Unit 1.1 Topic 8

The Fluid Mosaic Model.

The term fluid mosaic is used to describe the molecular arrangements in membranes. The main features include:

• a bilayer of phospholipid molecules forming a basic structure.
• various protein molecules floating in the phospholipid bilayer, some completely freely, some bound to other components or to structures within the cell.
• and some (extrinsic) proteins partially embedded in the bilayer on the inside or the outside face; other (intrinsic) proteins completely span the bilayer.

Glycoproteins and glycolipids.

Some of the phospholipid molecules making up the bilayer, and some of the proteins found in the membrane, also have small carbohydrate parts attached to them . Where phospholipid molecules have a carbohydrate part attached they are called glycolipids. Where protein molecules have a carbohydrate part attached they are called glycoproteins.

Membrane components and their roles.

Membrane stability and fluidity:

• Cholesterol gives the membranes of some eukaryotic cells mechanical stability. This steroid molecule fits between fatty acid tails and helps them make the barrier more complete, so substances like water molecules and ions cannot pass easily and directly through the membrane.

Membrane transport functions:

• Channel proteins allow the movement of substances across the membrane. Molecules of sugars such as glucose are too large and too hydrophilic to pass directly through the phospholipid bilayer. Instead they enter and leave the cells through…