Question
Describe the light reactions of photosynthesis. Explain the Z-scheme of electron transport from PSII to PSI, including the role of water splitting, plastoquinone, cytochrome b6f, plastocyanin, and ferredoxin.
(NEET, CBSE Class 11 — Photosynthesis in Higher Plants)
Solution — Step by Step
Photosystem II (P680) absorbs light. The reaction centre chlorophyll P680 gets excited and donates an electron. To replace this lost electron, water is split (photolysis): . This is the source of all the oxygen we breathe.
The excited electron from PSII passes through pheophytin, then to plastoquinone (PQ), then to the cytochrome b6f complex, and finally to plastocyanin (PC). During this transfer through cyt b6f, protons are pumped into the thylakoid lumen, building a proton gradient.
Plastocyanin delivers the electron to Photosystem I (P700). PSI absorbs light, re-excites the electron to a higher energy level, and passes it to ferredoxin (Fd). This second boost of energy is why we call it the Z-scheme — the electron energy goes up, down, then up again, tracing a Z shape.
Ferredoxin passes the electron to the enzyme NADP reductase, which combines 2 electrons + 1 H with NADP to form NADPH. This happens on the stromal side of the thylakoid membrane.
The proton gradient (high H in lumen, low in stroma) drives protons back through ATP synthase (CF-CF complex), producing ATP from ADP + Pi. This is photophosphorylation.
graph LR
A["H₂O → O₂ + H⁺ + e⁻"] --> B["PSII (P680)"]
B -->|"Excited e⁻"| C["Pheophytin"]
C --> D["Plastoquinone PQ"]
D --> E["Cyt b6f complex"]
E -->|"H⁺ pumped"| F["Plastocyanin PC"]
F --> G["PSI (P700)"]
G -->|"Re-excited e⁻"| H["Ferredoxin Fd"]
H --> I["NADP⁺ reductase"]
I --> J["NADPH"]Why This Works
The Z-scheme is called that because when you plot the redox potential of each carrier on the y-axis, the electron path traces a Z (or N) shape. PSII lifts the electron from a low energy state, it falls through the ETC (releasing energy for ATP), then PSI lifts it again to an even higher energy level — high enough to reduce NADP.
Two photosystems are needed because no single photon has enough energy to push an electron from water all the way to NADP. The two-step boost makes the process thermodynamically feasible.
The net products of the light reactions: ATP and NADPH (called “assimilatory power”). These are used by the Calvin cycle to fix CO into sugars.
Alternative Method — Cyclic Photophosphorylation
When the cell needs more ATP but not NADPH, electrons from PSI cycle back to the cytochrome b6f complex via ferredoxin, skipping NADP reduction entirely. This produces ATP without NADPH and without oxygen evolution. Only PSI participates.
Non-cyclic: both PSI and PSII, produces ATP + NADPH + O. Cyclic: only PSI, produces only ATP, no NADPH, no O. NEET often asks for the differences between these two.
Common Mistake
Students often say “PSI comes first because it is number I.” In reality, PSII acts first in the electron transport chain. The numbering is historical — PSI was discovered first. The functional order is PSII then PSI. Getting this sequence wrong mixes up the entire Z-scheme.