Stereochemistry is about the arrangement of atoms in space. Recall that two molecules with identical chemical formulas can still be isomers because the atoms are in a different spatial arrangement. One great example of how the different three-dimensional arrangement of atoms alters the properties of compounds is the similar composition yet completely different nature of starch and cellulose.
Starch and cellulose are carbohydrates which are polymers of monosaccharides or simply sugars. In this case, they are both polymers of glucose. Because they are made of the same molecule, starch and cellulose are stereoisomers.
Starch is contained in large amounts in wheat, potatoes, corn, rice etc. We eat quite a lot of food rich in starch, while cellulose is the most abundant organic polymer on Earth making a large content in cotton fiber (90%), wood (40–50%), and dried hemp is (60%).
So, what makes them so different?
Glucose, the building unit of starch and cellulose, is a six-membered ring that adopts a chair conformation and as a result, it has axial and equatorial groups. The only difference when making the polymer is that the oxygen, in starch, linking the units uses one equatorial and one axial bond while in cellulose it links them by two equatorial bonds. The human body contains an enzyme capable of hydrolyzing the axial bond in starch and making glucose while it leaves the equatorial bonds intact hence we cannot digest cellulose.
Do you know how to draw the chair conformations?
Do you remember stereoisomers such as enantiomers and diastereomers?