Lipids are a class of natural organic compounds in plants and animals, defined by a specific way they behave: they are soluble in nonpolar solvents. That is, lipids are not soluble in water but dissolve in solvents like gasoline, ether, carbon tetrachloride, or oil. The vast majority of lipids are colorless and mostly fats and oils. They are derived from living systems of plants, animals, or humans.
Lipids are one of the three broad classifications into which nourishing substances can be broken. Lipids, proteins, and carbohydrates are the three very general classifications. Fiber may be filling but is not called nourishment. Lipids are rich in energy, supplying twice the caloric value per unit weight than carbohydrates or proteins. Seeds contain lipids as energy storage substances to get the plant started.
Lipids may also be fat soluble. For example, humans can store some vitamins (A, D, E, and K) in body fat. Other vitamins are water soluble, and excess is passed in urine and must be replaced frequently.
Besides fat-soluble vitamins, hormones, waxes, oils, and many very important substances are also lipids, although they bear little similarity to one another in terms of their chemical formulations. Lipids also vary greatly in their molecular structure. Most lipid molecules are not electrically charged, nor is either end of the compound electrically polarized. They are nonpolar compounds, electrically neutral throughout.
Because there is no structural definition of a lipid, the exact definition is a bit vague, and a broad definition will include almost any organic compound that is not water soluble. Many can be volatile because their molecules are small, although mineral oils or waxes obtained from petroleum or paraffin are not lipids. Instead interest is focused on substances related to living plant and animalbiochemistry. And these substances are comprised of large molecules that are nonvolatile.
Many lipids are essential to good human health. Some serve as chemical messengers in the body. Others serve as ways to store chemical energy. There is a good reason that babies are born with “baby fat.” Seeds contain lipids for the storage of energy. People living in Arctic zones seek fatty foods in their diet.
Fat is a poor conductor of heat, so lipids can also function as an insulator. Their functions are as varied as their structures. But because they are all fat soluble, they all share in the ability to approach and even enter a body cell.
Lipids and cell membranes
Body cells have a membrane that is quite complicated but it can be represented by a double layer of lipids or lipids attached to proteins. Thus the behavior of lipids and lipidlike molecules becomes very important in understanding how a substance may or may not enter a cell. Many biochemical processes that occur in our bodies are becoming better understood as scientists learn more about the lipid-like layer around cells.
The lipid layer around a cell membrane allows pesticides and other lipid-like molecules to get into places other than those intended. This may cause problems because pesticides may change the way the cell membrane behaves.
Lipids associated with proteins are called lipoproteins. Lipids attached to sugars or carbohydrates are called glycolipids. There are also lipids attached to alcohols and some to phosphoric acids. Attachment to other compounds greatly alters a lipid’s behavior, often making one end of the molecule water soluble. Such new substances are bipolar and can become involved in aqueous chemistry. This is important because it allows lipids to move out of the intestine and into the bloodstream. In the digestion process, lipids are made water soluble either by being broken down into smaller parts or becoming bipolar through association with another substance. The breaking down is usually done via two different processes. One is hydrolysis, which means chemical reaction with water, and the other is called saponification.
Metabolism of lipids
The processes by which a lipid is broken down or by which it is built are quite complicated. The liver can convert fats into blood sugar, or glucose. Very specific and very effective enzymes are involved in the many steps of the processes. As a group, these enzymes are called lipases. There is one group of lipids that are not easily broken down. These nonsaponifiable lipids are the steroids and carotenoids—red or yellow pigments found cells involved in photosynthesis.