Monosaccharides
- The monomers of carbohydrates are monosaccharides
- Two monosaccharides can join to form a disaccharide
- Many monosaccharides join to form a polysaccharide
- Monosaccharides can join together via condensation reactions
- The new chemical bond that forms between two monosaccharides is known as a glycosidic bond
- Monosaccharides have the general formula CnH2nOn
- Where 'n' is the number of carbon atoms in the molecule
- Note that this formula only applies to monosaccharides
- Monosaccharide properties include:
- Colourless crystalline molecules
- Soluble in water
- There are different types of monosaccharide formed from molecules with varying numbers of carbon atoms, for example:
- Triose molecules contain 3 carbon atoms, e.g. glyceraldehyde
- Pentose molecules contain 5 carbon atoms, e.g. ribose
- Hexose molecules contain 6 carbon atoms, e.g. glucose
Ribose and glucose structure diagrams
Pentose sugars, such as ribose (top), can be recognised by their five-point carbon rings and hexose sugars, such as glucose (bottom) by their six-point carbon rings
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
- Glucose is the main substrate used in respiration, releasing energy for the production of ATP
- Glucose is produced during photosynthesis
- Glucose exists in two structurally different forms, alpha (α) glucose and beta (β) glucose, these structures are known as the isomers of glucose
- This structural variety results in different functions between carbohydrates
- This seemingly minor example of isomerism has far-reaching consequences on the functions of the polymers
Glucose structure diagrams
The straight chain structure of glucose can form rings of alpha glucose. Glucose also forms rings of beta glucose.
- Different polysaccharides are formed from the two isomers of glucose
- Starch and glycogen are made from molecules of alpha glucose
- Cellulose is made from molecules of beta glucose
Properties of glucose
- Glucose has several properties that are essential to its function in living organisms
- Stable structure due to the presence of covalent bonds which are strong and hard to break
- Soluble in water due to its polar nature
- Easily transportable due to its water solubility
- A source of chemical energy when its covalent bonds are broken
Examiner Tip
You should be able to recognise ring structures of hexose and pentose monosaccharides, and use glucose as an example of a hexose monosaccharide