Final answer:
All options presented are functions performed by membrane proteins except the role to 'serve as enzymes'; while some membrane proteins have enzymatic functions, this is not a universal role for all membrane proteins.
Step-by-step explanation:
The question asks which one of the following is not a function of membrane proteins:
- Attach the membrane to the cytoskeleton
- Form junctions between cells
- Provide cellular identification tags
- Serve as enzymes
All these options are indeed functions of membrane proteins, with one exception that is not typically described as a fundamental role of membrane proteins. While membrane proteins do attach the membrane to the cytoskeleton, form junctions between cells, and provide cellular identification tags (such as glycoproteins part of cell recognition sites), all membrane proteins do not serve as enzymes. It's important to understand that while some membrane proteins can have enzymatic functions, such as those in the electron transport chain, not all membrane proteins serve this function.
Therefore, the function that is not a general role of membrane proteins is to serve as enzymes.