Cholesterol's Hydrocarbon Tail
Cholesterol's Tail is a Hydrocarbon
Like the steroid ring structure, cholesterol's hydrocarbon tail is made up entirely of hydrogen and carbon.
The Hydrocarbon Tail is Non-Polar
In the lesson on the hydroxyl group, you learned that the electrons in a bond between oxygen and hydrogen are more attracted to the oxygen. The electrons behave like a cloud surrounding the two atoms, but the cloud is more dense towards the oxygen.
Another way of saying this is that there is a significant difference in electronegativity between hydrogen and oxygen.
The oxygen is more electronegative, pulling the electrons toward itself, so, although the hydroxyl group as a whole is electrically neutral, the side toward the oxygen has a slight negative charge and the side toward the hydrogen has a slight positive charge.
This is not the case in the hydrocarbon tail. The electronegativity of hydrogen and carbon are very similar, so the electron cloud is distributed evenly over the two atoms.
Carbon-hydrogen bonds are said to be non-polar because they do not have positive and negative poles within themselves.
Polar Molecules are Water-Soluble — Non-Polar Molecules are Fat-Soluble
While polar groups like the hydroxyl group and polar molecules like water can make relatively strong intermolecular bonds (although these bonds are much weak than the covalent intramolecular bonds) with each other because of the attraction between the partial positive and negative charges, non-polar groups and non-polar molecules form very weak attractions between each other.
For this reason, polar molecules dissolve in other polar molecules, and non-polar molecules dissolve in other non-polar molecules. This results in a division between water-soluble molecules (polar), and fat-soluble molecules (non-polar).
The Process of Dissolving
This might not be immediately obvious. Consider what it takes for something to dissolve in water:
Water molecules form large clusters. Most things that will dissolve in water are clustered together in the form of a crystal — salt, for example. In order for salt to dissolve in water, three things must happen:
The bonds between the sodium and chloride ions must be broken
The bonds holding the clusters of water molecules must be broken
Bonds must be formed between water molecules and the sodium and chloride ions, so that each ion is enveloped in its own "cage" of water molecules.
Remember that the attractive forces between polar molecules are strong, and that the attractive forces between non-polar molecules are weak. In order to break the strong, polar bonds between water molecules, the molecule or ion that is trying to dissolve must be either a polar molecule or a charged ion, both of which have strong attractive forces.
Dissolving oil, on the other hand, into water, will not work because the weak, non-polar bonds between fat molecules do not possess strong enough attractive forces to break up the water molecules. This is true even though the forces within the oil are weak and easy to break apart, because the oil will only break apart if it has something else to bond to, and water, hiding in its own cluster, is unavailable.
But dissolving two fat-soluble substances together will work, because, even though neither substance exerts a particular strong force in breaking apart the other, the forces holding each together are so weak that they break apart readily.
Cholesterol is Amphipathic
Since cholesterol contains a polar hydroxyl group, on the one hand, and a non-polar steroid ring structure and hydrocarbon tail, on the other, it has both a water-soluble region and a fat-soluble region.
Molecules that have both water-soluble and fat-solbule properties are called amphipathic. Cholesterol's properties as an amphipathic molecule are important to its function in the human body.
Although cholesterol contains a water-soluble portion, however, it still is not water-soluble enough to dissolve in the blood. It must therefore be transported in lipoproteins. To learn about lipoproteins, continue on to the next lesson.