Question: why is glucose not a direct source of cellular energy? 

Answer: Glucose is the material that energy is made out of (carb-wise).  Glucose (carbs), tri-glycerides (fats), and amino acids (proteins) all give you energy, and all three of them go through different processes to be turned into Acetyl CoA which then goes through the Krebs Cycle to be turned into Carbon Dioxide, and Hydrogen.  You breathe out the Carbon Dioxide, and the Hydrogen gets shipped (by means of Riboflavin and Niacin) to the Electron Transport Chain.  This handy dandy process leads to the creation of Adenosine Triphosphate (ATP), which IS the direct source of cellular energy.  The cell breaks one phosphate group off the ATP and the energy released from that bond is the energy that runs the body.  Because glucose goes through all those processes, it isn't technically the direct source of energy for the cell.
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