Final answer:
Membrane proteins can function as hormone or neurotransmitter receptors and as cell-recognition molecules, among other functions.
Step-by-step explanation:
Membrane proteins can function as receptors for hormones or neurotransmitters, acting as signal transducers, or as cell-recognition molecules that bind cells together, among other functions.
For example, receptors for hormones or neurotransmitters, such as the insulin receptor or the dopamine receptor, are membrane proteins that allow cells to respond to specific signals.