WAEC Biology 2021 Papers

Concept:The light stage of photosynthesis, also called the light-dependent stage, is driven by light energy and chiefly involves the photolysis of water.Explanation:Light energy is absorbed by chlorophyll and used to split water molecules ($\mathrm{H_2O}$).This splitting is called photolysis of water.It produces hydrogen ions ($\mathrm{H^+}$), electrons, and oxygen gas.The hydrogen ions combine with $\mathrm{NADP^+}$ to form $\mathrm{NADPH}$.During the light stage, ATP is synthesised, not broken down.The reduction of carbon (IV) oxide ($\mathrm{CO_2}$) to carbohydrate and the formation of phosphoglyceric acid both occur in the dark stage of photosynthesis (Calvin cycle).Thus, the process that occurs at the light stage is the splitting of water molecules to form hydrogen ions.Answer:Option A: splitting of water molecules to form hydrogen ions.