Question

In oxidative phosphorylation in mitochondria, high-energy electrons are transferred to the electron-transport chain from activated carriers like NADH. Stage 1 of photosynthesis also uses an electron-transport chain to pump protons and make ATP. In this case, where do the high-energy electrons come from?

Answer 1

The initial source of high-energy electrons in the light-dependent reactions of photosynthesis is water. These electrons are excited by sunlight in photosystem II and are used for the reduction of NADP+ to NADPH, while also contributing to the production of ATP via an electrochemical gradient.

Step-by-step explanation:

In the light-dependent reactions of photosynthesis, the initial source of high-energy electrons for the chloroplast electron transport chain comes from water molecules. In photosystem II (PSII), sunlight energy is used to extract electrons from water, resulting in the formation of oxygen gas and protons. These high-energy electrons then move through the electron transport chain to photosystem I (PSI), where they participate in the reduction of NADP+ to NADPH. This process is coupled with the pumping of protons across the thylakoid membrane, generating an electrochemical gradient utilized by ATP synthase to produce ATP in a process known as photophosphorylation.