Important Organic Molecules

Lipids 
Many lipids (lip'idz) are nonpolar, which means that, un­like water, their elecU'ons are evenly distributed. This differ­ence in structure makes lipids insoluble in water. For example, when water and oil are mixed together and set aside, they immediately begin to separate. This is true of fats, the most familiar lipids, such as lard, butter, and oil. In the body, fats act as long-term energy storage. Adipose tis­sue is composed of cells that contain many molecules of fat.
Fats (Triglycerides) 
A fat contains two types of unit molecules: glycerol (glis'er-ol) and fatty acids (fat'e as'idz). Each fatty acid has a hydrocarbon chain (carbon atoms with hydrogens attached) that ends in an acid group. Fatty acids are either saturated or unsaturated. Saturated fatty acids have no double bonds between the carbon atoms. The carbon chain is saturated, so to speak, with all the hydrogens that can be held. Unsaturated fatty acids have double bonds in the carbon chain wherever there is only one hydrogen atom per carbon atom. Unsaturated fatty acids are most often found in vegetable oils and account for the liquid nature of these oils. Vegetable oils are hydrogenated (hydrogens are added) to make margarine. Polyunsaturated margarine contains a large number of unsaturated, or double, bonds.
Glycerol is a compound with H - C - OH attached to three carbon atoms. When fat is formed by dehydration synthesis, the -OH groups react with the acid portions of three fatty acids so that three molecules of water are formed. The reverse of this reaction represents hydrolysis of the fat molecule into its separate components.
Phospholipids 
Phospholipids, as their name implies, contain a phosphate group. Essentially, phospholipids are constructed like fats, except in place of one fatty acid there is a phosphate group, or a grouping that contains both phosphorus and nitrogen. The phosphate group carries a charge and becomes the so­called polar "head" of the molecule. The rest of the mole­cule becomes the nonpolar "tail." The plasma membrane (cell membrane) is a phospholipid double layer of mole­cules in which the polar heads are attracted to and face the water. The nonpolar tails are water repelling and face each other.
Steroids 
Steroids are lipids with a structure that differs entirely from that of fats. Steroid molecules have a backbone of four fused carbon rings, but each one differs primarily by the arrangement of the atoms in the rings and the type of functional groups attached to them. Cholesterol is the precursor of several other steroids, such as al­dosterone, a hormone that helps to regulate the sodium level of blood, and the sex hormones, such as estrogen and testosterone, which help to maintain male and female characteristics.
Evidence has been accumulating for years that a diet high in saturated fats and cholesterol leads to deposits of fatty material inside the lining of blood vessels. These de­posits reduce blood flow and result in circulatory disor­ders. In familial hypercholesterolemia (fah-mne-al hi"per-ko-les-ter-ol-e'me-ah), the individual is unable to remove cholesterol from the bloodstream and suffers from heart attacks.
Lipids include nonpolar fats (long-term, energy-storage molecules that form from glycerol and three fatty acids) and the related phospholipids, which have a charged group. Steroids are lipids with an entirely different structure from that of fats.