4 Which protein component is mobile in between PS II and PS I ?
(1) PC                             (2) PQ
(3) Cytochrome F₆                (4) Ferrodoxin

The correct answer is (1) PC (Plastocyanin): PC is the mobile protein component between Photosystem II (PSII) and Photosystem I (PSI) in the photosynthetic electron transport chain. Here is a detailed explanation of every option and an SEO-optimized article.

Mobile Protein Component Between PSII and PSI

Introduction

Key phrase: mobile protein between PSII and PSI in photosynthesis

During photosynthesis, electrons flow through a series of protein complexes and mobile carriers, linking PSII and PSI inside the thylakoid membrane. Understanding these carriers is crucial for anyone studying photosynthetic energy conversion and plant biology exams.​

Explanation of Each Option

• (1) PC (Plastocyanin):

  • Correct. Plastocyanin is a small, copper-containing protein that shuttles electrons between the cytochrome b6f complex and PSI in the thylakoid lumen. It is highly mobile in the aqueous phase, delivering electrons to PSI for further transfer to ferredoxin.​

• (2) PQ (Plastoquinone):

  • PQ is a lipid-soluble molecule that transfers electrons from PSII to cytochrome b6f within the membrane’s fatty acid region but does not move between cytochrome b6f and PSI. PQ is mobile in the lipid bilayer and acts before plastocyanin in the electron transport chain.​

• (3) Cytochrome F6 (likely refers to Cytochrome b6f):

  • The cytochrome b6f complex is a membrane-bound protein complex, not a mobile carrier. It acts as a stationary intermediary facilitating electron transfer from PQ to PC and is essential for proton translocation.​

• (4) Ferredoxin:

  • Ferredoxin is the iron–sulfur protein receiving electrons from PSI and shuttling them onward to NADP+ for reduction. It acts after PSI and is not the bridge between PSII and PSI.​

Key Facts About Mobile Protein Components

• Plastocyanin (PC): Water-soluble, copper-binding, diffuses freely in the thylakoid lumen, directly transfers electrons to PSI.

• Plastoquinone (PQ): Lipid-soluble, shuttles electrons within membrane from PSII to cytochrome b6f, not mobile between PSII and PSI.

• Cytochrome b6f: Stationary protein complex, not a mobile carrier.

• Ferredoxin: Receives electrons from PSI, not involved in PSII-to-PSI electron shuttling.