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Botany · Photosynthesis in Higher Plants

Light Reaction — PSI, PSII & Z-Scheme

The light reaction is the photochemical stage of photosynthesis in which solar energy is converted into the chemical currencies ATP and NADPH, and molecular oxygen is released as a by-product. It is anchored in §11.5–11.6 of the NCERT Class XI Biology textbook and is among the highest-yield sections for NEET, contributing one or two direct questions per paper on topics such as the absorption maxima of P680 and P700, the sequence of electron carriers, and the functional asymmetry between the two photosystems.

NCERT Grounding

Section 11.5 of NCERT Class XI Biology states: "The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II (PS II). These are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction." Section 11.6 then details the electron transport: from PS II through cytochromes to PS I, and finally to NADP+ reduction — a pathway the textbook names the Z-scheme because of the characteristic shape it traces on a redox potential graph.

"This whole scheme of transfer of electrons … is called the Z scheme, due to its characteristic shape."

NCERT Class XI Biology, §11.6

The Two Photosystems

Each photosystem is a supramolecular complex embedded in the thylakoid membrane. It consists of a light-harvesting complex (LHC) — also called the antenna — made up of hundreds of accessory pigment molecules (chlorophyll b, xanthophylls, carotenoids) bound to proteins, and a single special chlorophyll a molecule that forms the reaction centre. The accessory pigments funnel absorbed photon energy towards this reaction centre by resonance energy transfer.

Photosystem I vs Photosystem II — key identifiers

PS II

Reaction centre: P680

Photosystem II

  • Absorption maximum at 680 nm (red light)
  • Located in thylakoid membrane (grana lamellae)
  • Primary acceptor: pheophytin (modified Chl a)
  • Oxidises water — O2 evolution site
  • Passes electrons to plastoquinone (PQ)
  • Discovered after PSI despite functioning first
VS

PS I

Reaction centre: P700

Photosystem I

  • Absorption maximum at 700 nm (far-red light)
  • Present in both grana and stroma lamellae
  • Primary acceptor: iron-sulphur protein (Fe-S)
  • Reduces NADP+ to NADPH via ferredoxin
  • Receives electrons from plastocyanin (PC)
  • Discovered first — hence the lower number

The reaction centre of PSII, P680, absorbs at 680 nm. On absorbing a photon, P680 is excited to a higher energy state and ejects a high-energy electron to its primary acceptor, pheophytin. This leaves P680 in an oxidised state (P680+), which is one of the strongest biological oxidising agents known — powerful enough to extract electrons from water molecules, a reaction with an exceptionally positive reduction potential.

Electron Transport Chain

The electron transport chain (ETC) of the light reaction consists of a series of protein-bound redox carriers arranged in order of increasing reduction potential. The overall direction of spontaneous electron flow is from carriers with more negative reduction potentials (higher energy) to those with more positive reduction potentials (lower energy) — releasing free energy at each step. This energy is coupled to proton translocation and ATP synthesis.

Non-cyclic electron flow — Z-scheme sequence

Water → NADPH
  1. Step 1

    Water splitting

    2H2O → 4H+ + 4e– + O2. Occurs on lumenal face of thylakoid. Electrons supplied to P680+.

    OEC (Mn cluster)
  2. Step 2

    PSII excitation

    P680 absorbs 680 nm photon. Excited electron passed to pheophytin (primary acceptor).

    P680 → Pheo
  3. Step 3

    Plastoquinone (PQ)

    PQ accepts electrons + H+ from stroma; diffuses through membrane to Cyt b6f. Carries both e– and H+.

    Mobile carrier
  4. Step 4

    Cytochrome b6f

    Transfers electrons from PQ to plastocyanin; releases H+ into lumen — key ATP-driving step.

    Proton pump
  5. Step 5

    Plastocyanin (PC)

    Copper-containing soluble protein in lumen; shuttles e– from Cyt b6f to oxidised P700+ of PSI.

    Cu protein
  6. Step 6

    PSI excitation

    P700 absorbs 700 nm photon. Excited electron passed to Fe-S primary acceptor, then to ferredoxin (Fd).

    P700 → Fe-S → Fd
  7. Step 7

    NADPH formation

    Ferredoxin reduces NADP+ via NADP+ reductase (FNR) enzyme on stromal face. NADPH released to stroma.

    NADP+ → NADPH

The Z-Scheme Explained

The Z-scheme is a graphical representation of non-cyclic electron flow plotted on a scale of reduction potential (E°') — sometimes called the redox potential scale — where more negative values represent higher-energy (more reducing) states, and more positive values represent lower-energy (more oxidising) states.

Figure 1 — Z-scheme (redox potential diagram) Z-Scheme of Photosynthesis — Redox Potential Diagram Reduction Potential (E°') more negative ↑ −1.0 V −0.6 V 0.0 V +0.6 V +0.82 V PS II region PS I region H₂O/O₂ P680* P680 Pheophytin PQ Cyt b6f PC P700* P700 Fd NADPH hv hv Z-shape 680 nm photon 700 nm photon O₂ ↑

Figure 1. Z-scheme of non-cyclic electron transport plotted on a reduction potential axis. Upward arrows (coloured) represent photon-driven excitation steps that boost electrons to higher energy states. Downward arrows represent spontaneous, energy-releasing electron transfer steps. The path from H2O through PSII, PQ, Cyt b6f, PC, PSI, Fd to NADPH traces the letter Z when the two photon-driven "lifts" are included.

The name "Z-scheme" arises purely from the visual shape of the pathway on this graph. There are two energy-input steps — the PSII excitation and the PSI excitation — each of which raises electrons to a higher (more negative) reduction potential. Between these two inputs, electrons flow spontaneously downhill through PQ and the cytochrome b6f complex, releasing energy that is coupled to proton translocation across the thylakoid membrane.

Mobile Electron Carriers — PQ, PC and Fd

Three mobile carriers connect the fixed protein complexes of the electron transport chain. Each has a distinct chemical nature and location, and these details are directly testable in NEET.

Key rule: Each mobile carrier spans a specific gap in the chain and cannot be substituted for another — loss of any one blocks the entire non-cyclic pathway.

Plastoquinone (PQ)

PSII → Cyt b6f

Location in chain

Nature: Lipid-soluble quinone; mobile in thylakoid membrane bilayer

Function: Accepts 2e– and 2H+ from PSII; diffuses to Cyt b6f; deposits both — releasing H+ into lumen (proton pump role)

NEET note: NEET 2020 tested the PQ → Cyt b6f direction specifically

NEET 2020 Q.7

Plastocyanin (PC)

Cyt b6f → PSI

Location in chain

Nature: Small, copper-containing soluble protein; located in thylakoid lumen

Function: Accepts 1e– at a time from Cyt b6f; shuttles to oxidised P700+ to reduce it before the next photon can drive PSI

Remember: PC contains copper (Cu); its blue colour in oxidised form is diagnostically useful

Ferredoxin (Fd)

PSI → NADP+ reductase

Location in chain

Nature: Small, iron-sulphur (Fe-S) protein; located on stromal face of thylakoid

Function: Accepts excited electrons from PSI primary acceptor; reduces NADP+ to NADPH via the enzyme NADP+ reductase (FNR)

In cyclic: Fd can return electrons to Cyt b6f rather than NADP+ — generating only ATP, not NADPH

Products of Non-Cyclic Photophosphorylation

Non-cyclic photophosphorylation is the coordinated operation of both PSII and PSI in series, linked by the electron transport chain. It is called "non-cyclic" because electrons originate from water molecules and are ultimately deposited on NADP+ — they do not return to their source. Three distinct products emerge:

ATP

Product 1 — from chemiosmosis

Synthesised by ATP synthase (CF0-CF1) as protons flow from the lumen back into the stroma. The proton gradient is built by PQ carrying H+ across the membrane and by water splitting on the lumenal side.

+ NADPH

Product 2 — from PSI

Formed when ferredoxin reduces NADP+ via NADP+ reductase on the stromal face of the thylakoid membrane. NADPH is the primary reductant used in the Calvin cycle to fix CO2.

O2

Product 3 — from water splitting at PSII

Released into the thylakoid lumen and then diffused out as molecular oxygen. The oxygen-evolving complex (OEC) at PSII contains a cluster of four manganese ions (Mn4CaO5) that catalyses the oxidation of two water molecules per four electrons removed. This is the source of all photosynthetic O2 in Earth's atmosphere.

Cyclic photophosphorylation, by contrast, involves only PSI. Electrons excited in P700 are passed to Fd and then cycled back through Cyt b6f and PC to P700+ rather than being used to reduce NADP+. This generates only ATP — no NADPH and no O2. Cyclic flow supplements ATP supply particularly when the Calvin cycle demands more ATP than NADPH relative to what non-cyclic flow provides (the Calvin cycle requires 3 ATP per 2 NADPH consumed).

Worked Examples

Worked Example 1

A molecule is described as a lipid-soluble quinone that accepts electrons from the reaction centre of PSII and delivers them to the cytochrome b6f complex. Identify this molecule and state one additional function it performs beyond simple electron transfer.

Answer: Plastoquinone (PQ). In addition to transferring electrons, PQ simultaneously transports protons (H+) from the stroma to the thylakoid lumen. When PQ accepts electrons from PSII it picks up two protons from the stromal side; when it donates those electrons to the Cyt b6f complex at the lumenal face, it releases the protons into the lumen. This vectorial proton transport contributes directly to the proton gradient across the thylakoid membrane that drives ATP synthesis through the CF0-CF1 ATP synthase.

Worked Example 2

In an experiment, a plant is illuminated only with light of wavelength greater than 680 nm (far-red light, beyond the absorption peak of PSII). Predict the effect on (a) oxygen evolution and (b) NADPH production. Justify your answer.

Answer: (a) Oxygen evolution ceases. P680 of PSII has an absorption maximum at 680 nm and cannot be efficiently excited by wavelengths beyond 680 nm. Without PSII excitation, water splitting does not occur, so no O2 is produced. (b) NADPH production drops to near zero in non-cyclic photophosphorylation. PSI (P700) can be excited by far-red light, but without electrons supplied from PSII through the ETC, P700+ has no electron donor and cannot sustain continuous NADPH production. However, cyclic photophosphorylation using only PSI can continue, producing small amounts of ATP but no NADPH. This situation — called the Emerson enhancement effect in reverse — demonstrates that both photosystems must work cooperatively for full photosynthetic output.

Worked Example 3

Place the following electron carriers in the correct sequence of non-cyclic electron flow, starting from the electron donor and ending at the final acceptor: Ferredoxin, Plastocyanin, Pheophytin, NADP+, Plastoquinone, Cytochrome b6f.

Correct sequence: H2O → P680 → Pheophytin → Plastoquinone (PQ) → Cytochrome b6f → Plastocyanin (PC) → P700 → Ferredoxin (Fd) → NADP+ (→ NADPH). The two photosystems provide two separate "boosts" of energy at the P680 and P700 nodes; between these, electrons flow spontaneously down an energy gradient.

Common Confusion & NEET Traps

Property Non-cyclic photophosphorylation Cyclic photophosphorylation
Photosystems involved Both PSI and PSII PSI only
Electron source Water (H2O splitting) P700 of PSI (electrons recycled)
Final electron acceptor NADP+ (→ NADPH) P700+ (electrons return to start)
ATP produced? Yes Yes
NADPH produced? Yes No
O2 evolved? Yes No
Probable location Grana lamellae Stroma lamellae

NEET PYQ Snapshot — Light Reaction & Z-Scheme

Direct questions from official NEET papers on photosystem absorption maxima and electron carrier sequence.

NEET 2023 · Q.115

The reaction centre in PS II has an absorption maxima at:

  1. 780 nm
  2. 680 nm
  3. 700 nm
  4. 660 nm
Answer: (2) 680 nm

Why: The reaction centre chlorophyll a of PSII is designated P680 precisely because its absorption maximum is at 680 nm. Option (3) — 700 nm — is the absorption maximum of PSI's reaction centre (P700). Option (1) — 780 nm — lies in the near-infrared and is outside the functional range of these reaction centres. Option (4) — 660 nm — is a distractor; 660 nm lies within the red absorption band of Chl a but is not the designated peak of either reaction centre chlorophyll. Candidates who confuse P680 with P700 select option (3).

NEET 2020 · Q.7

In light reaction, plastoquinone facilitates the transfer of electrons from:

  1. Cytb6f complex to PS-I
  2. PS-I to NADP+
  3. PS-I to ATP synthase
  4. PS-II to Cytb6f complex
Answer: (4) PS-II to Cytb6f complex

Why: Plastoquinone is the mobile lipid-soluble quinone that collects electrons from the primary acceptor of PSII and delivers them to the cytochrome b6f complex. Option (1) describes the role of plastocyanin (PC), not PQ. Option (2) describes the role of ferredoxin (and NADP+ reductase). Option (3) is incorrect — PS-I is not connected to ATP synthase directly. The NEET trap is confusing PQ (PSII → Cyt b6f) with PC (Cyt b6f → PSI).

Concept

Which of the following is NOT a product of non-cyclic photophosphorylation?

  1. ATP
  2. NADPH
  3. O2
  4. CO2
Answer: (4) CO2

Why: Non-cyclic photophosphorylation produces ATP (via chemiosmosis), NADPH (via ferredoxin and NADP+ reductase at PSI), and O2 (via water splitting at PSII). Carbon dioxide is neither produced nor consumed during the light reaction; it is fixed in the Calvin cycle (dark/biosynthetic phase) using ATP and NADPH as substrates.

FAQs — Light Reaction & Z-Scheme

Frequently asked questions from NEET aspirants on PSI, PSII, and the Z-scheme electron pathway.

Why is PSII numbered before PSI if electrons flow through PSII first?

The photosystems are named in the order of their discovery, not their functional sequence. PSI was isolated and characterised before PSII. In the actual electron transport pathway, PSII functions first — it oxidises water and passes electrons down the chain to PSI.

What is the absorption maximum of the PSII reaction centre?

The reaction centre chlorophyll a of PSII (called P680) absorbs maximally at 680 nm, which falls in the red region of the visible spectrum. This was confirmed in NEET 2023 Q.115.

What role does plastoquinone play in the Z-scheme?

Plastoquinone (PQ) is a mobile, lipid-soluble electron carrier embedded in the thylakoid membrane. It accepts electrons from PSII and transfers them to the cytochrome b6f complex. PQ also carries protons from the stroma side to the lumen, contributing to the proton gradient that drives ATP synthesis.

What are the final products of non-cyclic photophosphorylation?

Non-cyclic photophosphorylation produces three end products: ATP (from chemiosmosis), NADPH (formed when ferredoxin reduces NADP+ via NADP+ reductase), and O2 (released by splitting of water at PSII).

Why is the electron transport pathway called the Z-scheme?

When all the electron carriers are plotted on a redox potential (reduction potential) scale, the path from water through PSII, down the electron transport chain, up through PSI, and finally to NADPH traces a shape resembling the letter Z — hence the name Z-scheme.

What is the function of ferredoxin in the light reaction?

Ferredoxin (Fd) is an iron-sulphur protein that accepts excited electrons from the primary acceptor of PSI. It then passes these electrons to the enzyme NADP+ reductase (ferredoxin-NADP+ reductase), which uses them to reduce NADP+ to NADPH.

What is plastocyanin and where does it act in the Z-scheme?

Plastocyanin (PC) is a small, copper-containing soluble protein located in the thylakoid lumen. It shuttles electrons from the cytochrome b6f complex to the oxidised reaction centre of PSI (P700+), thereby completing the connection between the two photosystems.

Related Topics
Photosynthesis in Higher Plants Back to the full chapter notes and topic index. Splitting of Water Photolysis at PSII — the oxygen-evolving complex in detail. Chemiosmosis in the Chloroplast How the proton gradient built by PQ drives ATP synthase. Cyclic vs Non-Cyclic Photophosphorylation When only PSI operates and why only ATP is produced. Photosynthetic Pigments Chlorophyll a, b, carotenoids and their absorption spectra. Action and Absorption Spectrum Why P680 and P700 absorb at their specific wavelengths. Calvin Cycle — C3 Pathway How ATP and NADPH from the light reaction fix CO2.