2.10: Many complex carbohydrates are time-release packets of energy.

In contrast to the simple sugars, some carbohydrates contain more than one sugar unit or building block. When two simple sugars are bonded together they form a disaccharide; the joining of glucose and galactose, for example, makes the disaccharide lactose, which is the sugar found in milk. Much larger numbers of simple sugars may be joined together—sometimes as many as 10,000. In this case, the resulting carbohydrate is called a polysaccharide, or a complex carbohydrate (FIGURE 2-24). Depending on how the simple sugars are bonded together, polysaccharides may function as “time-release” stores of energy or as structural materials that may be completely indigestible by most animals. An example of such a structural material is the polysaccharide cellulose—the primary component of plant cell walls.

Like simple sugars, many disaccharides and polysaccharides are important sources of fuel for cells. Unlike simple sugars, however, disaccharides and polysaccharides must undergo some preliminary processing before the energy can be released from their bonds. Let’s look at what happens when we eat sucrose, common table sugar. Sucrose is the primary carbohydrate in plant sap and is composed of two simple sugars, glucose and fructose, linked together. To use sucrose, the body must first break the bond linking the glucose and fructose. Only then can the individual monosaccharides be broken down into their component atoms and the energy that was stored in their chemical bonds be harvested and used.

In plants, the primary form of energy storage is a complex carbohydrate called starch, found in roots and other tissues (see Figure 2-24). Starch consists of a hundred or more glucose molecules joined together in a line. Grains such as barley, wheat, and rye are high in starch content, and corn and rice are more than 70% starch. Although it is composed of glucose molecules linked together, starch does not taste sweet. Because of its molecular shape, it does not stimulate the sweetness receptors on the tongue. The glycogen that stores energy in your muscles and liver is also a complex carbohydrate, so it is sometimes referred to as “animal starch” (although it has a more branched structure than starch and carries more glucose units linked together).

Many complex carbohydrates are like “time-release” fuel pellets. The glucose molecules become available slowly, as the bonds between glucose units are broken. As a demonstration, put a piece of potato or bread into your mouth and let it rest on your tongue. Initially, it does not taste sweet. After a few minutes, you will begin to taste some sweetness as the chemicals in your saliva break the starch down into glucose.

The relative amounts of complex carbohydrates and simple sugars in foods cause them to have different effects when you eat them. Oatmeal (along with rice and pasta), for example, is rich in complex carbohydrates. Fresh fruits, on the other hand, are rich in simple sugars, such as fructose. Consequently, although fruit will give a quick burst of energy as the sugars are almost immediately available, the fuel will soon be gone from the bloodstream. Simple sugars in the oatmeal will become available only gradually, as the complex carbohydrates of the oats are broken down into their simple sugar components (FIGURE 2-25).