Because photosynthesis is a source of oxygen, it seems intuitive that photosynthesis evolved first: once enough O2 was in the air, then respiration would be able to arise in the newly oxygen-enriched atmosphere. However, some biologists have argued since the 1970's that respiration in fact evolved first.
The process of photosynthesis is energy-storing because the process converts light energy into chemical energy, which is stored in the bonds of glucose.
