Explain where the electron comes from and how it gains energy during the light reactions.  Describe how the energy in the electron is used to produce the energy carrying molecules NADPH and ATP

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ncchemist | eNotes Employee

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Photosynthesis consists of two main parts, the light dependent reactions and the light independent reactions.  The light dependent reactions happen first.  The electron that starts the whole photosynthetic cycle originates from chlorophyll, the green pigment found in the chloroplasts of plants.  The energy in sunlight electronically excites the chlorophyll molecule and one of its electrons is released.  This electron then is passed through a series of chemical reactions until it ultimately reduces a molecule of NADP to NADPH.  The chlorophyll later regains the lost electron via the photolysis of a water molecule.  During this process, water loses 2 electrons to produce molecular oxygen (O2) and two protons.  The oxygen is released to the air and the protons help form a proton gradient across the chloroplast membrane.  This proton gradient helps to power the enzyme ATPsynthase to convert ADP to ATP.  The NADPH and ATP are then used by the plant to power the light independent reactions, which in turn is where the plant's carbohydrate based food is produced.