Explain the light and dark reactions of photosynthesis.

Photosynthesis occurs in two coordinated stages: the light reactions and the dark reactions (Calvin cycle), each with distinct roles and locations within the chloroplast.

The light reactions take place in the thylakoid membranes and require light energy. Chlorophyll absorbs photons, exciting electrons that move through an electron transport chain. This process leads to the formation of ATP (energy currency) and NADPH (reducing power). During this stage, water undergoes photolysis, releasing oxygen as a byproduct along with protons and electrons.

The dark reactions occur in the stroma of the chloroplast and do not require direct light, but they depend on ATP and NADPH produced in the light reactions. In this phase, carbon dioxide is fixed into organic molecules through a series of enzyme-driven steps known as the Calvin cycle. The key enzyme RuBisCO catalyzes the initial fixation of CO₂. Through reduction and regeneration phases, glucose or other carbohydrates are synthesized.

Both reactions are interdependent. The light reactions provide the energy and reducing power required for carbon fixation, while the dark reactions utilize these products to synthesize organic compounds.

Overall, these two stages work together to convert light energy into chemical energy stored in glucose.