<Tr> <Td> Integral proteins or transmembrane proteins </Td> <Td> Span the membrane and have a hydrophilic cytosolic domain, which interacts with internal molecules, a hydrophobic membrane - spanning domain that anchors it within the cell membrane, and a hydrophilic extracellular domain that interacts with external molecules . The hydrophobic domain consists of one, multiple, or a combination of α - helices and β sheet protein motifs . </Td> <Td> Ion channels, proton pumps, G protein - coupled receptor </Td> </Tr> <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>

What are the functions of the parts of the cell membrane