What is most of the energy given out as during the complete...

The complete combustion of glucose, which produces CO₂ and H₂O as end products, yields energy most of which is given out as heat. If this energy is to be useful to the cell, it should be able to utilize it to synthesize other molecules that the cell requires. The strategy that the plant cell uses is to catabolize the glucose molecule in such a way that not all the liberated energy goes out as heat.