Glucose has two isomers, α-glucose and β-glucose, with structures:…

The idea

Alpha-glucose and beta-glucose are isomers: both are C₆H₁₂O₆ built on the same six-membered ring, and they differ only at carbon 1, where the positions of the hydrogen atom and the hydroxyl group are inverted. In alpha-glucose the hydroxyl group on carbon 1 sits below the ring; in beta-glucose it sits above. That single difference decides which polysaccharide can be built and what it is good for. Joining many glucose units by condensation gives a polysaccharide, with one glycosidic bond formed and one water molecule released per join. With alpha-glucose the hydroxyl groups line up directly, so the chain can coil into a helix or carry branches, giving amylose, amylopectin and glycogen. With beta-glucose the hydroxyl groups only align if alternate molecules are rotated through 180 degrees, so the chain is forced straight and unbranched, giving cellulose.

What earns the marks

• Stating that alpha-glucose and beta-glucose have the same molecular formula but a different arrangement of atoms.
• Identifying the difference as the inverted positions of the hydrogen and hydroxyl groups on carbon atom 1.
• Stating that many glucose units join by condensation, forming glycosidic bonds, to make a polysaccharide.
• Linking alpha-glucose to starch and glycogen and beta-glucose to cellulose.
• Stating that alternate beta-glucose molecules are rotated 180 degrees in the chain.

Common mistakes and how to put them right

• Claiming the two isomers have different molecular formulae or different numbers of carbon atoms.

• Naming the wrong carbon, most often carbon 4, as the one where the groups are inverted.

• Answering 'they are different shapes' with no reference to the hydroxyl group position.

• Stating that beta-glucose cannot form glycosidic bonds at all, rather than needing rotation first.

• Labelling a diagram without indicating which carbon is carbon 1.

Exam tips

• Number the carbons on any glucose diagram before you answer - carbon 1 is the one to the right of the ring oxygen.
• Use the word inverted or reversed about the hydrogen and hydroxyl groups; it is the phrasing the mark scheme rewards.
• Explain the consequence as well as the difference: alpha allows coiling and branching, beta forces a straight chain.