cholesterol and unsaturation

[nartnad]

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so if cholesterol, found in the lipid bilayer, causes animal cells to become more RIGID, why is it that hydrocarbons that are UNSATURATED cause increased fluidity and a lower melting point in chemical mixture?

 

[jay47]

so if cholesterol, found in the lipid bilayer, causes animal cells to become more RIGID, why is it that hydrocarbons that are UNSATURATED cause increased fluidity and a lower melting point in chemical mixture?

Actually cholesterol plays a dual role. It maintains plasma membrane rigidity (is that a word?) by binding slightly with the hydrophobic portions of phospholopids as well as the hydrophilic portions of the phospholipids (with its hydroxyl group). This helps to stabilize the phospholipids and keep the membrane stronger. BUT, it also maintaines membrane fluidity by stopping phospholipids from coming together and crystallizing. So it actually has a dual purpose.
http://www.cholesterol-and-health.com/Cholesterol-Cell-Membrane.html

Homeoviscous adaptation, or the modification of the celllar membrane in response to environmental changes (usually temperature) can occur using cholesterol. If temperatures increase, and the membrane becomes too fluid, then cholesterol levels can increase to counter the effects. The opposite can occur if temperatures decrease.

http://findarticles.com/p/articles/mi_qa3746/is_199804/ai_n8804570/pg_5/?tag=content;col1

As far as the melting point question goes, I believe that any impurity in a chemical compound causes decreased melting point because it disrupts the normal crystalline structure of the mixture. I don't think this is not true with saturated hydrocarbons, as I believe any impurity will extend the normal melting point range below and above that of the pure substance/mixture (although a mixture is by definition not pure??....)
http://www.owlnet.rice.edu/~chem217/MeltingPt.html