Re: how membranes are affected by a gradual change in temperature?

Hi Addy,

The Plasma Membrane (also known as the lipid bilayer, see Figure 1) is a 
dynamic structure and is not static.  It is composed of 2 layers 
(leaflets) of phospholipids with the hydrophobic tails arranged towards 
the middle of the bilayer and the polar head groups arranged towards the 
outside so as to interact with hydrophillic substances such as water.  
Also important to note is that the 2 layers of phospholipids are 
asymmetric, which is to say that there are different phospholipids in the 
inner leaflet than there are in the outer leaflet.  Proteins are found in 
the plasma membrane and are categorized in 3 ways: transmembrane, 
peripheral and surface proteins.  The phospholipids are in constant motion 
at physiological temperatures as are the proteins, but the motion is 
lateral throughout each leaflet.  The probability of a protein or 
phospholipid "flipping" to the other leaflet is low because it is 
energetically unfavorable.  As an aside, there are enzymes called 
flippases which actually do allow phospholipids to "flip" to the other 
leaflet.  The membrane will remain fluid as temperature is decreased until 
a critical temperature is reached at which time the membrane will 
crystallize and solidify.  This critical temperature depends on the 
membrane's fatty acid composition.  For example, a membrane that contains 
a large amount of phospholipids with double bonds in their hydrocarbon 
chains will have a lower crystallization temperature than a membrane which 
contains phospholipids with fewer or no double bonds.  The double bonds 
create a "kink" in the hydrocarbon tails which prevent the phospholipids 
from packing together tightly and thus crystallizing.