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lipid bilayer
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A composite diagram shows the structural formulas, space-filling models, and simplified depictions of lipid molecules including stearic acid and phosphatidylcholine, and how these assemble into a micelle and lipid bilayer, with labels identifying hydrophilic heads and hydrophobic tails. Simple diagram of a lipid bilayer showing two layers of phospholipid molecules with round hydrophilic heads facing outward and hydrophobic tails facing inward, illustrating a cellular membrane structure. Detailed diagram of a plasma membrane's lipid bilayer showing two layers of phospholipid molecules with hydrophilic heads facing outward and hydrophobic tails inward, embedded cholesterol, integral (intrinsic) proteins spanning the membrane, peripheral (extrinsic) proteins attached to the surface, and sugar side chains extending from the outer face.
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lipid bilayer

biology
Also known as: bilayer, lipoprotein bilayer
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A composite diagram shows the structural formulas, space-filling models, and simplified depictions of lipid molecules including stearic acid and phosphatidylcholine, and how these assemble into a micelle and lipid bilayer, with labels identifying hydrophilic heads and hydrophobic tails.
Lipid structure Structure and properties of two representative lipids. Both stearic acid (a fatty acid) and phosphatidylcholine (a phospholipid) are composed of chemical groups that form polar “heads” and nonpolar “tails.” The polar heads are hydrophilic, or soluble in water, whereas the nonpolar tails are hydrophobic, or insoluble in water. Lipid molecules of this composition spontaneously form aggregate structures such as micelles and lipid bilayers, with their hydrophilic ends oriented toward the watery medium and their hydrophobic ends shielded from the water.

The lipid bilayer is the fundamental structure of all cell membranes. It's a thin layer, typically 4 to 10 nanometers thick, that separates the cell from its external environment. This bilayer is composed of phospholipids and sterols, which are amphiphilic molecules. This means they have both water-attracting (hydrophilic) and water-repelling (hydrophobic) regions.1234

In the bilayer, the hydrophobic tails of the lipid molecules face inward, away from water, while the hydrophilic heads face outward, interacting with the aqueous environment inside and outside the cell. This arrangement creates a stable, fluid barrier that controls the passage of substances into and out of the cell. While lipid-soluble molecules can pass through, the bilayer effectively repels large, water-soluble molecules and charged ions, necessitating specialized transport proteins for their movement.15234

Summary created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
7 Britannica Sources
  1. cell - Membrane lipids
  2. lipid - Physical characteristics of membranes
  3. cell - Membrane proteins
  4. cell membrane: Top Questions
  5. cell membrane
  6. cell - Permeation
  7. lipid - Glycerophospholipids

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