Which product of the light-dependent reactions is essential for the progression to the dark reactions?

The product of the light-dependent reactions that is essential for the progression to the dark reactions (also known as the Calvin cycle) is ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).

During the light-dependent reactions of photosynthesis occurring in the thylakoid membranes of chloroplasts, light energy is used to split water molecules, generating oxygen, protons (H⁺), and electrons. These electrons are passed through an electron transport chain, leading to the formation of ATP through chemiosmosis. Simultaneously, another electron carrier, NADP⁺, is reduced to NADPH by accepting electrons and protons.

Both ATP and NADPH serve as energy-rich molecules produced during the light-dependent reactions. These energy carriers, along with the oxygen generated, provide the necessary energy and reducing power required for the Calvin cycle, or the dark reactions, which take place in the stroma of the chloroplast.

During the Calvin cycle, ATP and NADPH are utilized to convert carbon dioxide (CO₂) into carbohydrates like glucose. ATP provides the energy needed for the chemical reactions, while NADPH supplies the reducing power necessary to convert CO₂ into sugars, contributing to the synthesis of organic compounds required for plant growth and metabolism.