This is the last class of macromolecules. Unlike the previous groups, however, lipids are a chemically diverse group of molecules that all share a common property: they are (relatively) insoluble in water due to their hydrophobicity (and thus soluble in other lipids). While there are several different classes of lipids, we will mention only three major categories.

1. Triglycerides ( or neutral fat)s: This group is better known as the fats. As their name suggests, the triglycerides are composed of one molecule of glycerol and joined via ester bonds with three molecules of fatty acids. As is shown in Figure 12, fatty acids are long chains of carbon and hydrogen usually between 14-24 carbons long (and they always have an even number of carbons). Due to the abundance of C and H, fatty acids are very hydrophobic and are not very soluble in water. Fatty acids come in two major types, saturated and unsaturated. Saturated means that they are "completely saturated" with H and have no double bonds. Unsaturated means just that, not saturated with H and it contains double bonds (see Figure 12). Generally saturated fats have a high melting point so that they are a solid at room temperature. Saturated fats are found in animals. Unsaturated fats have a lower melting point and are a liquid at room temperature. Unsaturated fats are found in plants. Now you should understand why butter (made from animal fat) is a solid while fats from vegetables are oils. Margarine is made by hydrogenating (adding H) to unsaturated vegetable oils increasing the amount of saturation and thus the melting point (so it will be a solid). Just read the label - "partially hydrogenated corn oil."

The primary cellular function of fatty acids is as long term energy storage. Simply, the body will store a small amount of excess nutrients as glycogen, but after a certain point, these nutrients are converted to triglycerides for storage. Triglycerides are efficient energy storing molecules as more energy can be stored in a pound of fat than in a pound of glycogen.

2. Steroids: This second class of lipids has almost nothing in common with triglycerides other than the fact that they are hydrophobic. All steroids are derivatives of a common structure made up of 4 fused carbon rings. Each different steroid contains different chemical groups attached to these rings (cholesterol and cortisol are shown in Figure 13). We are concerned with two major functions of steroids: as a significant part of the structure of cell membranes and as hormones.

The most common steroid is cholesterol. It is found in the cell membrane of cells where it aids in maintaining the correct fluidity of the membrane. It also serves as the precursor molecule for the synthesis of other steroids including bile salts (aids in the digestion of fats) and steroid hormones (such as testosterone, estrogen, progesterone, and cortisol).

3. Phospholipids: This class of lipids are really derivatives of triglycerides. The are composed of a glycerol molecule with two fatty acids (a diglyceride). The third carbon contains a phosphate group and usually some added polar molecule (such as ethanolamine, serine or choline). This adds an interesting dimension to these lipids. Much of the molecule (the diglyceride) is hydrophobic just as in the triglycerides. However, remember that phosphate is highly charged (and don't forget the additional polar molecule) thus part of the molecule is hydrophilic! Any molecule that is hydrophilic at one end and hydrophobic at another is said to be amphipathic.

How do these molecules act in water? Since part of them is hydrophobic, this part will tend to aggregate together as do oils. The hydrophilic (the phosphate group) area, however, "likes" water and will try to dissolve in it. You could imagine that the phospholipids will form small solid spheres in water with the hydrophobic "tails" together in the center of the sphere and the hydrophilic phosphate "head" groups out facing the water. You will find that phospholipids serve as the basis for the structure of all the membranes of the cell.

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