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.
