is the fluid-filled space in chloroplasts surrounding the thylakoids, and thylakoids are the membrane-bound structures where light-dependent reactions of photosynthesis take place. These components are not specialized for nitrogen fixation as heterocysts are.