Question

In the light reactions, what is the initial electron donor? Where do the electrons finally end up?

Short answer

The initial electron donor is the water. NADP⁺ acquires the electron from the electron donor in the process of the electron transport chain. Finally, it gets reduced to NADPH.

Step-by-step solution

Description of light reactions

The initial step of photosynthesis is the light reaction.The sunlight that is obtained from the environment is converted into chemical energy in this process. The products of this reaction are ATP and NADPH.

The location in which this reaction occurs is the thylakoid that is present in the chloroplasts, particularly within the photosystem.

Description of NADP+

NADP⁺is abbreviated as Nicotinamide adenine dinucleotide phosphate. It is a cofactor molecule that is a non-protein component responsible for the progress of a chemical reaction.

Explanation about initial electron donor and fate of electrons

The splitting up of water molecules results in two electrons and two hydrogen and oxygen atoms. These electrons are transferred to cofactor NADP⁺in the electron transport chain via the ferredoxin molecules in the photosystem.

Ferredoxin molecules present in the chloroplast contain iron. This iron molecule transfers the electrons to the final electron acceptor. The electrons finally get transferred to the NADP⁺ by the activity of an enzyme known as NADP⁺ reductase. The reductase enzyme facilitates the reduction reaction and yields NADPH.

Hence, the travel of electrons in the electron transport chain starts from the water and finally reduces NADP⁺ to NADPH.