<Tr> <Td> Lipid anchored proteins </Td> <Td> Covalently bound to single or multiple lipid molecules; hydrophobically insert into the cell membrane and anchor the protein . The protein itself is not in contact with the membrane . </Td> <Td> G proteins </Td> </Tr> <Tr> <Td> Peripheral proteins </Td> <Td> Attached to integral membrane proteins, or associated with peripheral regions of the lipid bilayer . These proteins tend to have only temporary interactions with biological membranes, and once reacted, the molecule dissociates to carry on its work in the cytoplasm . </Td> <Td> Some enzymes, some hormones </Td> </Tr> <P> The cell membrane has large content of proteins, typically around 50% of membrane volume These proteins are important for cell because they are responsible for various biological activities . Approximately a third of the genes in yeast code specifically for them, and this number is even higher in multicellular organisms . Membrane proteins consist of three main types: Integral proteins, peripheral proteins, and lipid - anchored proteins . </P> <P> As shown in the adjacent table, integral proteins are amphipathic transmembrane proteins . Examples of integral proteins include ion channels, proton pumps, and g - protein coupled receptors . Ion channels allow inorganic ions such as sodium, potassium, calcium, or chlorine to diffuse down their electrochemical gradient across the lipid bilayer through hydrophilic pores across the membrane . The electrical behavior of cells (i.e. nerve cells) are controlled by ion channels . Proton Pumps are protein pumps that are embedded in the lipid bilayer that allow protons to travel through the membrane by transferring from one amino acid side chain to another . Processes such as electron transport and generating ATP use proton pumps . A G - protein coupled receptor is a single polypeptide chain that crosses the lipid bilayer seven times responding to signal molecules (i.e. hormones and neurotransmitters). G - protein coupled receptors are used in processes such as cell to cell signaling, the regulation of the production of cAMP, and the regulation of ion channels . </P>

Two other important functions of the cell membrane