"Ribosomes themselves are constructed from proteins along with ribosomal RNA (rRNA). Prokaryotic ribosomes are found in the cytoplasm. They are called 70S ribosomes because they have a size of 70S, whereas eukaryotic cytoplasmic ribosomes have a size of 80S. Although they are the same size, bacterial and archaeal ribosomes have different proteins and rRNA molecules, and the archaeal versions are more similar to their eukaryotic counterparts than to those found in bacteria. Prokaryotic ribosomes are composed of two subunits: the 30S small subunit and the 50S large subunit, each of which is composed of protein and rRNA components. As single-celled organisms living in unstable environments, some prokaryotic cells have the ability to store excess nutrients within cytoplasmic structures called inclusions. Storing nutrients in a polymerized form is advantageous because it reduces the buildup of osmotic pressure that occurs as a cell accumulates solutes. Various types of inclusions store glycogen and starches, which contain carbon that cells can access for energy. Volutin granules, also called metachromatic granules because of their staining characteristics, are inclusions that store polymerized inorganic phosphate that can be used in metabolism and assist in the formation of biofilms. Microbes known to contain volutin granules include the archaea Methanosarcina, the bacterium Corynebacterium diphtheriae, and the unicellular eukaryotic alga Chlamydomonas. Sulfur granules, another type of inclusion, are found in sulfur bacteria of the genus Thiobacillus; these granules store elemental sulfur, which the bacteria use for metabolism. Occasionally, certain types of inclusions are surrounded by a phospholipid monolayer embedded with protein. Polyhydroxybutyrate (PHB), which can be produced by species of Bacillus and Pseudomonas, is an example of an inclusion that displays this type of monolayer structure. Industrially, PHB has also been used as a source of biodegradable polymers for bioplastics."