Final answer:
Substance A in photosynthesis refers to the reactant carbon dioxide (CO2), which is converted into glucose along with water using light energy from the sun.
Step-by-step explanation:
Substance A in photosynthesis refers to the reactant carbon dioxide (CO2), which plays a crucial role in the process.
In photosynthesis, carbon dioxide is converted into glucose (C6H12O6) along with water (H2O) using light energy from the sun. The chemical equation for photosynthesis is 6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2.