Summary
Highlights
The cell membrane, also called the plasma membrane or plasmalemma, forms the outer protective wall of the cell. It's crucial for maintaining cell shape and size, and it regulates all exchange between the cell's interior and exterior.
The cell membrane is very thin and pliable, approximately 10 nanometers thick. It's primarily composed of lipids (40%), proteins (55%), and carbohydrates (5%).
Lipids are arranged in a bilayer, mainly consisting of phospholipids and cholesterol. Phospholipids have a hydrophilic phosphate head and a hydrophobic lipid tail, arranging themselves in a way that the heads face the water and tails are inside. These molecules move freely, giving the membrane a fluid state, known as the fluid mosaic model. Cholesterol controls the membrane's fluidity, decreasing it at modest concentrations (making it more rigid) and increasing it at high concentrations. The lipid bilayer is semipermeable, allowing lipid-soluble substances to pass through, but not water or large molecules.
Cell membrane proteins are mostly glycoproteins, categorized as integral (tightly integrated with the bilayer) or peripheral (loosely attached). They serve various functions: transport, receptors for signals, second messengers in intracellular signaling, enzymes, adhesion molecules for cell attachment, submembrane cytoskeleton for strength, and antigens.
Most carbohydrates are found in combination with proteins and lipids as glycoproteins and glycolipids, forming an outer coating called the glycocalyx. Functions include making the cell surface electronegative to repel negatively charged objects, facilitating cell-to-cell attachment, acting as receptors, and participating in immune reactions.
The cell membrane's central lipid bilayer, made of phospholipids and cholesterol, is fluid and permeable only to lipid-soluble substances. Proteins attached to it function as transport channels, receptors, enzymes, adhesion molecules, and antigens. Carbohydrates form the glycocalyx, contributing to the cell's negative charge, cell adhesion, receptor functions, and immune responses.