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Chemistry LibreTexts

21.03: Monosaccharides

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    177494
  • The brain is a marvelous organ. And it's a hungry one, too. The major fuel for the brain is glucose—a carbohydrate. The average adult brain represents about \(2\%\) of our body's weight, but uses \(25\%\) of the glucose in the body. Moreover, specific areas of the brain use glucose at different rates. If you are concentrating hard (taking a test, for example), certain parts of the brain need a lot of extra glucose, while other parts of the brain only use their normal amount.

    Monosaccharides

    Some foods that are high in carbohydrates include bread, pasta, and potatoes. Because carbohydrates are easily digested, athletes often rely on carbohydrate-rich foods to enable a high level of performance.

    Carbohydrate food sources
    Figure \(\PageIndex{1}\): Foods that serve as carbohydrate sources.

    The term carbohydrate comes from the fact that the majority contain carbon, hydrogen, and oxygen in a ratio of 1:2:1, making for an empirical formula of \(\ce{CH_2O}\). This is somewhat misleading, because the molecules are not actually hydrates of carbon at all. Carbohydrates are monomers and polymers of aldehydes and ketones that have multiple hydroxyl groups attached.

    Carbohydrates are the most abundant source of energy found in most foods. The simplest carbohydrates, also called simple sugars, are plentiful in fruits. A monosaccharide is a carbohydrate consisting of one sugar unit. Common examples of simple sugars or monosaccharides are glucose and fructose. Both of these monosaccharides are referred to as hexoses, since they have six carbons. Glucose is abundant in many plant sources, and makes up sweeteners such as corn sugar and grape sugar. Fructose is found in many fruits, as well as in honey. These sugars are structural isomers of one another, with the difference being that glucose contains an aldehyde functional group, whereas fructose contains a ketone functional group.

    Linear forms of monosaccharides glucose and fructose
    Figure \(\PageIndex{2}\): Glucose and fructose are monosaccharides, or simple sugars.

    Glucose and fructose are both very soluble in water. In aqueous solution, the predominant forms are not the straight-chain structure shown above. Rather, they adopt a cyclic structure (see figure below). Glucose is six membered ring, while fructose is a five-membered ring. Both rings contain an oxygen atom.

    Cyclic form of monosaccharides glucose and fructose
    Figure \(\PageIndex{3}\): The cyclic form of sugars is the favored form in aqueous solution.

    Another important group of monosaccharides are the pentoses, containing five carbons in the chain. Ribose and deoxyribose are two pentoses that are components of the structures of DNA and RNA.

    Structure of the monosaccharides ribose and deoxyribose
    Figure \(\PageIndex{4}\): Ribose and deoxyribose.

    Summary

    • Carbohydrates are monomers and polymers of aldehydes and ketones that have multiple hydroxyl groups attached.
    • A monosaccharide is a carbohydrate consisting of one sugar unit.
    • Common examples of simple sugars or monosaccharides are glucose and fructose.
    • Another important group of monosaccharides are the pentoses, containing five carbons in the chain; DNA and RNA are partly comprised of pentoses.

    Contributors and Attributions

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.

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