A cell membrane protects a cell and provides it structural support, but it is still necessary for the cell to interact with its outside environment. Along the surface of the cell, important proteins are arranged that facilitate these functions and help link individual cells to the community of cells that compose the larger organism.
Cell surface proteins are proteins that are embedded in or span the layer of cell membranes of more complex organisms. These proteins are integral to the way in which a cell interacts with the environment around it, including other cells. Some of these proteins, especially ones that are exposed to the external side of the membrane, are called glycoproteins because they have carbohydrates attached to their outer surfaces.
A passive transporter allows solutes to flow into or out of the cell, provided that there is a greater concentration on the other side of the membrane. This protein has a molecular gate that can open and close in controlled ways. An active transport, on the other hand, actively pumps a solute through a channel. This requires an energy input.
A recognition protein can identify other cells as belonging to the tissue and body or as foreign to the body. Communication proteins can form contacts between adjoining cells in order to facilitate cell to cell communications through which signals can flow. An adhesive protein allows cells to stick to other cells or proteins that are part of tissue.
A receptor protein allows communication with substances that serve as signaling molecules such as hormones. These molecules bind with the receptor protein and change the activities within a cell, allowing it to fulfill other functions corresponding to the needs of the organism. Receptor proteins are docked along the outside of the cell.
One of the major activities of many proteins is to catalyze reactions within the cell that would normally take much longer or never happen at all. These proteins are known as enzymes. Enzymes along the cell membrane can catalyze reactions that are related directly to the cell membrane.