Phospholipid bilayer

A double layer of phospholipids with water-attracting heads facing the surrounding water and water-repelling tails facing inward. It forms the basic barrier of cell membranes.

Amphipathic molecule: A molecule with distinct water-attracting and water-repelling regions. Each phospholipid’s opposing affinities drive its orientation at the bilayer-water interface.
Phospholipid: A lipid with a phosphate-containing polar head and hydrophobic hydrocarbon tails. Its polar head and nonpolar tails create the bilayer’s defining two-sided arrangement.
Plasma membrane: The membrane surrounding a cell, separating its interior from the external environment. It is the cell’s principal bilayer boundary and controls exchange with its surroundings.
Monolayer: A single molecular layer at an interface or on a surface. A monolayer has one molecular sheet, whereas the bilayer places hydrophobic regions between two leaflets.
Gorter and Grendel experiment: A 1925 experiment comparing extracted red-blood-cell lipids with the cells’ surface area. Its approximate two-to-one area result supported the proposal that membranes contain two lipid layers.
Hydrophobic effect: The tendency of nonpolar substances to cluster in water, reducing disruption of water’s hydrogen-bond network. It pushes phospholipid tails away from water and helps assemble the bilayer.
Fatty acid: A carboxylic acid with a hydrocarbon chain, found in many lipids. Fatty-acid chains form the hydrophobic tails whose length and saturation affect packing.
Organelle membrane: A membrane that surrounds or partitions an organelle within a eukaryotic cell. Bilayers create compartments that maintain distinct chemical environments inside cells.
Lipid monolayer: A single layer of lipids arranged at an interface, with polar heads and hydrophobic tails oriented by the adjoining phases. It forms at oil-water or air-water interfaces rather than enclosing an aqueous compartment as a bilayer does.
Evert Gorter: A Dutch physician and physiologist who studied red blood cells and membrane structure. He co-authored the area-comparison experiment that helped establish the bilayer model.

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