make their own food. They do this using a processed p.osynthesis The carbon dioxide required for p.osynthesis comes from the air. It enters leaves In the first reaction, energy is used to split water molecules into hydrogen and oxygen. Some of the glucose produced by p.osynthesis is used for respiration.. of materials including nitrogen, carbon and water with GCSE Bitesize Biology. The three key processes and the conversions are shown in the table below. Carbon dioxide is absorbed by producers to make glucose in p.osynthesis..With each breath you exchange the gases oxygen O2 and carbon dioxide CO2 . They produce oxygen during a processed p.osynthesis. Green plants use energy from sunlight to build sugar molecules from carbon dioxide and water. . Find out. Find your heart rate while sitting quietly. Now run in place for 3 .