2.8–2.11: Carbohydrates are fuel for living machines.

Worldwide, more corn is produced (by weight) than any other grain.

Hardly a day goes by without an item in a magazine or newspaper or on TV talking about whether carbohydrates are good or bad for us. Sports drinks such as Gatorade are filled with carbs, and in the days before a big game or race, athletes often “carbo-load” by eating bowls of pasta. Yet other dietary supplements and “power” bars tout that they are low-carb and are effective in weight loss by causing the body to resort to using fat for energy. Meanwhile, nutritionists, doctors, and many diet programs exhort people to increase the amount of fiber—another type of carbohydrate—in their diet. What exactly are they all talking about?

Carbohydrates are one type of macromolecule—a large molecule made up from smaller building blocks or subunits. Four types of macromolecule are essential to the building and functioning of living organisms: carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates are molecules that contain carbon, hydrogen, and oxygen: they are the primary fuel for running all of the cellular machinery and also form much of the structure of cells in all life forms. Sometimes they contain atoms of other elements, but they must have carbon, hydrogen, and oxygen to be considered a carbohydrate (FIGURE 2-20). Additionally, a carbohydrate generally has approximately the same number of carbon atoms as it does H₂O units. For instance, the best-known carbohydrate, glucose, has the composition C₆H₁₂O₆ (6 carbons and, as a little math will show, 6 H₂O units; notice that 6 × H₂ = H₁₂ and 6 × O = O₆). A carbohydrate called maltose has the composition C₁₂H₂₂O₁₁.

Figure 2.20: All carbohydrates have a similar structure and function.

Carbohydrates are classified into several categories, based on their size and their composition. The simplest carbohydrates are the monosaccharides, or simple sugars. The simple sugars contain anywhere from three to six carbon atoms and, when they are broken down, the products usually are not carbohydrates. Two common monosaccharides are glucose, found in the sap and fruit of many plants, and fructose, found primarily in fruits and vegetables, as well as in honey. Fructose is the sweetest of all naturally occurring sugars. The suffix -ose tells us that a substance is a carbohydrate.

Carbohydrates function well as fuels because of their many carbon-hydrogen bonds; as those bonds are broken down and other, more stable bonds (primarily between carbon and oxygen) are formed, a great deal of energy is released that organisms can use.

In the next section we investigate glucose, the chief carbohydrate used by organisms to fuel their activities.