Superman instantly becomes very weak when he comes into contact with Green Kryptonite. With enough exposure, the “Man of Steel” could even die. But why? Kryptonite has this effect on Superman because of the way the former interacts with his cells.
Much of Superman’s power comes from the planet Earth’s yellow sun. Think of Superman as a plant that uses photosynthesis to make its own food. Through photosynthesis, plants use the sun’s energy sun to turn water and carbon dioxide into oxygen and sugar.
It’s not clear however how Superman’s cells use sunlight or the difference of getting light from a yellow sun as compared with, say, a red sun.
There are some possible explanations why Green Kryptonite keeps Superman from getting power from the sun. One is that the radiation from kryptonite might displace the solar radiation that is responsible for Superman’s powers. The radiation also may interrupt some other organic process within Superman’s body.
Thus, Superman cannot take advantage of the powers given to him by the Earth’s yellow sun.