8 Electrons excited from PS II are accepted by-
(1) PQ                    (2) Pheophytin
(3) PC                   (4) Cyt b₆

Electron Acceptance in Photosystem II

Introduction

Key phrase: electron acceptor after PSII excitation pheophytin

During oxygenic photosynthesis, absorption of light by Photosystem II triggers electron transfer through a defined pathway. Pinpointing the primary electron acceptor is key for understanding photosynthetic energy conversion and electron chain mechanics in chloroplasts.

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Explanation of Each Option

• (1) PQ (Plastoquinone):

  • Plastoquinone is an essential electron carrier, but it receives electrons later in the chain, after they have already been accepted by pheophytin and transferred through the PSII complex. PQ acts downstream, accepting electrons from the reduced primary and secondary quinone binding sites in PSII.

• (2) Pheophytin:

  • Correct. Pheophytin is the primary electron acceptor in PSII that directly receives electrons from the excited P680 chlorophyll molecule. This molecule creates a stable charge separation that initiates the electron transfer chain, allowing energy to move through subsequent carriers.

• (3) PC (Plastocyanin):

  • Plastocyanin is a mobile electron carrier involved after the cytochrome b6f complex and accepts electrons during transport between PSII and PSI, not directly from excited PSII.

• (4) Cyt b6 (Cytochrome b6f):

  • Cytochrome b6f is a later component in the electron transport chain, receiving electrons after plastoquinone has shuttled them away from PSII. It does not directly accept electrons from PSII.

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Key Steps in PSII Electron Transfer

• P680 (chlorophyll) absorbs light → becomes excited, donates electron.

• Pheophytin is the first electron acceptor after excitation.

• Electrons move to plastoquinone (QA and QB), then to cytochrome b6f, and onwards.