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Defines and illustrates cyclic and straight chain structures of important monosaccharides.

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Photograph of a human brain

Credit: Courtesy of the National Institutes of Health
Source: http://commons.wikimedia.org/wiki/File:Human_brain_NIH.png
License: CC BY-NC 3.0

What’s on your mind?

The brain is a marvelous organ. And it’s a hungry one, too. The major fuel for the brain is the carbohydrate glucose. 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. Something to think about.


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

Credit: Image copyright Elena Schweitzer, 2014
Source: http://www.shutterstock.com
License: CC BY-NC 3.0

Foods that serve as carbohydrate sources.[Figure2]

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 CH2O. 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 sweetners such as corn sugar or grape sugar. Fructose occurs in a great many fruits and is also found 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

Credit: CK-12 Foundation - Joy Sheng
License: CC BY-NC 3.0

Glucose and fructose are monosaccharides, or simple sugars.[Figure3]

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 a six-membered ring, while fructose is a five-membered ring. Both rings contain an oxygen atom.

Cyclic form of monosaccharides glucose and fructose

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

The cyclic form of sugars is the favored form in aqueous solution.[Figure4]

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

Credit: Ribose: User:NEUROtiker/Wikimedia Commons; Deoxyribose: User:Yikrazuul/Wikimedia Commons
Source: Ribose: http://commons.wikimedia.org/wiki/File:Beta-D-Ribofuranose.svg; Deoxyribose: http://commons.wikimedia.org/wiki/File:Deoxyribose_structure.svg
License: CC BY-NC 3.0

Ribose and deoxyribose[Figure5]


  1. What functional groups are on glucose?
  2. What functional groups are on fructose?
  3. Which form is favored in aqueous solution?

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Image Attributions

  1. [1]^ Credit: Courtesy of the National Institutes of Health; Source: http://commons.wikimedia.org/wiki/File:Human_brain_NIH.png; License: CC BY-NC 3.0
  2. [2]^ Credit: Image copyright Elena Schweitzer, 2014; Source: http://www.shutterstock.com; License: CC BY-NC 3.0
  3. [3]^ Credit: CK-12 Foundation - Joy Sheng; License: CC BY-NC 3.0
  4. [4]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0
  5. [5]^ Credit: Ribose: User:NEUROtiker/Wikimedia Commons; Deoxyribose: User:Yikrazuul/Wikimedia Commons; Source: Ribose: http://commons.wikimedia.org/wiki/File:Beta-D-Ribofuranose.svg; Deoxyribose: http://commons.wikimedia.org/wiki/File:Deoxyribose_structure.svg; License: CC BY-NC 3.0

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