Z Scheme
Z திட்டம்
Stop confusing the Z-scheme with cyclic photophosphorylation. In the Z-scheme, electrons never return to PS II. If they did, you wouldn't get NADPH, and the Calvin cycle would starve. Precision is survival.
The idea
The Z-scheme describes the non-cyclic electron transport chain. Light energy excites PS II (P680), triggering photolysis of water. Electrons move through the cytochrome b6f complex to PS I (P700). Finally, NADP+ reductase reduces NADP+ to NADPH + H+.
Why it matters
It facilitates the continuous flow of electrons from water to NADP+, generating both ATP and NADPH.
NEET lens
NEET focuses on the sequence: PS II -> Plastoquinone -> Cytochrome b6f -> Plastocyanin -> PS I -> Ferredoxin -> NADP+. Remember, photolysis occurs only at PS II. Do not confuse the redox potential scale shape with the actual physical path.
Clinical significance
Herbicides like DCMU block electron flow between PS II and plastoquinone. This halts the Z-scheme, starving the plant of ATP and NADPH. In human toxicology, understanding this mechanism is vital for identifying plant-based poisoning.
Introduction
The Z scheme (non-cyclic photophosphorylation) describes the pathway of electron flow in light reactions — from water through PSII → plastoquinone → cytochrome b6f complex → plastocyanin → PSI → ferredoxin → NADP⁺ reductase → NADPH.
Core concept
Z scheme steps: Water (E₀ = +0.82V) → PSII (P680, excited by light) → plastoquinone (PQ) → cytochrome b6f complex (H⁺ pumped into lumen) → plastocyanin (PC) → PSI (P700, excited by light again) → ferredoxin (Fd) → NADP⁺ reductase (FNR) → NADPH. Shape of the curve (high to low to high to low energy) resembles letter Z when plotted on redox potential vs electron flow graph.
Go deeper
Two light excitation events are needed (PSII and PSI) because: thermodynamic requirement — to raise electrons from the energy level of water (+0.82V) to the level needed to reduce NADP⁺ (-0.32V) requires energy input at two points. One excitation is not enough.
From Class 10
During light reactions, electrons flow from water through a chain of molecules to finally reduce NADP⁺. This electron flow pathway is called the Z scheme.