Answer
Glucose breaks apart over time so it is important that it be converted to a more stable form for later use by the cell. Conversion of a less stable form into a more stable form (glycogen) requires an input of energy.
Work Step by Step
Ordered energy sources like glucose have a tendency towards entropy; they tend to break apart. Energy would be needed to put it back together. The analogy used in the book is that of a clean room. A clean room quickly becomes messy and energy is needed to clean it back up again.
