Photosynthesis in Higher Plants · Lesson 4 of 10
Linear Electron Flow, the Z-Scheme and Water Splitting
“Trace non-cyclic electron transport from water through PS II and PS I to NADPH, explaining oxygen evolution.”
• Trace the order of carriers in linear photosynthetic electron flow. • Explain why PS II needs replacement electrons from water. • Interpret a Z-scheme energy diagram. • Describe formation of NADPH and why released oxygen is water-derived.
What happens after the reaction centre absorbs light?
The photosystem reaction centre is part of an electron-transfer chain, not simply a store of absorbed light. In the common linear pathway, PS II absorbs light and transfers an excited electron from P680 to a primary acceptor. That electron then moves through several carriers in the thylakoid membrane. At PS I, a second photochemical excitation increases the energy of the electron again, allowing it to pass to ferredoxin and ultimately contribute to NADP⁺ reduction. The chain links light absorption to formation of chemical reducing power.
The textbook calls the overall energy diagram a Z-scheme because, when the participating carriers are plotted on a suitable redox-potential scale, the light-driven rises and carrier-mediated descents create a zigzag shape. A Z-scheme is a diagram of changes in electron-transfer energy, not the literal spatial route taken by an electron within a chloroplast. The two upward transitions represent the work done by light at the two photosystems; between them electrons move along redox-transfer pathways.
The intersystem chain links PS II to PS I
After charge separation in PS II, electrons are conveyed through the plastoquinone pool and the cytochrome b₆f complex and then via plastocyanin to PS I. These steps are associated with proton movement and help build the gradient later used to synthesise ATP. The textbook often condenses these carriers into a generic electron transport system, but naming the major components makes the link to chemiosmosis intelligible. The electrons are not consumed simply by moving; they are transferred from one donor to another acceptor.
A common error is to imagine that PS I always supplies its own new electrons directly from water. In linear electron flow, PS I obtains replacement electrons from the transport chain originating at PS II. The two reactions are therefore coupled. If the intersystem supply fails, PS I can no longer continue an ordinary sustained flow from water to NADP⁺, although cyclic routes using PS I can operate under certain conditions.
How PS II replaces the electrons it loses
Oxidised PS II requires replacement electrons. The oxygen-evolving complex associated with its lumen-facing region catalyses the oxidation of water. The overall balanced reaction is 2H₂O → O₂ + 4H⁺ + 4e⁻. Four electrons become available to replenish the photosynthetic charge-separation system, four protons are released into the thylakoid lumen, and one molecule of oxygen is formed. This reaction explains both the continuing electron supply and the origin of evolved oxygen gas.
Oxygen is a net by-product of water oxidation; it does not arise from a direct splitting of atmospheric carbon dioxide in the Calvin cycle. The released protons contribute to the acidification of thylakoid lumen, which helps create an electrochemical gradient across the thylakoid membrane. Thus water splitting supplies electrons for reduction reactions and simultaneously contributes to the proton-motive force for ATP synthesis.
How electrons reach NADPH
Light excites the reaction centre of PS I, P700, and its transferred electrons pass to a sequence of acceptors including ferredoxin. Ferredoxin–NADP⁺ reductase then catalyses reduction of NADP⁺ on the stroma-facing side of the thylakoid system. The simplified reduction can be represented as NADP⁺ + 2e⁻ + H⁺ → NADPH. These reducing equivalents subsequently help convert carbon-fixation intermediates into carbohydrate precursors.
NADP⁺ and NADPH are not the same molecules as ADP and ATP. ATP serves primarily as chemical energy currency for biosynthetic work; NADPH supplies high-energy electrons for reduction reactions. Photosynthesis requires both, which is why the light reactions combine electron flow and a proton gradient rather than producing only one kind of energy carrier.
| Stage | Electron donor | Electron acceptor or next destination | Important consequence |
|---|---|---|---|
| Water splitting | H₂O | Oxidised PS II | O₂ and lumen H⁺ released |
| PS II excitation | P680 | Primary PS II acceptor | Photoinduced electron transfer |
| Intersystem chain | PS II acceptors | PS I via plastocyanin | Supports proton gradient |
| PS I excitation | P700 | PS I primary acceptors | Second photochemical energy boost |
| NADP⁺ reduction | Ferredoxin | NADP⁺ | NADPH produced in stroma |
Why non-cyclic photophosphorylation involves both photosystems
When electrons travel from water through PS II and PS I to NADP⁺, they do not return to the original reaction centre. This is called non-cyclic or linear electron flow. The resulting electron transfers generate NADPH and help create a proton gradient for ATP synthesis; water oxidation evolves O₂. 'Non-cyclic' refers to the electron path and must not be confused with the cyclic nature of the Calvin pathway, which regenerates RuBP rather than recycling light-reaction electrons.
Water → PS II/P680 → plastoquinone → cytochrome b₆f → plastocyanin → PS I/P700 → ferredoxin → NADP⁺ reduction. Light excites both photosystems, while water supplies replacement electrons at PS II.
Quick Quiz
Quiz
Which photosystem first receives electrons from water in linear electron flow?
Which electron flow best defines the standard non-cyclic pathway?
What happens to the four protons released by oxidation of two waters?
What is the direct role of ferredoxin–NADP⁺ reductase?
Why does a Z-scheme have two sharp rises in electron energy?
Practice Problems
- Trace non-cyclic flow from water to NADPH with the major carriers.
- Balance the water-oxidation reaction and explain where its products go.
- Why are two photosystems needed in the usual linear pathway?
- Explain why the Z-scheme is an energy representation rather than a cell map.
- Compare the roles of NADPH and ATP in later carbohydrate production.
Model Answers
1. Water oxidation replenishes PS II; excited electrons pass through plastoquinone, cytochrome b₆f and plastocyanin to PS I. PS I re-excites them and passes them to ferredoxin and FNR, producing NADPH.
2. 2H₂O → O₂ + 4H⁺ + 4e⁻. Electrons replenish PS II; H⁺ accumulates in the thylakoid lumen; O₂ diffuses away as a product.
3. PS II supplies powerful oxidising chemistry to withdraw electrons from water; PS I adds another light-driven energy input for their reduction of NADP⁺.
4. Its zigzag reflects electron-energy/redox changes during the sequence of acceptors; anatomical location is represented separately by a membrane diagram.
5. NADPH provides reducing electrons; ATP provides energy and phosphorylation for Calvin-cycle reactions. Both are produced using light-reaction mechanisms.
Key Takeaways
• Linear flow runs water → PS II → transport chain → PS I → NADP⁺, yielding reducing power as NADPH. • The Z-scheme shows two light-driven electron-energy rises. • PS II-linked water oxidation produces O₂, four protons and four electrons per two waters. • Cytochrome-linked transfers help build the proton gradient needed for ATP production. • NADPH supplies reducing power while ATP powers carbon-fixation chemistry.