Tutorial developed by Ross Feldberg, Dept. of Biology, Tufts University
Background on Lipids and Membranes
This tutorial will show you a few structures of molecules that make up the lipid bilayer as well as a representation of a portion of a bilayer (derived from the membrane tutorial by Eric Marz) and a protein (bacteriorhodopsin) that spans a membrane. The goal of this tutorial is to give you a better feeling for how double bonds affect fatty acid structure, how triglycerides assemble to form a bilayer and the properties of a protein that spans a membrane.
Fatty acids are long chain hydrocarbons with a carboxylic acid functional group at one end. As such, the hydrocarbon tail is quite hydrophobic and tends to associate with other hydrocarbon tails, while the acid end is charged and able to interact with water. Stearic acid is a C18 molecule with no double bonds. Fatty acids with no double bonds are said to be "saturated".
Oleic Acid is the C18 acid that has one double bond (between carbons 9-10, numbering from the carboxylate group as 1). Fatty acids containing double bonds are termed "unsaturated" and the double bond introduces a bend in the molecule which interferes with the close packing of adjacent fatty acids. This decrease in van der Waals interactions results in a lipid phase which is less rigid and more fluid (e.g. compare animal fats (solids) with vegetable oils (liquids). This difference is explained by the fact that vegetable fatty acids have more double bonds.
The basic building block of membranes are the glycerophospholipids. These are composed of the three carbon alcohol, glycerol which has two of its alcohol groups esterified to fatty acids while the third alcohol is esterified to a molecule of phosphoric acid. Usually, the phosphoric acid is in turn linked to some other group such as ethanolamine (- CH2CH2NH3+) or choline (CH2CH2N(CH3)3). The structure shown here is 1-stearoyl- 2-oleoyl-3-phosphatidylcholine. The phosphoric acid end of the structure is charged and thus quite polar, while the fatty acid tail portion of the molecule is quite hydrophobic.
This is a view of a slice through a lipid bilayer. Note how the charged phosphate groups (orange) form charged surfaces on both sides of the bilayer which can interact with water while the fatty acids form a hydrophobic core to the membrane which acts as a barrier to polar molecules. Large polymers generally can't get across the membrane, while small polar molecules are carried across by proteins which span the membrane. The polar ends of this membrane carry a negative charge from the phosphate group as well as a positive charge from the choline group -N(CH3)3+. You can see the nirogen if you change the display to wireframe.
Choleserol is a member of the steroid class of lipids. Steroids consist of four fused hydrocarbon rings, three of which are six-membered and one of which is five-membered. Substituents off of this fused ring core determine the specific steroid. Cholesterol has a -OH group at position 3 of ring A, a CH3 at position 10 where rings A and B join, another CH3 at position 13 where rings C and D join and a eight membered hydrocarbon chain extending from the five membered ring D. Cholesterol is an important constituent of membranes and gives rise to the steroid hormones, such as estradiol. The chemical structure drawn flat on a page does not really show the structure of this molecule. The model shown here has the ring system thickened. How do the various methyl groups project with reference to the fused ring system?