Biological membrane

A thin, selectively permeable structure, usually composed of lipids and proteins, that encloses cells or compartments within them.

Lipid bilayer: A double layer of amphipathic lipids that forms the basic barrier of most biological membranes. Its hydrophobic interior blocks most water-soluble substances from crossing freely.
Phospholipid: A lipid with a phosphate-containing head and hydrophobic tails that commonly forms biological membranes. Its water-attracting head and water-avoiding tails drive bilayer formation.
Mitochondrial membrane: A membrane enclosing or partitioning a mitochondrion, including its outer and inner membranes. The inner membrane supports the proton gradient used to make ATP.
Cell wall: A rigid or flexible extracellular layer that supports and protects cells in many organisms. Unlike a membrane, a cell wall generally provides structural support rather than the primary selective barrier.
Membrane protein: A protein embedded in or attached to a biological membrane, where it performs transport, signaling, or structural roles. Proteins provide selective pathways and specific functions the lipid bilayer cannot perform alone.
Cholesterol: A sterol found in animal membranes that affects lipid packing and membrane fluidity. It helps tune membrane flexibility and permeability.
Plasma membrane: The membrane that separates a cell’s interior from its external environment. It controls exchange and communication at the cell boundary.
Lipid monolayer: A single layer of amphipathic lipids at an interface between water and a nonpolar phase. It contrasts with the two-leaflet bilayer that forms most cellular membranes.
Membrane fluidity: The ease with which lipids and proteins move within a membrane’s plane. Lateral movement lets membranes reorganize while remaining intact.
Glycolipid: A lipid with a covalently attached carbohydrate group, often exposed on cell surfaces. Its outward-facing sugars contribute to recognition and surface interactions.

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