What do proteins do in the cell membrane?
Receptor proteins: receives chemical signals from outside of the cell. This causes some sort of reaction by the cell, such as a change in the cell's electrical activity.
Channel proteins: allow for facilitated diffusion by certain materials down the concentration gradient. A notable example of a channel protein is an aquaporin, which help water diffuse into and out of the cell.
Transport proteins: the main component of active transport. It moves more complex into and out of the cell, typically acting as a sort of "pump". It requires energy.
Glycoproteins: these have many functions. They include: structural (collagen), protection (high weight polymers in epithelial cells), reproduction (increase sperm cell's attraction to egg), cell-to-cell adhesion, functional as hormones, enzymes, carriers (transport), inhibitors, freezing-point depression in antarctic fish, vision (retinal rods), and even are immunologically beneficial.