What do photosynthesis and cellular respiration have to do with the law of conversation of mass and chemical reactions?

In plants, photosynthesis, occurring in chloroplasts, is an anabolic (bond-building) process whereby CO2 and H2O combine with the use of light (photon) energy. This yields O2 and sugar (i.e. glucose). This occurs in 2 phases: light-dependent and dark (Calvin cycle) reactions, which both continually recycle ADP/ATP and NADP/NADPH.
The catabolic (bond-breaking) process in plants is cellular respiration, in which glucose is broken down with O2 by glycolysis (cytoplasm only) and mitochondrial reactions (Krebs cycle and E.T.C.) to yield CO2 and H2O. These reactions recycle ADP/ATP and NAD/NADH. The CO2 and water produced by cellular respiration feed into the photosynthetic processes, and in turn, the O2 and glucose resulting from photosynthesis supply the respiratory reactions.