tem I, which uses them to maintain the flow of electrons to P700, and restores its function. P680 in Photosystem II is now electron deficient because it has donated electrons to P700 in Photosystem I. P680 electrons are replenished by the water that has been absorbed by the plant roots and transported to the chloroplasts in the leaves. The movement of electrons in Photosystems I and II and the action of an enzyme split the water into oxygen, hydrogen ions, and electrons. The electrons from water flow to Photosystem II, replacing the electrons lost by P680. Some of the hydrogen ions may be used to produce NADPH at the end of the electron transport chain, and the oxygen from the water diffuses out of the chloroplast and is released into the atmosphere through pores in the leaf [8]. The transfer of electrons in a step-by-step fashion in Photosystems I and II releases energy and heat slowly, and protects the chloroplast and cell from a harmful temperature increase [6]. It also provides time for the plant to form NADPH and ATP. In the words of American biochemist and Nobel laureate Albert Szent-Gyorgyi, "What drives life is thus a little electric current, set up by the sunshine." [3] The chemical energy required for the light-independent reaction is supplied by the ATP and NADPH molecules produced in the light-dependent reaction. The light-independent reaction is cyclic; it begins with a molecule that must be regenerated at the end of the reaction in order for the process to continue. In the Calvin cycle (Figure 13), named after the American chemist Melvin Calvin who discovered it, the light-independent reaction uses the electrons and hydrogen ions associated with NADPH and the phosphorous associated with ATP to produce glucose [9]. These reactions occur in the stroma, the fluid in the chloroplast surrounding the thylakoids, and each step is controlled by a different enzyme. The light-independent reaction requires...
