English: A set of three images illustrating the carboxysome, a bacterial organelle involved in carbon fixation. (Left, above) A thin-section electron micrograph of H. neapolitanus cells with carboxysomes inside. In one of the cells shown, arrows highlight the carboxysomes. (Left, below) Purified carboxysomes (material courtesy of S. Heinhorst and G. Cannon) as visualized by cryo-electron microscopy (courtesy of M. Yeager and K. Dryden). (right) A model for the structure of the carboxysome. The shell is composed of several hundred hexameric protein building blocks and 12 pentameric building blocks. RuBisCO, the main interior enzyme is shown packed inside in a regular arrangement for simplicity, though the actual organization of the enzymes inside is not yet understood. The other key enzyme, carbonic anhydrase, which is present in lesser amounts, is not illustrated. Scale bars are 100 nm. (image by T. Yeates).