Cyanobacteria are gram-negative bacteria that are photosynthetic, having chlorophyll a, that unites these prokaryotic organisms with the eukaryotic organisms, collectively called algae. As prokaryotes, the internal structure has no membrane-bound organelles, such as the chloroplasts, nucleus, and mitochondria. However, they do have structures including ribosomes, cyanophycean granules, polyhedral bodies, polyphosphate bodies, and some genera have gas vesicles. They also have membranes, such as the thylakoids embedded with chlorophyll molecules for light capture. Many of these structures are visible with light microscopy and aid in determining the genus of an organism. The internal cellular material is contained by a cytoplasmic membrane, which is then surrounded by a complex layered cell wall composed of mucopeptides, carbohydrates, amino acids, and fatty acids. Cyanobacteria also have extracellular polymeric substances that are often apparent and are composed of polysaccharides, proteins, lipids, and nucleic acids that are secreted by the cells and are used to determine a genus.