plasmalemma

Plasmalemma, commonly known as the plasma membrane, is the biological membrane that encloses the cytoplasm of a cell. It defines the cell’s boundaries, separates the interior from the external environment, and in plants and fungi lies just inside the cell wall. In muscle and nerve tissues, the specialized membrane beneath the muscle fiber or neuron is often referred to as the sarcolemma or plasmalemma.

Structure and composition: The plasmalemma is a phospholipid bilayer with embedded proteins and cholesterol, forming a fluid mosaic. Phospholipids provide a hydrophilic head and hydrophobic tails, producing membrane asymmetry. Integral membrane proteins span the bilayer, while peripheral proteins associate with one face. Carbohydrates are typically attached to lipids or proteins on the extracellular surface, forming the glycocalyx, which participates in recognition and protection. The membrane exhibits lateral mobility of its components and contains microdomains such as lipid rafts that organize signaling molecules.

Functions: It serves as a selective barrier that controls the movement of ions, nutrients, and waste through channels, carriers, and pumps. It participates in signal transduction via membrane receptors and supports cell communication. The plasmalemma enables cell–cell and cell–matrix adhesion through adhesion molecules, and it participates in endocytosis and exocytosis, mediating secretion and uptake. It also maintains cell shape and participates in the generation and propagation of electrical signals in excitable cells through membrane potential.

Variants and notes: In plants, plasmolysis under osmotic stress highlights the boundary between the plasmalemma and the cell wall. The term is also used for specialized membranes in various tissues, reflecting functional adaptations of the same basic structure.