The membrane is a dynamic structure which is always moving, this is because the phospholipids can move within the plasma membrane with or without a catalyst.

The plasma membrane separates the interior of a cell from its outside environment and can be best represented by the fluid mosaic model. The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components that are fluid and dynamic. The plasma membrane is also composed of a phospholipid bilayer and those phospholipid molecules can move around without the need for a catalyst. For example, a phospholipid can “flip-flop” to the opposite layer. This is known as transbilayer diffusion and is very slow. Another example is when a phospholipid moves side to side within its layer. This is very fast and is known as lateral diffusion.

Phospholipid movement can also be catalyzed using proteins. All catalyzed movements are quite fast. Flippase brings a phospholipid from the outer leaflet to the inner leaflet, and floppase brings a phospholipid from the inner leaflet to the outer leaflet. Both flippase and floppase require ATP. Scramblase brings a phospholipid from the outer leaflet to the inner leaflet AND a phospholipid from the inner leaflet to the outer leaflet. Scramblase does not require ATP.

Key Points

• Phospholipids can be moved between and within their layers without a catalyst (transbilayer and lateral diffusion).

• Phospholipids can also be moved between and within their layers with a protein catalyst such as flippase and floppase along with ATP. Scramblase is a protein catalyst that does not require ATP.

Key Terms

transbilayer diffusion: a phospholipid can “flip-flop” to the opposite layer, is very slow

lateral diffusion: phospholipid moves side to side within its layer, is very fast

flippase: brings a phospholipid from the outer leaflet to the inner leaflet

floppase: brings a phospholipid from the inner leaflet to the outer leaflet

scramblase: brings a phospholipid from the outer leaflet to the inner leaflet AND a phospholipid from the inner leaflet to the outer leaflet

fluid mosaic model: describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins

plasma membrane: the semipermeable barrier that surrounds the cytoplasm (inside contents) of a cell

phospholipids: a major component of cell membranes consisting of two hydrophobic fatty acid tails and a hydrophilic head consisting of a phosphate group