Cell Membranes

Structure and Composition of Cell Membrane

Role of the Cell Membrane

Cells have a unique cell membrane that serves many functions, including controlling the passage of substances into and out of the cell and cell communication.
Both prokaryotes and eukaryotes have a cell membrane. This cell structure is a semipermeable membrane found outside the cytoplasm, or gel-like interior, of the cell. It is helpful to think of the cell membrane as a boundary that forms between the cell's internal and external environment. The cell membrane has many functions. First, it defines a cell by enclosing cellular contents within the cell.
Despite the difference in structure of prokaryotic and eukaryotic cells, they both have a cell membrane (a semipermeable membrane found outside the cytoplasm of a cell) that surrounds the inner contents of the cells.
Second, it creates a barrier that selectively controls what enters the cell from the outside environment and what substances leave the cell. Because of selective permeability, only specific molecules are able to cross the cell membrane. Essential molecules such as nutrients are able to enter the cell, while waste is able to exit the cell. Molecule size also plays a role in this permeability. Small molecules such as water and oxygen are able to freely cross the cell membrane, but larger molecules such as sugars are regulated.

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

The cell membrane structure is a lipid bilayer with proteins, carbohydrates, and other lipids embedded within.
Cell membranes all have the same basic structure, composed of two facing layers of lipids. However, cells may have a variety of other types of membrane components. Different cell types have unique sets of structures embedded within the basic membrane structure. Some cells may have membrane components found only in that specific cell type. In some cases, it is not the identity of the component but the number of specific components found on a certain type of cell that distinguishes it from other cells. A phospholipid is a lipid molecule composed of two fatty acid tails. Both tails are attached to a phosphate head by a glycerol molecule. Phospholipids create a specific structural design of the cell membrane. Their behavior in aqueous solutions assembles them into bilayers. Within each bilayer, the hydrophilic (water-loving) phospholipid heads that face outside the cell contact the extracellular fluid (watery solution outside cells), and the hydrophilic phospholipid heads that face inside the cell contact the cytoplasm (contents of the cell enclosed by the cell membrane,). The hydrophobic (water-fearing) tails of each layer face each other because the tails are resistant to water.
The cell membrane consists of a phospholipid bilayer that contains both hydrophobic, or water-fearing, fatty acid tails and hydrophilic, or water-loving, heads. The hydrophilic heads are found toward the more aqueous sides of the cell (inside and outside).
Cholesterol is a lipid component found in some cell membranes that associates with the head of phospholipids, and sits between adjacent phospholipids. It contributes to cell membrane fluidity by preventing adjacent fatty acids from getting too close and effectively solidifying. Cholesterol also plays a role in cell signaling by stabilizing the association of signaling molecules with the membrane, allowing them to perform their function.
The lipid bilayer is a thin polar membrane made of two layers of lipid molecules. Many different substances make up the cell membrane and contribute to its selectively permeable nature.
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

Proteins associated with cell membranes are either associated with one side of the membrane or are embedded in it and have several roles in membrane activity.
Proteins of the cell membrane can be either embedded in the membrane or associated with one side of the membrane. A peripheral protein is a protein that is not embedded within the lipid bilayer but is closely associated with the membrane through non-covalent interactions with other membrane components. Peripheral proteins are often involved in cell communication and enzymatic activity. An integral protein is a protein that is embedded within the cell membrane. Some integral proteins are inserted into one side of the membrane but do not go all the way across the membrane. Other integral proteins traverse the entire lipid bilayer and are called transmembrane proteins. Transport proteins are classified as a type of integral protein. There are two types of transport proteins: carrier proteins and channel 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 fluid mosaic model describes the nature of the cell membrane in that the membrane is not rigid and it contains many components.
The fluid mosaic model describes the changeable structure of the cell membrane. The term fluid refers to the nature of the lipid bilayer itself. The phospholipids do not form rigid sheets; rather, the lipids form a flexible film of movable components. This is somewhat similar to a layer of soap bubbles, where each bubble can move in relation to the rest of the bubbles but is still connected to the whole. Because the lipids "flow" past each other, the membrane resembles a fluid.

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.