Role of the Cell Membrane
Last, the cell membrane aids in cell communication. Receptors are proteins that are found on the surface of cell membranes or inside the cell. Receptors can serve as either receivers of extracellular signals or activators of intracellular processes. For example, some cells have receptors for hormones that play a role in the regulation of biological processes in which the cell takes part. The binding of the hormone to the receptor changes the activity of the cell. Cells also have receptors that can bind to circulating immune cells, helping the immune system detect the presence of disease-causing organisms or viruses.
Cell Membrane Structure
A carbohydrate is an organic molecule that contains carbon, hydrogen, and oxygen and provides energy to cells. Carbohydrates are commonly found on the exterior surface of the cell membrane. Carbohydrates can form hybrid molecules such as glycoproteins (carbohydrates bound to proteins) and glycolipids (carbohydrates bound to lipids). Glycoproteins and glycolipids are both found in cell membranes, often as receptors for cell signaling. The purpose of the carbohydrate portion of these molecules is to aid in cell-to-cell recognition by binding to complementary structures on adjacent or circulating cells, such as immune cells.
Proteins are found both embedded within the lipid bilayer and associated with the surface of the membrane facing the inside of the cell. Many cell membrane proteins function in cell signaling and in transport of substances across the membrane. Some proteins are involved in the cell shape and structure of tissue by binding to the cell's cytoskeleton, to the extracellular matrix, or to adjacent cells. Membrane proteins can also be enzymes that are involved in cellular processes, particularly in the membranes of the mitochondrion and the chloroplast.
Cell Membrane Proteins
A carrier protein physically binds to a molecule and facilitates its transport across the cell membrane's lipid bilayer. Carrier proteins have a specific binding site for an ion or molecule. When the binding site is occupied, the carrier protein changes shape. The change in shape allows the bound substance to have access to the opposite side of the cell. One example of a carrier protein is the glucose transporter protein, which moves glucose across the cell membrane.
A channel protein, also known as an ion channel, creates a pathway that has a hydrophilic interior for ions (polar molecules) to pass through. The hydrophilic interior of these proteins is exposed to the cytoplasm and extracellular fluids. They have a hydrophilic channel within their core that creates an opening in the cell membrane. Some channels can be opened or closed in response to a stimulus, and these are called gated channels. Gated channels are critical to the functioning of nerve cells by allowing the controlled movement of ions that propagate the signal. Sodium and calcium ion channels are examples of channel proteins.
Fluid Mosaic Model of Cell Membrane Structure
The term mosaic refers to the membrane in which other component molecules such as proteins, cholesterol, and carbohydrates are embedded or with which they are associated. Many of these component molecules can change location within the plane of the membrane. In some cases, the movement of membrane components is driven by specific intracellular and extracellular signals. Certain proteins may also be localized to a particular part of the cell membrane, either for functional or structural reasons. For example, carrier proteins are often grouped together on the membrane to more efficiently transport material across the membrane and reduce the energy needed for this transport.