In a transverse wave, this is the magnitude of height of a wave from origin to crest. in a longitudinal wave, this is corresponds to how much the wave is compressed, as compared to areas of little compression.

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.