Photosynthesis is the process in which plants take in carbon dioxide and produce oxygen. They take in the carbon dioxide through the stoma cell, and light energy through the chlorophyll, which is located in the chloroplast cells. They take the carbon dioxide and the solar energy, and they turn that into glucose. The plant produces oxygen much like we produce carbon diaoxide. We don't need the carbon dioxide, and they don't need the oxygen. We need photosynthesis because we need oxygen, and we produce carbon dioxide. Plants, and some bacteria, need carbon dioxide and produce oxygen. We need plants to help maintain the balance, because we depend on them so we can live.
Cellular respiration is the process in which plants take in the carbon dioxide from the air, and water from its roots, and breaks apart the molecules, and turns them into glucose. There is some extra oxygen that the plant does not use to produce the glucose, and that is released as a waste product, providing humans and animal ps with the oxygen we need to survive.