Amphipatic materials aggregate in water to form larger particles, with the polar heads orientated towards the water and the apolar tails gathered together in a waterfree zone. This is called hydrophobic interaction.

Micelles (left), and bilayers (middle) that can also be spherical (middle right), result from these interactions.

Cell membranes are built up out of such an amphipatic phospholipid bilayer. Two aqueous phases, cell content and extra-cellular water, are separated from each other by an apolar layer (fatty acid tails) originating from phospholipids (2) and cholesterol (3). Globular proteins (1), glycoproteins (6) which can contain a helical structure (5) and are bonded to sugar chains (7), and glycolipids (4) protrude through this layer.

The globular proteins are chiefly responsible for the transport of polar compounds through the membrane.