Structure of Plasma Membrane

The fluid mosaic model

The plasma membrane is the thin boundary, only about seven nanometres thick, that encloses every living cell and separates the cytoplasm from the outside environment. Biologists describe it using the fluid mosaic model. It is called fluid because the molecules can drift sideways within the layer, and mosaic because many different molecules are scattered throughout it like tiles in a mosaic picture. Its main jobs are to control which substances enter and leave the cell and to allow cells to recognise one another.

The phospholipid bilayer

The main framework is a double layer of phospholipid molecules. Each phospholipid has a hydrophilic (water-loving) phosphate head and two hydrophobic (water-hating) fatty acid tails. In watery surroundings the heads point outward toward the water while the tails tuck inward, away from water. This arrangement forms a stable barrier that lets few water-soluble substances pass freely.

Membrane proteins and other molecules

Proteins are embedded in or attached to the bilayer:

• Carrier proteins change shape to move specific particles across the membrane.
• Channel proteins form water-filled pores for ions and water.
• Receptor proteins recognise signalling molecules such as hormones.

Cholesterol molecules sit between the phospholipids and control how fluid the membrane is, keeping it stable across a range of temperatures. Short carbohydrate chains attach to proteins to form glycoproteins and to lipids to form glycolipids; these act as cell markers for recognition, for example allowing the immune system to tell the body's own cells from invaders. Because the tails avoid water, the bilayer also seals itself if it is torn.