<P> Compared to other classes of proteins, the determination of membrane protein structure s has remained a challenge in large part due to the difficulty in establishing experimental conditions where the correct conformation of the protein in isolation from its native environment is preserved . </P> <P> Membrane proteins perform a variety of functions vital to the survival of organisms: </P> <Ul> <Li> Membrane receptor proteins relay signals between the cell's internal and external environments . </Li> <Li> Transport proteins move molecules and ions across the membrane . They can be categorized according to the Transporter Classification database . </Li> <Li> Membrane enzymes may have many activities, such as oxidoreductase, transferase or hydrolase . </Li> <Li> Cell adhesion molecules allow cells to identify each other and interact . For example, proteins involved in immune response . </Li> </Ul> <Li> Membrane receptor proteins relay signals between the cell's internal and external environments . </Li>

What are the five functions of proteins in the cell membrane
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