Final answer:
Cyanobacteria are considered prokaryotes because they do not have membrane-bound organelles and perform photosynthesis within infolded plasma membrane regions called thylakoids, using photosynthetic pigments including chlorophyll a.
Step-by-step explanation:
Cyanobacteria are classified as prokaryotes because they lack membrane-bound organelles such as nuclei, mitochondria, and chloroplasts. Photosynthetic membranes in prokaryotes, including cyanobacteria, are infolded regions of the plasma membrane, known as thylakoids. Unlike eukaryotic photosynthetic organisms that have distinct organelles (chloroplasts) for photosynthesis, cyanobacteria utilize these thylakoids for the same purpose. These organisms contain photosynthetic pigments like chlorophyll a, phycocyanin, and cyanophycin, which are crucial for photosynthesis. It's important to note that the photosynthesis in cyanobacteria is oxygenic, similar to that in eukaryotes. The historical endosymbiotic theory suggests that such cyanobacteria were the precursors of chloroplasts in eukaryotic plants.