Lipid

Lipids are a class of natural, organic compounds in plants and animals, defined by a specific way they behave: they are soluble in non-polar 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.

Lipids comprise 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 are derived from living systems of plants, animals, or humans. Substances that are lipids may also be called fat-soluble. For example, this designation is frequently applied to those vitamins in our food that a human can store in body fat . This is contrasted with the vitamins that are not lipid-like, but are instead water soluble. Excesses of the water soluble vitamins are passed in one's urine and must be replaced frequently. The fat-soluble, lipid-like vitamins do not need to be taken daily. (The common known fat-soluble vitamins are vitamins A, D, E, and K.)

Typical lipids

Besides the lipid vitamins that are fat-soluble, hormones , waxes, oils, and many very important substances are also examples of lipids. These examples 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 the least bit electrically polarized. They are non-polar compounds, electrically neutral throughout.

Because there is no structural definition of lipids, the exact definition of a lipid is a bit vague and a few scientists seeking a broad definition will include almost any organic compound that is not water soluble. Many of these can be volatile because their molecules are small. Most scientists do not include mineral oils or waxes obtained from petroleum or paraffin. Instead interest is focused on substances related to living plant and animal biochemistry . And these substances are comprised of large molecules that are non-volatile.

Many lipids are essential to good human health. Some of them 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 lipid-like 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.

Another insight to be gained by understanding the lipid layer around a cell membrane deals with problems associated with pesticides or other lipid-like molecules that get into places other than those intended. The problems arise because many pesticides are lipid-like and may change the way the cell membrane behaves.

Lipids that are found associated with proteins go by the term lipoproteins. Lipids attached to sugars or carbohydrates are called glycolipids. There are also lipids attached to alcohols and some to phosphoric acids. The attachment with other compounds greatly alters the behavior of a lipid, 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 one's intestine and into the blood stream. In the digestion process, lipids are made water soluble by either 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 called 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 non-saponifiable lipids are the steroids and carotenoids. Carotenoids are red or yellow pigments found cells involved in photosynthesis .

Resources

books

Gebo, Sue. What's Left to Eat? New York: McGraw Hill, 1992.

Holme, David J., and Hazel Peck. Analytical Biochemistry. Essex, England: Burnt Mill, Harlow, 1993.

Sullivan, Darryl, and Donald E. Carpenter, eds. Methods of Analysis for Nutritional Labeling. Arlington, VA: AOAC International, 1993.

Donald H. Williams

KEY TERMS

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Enzyme

—Biological molecule, usually a protein, which promotes a biochemical reaction but is not consumed by the reaction.

Metabolism

—The process by which food material is broken down and used in the construction of new material.

Molecule

—The smallest unit of a compound having the properties of the compound. A molecule is made up of more than one atom. Water, H2O, is a molecule composed of three atoms.

Organic

—Substances associated with living systems is the old, but common definition. Now chemists apply it to most compounds that contain carbon atoms, especially rings or chains of carbon atoms.

Polar/non-polar

—Characterized by having opposite ends, as a magnet has a north and south pole. When applied to compounds it means that one end of the molecule, or individual part that comprises the compound, has an abundance of electrical charge and the other end has a shortage of electrical charge. Something that is non-polar is electrically neutral throughout the molecule. The compound has no positive or negative end.

Proteins

—Important nitrogen containing organic compounds that are most easily identified as the building material of a body's meat, skin, and finger nails.

Solubility

—The amount of a material that will dissolve in another material at a given temperature.

Volatile

—Readily able to form a vapor at a relatively low temperature.