Cell Membrane

The cell membrane, also known as the plasma membrane or cytoplasmic membrane, is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. The basic function of the cell membrane is to protect the cell from its surroundings. It consists of the phospholipid bilayer with embedded proteins. The membrane also contains lipid rafts and is connected to the cytoskeleton.

Structure

The cell membrane is primarily composed of a mix of proteins and lipids. Depending on the membrane's location and role in the body, lipids can account for anywhere between 20 to 80 percent of the membrane, with the remainder being proteins. The lipids give the membrane its fluid properties, while proteins help to maintain its chemical climate and assist in transferring molecules across the membrane.

Phospholipid Bilayer

The most striking feature of the cell membrane is its phospholipid bilayer. Phospholipids are a type of lipid molecule that form two layers. Each molecule has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails. The hydrophobic tails of phospholipid molecules are repelled by water and, in the bilayer, face inward, shielded from water, while the hydrophilic heads face outward, where they interact with water.

Proteins

Proteins embedded within the phospholipid bilayer carry out the specific functions of the cell membrane, including selective transport of molecules and cell-to-cell communication. The proteins are classified as integral (or intrinsic) and peripheral (or extrinsic) proteins. Integral proteins span the membrane and can form channels through which certain substances can pass. Peripheral proteins are attached to either the outer or inner surface of the membrane and are typically involved in signaling or maintaining the cell's structure.

Function

The primary function of the cell membrane is to protect the cell from its surroundings. It also has the following functions:

• Transportation: The cell membrane controls the movement of substances in and out of cells and organelles. In this way, it is selectively permeable to ions and organic molecules.
• Receptors: Membrane proteins often serve as receptors and provide binding sites for hormones or other trigger molecules.
• Enzymatic activity: Membrane proteins can have enzymatic activity, performing specific biochemical reactions at the membrane surface.
• Cell signaling: The cell membrane plays a crucial role in cell signaling and communication, often through receptor proteins.

Clinical Significance

Alterations in the cell membrane's structure or function can result in various diseases, including cystic fibrosis, an inherited disorder that severely affects breathing and digestion, and Type 2 diabetes, which involves insulin resistance.

See Also

• Cell biology
• Lipid bilayer
• Membrane biology