Question

In photosystem-I, the first electron acceptor is-
a. Plastocyanin
b. An iron-sulphate protein
c. Ferredoxin
d. Cytochrome

Answer 1

Photo systems are a group of pigments molecules which take part in the conversion of light energy into the chemical energy. There are two photosystems I and II. In both photosystems there is a reaction centre, it is a molecule of chlorophyll ‘a’ which absorbs light energy. The PS I functions most efficiently at a wavelength of 700nm, hence called P700. And the PS II functions most efficiently at 680nm wavelength of light, thus known as P680.

Complete answer:
The primary electron acceptor in photosystem-I is Ferredoxin which is an iron-sulphate protein. The process of cyclic photophosphorylation involves only PS I and this process takes place in the membrane of stroma lamellae. When only PS I is functional, the electron is circulated within the photo system and the phosphorylation occurs, due to cyclic flow of electrons.

After PS I receives light, the electron from the reaction centre gets ejected and it reaches to the first electron acceptor which is Ferredoxin, an iron-sulphate complex present on the stromal side of the lumen. NADP reductase enzyme catalyses this reaction. From here this electron moves to plastoquinone where one molecule of ATP is synthesized from one ADP and inorganic phosphate. Electron then reaches the Cytochrome b6 f complex from plastoquinone. The electron then goes to Plastocyanin and moves back to PSI.

Hence, the correct answer is option (B) and (C).

Note: In PhotoSystem I:
• There is no requirement of an external source of electrons.
• No oxygen is evolved here because no photolysis of water takes place.
• Only ATP is synthesised here in PS I.
• It can easily operate under anaerobic conditions, where no Carbon dioxide is available and in low light intensity.
• Except in certain bacteria, PS I does not get involved in photosynthesis.