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Carbohydrates get their name from the fact that they are composed of only three types of atoms in particular combinations. The term literally means “carbon and water”, belying their atomic composition: [C(H2O)]n. This means that only carbon, hydrogen, and oxygen are present, and there is usually twice as much hydrogen as oxygen or carbon. In animals, the major function of carbohydrates is to provide energy for the organism. The fundamental carbohydrate subunit is the monosaccharide (see Figure 2.1). Monosaccharides can exist alone, or can be polymerized into larger disaccharides (see Figure 2.2) and polysaccharides.

Monosaccharides: These are the simplest carbohydrate subunits found in nature. Along with disaccharides, they have a sweet taste and have thus been referred to historically and nutritionally as sugars orsimple carbohydrates. Many nutritional monosaccharides are six-carbon compounds, such as glucose (the body’s favorite fuel molecule), fructose (“fruit sugar”), and galactose. Other important monosaccharides are the five-carbon sugars ribose and deoxyribose, part of the nucleotides that compose DNA and RNA.
Disaccharides: Composed of two monosaccharide units joined by a glycosidic bond, these are also recognized as sugars or simple carbohydrates nutritionally. Sucrose, or common table sugar, consists of one glucose and one fructose subunit. Lactose (“milk sugar”) is made up of one glucose and one galactose subunit. Maltose (“malt sugar”) is composed of two glucose subunits. All of these molecules must be broken down into their constituent monosaccharides before they can be utilized by the body.
Polysaccharides: Polysaccharides are made up of many, often hundreds, of monosaccharide subunits. In nature, all of the important polysaccharides are glucose polymers, differing only in their physical arrangement and the type of bonds that join the subunits. Because they do not taste sweet, they are referred to nutritionally as complex carbohydrates, and include such compounds as starch and fiber. Starch is an energy storage molecule found in plants, and often makes up a large part of the human diet in the form of grains and vegetables. Because it is a polymer of glucose, starch is broken down into glucose subunits to be used as fuel in our bodies. Fiber, or cellulose, is a structural polysaccharide, composing the cell walls of plants. Due to the nature of the glycosidic bonds joining the glucose subunits, however, most animals (including humans) are unable to digest it. Glycogen is very similar to starch, and is sometimes referred to as “animal starch”. Animals often store excess glucose in this form in their livers and muscles as an energy reserve.
Glucose, a monosaccharide
Maltose, a disaccharide

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