Carbohydrates and Monosaccharides

Study Notes

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 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

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'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+)

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).

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

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.