In photosynthesis, the suns energy is captured by chlorophyll in the chloroplasts in plant leaves. This energy is used to split water apart in a process called photolysis. The hydrogen from the split is used, along with the energy derived from splitting the hydrogen off the water molecule, to combine with carbon dioxide to form PGAL. PGAL's combine to form glucose, which is why photosynthesis occurs. The oxygen that was split from the water molecule is released. By tagging the oxygen from the water, scientists can trace its' path.