Explanation

The Calvin cycle proceeds in three stages (1) carboxylation, during which $CO_{2}$ combines with ribulose 1, 5-bisphosphate; (2) reduction, during which carbohydrate are formed at the expense of the photochemically made ATP and NADPH; and (3) regeneration during, $CO_{2}$ acceptor is ribulose 1, 5-bisphosphate is formed again so that the cycle continues. Regeneration of the ${CO}_{2}$ acceptor molecule, RuBP is crucial if the cycle is to continue uninterrupted. The regeneration steps require one ATP for phosphorylation to form RuBP