Now about the Answer to your question ==== Carbon dioxide ,along with water play the most important role in photosynthesis . Let me make it simple for you. 1 ) CO2 gas from the atmosphere enters the leaf through tiny holes called stomata. 2) Water also enters the leaf from the roots with the help from xylem tissue. 3) This water (H2O) is split in to Hydrogen ( H2 ) and Oxygen (O2) .