Monosaccharides

Monosaccharides are monomers – single sugars named according to the number of carbon atoms in the molecule. The numbers on the diagrams below show the number given to each carbon atom in the molecules.

All carbohydrates contain the elements carbon, hydrogen and oxygen.

Monosaccharides have the general formula:

(CH2​O)n​ or Cn​(H2​O)n​ or Cn​H2n​On ​where n = the number of atoms

e.g., (CH2​O)6 ​= C6​(H2​O)6 ​= C6​H12​O6​.

Structural isomers

Molecules with the same molecular formula but with different arrangements of their atoms are called structural isomers.

In the diagram, glucose, galactose and fructose all have the same molecular formula, CX6​HX12​OX6​. So, they are structural isomers. However, ribose and deoxyribose have different molecular formulae so cannot be structural isomers:

• ribose, CX5​HX10​OX5​

• deoxyribose, CX5​HX10​OX4​.

Many monosaccharides can exist in straight chain forms or ring forms, and can also show alpha (α) and beta (β) isomerism. The only difference in the alpha (α) and beta (β) ring isomers is the position of the OH group on carbon atom 1.

To remember which ring form is which, use ABBA:

Alpha OH Below – Beta OH Above

Different isomers of glucose are shown in the diagram.

You need to be able to recognise the following hexose sugars.

Fructose has a central ring of four carbon atoms and one oxygen atom, with a CHX2​OH at carbon atoms 1 and 6.

Glucose and galactose all have a central ring of five carbon atoms and one oxygen atom, with a CHX2​OH group at carbon atom 6. They can also exist in alpha and beta forms. Use the table below to tell the difference between the different sugars.