BiologyNEB 2082
How do different photosynthetic pigments contribute to capturing light energy? Explain how cyclic and non cyclic photo phosphorylation generate ATP and NADPH, and illustrate these with appropriate…
8How do different photosynthetic pigments contribute to capturing light energy? Explain how cyclic and non-cyclic photo-phosphorylation generate ATP and NADPH, and illustrate these with appropriate charts. [2+3+3]
Answer
Photosynthetic Pigments and Their Role in Light Capture
Photosynthetic organisms contain several pigment molecules that broaden the range of solar radiation that can be harvested. The three major groups in higher plants are chlorophyll a, chlorophyll b, and carotenoids (including β‑carotene and xanthophylls).
| Pigment | Peak absorption (nm) | Contribution to light capture |
|---|---|---|
| Chlorophyll a | 430 (blue) & 662 (red) | Primary electron donor in the reaction centre (P680 in PS II, P700 in PS I). Captures the highest‑energy photons and initiates charge separation. |
| Chlorophyll b | 453 (blue) & 642 (red) | Extends the absorption range toward shorter wavelengths; transfers excitation energy to chlorophyll a via Förster resonance energy transfer. |
| Carotenoids | 450‑500 (blue‑green) | Absorb light that chlorophylls miss; protect the photosystems by dissipating excess energy as heat and quenching singlet oxygen. |
The combined absorption spectrum of a leaf is the superposition of these individual spectra, allowing plants to use ≈ 400–700 nm (the photosynthetically active radiation, PAR).
The figure shows that chlorophyll a dominates at the red edge, chlorophyll b fills the blue‑green gap, and carotenoids provide a broad shoulder in the 450‑500 nm region, together ensuring maximal capture of incident photons.
Photophosphorylation Pathways
Photosynthetic light reactions convert the captured photon energy into chemical energy in the form of ATP and NADPH. Two distinct electron‑transport routes operate:
| Feature | Non‑cyclic (linear) photophosphorylation | Cyclic photophosphorylation |
|---|---|---|
| Primary photosystem | PS II → PS I (both active) | Only PS I |
| Electron source | Water (H₂O) → O₂ + 4 e⁻ | No external donor; electrons return to PS I |
| End electron acceptor | NADP⁺ → NADPH | No NADP⁺ reduction |
| ATP yield per photon | ~1.5 ATP (via chemiosmosis) + 1 NADPH | ~2 ATP (higher ATP/ NADPH ratio) |
| Role | Provides both reducing power and ATP for the Calvin cycle | Adjusts ATP/NADPH balance when NADPH demand is low |
Non‑Cyclic Photophosphorylation (Linear Electron Flow)
- Photon absorption by P680 (PS II) excites an electron.
- The excited electron is transferred to plastoquinone (PQ) and then to the cytochrome b₆f complex.
- Proton pumping across the thylakoid membrane by the cytochrome b₆f complex creates a proton gradient.
- Electrons continue to plastocyanin (PC) and reach P700 (PS I).
- A second photon excites P700; the electron is passed to ferredoxin (Fd) and finally to NADP⁺ reductase, reducing NADP⁺ to NADPH.
- The proton gradient drives ATP synthase, synthesising ATP from ADP + Pi.
Cyclic Photophosphorylation (Only PS I)
- Light excites P700; the electron is transferred to ferredoxin as in the linear route.
- Instead of reducing NADP⁺, the electron is diverted back to the plastoquinone pool via the ferredoxin‑plastoquinone reductase (FQR).
- The electron then follows the same path through cytochrome b₆f, plastocyanin, and returns to P700.
- Each turn pumps additional protons, increasing the electrochemical gradient and yielding more ATP without producing NADPH.
The two pathways are illustrated in the flow diagram below.
Key points from the diagram
- Both pathways share the PQ‑cytochrome b₆f‑PC‑PSI segment, which is the main site of proton translocation.
- In the linear route, electrons are ultimately accepted by NADP⁺, generating NADPH.
- In the cyclic route, electrons are recycled to P700, producing additional ATP without NADPH formation.
Energy Yield Summary
- Non‑cyclic: For every 8 photons (4 absorbed by PS II, 4 by PS I) → 2 NADPH + ≈ 3 ATP (≈ 1.5 ATP per photon).
- Cyclic: For every 4 photons absorbed by PS I → ≈ 2 ATP (no NADPH).
The plant balances the two routes to meet the Calvin‑cycle demand of 3 ATP : 2 NADPH. When the ATP requirement exceeds NADPH, cyclic photophosphorylation is up‑regulated.
Figure 1 illustrates the complementary absorption of chlorophyll a, chlorophyll b, and carotenoids, while Figure 2 (mermaid flowchart) visualises the electron flow and proton‑gradient formation in both photophosphorylation pathways, highlighting where ATP and NADPH are produced.
Discussion
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