Final answer:
A lack of water in the chloroplast can stop the production of ATP and NADPH as water is the source of electrons needed for the light-dependent reactions of photosynthesis.
Step-by-step explanation:
If the chloroplast has stopped producing ATP and NADPH, one of the key causes could be a lack of water as water is the initial source of electrons in the chloroplast electron transport chain. This electron transport chain is essential for the light-dependent reactions of photosynthesis, where energy from sunlight is used to extract electrons from water molecules at photosystem II, with oxygen as a by-product. The electrons then move through the electron transport chain to photosystem I, where they reduce NADP+ to NADPH. This movement of electrons also creates a proton gradient used by ATP synthase to generate ATP. Therefore, without water, the chloroplast cannot extract electrons required to initiate this process, which results in the cessation of ATP and NADPH production.