Answer
The seven roles played by membrane proteins are:
Receptors
Second-messenger systems
Enzymes
Channel proteins
Carriers
Cell-identity markers
Cell-adhesion molecules
Work Step by Step
Proteins usually do something.
Receptors - Chemical signals cannot enter a cell, they must bind to a receptor protein on the cell's surface.
Second-messenger systems - When a chemical signal activates a receptor, sometimes the receptor protein produces a second messenger in the cell to carry out the required action. Several proteins may be used in this second-messenger system.
Enzymes - various enzymes are located on the plasma membrane. They can carry out the final steps of starch and protein digestion, help produce second messengers, and break down hormones and signaling molecules.
Channel proteins - These allow for water and hydrophilic solutes to move through the membrane by diffusion. These channels can be open or gated.
Carriers - Carries are transmembrane proteins that bind to larger molecules, like glucose, and actively transport them to the other side of the membrane.
Cell-identity markers - Glycoproteins contribute to the glycocalyx.
Cell-adhesion molecules - These proteins are like Velcro. They help cells adhere to one another and to extracellular material.
