OCR A Level Biology:复习笔记2.2.3 Monosaccharides


  • Sugars can be classified as reducing or non-reducing; this classification is dependent on their ability to donate electrons
  • Reducing sugars can donate electrons (the carbonyl group becomes oxidised), the sugars become the reducing agent
    • Thus reducing sugars can be detected using Benedict’s test as they reduce the soluble copper sulphate to insoluble brick-red copper oxide


  •  Examples of reducing sugars include: glucose, fructose and galactose
    • Fructose and galactose have the same molecular formula as glucose however they have a different structural formula
    • The different arrangement of atoms in these monosaccharides gives them slightly different properties


  • Non-reducing sugars cannot donate electrons, therefore they cannot be oxidised
    • To be detected non-reducing sugars must first be hydrolysed to break the disaccharide into its two monosaccharides before a Benedict’s test can be carried out
    • Example: sucrose


The mnemonic to remember the definitions for oxidation and reduction


  • There are different types of monosaccharide formed from molecules with varying numbers of carbon atoms, for example:
    • Trioses (3C) eg. glyceraldehyde
    • Pentoses (5C) eg. ribose
    • Hexoses (6C) eg. glucose


  • The most well-known carbohydrate monomer is glucose
  • Glucose has the molecular formula C6H12O6
  • Glucose is the most common monosaccharide and is of central importance to most forms of life
    • The main function of glucose is as an energy source
    • It is the main substrate used in respiration, releasing energy for the production of ATP
    • Glucose is soluble and so can be transported in water


  • Glucose exists in two structurally different forms – alpha (α) glucose and beta (β) glucose and is therefore known as an isomer
    • This structural variety results in different functions between carbohydrates


Straight chain and ring structural formula of alpha & beta glucose


  • Different polysaccharides are formed from the two isomers of glucose

Structure of Polysaccharides Table


Ribose and Deoxyribose

  • Sugars that contain five carbon molecules are described as pentose sugars
  • Ribose and deoxyribose are important pentose sugars found in the nucleotides that make up RNA and DNA
  • Ribose and deoxyribose are very similar in terms of structure
    • Deoxyribose has lost one oxygen atom at carbon number 2


The structural formula of ribose and deoxyribose

Exam Tip

Become familiar with the OILRIG mnemonic to remember what happens to a molecule when electrons are lost from it (oxidation) or gained by it (reduction).You must be able to recognise and draw the isomers of α and β glucose.