Photosynthesis is the process carried out by plants to harvest energy from sunlight. The most important molecule in photosynthesis is called chlorophyll; it’s what collects sunlight and gives plants their green color. Carbon dioxide (CO2) is taken in through cells called stomata, and the plants also draw water up through the roots and into the leaves. The reaction catalyzes when chlorophyll absorbs light and makes ATP along with another molecular energy source called NADPH (nicotine adenine dinucleotide phosphate). In the process, CO2 is used up and water molecules are split, releasing O2 gas, which other organisms (like humans and animals) can then breathe.