Guillermo was finding it difficult to walk up the steps to his family's apartment. His doctors think that his cells might not be getting enough oxygen or glucose. why does having low levels of glucose or oxygen in his cells make it difficult for Guillermo to walk up the stairs

Oxygen and glucose are both essential for energy production

Glycolysis is the break down of sugar with or without the presence of oxygen to provide energy to our cells. The energy for the cell is in the form of ATP. In the presence of oxygen the process is much more efficient and is the last step in oxidative phosphorylation. In this process the Oxygen will pick up the protons that have driven the proton motif force that was used to turn the ATPase that was used to make ATP. The oxygen picks up these protons and keeps the cell pH stable and this is called metabolic water. The Dr should check the blood for the presence of both glucose and oxygen.