In general, insulin acts to reduce extracellular (including blood plasma) levels of glucose by interacting in some way yet unknown with various cell membranes. In adipose (fatty) tissue it facilitates the cellular uptake of glucose and its subsequent conversion to fatty acids, and it inhibits the breakdown of fatty acids to simpler compounds. In muscle it again facilitates the transport of glucose into cells and in addition stimulates its conversion to glycogen. It also increases protein synthesis in muscle. In the liver, insulin facilitates glucose catabolism and its conversion to glycogen and inhibits its synthesis from simpler compounds.