Photosystems and the Electron Transport Chain in Photosynthesis

The photosystems and electron transport chain form the foundation of light-dependent reactions in photosynthesis. Located in the thylakoid membrane of chloroplasts, these protein-pigment complexes work together to convert light energy into chemical energy in the form of ATP and NADPH.

 

🔗 Related: The Biochemistry of the Calvin Cycle: Enzymes, Regulation, and Energy Balance

🔆 Photosystem II (PSII)

PSII is the first protein complex in the light reactions. When chlorophyll molecules in PSII absorb photons, electrons become excited and are transferred to a primary electron acceptor. To replace these lost electrons, water is split (photolysis), producing O₂ and H⁺:

2 H₂O → 4 H⁺ + 4 e⁻ + O₂

This process occurs at the oxygen-evolving complex (OEC) within PSII.

 

🔁 Plastoquinone and the Cytochrome b6f Complex

Excited electrons from PSII are passed to plastoquinone (PQ), which also picks up protons from the stroma. PQ shuttles electrons to the cytochrome b₆f complex, where protons are pumped into the thylakoid lumen, generating a proton gradient. This electrochemical gradient is essential for ATP synthesis.

🔀 Plastocyanin and Photosystem I (PSI)

After the cytochrome complex, electrons are transferred to plastocyanin (PC), a copper-containing protein that carries them to photosystem I. PSI re-energizes electrons using a second photon of light and passes them down to ferredoxin (Fd).

 

⚡ ATP and NADPH Production

Ferredoxin transfers the high-energy electrons to NADP⁺ reductase, which forms NADPH by combining NADP⁺, electrons, and a proton:

NADP⁺ + 2 e⁻ + H⁺ → NADPH

Simultaneously, protons in the lumen flow back into the stroma via ATP synthase, driving the formation of ATP from ADP and inorganic phosphate:

ADP + Pi → ATP

🔄 Cyclic vs. Noncyclic Electron Flow

In noncyclic electron flow, both ATP and NADPH are produced, and electrons travel linearly from water to NADP⁺. In contrast, cyclic flow uses PSI alone, cycles electrons back to the cytochrome complex, and produces ATP only—no NADPH or O₂.

 

📌 Summary

The photosystems and electron transport chain are central to converting solar energy into chemical potential. Through orchestrated electron transfers, proton gradients, and redox reactions, plants generate the ATP and NADPH required for the Calvin cycle and life itself.