Final answer:
In photosynthesis, the initial source of electrons for the chloroplast electron transport chain is water, and each molecule of NADPH carries two electrons. The stepwise transfer of electrons through the electron transport chain facilitates ATP synthesis through proton gradient generation.
Step-by-step explanation:
The source of electrons passed to NADP+ at the end of the electron transfer chain in photosynthesis is water (H2O). During the light-dependent reactions of photosynthesis, photosystem II (PSII) absorbs light energy, which is used to extract electrons from water molecules. These electrons are then transferred through the chloroplast electron transport chain to photosystem I (PSI), where NADP+ is reduced to NADPH, a vital electron carrier for subsequent biosynthetic reactions, namely the Calvin cycle.
To calculate the number of electrons transferred to NADP+ to form NADPH, one must recognize that each NADPH molecule carries two electrons. Hence, for each NADPH synthesized, two electrons are transferred from the excited chlorophyll in PSI to NADP+.
The free energy change during the electron transport can be calculated using the free energies per electron provided. For instance, if the electrons come from a molecule like NADH with a potential of -0.32V and are transferred to oxygen with a potential of +0.82V, the stepwise transfer of electrons through the electron transport chain allows for the generation of ATP through the creation of a proton gradient across the membrane.