There are multiple levels of metabolic regulation. In intrinsic regulation, the metabolic pathway self-regulates to respond to changes in the levels of substrates or products; for example, a decrease in the amount of product can increase the flux through the pathway to compensate.[101] This type of regulation often involves allosteric regulation of the activities of multiple enzymes in the pathway.[103] Extrinsic control involves a cell in a multicellular organism changing its metabolism in response to signals from other cells. These signals are usually in the form of soluble messengers such as hormones and growth factors and are detected by specific receptors on the cell surface.[104] These signals are then transmitted inside the cell by second messenger systems that often involved the phosphorylation of proteins.[105]