If the reaction between a glucose molecule and oxygen releases energy, what must occur during the reaction in which glucose is produced?
In other words, I think this is asking what do u need to make glucose.
You are also talking about the basic, life-enabling cycle of respiration---->photosynthesis---->respiration
water + carbon dioxide + energy --> glucose + oxygen
Life requires energy; thing tend toward entropy (randomness) in the universe. Life is a case of going against entropy, and requires energy to do so. The energy to do this is basically stored by plants, during the process of photosynthesis, in the complexity of the structure of glucose. When glucose is broken down during respiration, energy is released.
I'm not a chemist, but I think the question contains its own answer. If energy is released later, it must first be stored. Therefore, what must occur during the reaction in which glucose is produced is the storage of energy. That storage would make the reaction in which glucose is produced an endothermic reaction, right? (I loved chemistry in high school and college, but it's been years and years since I've read anything in that area.)
I think, too, that this question alludes to the use of ATP (see the link below) to store and release energy in living cells.
The reaction that you are talking about happens in the second stage of a plant's production of energy. This process is known as the Calvin cycle.
During the Calvin cycle the electrons begin to reactivate. Carbon molecules affix themselves to the glucose. The molecule that is created dissolves in the stroma creating an unstable 6 carbon molecule. The molecule breaks down. It then sends out two molecules which are converted into a 3 carbon molecule which converts into sugar phosphate. The phosphate then breaks the bond and bonds with hydrogen. This creates the molecule glucose.