I had a practice passage today on fluidity and saturated/unsaturated fats. I think I might have mixed up the two so I just want to see if my reasoning is correct or not.
Saturated fats: Higher heat of combustion, no double bonds, more rigid at room temperature (example: animal fat)
Unsaturated Fats: Lower heat of combustion, contains double bonds, a fluid at room temperature (vegetable oil)
The question referred to Phase Transition Temperature (TBR Test 6, Passage 1 Bio section). It said that if the higher the phase transition temperature, the more rigid the molecule.
Does something with a longer bonds tend to be more rigid? And if something has kinks like a unsaturated fatty acid how come its more fluid?
regarding the first question, I think what you are trying to ask is why are LONGER hydrocarbons (not longer bonds) more rigid than shorter chained hydrocarbons and by more rigid you are implying that they have a higher BP?
is this what you are asking?
if so, longer hydrocarbons have a higher boiling point because they have greater surface area and thus there are a greater number of Van der Waal forces within a longer hydrocarbon chain compared to a shorter chain, which results in a higher BP. Although van der waal forces are considered relatively weak compared to the other types of attractive forces (dipole-dipole, H-bonding), they still make a difference with regard boiling point of a hydrocarbon, and thus they affect hydrocarbons that make up biological membranes.
#2) Unsaturated fatty acids have double bonds in the chain which create kinks. Kinks significantly reduce a chains ability to stay linear and fold according, since they prevent free bond rotation around the C-C bond. An example that I used in tutoring that helped students out, which I read in a book was that, you can think of hydrocarbons each as a pencil. Now, say you want to stuff all of these pencils in a box, and that each pencil fits nicely in to one portion of the box, which allows for perfect packing within the box.
well, if a few of the pencils were to be bent (ie, a few hydrocarbons contained double bonds) it would be harder to pack those pencils in that box, because the bending disrupts their ability to nicely position within the box. You can think of the box with a perfect arrangement as a lipid that is solid at room temperature, and a box that does not have a perfect packing arrangement as a lipid that is liquid at room temp.
essentially, double bonds which creates kinks lowers the BP and contribute to membrane fluidity because of a decreased ability of phospholipids to pack next to one another within the membrane. This is why cholesterol is added to lipids in sections where there are a significant number of kinks, to maintain the integrity of the membrane and not allow to be disrupted if there are too many kinds.
these pictures should reinforce what i am talking about:
http://telstar.ote.cmu.edu/biology/MembranePage/images/representation.jpg
http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/LIPIDS/Fig11_1abFattyAcids.GIF
http://www.cytochemistry.net/Cell-biology/cholesterol.jpg
