What is the relationship between the light and dark reactions of photosynthesis?

Question

My main concern is why are the dark reactions (those that produce hexose) needed? As far as I understand, the sole purpose of photosynthesis is to synthesize carbohydrates, which are only used for energy storage. So, why don’t plants store energy directly as ATP (by photophosphorylation) and not as carbohydrates? (Of course, that would be too bad for other creatures.)Elaborating my question:1. Plants use chlorophyll to convert light energy into chemical energy of ATP and NADPH by photophosphorylation and chemiosmosis.2. Then plants use the ATP and NADPH to synthesize glucose by the Calvin cycle.So, what is the need of step 2? Why don‘t plants just use that ATP and NADPH for their metabolism?---_PS:_ although @David has provided a nice answer to the _current_ form of the question, the selected answer by @christian still remains selected since it answered my _original_ query i.e. _why excess ATP is not feasible for plants_ instead of _why synthesis of carbohydrates is important_. Any further answers/comments shall address the latter one, although none of them can be selected as correct since the latter query is not the original one.

Answer 1

Formation of ATP and NADPH occurs in presence of light this it forms light reaction part of photosynthetic mechanism. As light is not present during night time using these ATP and NADPH directly is not feasible for plants.

Reasons:-

1. The energy of these ATP and NADPH is stored in the form of C-C covalent bond of Glucose. Now these glucose molecules can be used as per requirement by the plants wherever and whenever required.

2. All parts of plant do not perform photosynthesis but requires energy to carry out their metabolic processes.

3. Probably the most important reason that ATP and NADPH is mainly made up of nitrogen and phosphorus which is usually deficient in the soil. In case plant uses ATP NADPH directly it'll require large quantities of N and P.

4. There is no transporter of phosphorylated molecules in the membrane.