What two elements are needed to initiate the light-dependent reactions of photosynthesis?

The light-dependent reactions of photosynthesis primarily require sunlight and water to initiate the process. Sunlight provides the energy needed to excite electrons in the chlorophyll molecules within the thylakoid membranes of chloroplasts. This energy is crucial for the conversion of light energy into chemical energy.

Water serves as an electron donor in these reactions; it is split through a process called photolysis, which releases oxygen as a byproduct and provides protons and electrons necessary for the formation of ATP and NADPH. Thus, sunlight and water work together to drive the essential light-dependent reactions that ultimately lead to the production of energy-rich molecules used in the subsequent light-independent reactions (Calvin cycle).