What is the purpose of performing a dark reaction? Explain why it is important to include the dark reaction in the PSII activity assay. Solution Dark reactions also called Calvin Benison Cycle, and it occurs in the stroma. This Dark reactions make use of these organic energy molecules (ATP and NADPH) produced in Light reactions. ATP provides the energy while, NADPH provides the protons required to fix the CO2(carbon dioxide) into carbohydrates. PSII activity is important the dark reaction. Because hydrolysis of water is linked with PSII..