Melvyn Bragg and his guests discuss photosynthesis, the process by which green plants and many other organisms use sunlight to synthesise organic molecules. Photosynthesis arose very early in evolutionary history and has been a crucial driver of life on Earth. In addition to providing most of the food consumed by organisms on the planet, it is also responsible for maintaining atmospheric oxygen levels, and is thus almost certainly the most important chemical process ever discovered. With: Nick Lane Reader in Evolutionary Biochemistry at University College London Sandra Knapp Botanist at the Natural History Museum John Allen Professor of Biochemistry at Queen Mary, University of London. Producer: Thomas Morris.