Does chemical reactions happen between atoms and nucleus of an atom

Inside each atom is a nucleus which is a collection of protons and neutrons linked together by nuclear bonds. Chemical reactions are the making, breaking, and rearranging of bonds between atoms in molecules. Chemical reactions do not change the nuclear structure of any atoms.

The answer is water molecules are split.cellular respiration and photosynthesis are complement  reactions to each other. these are reverse reactions. in cellular respiration glucose and oxygen yield water and carbon dioxide. so, when respiration begins,  glucose molecules are split. in the opposite reaction - in photosynthesis, carbon dioxide and water yield glucose and oxygen. thus,  when respiration begins, water molecules are split.