Lipids are a group of organic compounds that can be divided into the subcategories of fats and oils, waxes, phospholipids, and steroids. Most of these compounds are water insoluble (i.e. hydrophobic) due to their lack of polar functional groups. Each are found within living organisms in various quantities, and each serve specific purposes within the human body.
Fats and Oils are both polymers of two units: fatty acids and glycerol.
Fatty acids are long hydrocarbon chains with a carboxyl group on one end. The average fatty acid contains 15 to 18 atoms of carbon, thus making them look like this:
H H H H H H H H H H H H H H H
H-C-C-C=C-C=C-C-C=C-C-C-C-C-C-C-C-O-O-
H H H H H H H H H H H
Incidentally, the carboxyl group is polar, making the fatty acid as a whole hydrophilic, or water soluble. In addition, you will notice double bonds between some of the carbon atoms in the above chain. That means this particular fatty acid is unsaturated, as it is not "filled up" with hydrogen atoms. This is in contrast to "full" or saturated fatty acids that contain no such double bonds and possess a constant ratio of carbon to hydrogen.
The second monomer found in fats and oils is glycerol. Glycerol is a type of sugar, specifically an alcohol containing 3 hydroxyl functional groups. (Hydroxyl groups have the potential to bond covalently with carboxyls through the process of dehydration synthesis.) Glycerol looks like:
H H H
H-C-C-C-H
O O O
H H H
The simplest type of fat is the combination of three fatty acids and one glycerol - a triglyceride. The chemical equation for this combination (for a fully saturated version) is:
3(CH3(CH2)18COOH)+C3H5OH3 → 3(H2O)+ C57H107O6
The resulting molecule looks like:
H H H H H H H H H H H H H H H H H H H H
H-C-O-O-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-H
| H H H H H H H H H H H H H H H H H H H
|
| H H H H H H H H H H H H H H H H H H H
H-C-O-O-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-H
| H H H H H H H H H H H H H H H H H H H
|
| H H H H H H H H H H H H H H H H H H H
H-C-O-O-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-H
H H H H H H H H H H H H H H H H H H H H
Triglycerides, like most fats and oils are neutral because they lack polar groups sticking out. Again, they are water insoluble, e.g. cooking oil will not mix with water.
Unsaturated fats, or oils as they are known, are liquid at room temperature because they have a lower melting point. This is due to the fact that there are double bonds in their hydrocarbons, which create kinks in the chain. This, in turn, prevent close packing of the molecules, making them liquid at normal temperatures. Thus, you can tell a saturated from an unsaturated fat by considering their phase state at room temperature. E.g., butter, Crisco, lard are saturated; olive oil is unsaturated.
Fats are used by organisms primarily as insulation and energy storage. Their structure makes them especially efficient at storing energy; the many carbon-hydrogen bonds are rich in chemical energy. Molecule for molecule, fats are more than 2 times as efficient as glycogen (or animal starch at storing energy. Since there can be no water mixed in with the hydrophobic fat droplets in, for example, a human body, they are even more efficient in terms of heaviness. Weight for weight, fat is more than 6 times as efficient as starch at storing energy!
Waxes are another type of lipid. These occur when a long-chain fatty acid bonds with a long-chain alcohol, creating a rigid molecule that is solid at room temperature. They have relatively high melting points. Waxes are hydrophobic and resistant to degradation. They are used in the human body, for example in the ear where they trap dust and other small particles before they reach the eardrum.
Phospholipids are constructed like neutral fats, but have polar phosphate groups attached to them. In place of the third fatty acid in a triglyceride, phospholipids have a hydrocarbon chain with a phosphate group connecting it to the glycerol and an ionized nitrogen grouping at the other end. While the other two fatty acids are hydrophobic, and form the "tails" of the molecule, this hydrocarbon remains hydrophilic due to this ionized end, thus forming the polar "head" of the phospholipid.
Phospholipids have an interesting property: when placed in water they form a phopholipid bilayer. That is, they line up in a double layer, two molecules thick, with the polar, hydrophilic heads on the outside, and the hydrophobic tails meshing on the inside. The resulting structure is a semipermeable membrane, precisely what is found in every living cell! A bubble of this phospholipid bilayer serves to isolate an exterior solution from the interior.
Exterior
__ __ __ __ __ __ __ __ __ __
Polar heads .../ \/ \/ \/ \/ \/ \/ \/ \/ \/ \...
\__/\__/\__/\__/\__/\__/\__/\__/\__/\__/
| | | | | | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | | | | | |
Nonpolar tails ||||||||||||||||||||||||||||||||||||||||
||||||||||||||||||||||||||||||||||||||||
| | | | | | | | | | | | | | | | | | | |
|_| |_| |_| |_| |_| |_| |_| |_| |_| |_|
Polar heads.../ \/ \/ \/ \/ \/ \/ \/ \/ \/ \...
\__/\__/\__/\__/\__/\__/\__/\__/\__/\__/
Interior
The last major class of lipid found in organisms is the steroid. These molecules have a somewhat different structure, being composed not of carbon chains, but of rings. Specifically, a steroid consists of precisely four fused carbon rings. Their specific structure and function varies according to what functional groups are attached to these rings. The molecule cholesterol is precursor to several important steroids, including the sex hormones that determine human primary and secondary sex characteristics. However, as any nutritionist can tell you, high cholesterol is a Bad Thing. This is actually because this fatty material is deposited on the linings of blood vessels, decreasing blood flow. In addition, cholesterol can become integrated into the phospholipid bilayers of cell membranes, making them rigid. This is detrimental to membrane structure.
In general, lipids are a very important class of organic molecule, and are diverse in both structure and function.
Source:
Mader, S.S. Biology 5th ed. McGrawHill, 1996.