What is/are used to convert ADP into energy-rich compound ATP by adding one inorganic phosphate?
Electrons
Adenosine diphosphate (ADP) is converted into Adenosine triphosphate (ATP) by addition of one inorganic phosphate group. This addition requires a high amount of energy. This energy is incorporated in terms of electrons.
When the photosystems containing chlorophyll molecules absorb light energy, they get excited to higher levels. But at higher levels, they are highly unstable and so quickly come down to stable levels by giving off that extra energy in the form of electrons. These electrons ultimately are captured by ADP and addition of inorganic phosphate becomes possible to get ATP.