There are many different types of proteins positioned throughout the cell membrane, with each serving a different function. They are responsible not only for transporting nutrients and ions across the cell membrane, but also for carrying out a wide variety of other processes essential to cell functioning. The proteins found in the cell membrane can be classified into categories based on functional class, including channel proteins, transport proteins, recognition proteins, receptor proteins, junction proteins, and enzymes.
Various proteins move through the phospholipid barrier using ion channels or pumps. Their passage through the phospholipid bilayer is determined by the protein and its function.
- Channel protein: These proteins act as tiny pores in the cell membrane, allowing only a few specific molecules, such as ions and water, to pass through the membrane. They may be gated or non-gated, and gated channel proteins need a signal to open for the passage of water and ions. Channel proteins, such as actin and myosin, are activated by an electrical signal from a neurotransmitter, such as acetylcholine.
- Carrier protein: Carrier, or transport, proteins assist in the movement of larger molecules, such as amino acids and glucose, through the cell membrane. They bind to the molecules in a process called active transport. Some carrier proteins transport only one specific type of molecule, while others may carry two ions in one direction or two ions moving in different directions.
- Cell recognition protein: A cell recognition protein is a glycoprotein, a carbohydrate bonded by a protein, that extends outward from the cell membrane and is responsible for distinguishing one type of cell from another.
- Receptor protein: These proteins are found on the outer cell surface and allow specific molecules to bind to them. Once bound, the protein changes its shape, causing a cellular response.
- Enzymatic protein: Enzymes are special proteins that participate in metabolic reactions.
- Junction protein: Junction proteins assist in cell-to-cell adhesion and communication.