The fluid-mosaic model of membrane structure Polysaccharide attached to protein forming a glycoprotein Phospholipid bilayer Protein Cholesterol

The structure of the plasma membrane “FLUID” – the molecules can move within the membrane “MOSAIC” – other molecules eg proteins are embedded within the phospholipid bilayer

Fluid Mosaic Model Branching carbohydrate portion of protein which acts as an antigen Glycolipid Hydrophilic heads of phospholipid molecules Protein molecule lying on surface Pore Hydrophilic tails of phospolipid molecules Protein molecule spanning the phospolipid bilayer Protein molecule partly embedded

MEMBRANE PROTEINS Can be: Extrinsic – on the surface eg receptor proteins for hormones, cell recognition molecules (eg glycoproteins)

Or can be: Intrinsic – span the membrane between the outside and inside of the cell eg carrier proteins/channel proteins

Both these types of proteins are globular proteins with a specific shape which only fits molecules with a complementary shape eg insulin receptors only fit insulin, sodium channels only allow sodium through.

Electron micrograph of the plasma membrane of two adjacent epithelial cells glycoprotein coat plasma membrane

PHOSPHOLIPIDS These are polar molecules with hydrophilic (‘water loving’) phosphate heads (PO4-) and hydrophobic (‘water hating’) fatty acid tails.

These arrange themselves in a bilayer with the hydrophilic heads on the outside and the hydrophobic tails on the inside THINK!!!! Why do they arrange themselves this way?

Fluid – Mosaic Model (Singer & Nicholson, 1972) Carbohydrate attached to a lipid : glycolipid Carbohydrate attached to a protein : glycoprotein lipid bilayer hydrophobic regions of protein hydrophilic region of protein hydrophilic channel, acts as a pore through which ions & certain polar molecules may pass EXT INT In this model the lipid bilayer remains unchanged as the unit membrane, but it is regarded as a dynamic structure in which proteins can float in the lipid, some moving about freely, while others are fixed in position.

The phospholipids and proteins regulate the movement of substances through the membrane (membrane is selectively permeable)

The phospholipid molecules allow small, uncharged or lipid-soluble molecules through (O2, CO2, H2O) Larger, charged and water soluble molecules can only cross the membrane via the carrier and channel proteins (e.g. Glucose: C6H12O6 and ions such as Na+, Cl-, K+).