2.2.2

Carbohydrates

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Types of Monosaccharides

Carbohydrates are made of carbon, hydrogen and oxygen atoms. They are made from monosaccharides, which are simple sugars containing three to seven carbon atoms.

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Examples of monosaccharides

  • Monosaccharides are simple sugars. Examples include:
    • Glucose.
    • Galactose (found in milk).
    • Fructose (found in fruit).
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Glucose

  • Glucose is a hexose sugar that has the chemical formula C6H12O6.
  • Glucose is an important source of energy in humans.
  • During cellular respiration, the energy released from glucose helps to make adenosine triphosphate (ATP).
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Alpha vs beta glucose

  • Alpha- and beta-glucose are isomers. Isomers have the same molecular formula but a different arrangement of atoms in space.
  • The carbon atoms are numbered from 1 – 6 and the OH (hydroxyl) groups are in a different orientation around C1.
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Hexose vs pentose sugars

  • Glucose is an example of a hexose sugar, as it has six carbons in its structure.
  • Pentose sugars are monosaccharides that have five carbon atoms in their structure, for example, ribose.
    • Ribose is one of the three main components of the nucleotides found in RNA.

Disaccharides and Polysaccharides

When two monosaccharides join via a condensation reaction, they form a disaccharide. When more than two monosaccharides join together, they form a polysaccharide chain.

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Examples of disaccharides

  • Glucose + glucose → maltose.
  • Glucose + fructose → sucrose.
  • Glucose + galactose → lactose.
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Functions of disaccharides

  • Sucrose is common table sugar.
  • Lactose is the sugar found in milk.
    • Lactose intolerance is a common problem where the body is unable to digest lactose.
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Polysaccharides

  • Polysaccharides are made up of two or more monosaccharides joined together by glycosidic bonds.
  • The chain may be branched or unbranched.
  • The chain may contain different types of monosaccharides.
  • Starch, glycogen, cellulose and chitin are examples of polysaccharides.
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Glycosidic bonds

  • Monosaccharides such as glucose can form covalent glycosidic bonds with neighbouring monosaccharide molecules to form carbohydrate polymers known as polysaccharides.
  • -OH groups from neighbouring monosaccharides undergo a condensation reaction to form an O-link between the two monosaccharides, with water being released.
  • To break a glycosidic bond, the reverse reaction occurs during which water is added.
    • This is called a hydrolysis reaction.
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Reducing vs non-reducing sugars

  • Maltose, glucose, and fructose are reducing sugars.
  • Sucrose is a non-reducing sugar.

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