Question

1. Describe the fluid mosaic model of membrane structure. What experimental methods support this model?
2. With regard to membrane fluidity, how can molecules move within membranes? How CAN’T they move?
3. What types of molecules increase or decrease membrane fluidity?
4. How do alpha-helices allow proteins to pass through membranes?
5. Describe the differences between integral membrane proteins, peripheral membrane proteins and lipid-anchored proteins with regard to how they interact with the membrane.

Answer 1

Describe the fluid mosaic model of membrane structure.

The fluid mosaic model represents the current understanding of the structure of biological membranes. According to this model, the plasma membrane contains a biomolecular lipid bilayer, both surfaces of which are interrupted by protein molecules. The name of the model is derived from the fact that the phospholipids in the membrane are in a fluid state and that proteins are dispersed among the lipid molecules in the membrane forming a mosaic pattern.

Membrane phospholipids have a bilayer of two adjacent layers. In each layer, the phosphate ends of the Lipid molecules extend towards the membrane surface and the fatty acids ends extend inward. The charged phosphate ends of the molecules are hydrophilic and thus can interact with the watery environment. The fatty acid ends consisting largely of nonpolar hydrocarbon chains are hydrophobic and form a barrier between the cell and its environment. Some membrane also contains other lipids called hopanoids.

Interspersed among the liquid molecules are protein molecules. Some proteins are attached at the polar surfaces of the lipid ie, the extrinsive proteins or peripheral proteins while others partially penetrated bilayer or span the membrane entirely to stick out or both sides are called integral proteins or intrinsic proteins or transmembrane proteins.

What experimental methods support this model?

Freeze-fracture membrane useful technique for revealing integral membrane proteins because this method can separate a membrane along the middle of the phospholipid bilayer. Cell fusion experiments support the lateral mobility of membrane proteins. Experiments show that lipids can spin and move laterally but essentially are prevented from moving from one leaflet or layer to another one.