Question

In photosystem- I, the first electron acceptor is
A. An iron-sulphur protein
B. Plastoquinone
C. Cytochrome
D. Plastocyanin

Answer 1

Hint: The photosystem- I receive electrons from the plastocyanin or the cytochrome on the luminal side of the thylakoid membrane and also use light energy to transfer them across the membrane to the ferredoxin on the stromal side. Also, it can function in a cyclic electron transport pathway.

Complete answer:
The photosystem- I, is an integral membrane protein complex that absorbs the solar energy and mediates electron transfer from the plastocyanin to the ferredoxin.
The ferredoxin is the iron-containing sulphur protein, which is also the first pigment that accepts the electrons. The photosystem-I is also known as the plastocyanin-ferredoxin oxidoreductase. It is one of the two photosystems in the photosynthetic light reactions of the algae, plants, and cyanobacteria. The photosystem-I is an integral membrane protein complex that uses light energy to catalyze the transfer of the electrons across the thylakoid membrane from the plastocyanin to the ferredoxin. Eventually, the electrons that are transferred by the Photosystem I are used in producing high energy. The combined action of the entire photosynthetic electron transport chain also produces a proton-motive force that helps to generate the ATP molecule. The iron-sulphur proteins are the proteins that are characterized by the presence of iron-sulphur clusters containing sulfide-linked di-, tri-, and tetra iron centers in the variable oxidation states. The photosystem- I system comprises more than hundreds of cofactors which is significantly more than that of photosystem- II.
Therefore, the correct answer is Option (A).

Note: The iron-sulphur clusters are found in a variety of metalloproteins, such as the ferredoxins, as well as the NADH dehydrogenase, hydrogenases. The iron-sulphur clusters are best known for their role in the oxidation-reduction reactions of the electron transport in the mitochondria and the chloroplasts.