Carbohydrates and Monosaccharides

Carbohydrates

• Carbohydrates are molecular compounds made from three elements: carbon, hydrogen, and oxygen
• They can be classified into three types: monosaccharides, disaccharides, and polysaccharides
• Carbohydrates are:
  • A source of energy for the body (e.g. glucose)
  • A store of energy (e.g. starch in plants)
  • Building blocks for polysaccharides (giant carbohydrates)
  • Components of other molecules (e.g. DNA, RNA, glycolipids, glycoproteins, ATP)

Monosaccharides

• Monosaccharides are the simplest carbohydrates and are often called single sugars
• They are the building blocks from which all bigger carbohydrates are made
• They have the general molecular formula (CH2O)n, where n can be 3, 5, or 6
• Monosaccharides can be classified according to the number of carbon atoms in a molecule:
  • Trioses (n = 3), e.g. glyceraldehyde
  • Pentoses (n = 5), e.g. ribose and deoxyribose
  • Hexoses (n = 6), e.g. fructose, glucose, and galactose

Structural Isomers

• Molecules that have the same molecular formula but different structural formulae are called structural isomers
• Examples of structural isomers include molecules with the molecular formula C5H10O5 and C6H12O6

Glyceraldehyde

• Glyceraldehyde's molecular formula is C3H6O3
• Its structural formula shows it contains an aldehyde group (-CHO) and two hydroxyl groups (-OH)
• It can also be classified as an aldose due to the presence of an aldehyde group
• It is a reducing sugar and gives a positive test with Benedict's reagent
• The aldehyde group is oxidised to a carboxylic acid and Benedict's reagent is reduced (Cu2+ to Cu+)

Classification of Monosaccharides

• Glyceraldehyde is a triose, having three carbon atoms in each molecule.
• It is also an aldose, due to the presence of an aldehyde group.

Structural Forms of Monosaccharides

• Pentoses and hexoses can exist in two forms: cyclic and non-cyclic.
• Non-cyclic forms show the presence of an aldehyde group (aldoses) or a ketone group (ketoses).

Examples of Monosaccharides

• Fructose is a ketose, containing a ketone group.
• Ribose and glucose are aldoses, containing an aldehyde group.

Characteristics of Aldoses and Ketoses

• Aldoses are reducing sugars, while ketoses are non-reducing sugars.
• This classification is important in understanding the reaction of sugars with Benedict's reagent.

Representation of Cyclic Forms

• There are three ways to represent the cyclic form: Haworth formula, chair representation, and stereo projection.
• In water, pentoses and hexoses mainly exist in the cyclic form, which is essential for forming larger saccharide molecules.