Long distance runners tend to "carb-load" and eat large amounts of bread and pasta the night before a long run. This is because the glucose molecules that are contained within the starch can be used for energy. Cellulose (or fiber) is also made of many glucose molecules, yet this would not be beneficial to eat prior to a long run. Explain why starchy foods are better to eat in this situation compared to cellulose, in terms of their chemical differences.