Final answer:
CO2 is stored in the stroma of the chloroplast and utilized in the Calvin cycle, while H2O is housed in the thylakoid lumen where the light-dependent reactions occur.
Step-by-step explanation:
In the process of photosynthesis, carbon dioxide (CO2) and water (H2O) are critical reactants. CO2 enters the plant leaves through openings called stomata and is then diffused into the mesophyll cells, finding its way into the stroma of the chloroplast, which is the site of the light-independent reactions. Water, on the other hand, is absorbed by the roots and transported to the leaves where it is temporarily stored in the inner compartments inside the thylakoids, called the thylakoid space or lumen. The lumen is the site of the first part of photosynthesis, known as the light-dependent reactions, where water is split to release oxygen and provide electrons for the electron transport chain.