4.1.2—Membrane component arrangement

A cell-surface membrane contains several components whose positions and chemical properties give the membrane its barrier, transport, stability and recognition functions. The phospholipid bilayer is the base; cholesterol, proteins and carbohydrate-containing molecules add specialised roles.

• Phospholipids: Their hydrophilic heads face the aqueous surroundings and hydrophobic tails face inward, forming the bilayer and a barrier to many water-soluble substances. Individual molecules can move within their monolayer.
• Cholesterol: It fits between phospholipids with its hydrophilic region near the surface and hydrophobic region among the tails. This helps regulate fluidity, limits excessive permeability and adds stability.
• Membrane proteins: Intrinsic proteins are embedded in one or both monolayers; many span the membrane as transmembrane proteins. Extrinsic proteins associate with the inner or outer surface. Transport proteins provide selective routes for particular ions or polar molecules.
• Glycolipids and glycoproteins: Carbohydrate chains project from the outer surface. They can act as receptors and cell markers, supporting specific binding and cell-to-cell recognition.

The hydrophobic phospholipid core blocks many polar substances, while specific transmembrane proteins provide controlled hydrophilic routes for substances that cannot cross the core unaided. Cholesterol tunes how tightly the lipid region behaves, and outward-facing carbohydrate chains expose molecular identities for recognition and receptor binding.

Not every membrane protein is a transport protein, and carbohydrate chains are not distributed equally on both sides: glycolipids and glycoproteins have their carbohydrate chains facing the extracellular side. This component card explains the parts and their jobs; the overall fluid-mosaic definition belongs to the neighbouring model card.