Question
Explain the light reactions of photosynthesis with the Z-scheme of electron transport. How do PSII and PSI work together to produce ATP and NADPH?
Solution — Step by Step
flowchart LR
A[H2O] -->|Photolysis| B[PSII - P680]
B -->|e- excited| C[Pheophytin]
C --> D[Plastoquinone PQ]
D --> E[Cyt b6f complex]
E -->|e-| F[Plastocyanin PC]
F --> G[PSI - P700]
G -->|e- excited| H[Ferredoxin]
H --> I[NADP+ reductase]
I --> J[NADPH]
E -->|H+ pumped| K[ATP via chemiosmosis]Photosystem II absorbs light at 680 nm. The reaction centre chlorophyll P680 gets excited and loses an electron to the primary acceptor (pheophytin). The electron vacancy in P680 is filled by splitting water: . This photolysis of water releases O as a byproduct.
The excited electron passes through plastoquinone (PQ), the cytochrome bf complex, and plastocyanin (PC). As electrons move through the Cyt bf complex, H ions are pumped from the stroma into the thylakoid lumen, creating a proton gradient.
Photosystem I absorbs light at 700 nm. P700 gets excited and passes its electron to ferredoxin (Fd). The electron vacancy in P700 is filled by the electron arriving from PSII via plastocyanin.
Ferredoxin passes the electron to NADP reductase, which combines 2 electrons + H + NADP to form NADPH. This happens on the stromal side of the thylakoid membrane.
The H gradient (high in lumen, low in stroma) drives H through ATP synthase (CF-CF complex), producing ATP. This is photophosphorylation.
Why This Works
The Z-scheme ensures that light energy is used twice (at PSII and PSI) to boost electrons to a high enough energy level to reduce NADP. The “Z” shape comes from plotting the redox potential of each carrier — electrons go up (light absorption), down (electron transport), up again (light absorption), and finally reduce NADP.
Alternative Method
Cyclic photophosphorylation uses only PSI. Excited electrons from P700 pass through ferredoxin but instead of going to NADP, they return to PSI via the Cyt bf complex. This produces only ATP (no NADPH, no O). It supplements ATP when the Calvin cycle demands more ATP than NADPH.
Common Mistake
Students write “PSI comes first.” Despite the numbering, PSII acts first in the Z-scheme, and PSI acts second. PSII was named first chronologically in research, not by order of function. Also, photolysis of water occurs at PSII, not PSI — water replaces electrons lost by P680.