The involvement of phospholipids, cholesterol and proteins is occured in the fluidity of membrane. Lipid packaging and it's components influence the fluidity of the membrane. The fluidity of membrane is crucial for membrane functions. The membrane behaves as two dimensional fluids.
1.2. Membrane Fluidity
The ratio of saturated and unsaturated fatty acids determines the fluidity in the membrane at cold temperature. Factors affecting the fluidity of biomembrane
- Temperature : At low and high temperatures cholesterol increases and decreases the fluidity of membrane respectively. When fluidity increases, it prevent membrane lipids packing close together. Temperature decides what can enter or leave the cell. Cell functions it's best at normal physiological temperature, which is 98.6 degrees Fahrenheit in warm-blooded animals like human.
- Lipid Composition : If saturated fatty acids are compressed by decreasing temperatures, they press in on each other, making a dense rigid membrane. If unsaturated fatty acids are composed, the kinks push adjacent phospholipid molecules away, which helps maintain fluidity in the membrane.
- Different form of fatty acid : Trans form of fatty acid are more packed than cis form of fatty acid. Thus trans fatty acid are solid and cis fatty acid are liquid.
- Cholesterol : Cholesterol serves as a buffer, preventing lower temperatures from inhibiting fluidity and preventing higher temperatures from increasing fluidity. Cholesterol helps to separate the phospholipids in our cell plasma membranes at the high concentrations so that the chains of fatty acids cannot come together and crystalize. So, cholesterol prevents extremes wheather too fluid or too firm in the consistency of the cell membrane.
1.3. Lipid Rafts :
Some micro domains of cholesterol and spingolipids from discrete membrane are known as lipid rafts. These clusters move laterally within the plasma membrane and get associate with specific membrane proteins. Two types of lipid rafts are (1) planar (Non coveolar) (2) Caveolae. Caveolae are flask shaped invagination of plasma membrane that contain caveolin protein. Caveolin is an intrinsic cholesterol binding membrane protein. Although the function of lipid rafts remain to be fully understood, they play an important role in processes such as cell movement (Cholesterol trafficking) and the uptake of extracellular molecules by endocytosis as well as in cell signaling. The lipid composition of a bilayer also influences its thickness which in turn may play a role in localizing proteins to a particular membrane. It is found that spingomyelin associates into a more gel-like & thicker bilayer than phospholipids do. Similarly, cholesterol and other molecules that decrease membrane fluidity increase membrane thickness. Because Spingomyelin tails are already optimally stabilized, the extra addition of cholesterol has no effect on the thickness of a spingomyelin bilayer. Lipid rafts are the centers of organizing for the assembly of signaling molecules, influencing membrane fluidity and membrane protein trafficking and regulating neurotransmission and receptor trafficking lipid rafts are more ordered and tightly packed than it's surrounding stsucture of biomembrane. Lipid rafts are present in plasma membranes and other parts of cells like Lysosomes and Golgi complex.