4.1.3—Role of membrane components

The cell-surface membrane combines a hydrophobic phospholipid barrier with specialised proteins and outward-facing carbohydrate chains. This arrangement separates the cell from its surroundings while allowing controlled transport and specific recognition.

• Selective barrier: Hydrophilic heads face the aqueous surroundings and hydrophobic tails form the bilayer core. Polar molecules and ions cannot cross this core easily, so the lipid arrangement limits unassisted passage.
• Channel proteins: A transmembrane protein can form a hydrophilic pore. The pore provides a route for particular ions or polar molecules to cross by facilitated diffusion when a concentration gradient drives net movement.
• Carrier proteins: A specific solute binds to a carrier, which changes shape and exposes the binding site to the other side. This supports facilitated diffusion when movement is down a concentration gradient; the protein is not an open pore.
• Receptors and glycoproteins: Receptor proteins bind particular signalling molecules. Glycoproteins have carbohydrate chains on the outer surface, which can contribute to receptor function and cell-surface recognition.
• Structure→function boundary: Channel and carrier proteins are transport routes; receptor/glycoproteins are mainly recognition or signalling components. A membrane can therefore be selective without being an impermeable wall.

Because the bilayer core is hydrophobic, it blocks many water-soluble substances unless a suitable protein route is available. A channel offers a hydrophilic passage, whereas a carrier alternates access after specific binding. Receptor binding instead converts an external signal into cell-surface recognition; it is not itself a transport event.

Facilitated diffusion still follows a concentration gradient and does not mean that every polar substance can use every protein. Do not treat a receptor as a channel, or a glycoprotein carbohydrate chain as if it were exposed on the cytoplasmic side.