Photosynthesis: PS-I and PS-II Reactions

Photosynthesis: Light-Dependent Reactions
Photosystem II (PS-II) and Photosystem I (PS-I)

Overview of Light-Dependent Reactions

The light-dependent reactions of photosynthesis occur in the thylakoid membranes of chloroplasts. They convert light energy into chemical energy (ATP and NADPH) and release oxygen as a byproduct.

These reactions involve two main photosystems:

• Photosystem II (PS-II) – P680 reaction center
• Photosystem I (PS-I) – P700 reaction center

The two photosystems work together in the Z-scheme (non-cyclic electron transport) to produce ATP and NADPH.

Photosystem II (PS-II)

Location: Thylakoid membrane

Reaction Center: P680 (absorbs light at 680 nm)

Key Steps in PS-II:

1. Light energy is absorbed by antenna pigments (chlorophyll a, b and carotenoids) and transferred to the P680 reaction center.
2. P680 becomes excited (P680*) and donates a high-energy electron to the primary electron acceptor (pheophytin).
3. The oxidized P680⁺ is a very strong oxidant. It regains electrons by splitting water molecules in the Oxygen-Evolving Complex (OEC):
   2H₂O → 4H⁺ + 4e^- + O₂ (Photolysis of water)
4. Electrons flow from PS-II through the electron transport chain:
   Pheophytin → Plastoquinone (PQ) → Cytochrome b₆f complex → Plastocyanin (PC)
5. As electrons move through the cytochrome b₆f complex, protons (H⁺) are pumped into the thylakoid lumen, creating a proton gradient.
6. This proton gradient drives ATP synthesis via ATP synthase (photophosphorylation).

Important Points about PS-II:

• PS-II is the first photosystem in the Z-scheme.
• It is responsible for oxygen evolution (the source of atmospheric O₂).
• It performs non-cyclic photophosphorylation when working with PS-I.

Photosystem I (PS-I)

Location: Thylakoid membrane

Reaction Center: P700 (absorbs light at 700 nm)

Key Steps in PS-I:

1. Light energy is absorbed by antenna pigments and funneled to the P700 reaction center.
2. P700 becomes excited (P700*) and donates a high-energy electron to the primary acceptor (A₀, a special chlorophyll).
3. Electrons then pass through: A₁ (phylloquinone) → Iron-sulfur proteins (Fe-S) → Ferredoxin (Fd).
4. From Ferredoxin, electrons are transferred to NADP⁺ reductase, reducing NADP⁺ to NADPH:
   2Fd(red) + 2H⁺ + NADP⁺ → 2Fd(ox) + NADPH
5. The oxidized P700⁺ receives electrons from plastocyanin (PC), which comes from PS-II.

Important Points about PS-I:

• PS-I receives electrons from PS-II via the electron transport chain.
• It is responsible for the production of NADPH, the reducing power used in the Calvin cycle.
• PS-I can also operate independently in cyclic electron flow (only ATP is produced, no NADPH or O₂).

Comparison: PS-II vs PS-I

Feature	Photosystem II (PS-II)	Photosystem I (PS-I)
Reaction Center	P680	P700
Absorption Peak	680 nm	700 nm
Primary Function	Water splitting & electron donation	NADPH production
Electron Source	Water (H2O)	Plastocyanin (from PS-II)
Electron Acceptor	Pheophytin → Plastoquinone	Ferredoxin → NADP⁺
Oxygen Production	Yes (Photolysis)	No
ATP Production	Yes (via proton gradient)	Yes (in cyclic flow)
NADPH Production	No	Yes

Z-Scheme Summary (Non-cyclic Electron Flow)

H₂O → PS-II → Plastoquinone → Cyt b₆f → Plastocyanin → PS-I → Ferredoxin → NADP⁺
Products: O₂ + ATP + NADPH

Must Read Photosynthesis

Light reactions power the dark reactions (Calvin cycle) by providing ATP and NADPH.