Curiosity › Science › Biology and Medicine › Where do electrons come from in the light-dependent reactions? › Reply To: Where do electrons come from in the light-dependent reactions?
When light is incident on the photosystem II, the electrons in the chloroplast molecules are excited. And when this energy is transferred to the reaction centre (via resonant energy), the electron acceptors remove the excited electron from the (special pair) chlorophyll and initiates the electron transport chain process.
Also, due to photolysis, the water molecules are split into O2 atoms and H2+ ions. The electrons lost by the chlorophyll pigments in the photosystem II is replaced by these electrons lost (by the H2 atoms) in the splitting of the water. The H2+ ions then form the hydrogen ion gradient coupled with the electron chain, whereas the O2 atoms combine to form oxygen molecule that is released as a by-product.
Further down the electron transfer chain, the electrons from photosystem II reaches photosystem I and is accepted by the already electron deficit chlorophyll molecules (The photosystem I would also lose electrons the same was like photosystem II) in the reaction centre of photosystem I. The lost electrons from photosystem I will go into making NADPH, further leading to create ATP.
So to answer your question, the electrons come from a) excitation of electrons in the photosystem II reaction centre, b) splitting of water molecules due to photolysis, c) excitation of electrons in the photosystem I reaction centre.