Guillermo was finding it difficult to walk up the steps to his family's apartment. His doctor tested his cells and found that they contained low levels of glucose. Based on these results, the doctor thinks there might be a problem with Guillermo's digestive system. Do you agree or disagree with the doctor? How might a problem with Guillermo's digestive system lead to low levels of glucose in his cells? Why does having low levels of glucose in his cells make it difficult for Guillermo to walk up steps?