Monosaccharides (OCR A Level Biology): Revision Note

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