Filamentous protein structures, microtubules, and actin filaments form a cytoskeleton that serves as a framework for the cell's interior. They occur in pairs. Centrioles duplicate prior to cell division and function as part of a microtubule organizing center. Centrioles also give rise to basal bodies that direct the formation of cilia and flagella. Cilia and flagella are hairlike projections of cells responsible for locomotion.
Cellular metabolism includes all the chemical reactions that occur in a cell.
Every cellular reaction requires a specific enzyme. Metabolic pathways require enzymes to catalyze a series of reactions in sequence. Enzymes are named for their substrates and have an active site specific for the substrate. Many enzymes have nonprotein molecules, called coenzymes, to assist them. Some coenzymes, like NAD, remove hydrogen from substrates.