What is the basic structural difference …

Starch and cellulose are also composed of the same substance but with different structures. Both starch and cellulose are polysaccharides,that is, both molecules are made up of a lot of sugar molecules. In particular, starch and cellulose are made of the sugar molecule glucose.

Glucose is a type of sugar made of carbon (C), hydrogen (H), and oxygen (O). These elements form a ring with one of the carbons sticking off the end, kind of like a ball of yarn with an unraveled string. There are also alcohol (OH) groups attached to the carbons. There are six carbons numbered from 1 to 6, with the 'unraveled string' hanging off taking the 6th position. Glucose is a 3-D molecule; this means the attached elements can be pointed in different directions at any given time. There are two main forms of glucose: alpha and beta. In alpha glucose, the alcohol attached to carbon 1 is down. In the beta glucose, the alcohol attached to carbon 1 is up.

The structure formed when the molecules share oxygen is determined by which form of glucose is linked together. Starch contains alpha glucose, while cellulose is made of beta glucose. This difference may seem minor, but it plays a big role in the 3-D shape of the molecule. So starch and cellulose have different 3-D structures.