As we’ve seen, all antibodies have a similar structure and function; enzymes are structurally varied, but all have a catalytic function. In contrast, although all membrane proteins are located at the membrane, they otherwise are both structurally and functionally diverse. As we noted in Chapter 2 and discuss in more detail in Chapter 5, every biological membrane has the same basic phospholipid bilayer structure. Associated with each membrane is a set of membrane proteins that enables the membrane to carry out its distinctive activities (Figure 3-32). The complement of proteins attached to a membrane varies depending on cell type and subcellular location.