Membranes and Membrane Transport

Every cell is enclosed by a plasma membrane that separates its interior from the outside world and controls what crosses the boundary. This lesson explains the structure of membranes and the range of mechanisms cells use to move substances across them.

The fluid mosaic model

The accepted model of membrane structure is the fluid mosaic model, proposed by Singer and Nicolson in 1972. The membrane is built from a phospholipid bilayer. Each phospholipid has a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails. In water, phospholipids spontaneously arrange so that the heads face the watery environments inside and outside the cell while the tails point inward, away from water. The result is a stable two-layered sheet.

The membrane is described as