An acronym for ribulose bisphosphate carboxylase/oxygenase. Photosynthesis is the process of fixing atmospheric carbon dioxide and transforming it into organic carbon. Rubisco is the enzyme which actually does the trick. Specifically, rubisco attaches CO2 to ribulose bisphosphate, a five carbon sugar. It then splits the molecule into two 3-carbon phosphoglycerates which feed into a number of different metabolic pathways. Rubisco is unusually slow and inefficient. It fixes only about three carbon dioxide molecules per second, compared to 1000+ for an average metabolic enzyme. It is also easily confused by other substrates, notably oxygen, and makes a remarkable number of errors. Perhaps the evolved design, as bad as it is, can be no better. Rather than improving the process, plants simply make enormous quantities of enzyme. Rubisco is, in fact, the most common protein on earth. As much as 50% of the mass of each chloroplast is rubisco. Plants and algae build rubisco in compact octamers, with each monomer containing two peptides. The active site contains a magnesium ion bound by three amino acids. One of these is a uniquely modified lysine with an extra carboxyl group added to the end of its side chain. In plant cells, this activator group, is attached to rubisco during the day, turning the enzyme "on," and removed at night, turning the enzyme "off." The exposed side of the magnesium ion binds to both ribulose bisphosphate and the substrate carbon dioxide molecule.