Monosaccharides (Edexcel A (SNAB) A Level Biology)

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Cara Head

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Cara Head

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Monosaccharides: Structure

  • Carbohydrates are one of the main carbon-based compounds in living organisms
  • All molecules in this group contain C, H and O
    • Carbon atoms are key to the structure of organic compounds because
      • Each carbon atom can form covalent bonds; this makes the compounds very stable
        • Covalent bonds are so strong they require a large input of energy to break them
      • Carbon atoms can form covalent bonds with oxygen, nitrogen and sulfur
      • Carbon atoms can bond to form straight chains, branched chains, or rings
  • Carbon compounds can form small, single subunits, or monomers, that bond with many repeating subunits to form large molecules, or polymers
    • This is a process called polymerisation
  • The three types of carbohydrates are monosaccharides, disaccharides, and polysaccharides

Monosaccharides

  • Monosaccharides are the monomers of carbohydrate; they can join together to make carbohydrate polymers
    • Monosaccharides are simple carbohydrates
    • Monosaccharides are sugars
  • There are different types of monosaccharide formed from molecules with varying numbers of carbon (C) atoms, for example
    • Triose (3C) eg. glyceraldehyde
    • Pentose (5C) eg. ribose
    • Hexose (6C) eg. glucose
  • Glucose is a well known example of a monosaccharide
    • Glucose is a hexose sugar
    • The six carbons that make up glucose form a ring structure
      • Carbons 1-5 form a ring, while carbon 6 sticks out above the ring
  • Glucose comes in two forms; alpha (alpha) and beta (beta)
    • The forms of glucose are almost identical; they differ only in the location of the H and OH groups attached to carbon 1
      • Alpha glucose has the H above carbon 1 and the OH group below
        • Remember = alpha has the H above
      • Beta glucose has the H below carbon 1 and the OH group above
        • Remember = beta has the H below

Structure of α-D-glucose

Structure of β-D-glucose

Alpha glucose (top) has the hydrogen above carbon 1 and the OH group below, while beta glucose (bottom) has the hydrogen below carbon 1 and the OH group above

Monosaccharides: Function

  • The main function of monosaccharides is to store energy within their bonds
    • When the bonds are broken during respiration, energy is released
  • The structure of glucose is related to its function as the main energy store for animals and plants 
    • It is soluble so can be transported easily
    • It has many covalent bonds which store energy
  • Monosaccharides can combine through condensation reactions to form larger carbohydrates
  • Some monosaccharides are used to form long, structural fibers, which can be used as cellular support in some cell types

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding