The Light Reaction and the Z-Scheme: Following One Electron from Water to NADPH
You have memorised that photosynthesis makes oxygen and sugar, but NEET keeps asking sharper questions: where exactly does the oxygen come from, why is the light reaction drawn as a sideways "Z", and what do PS I and PS II really do? Once you see the light reaction as a story of electrons being pushed uphill by light and then sliding down, all of it falls into place.
This concept matters for NEET (Class 11, Plant Physiology: Photosynthesis in Higher Plants).
Quick Summary
The light reaction happens in the thylakoid membranes of the chloroplast and produces ATP, NADPH and O₂.
Two photosystems work in series: PS II (reaction centre P680) and PS I (reaction centre P700).
Water is split at PS II; its electrons replace those that PS II loses, and O₂ is released as a by-product.
Non-cyclic flow makes ATP and NADPH; cyclic flow (PS I only) makes ATP only.
ATP is made by chemiosmosis: a proton gradient builds up inside the thylakoid lumen and drives ATP synthase.
Start with what you already know
Think of a battery. To make a current flow you need something that gives up electrons easily and something that wants them. Light is the "charger" here: it lifts electrons in chlorophyll to a much higher energy level, from where they can roll downhill through a chain of carriers, releasing energy that the cell captures as ATP and NADPH.
Chlorophyll molecules are packed into units called photosystems. Each has many antenna pigments that collect light and funnel the energy to one special chlorophyll a molecule, the reaction centre. In PS II this centre absorbs best at 680 nm (called P680); in PS I it absorbs best at 700 nm (P700). The numbers are simply wavelengths in nanometres (nm).
The Z-scheme, step by step
Plot energy on the vertical axis and the electron's journey runs left to right like a sideways Z, hence the name.
PS II absorbs light. P680 is excited and gives an electron to a primary acceptor. P680 is now short of an electron and becomes a very strong oxidising agent.
Water is split. To refill P680, water on the lumen side of the thylakoid is broken down: 2H₂O → 4H⁺ + 4e⁻ + O₂. This is photolysis. The electrons go to P680, the H⁺ stay in the lumen, and O₂ is released. The manganese-containing oxygen-evolving complex, with Ca²⁺ and Cl⁻ as cofactors, does this job.
Downhill transport. The excited electron passes from the acceptor through plastoquinone (PQ), the cytochrome b₆f complex and plastocyanin (PC). While PQ is carrying electrons, it also moves protons from the stroma into the lumen, adding to the gradient.
PS I absorbs light. Electrons arriving through plastocyanin fill the gap left in P700 after it is excited by light and passes its own electron to another acceptor.
Uphill again, then to NADP⁺. The electron from P700 goes through ferredoxin and the enzyme NADP reductase, which uses it, with H⁺ from the stroma, to reduce NADP⁺ to NADPH.
Why ATP forms: chemiosmosis
Protons pile up in the lumen from three sources: water splitting, the PQ shuttle, and the removal of H⁺ from the stroma when NADP⁺ is reduced (this lowers the stroma's H⁺ count, which steepens the gradient). The thylakoid membrane is impermeable to protons, so they can only return through the CF₀ channel of ATP synthase. The CF₁ head uses that flow to join ADP and inorganic phosphate into ATP. This is photophosphorylation.
Cyclic versus non-cyclic
In non-cyclic photophosphorylation both photosystems work, and you get ATP, NADPH and O₂. In cyclic photophosphorylation only PS I is involved: the excited electron goes from the acceptor to the cytochrome complex and back to P700. Because it returns to where it started, there is no water splitting, no O₂ and no NADPH; only ATP is made. Cyclic flow occurs mainly in stroma lamellae, which lack PS II and NADP reductase, and it is useful when the cell needs extra ATP.
Worked example
How many photons-worth of electron transfers and how much NADPH does it take to release one molecule of O₂?
Splitting water gives O₂ only when two water molecules are used: 2H₂O → 4H⁺ + 4e⁻ + O₂.
So one O₂ means 4 electrons are released.
Each NADPH carries 2 electrons (NADP⁺ + 2e⁻ + H⁺ → NADPH, with the second H⁺ coming from the medium).
Therefore 4 electrons make 4 ÷ 2 = 2 NADPH.
Each electron passes through both photosystems, so it needs two photon absorptions. Total for 4 electrons: 4 × 2 = 8 photons.
Answer: one O₂ goes with 2 NADPH and a minimum of 8 photons.
Mistakes students actually make
Saying the O₂ comes from CO₂. It comes from water, as shown by experiments with heavy-oxygen water.
Reversing the order: electrons flow PS II → PS I, even though PS I is numbered first (it was discovered first).
Thinking water splitting happens in the stroma. It happens on the lumen side.
Claiming cyclic flow makes NADPH or O₂.
Mixing up where the proton gradient is: protons accumulate inside the lumen, and ATP is made on the stroma side.
How NEET asks it
Expect direct statement questions ("P680 is the reaction centre of..."), match-the-column items (PS I / PS II with P700 / P680, or cyclic / non-cyclic with products), assertion-reason on the source of O₂, and diagram-based questions where you must identify the carrier that comes after plastoquinone. Assertion questions about the location of ATP synthase CF₀ and CF₁ also recur.
Pro Tip: Remember the Z-scheme as "water feeds PS II, PS II feeds PS I, PS I feeds NADP⁺". Every time a question asks what is produced, trace the electron: if it came from water you get O₂, if it ended on NADP⁺ you get NADPH, and if it only looped inside PS I you get just ATP. Following one electron beats memorising a list.
Frequently Asked Questions
Where does the light reaction occur?
It occurs in the thylakoid membranes (grana and stroma lamellae) of the chloroplast. The Calvin cycle that follows takes place in the stroma, which is why the two stages are separated in space.
Why are there two photosystems instead of one?
One photon does not give an electron enough energy to travel from water all the way to NADP⁺. PS II lifts the electron enough to power the transport chain, and PS I lifts it again so it can reduce NADP⁺.
What is the role of cyclic photophosphorylation?
It supplies extra ATP without consuming water or producing NADPH. The Calvin cycle needs more ATP than NADPH, so cyclic flow helps balance the ratio.
Is PS I the same as P700?
No. PS I is the whole complex of proteins and pigments, while P700 is the specific chlorophyll a pair at its reaction centre. The same applies to PS II and P680.
Which light reaction products does the Calvin cycle use?
The Calvin cycle uses ATP and NADPH. The O₂ is released to the atmosphere. Without a steady supply of ATP and NADPH, carbon fixation soon stops even if CO₂ is plentiful.
Stuck on the Z-scheme or any step above? Drop your doubt in the comments, and share this with a fellow NEET aspirant who still mixes up PS I and PS II.
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