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With respect to long term energy storage, some energy is stored in cellulose, the other primary poly-carbohydrate plant cells make - but cellulose is primarily a structural molecule produced to maintain the structural integrity of the plant, not an energy storage molecule. Basically they make mono-carbohydrates, don't need them for current energy needs, so they link them together and store them.
With respect to moderate term energy storage, the plant produces mono-carbohydrates like glucose, fructose, sucrose and a whole host of other less common ones. Technically energy is stored in the chemical bonds between atoms and so it is an energy storage form. But these molecules are more an energy transit form - they are stable, soluble and easily transported and are useful for moving energy from one part of the plant (like the leaves, where they are produced) to other parts of plants (like the roots) where they are needed.
And with respect to very short term energy storage, plants produce the usual set of high energy intermediates - ATP, GTP, etc. They do this by breaking down glucose, fructose and other mono-carbohydrates and storing the energy released in one very high energy bond between the ADP molecule and the third phosphate group. Just glycolysis, TCA and electron transport. And they use ATP to shuttle energy around the cell (as opposed to the entire plant) and do stuff like pump molecules against their concentration gradients, signal, catalyze other reactions, etc.
As I said, plants are not really my strong suite and I would be happy if one of the other people in biology expanded on that.
I think I understand and wouldn't want to bother you much since it's not ur specific area of dealing but i'd like to ask though that if the lesser known stable energy molecules don't really store the energy but just transfer the energy from one place to another, wouldn't that mean that they're NOT storage molecules and hence do not store energy after photosynthesis? and essentially the energy [all of it] will be stored in a complex sugar aka starch... right? cos i'm not exactly sure or clear whether the structure of starch includes these lesser known molecules, or they're different structures altogether... :/
It depends how you defined "stored energy."
There is certainly energy stored in the bonds of single carbohydrate monomers like glucose and all the other "-oses." The leaf cells make glucose. In fact, they make far more of it than they could use. The root cells can't make any. So they can get no energy from it. Instead, the leaf cells use the glucose they make as mailing packages for energy which they send to the root and the root then uses.