MEMBRANE FLUIDITY

The membrane consists of a phospholipid bilayer which tends to be packed closely together at low temperatures.

-less molecular motion due to low temp

-van der Waals forces between hydrocarbon tails cause attraction and keep tails closer together

-as temperature is increased, motion increases and the membrane becomes more fluid

Structural features which increase fluidity:

1. Unsaturated fatty acid

-the fatty acid on the central carbon is unsaturated and so has a "kink" in its shape, preventing close alignment of the tails

-cells growing at lower temperatures often have a higher concentratin of unsaturated fatty acids in their membrane lipids

2. Cholesterol

-the cholesterol 'head' aligns itself with the phospholipid heads and the 4 nonpolar rings fit between the hydrocarbon tails of the phospholipids

- this disturbs the orderly packing making the membrane more fluid

-at high temperatures, cholesterol decreases fluidity by preventing too much motion

3. Hydrogen bonding

-the phospholipid heads are held in position by hydrogen bonding with surrounding water

-these bonds are transient, allowing constant motion

Proteins

-many proteins are able to move about the membrane

-some membrane functions require that the proteins be able to move (i.e. hormone receptors cluster together when the associated hormone binds)

-some proteins are anchored in a fixed position by the cytoskeleton (see the fluid mosaic model below)

-the experiment performed by Fry and Edidin demonstrated the fluidity of the membrane (coloured spots are fluorescent antibodies used as markers on the surface of a mouse membrane and on a human cell membrane...see class notes)