Carbon is considered the primary element of life, though in reality, there are four major elements that make up the majority of most biomolecules: carbon, oxygen, nitrogen, and hydrogen. Of these, however, carbon represents the largest proportion of any given biomolecule by weight, and forms the skeleton of the molecules of life.
There are two major reasons that carbon is the backbone element of the molecules of life. First, compared to other elements that form long lasting, stable bonds, carbon has the greatest variety of bonding patterns available to it. Carbon can form bonds to anywhere from two to four other atoms, in a variety of shapes and patterns, which affords great variety to the molecules of life. Second, carbon forms some of the most stable, longest lasting of the chemical bonds, making the molecules of life very tenacious.
Carbohydrates, lipids -- also known as fats -- nucleic acids, and proteins are the four major classes of biomolecules, or molecules that form the structural and functional components of cells. Each of these types of molecules consists of a carbon skeleton, or backbone, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." Generally speaking, carbon isn't responsible for the majority of the chemistry in which a biomolecule engages -- it's more of a scaffold to which the more reactive elements, including nitrogen and oxygen, are bonded.
Importance In Reactions
There are specific cases in which carbon is very important to a molecule's reactivity, however. For instance, carbohydrates all consist of one or more building block units called monosaccharides. In the case of a carbohydrate made up of two or more monosaccharides, the identity of the carbohydrate depends in part upon the individual monosaccharides bonded to one another, and in part upon the orientation of the bond. How one monosaccharide attaches to a particular carbon on another monosaccharide can make the difference between a carbohydrate being digestible, like starch, and indigestible, like fiber.
While your cells reuse and recycle the carbon from many of the nutrient molecules you take in, when you burn a molecule for energy, you eliminate the carbon in the form of carbon dioxide. Still, the carbon serves an important role even on its way out of the body. Carbon dioxide in the blood reacts with water, explains Dr. Lauralee Sherwood in her book "Human Physiology," forming bicarbonate. This compound helps maintain the acidity of the blood.