This MCQ module is based on: Light Reaction Electron Transport
TOPIC 47 OF 90
Light Reaction Electron Transport
🎓 Class 11
Biology
CBSE
Theory
Ch 11 – Photosynthesis in Higher Plants
⏱ ~14 min
🌐 Language:
🧠 AI-Powered MCQ Assessment
▲
📝 Worksheet / Assessment
▲
This assessment will be based on: Light Reaction Electron Transport
Upload images, PDFs, or Word documents to include their content in assessment generation.
Light Reaction and the Electron Transport
In Part 1 we established where photosynthesis happens and which pigments capture the light. We now follow the light energy itself — from the moment a photon strikes a pigment molecule to the moment its energy is banked in the chemical bonds of ATP and in the reducing power of NADPH.
What the light reaction delivers. Light reactions, or the ‘photochemical’ phase, include light absorption, water splitting, oxygen release, and the formation of the high-energy chemical intermediates ATP and NADPH. Several protein complexes are involved in the process.
11.5 What is Light Reaction?
The pigments are organised, not scattered
The four pigments you met in Part 1 do not float free in the thylakoid membrane. They are organised into two discrete photochemical light harvesting complexes (LHC) within Photosystem I (PS I) and Photosystem II (PS II).
A naming trap worth remembering. The two photosystems are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction. PS II acts first, PS I acts second. Almost every examination question on the Z scheme is testing whether you know this.
The LHC are made up of hundreds of pigment molecules bound to proteins. Each photosystem has all the pigments — except one molecule of chlorophyll a — forming a light harvesting system also called the antenna. These pigments help make photosynthesis more efficient by absorbing different wavelengths of light.
That single excepted chlorophyll a molecule forms the reaction centre. The reaction centre is different in the two photosystems:
| Photosystem | Reaction centre chlorophyll a | Absorption peak | Position in the sequence of working |
|---|---|---|---|
| PS I | P700 | 700 nm | Acts second |
| PS II | P680 | 680 nm (red light) | Acts first |
Notice the logic of the arrangement. A single chlorophyll a molecule can only absorb the narrow band of wavelengths it is tuned to, and photons arriving at any other wavelength would be wasted. By surrounding that one molecule with hundreds of assorted pigments, the plant collects photons across a wide slice of the visible spectrum and delivers their energy to a single point where the photochemistry — the actual ejection of an electron — can take place.
🎯 Interactive: Explore the two photosystems
Where it sits: Grana and stroma lamellae — in both photosystems
Hundreds of pigment molecules bound to proteins. Contains all the pigments except one chlorophyll a. Absorbs different wavelengths and funnels the energy to the reaction centre.
11.6 The Electron Transport
Step 1 — PS II absorbs 680 nm light
In Photosystem II the reaction centre chlorophyll a absorbs 680 nm wavelength of red light, causing electrons to become excited and jump into an orbit farther from the atomic nucleus. These electrons are picked up by an electron acceptor which passes them to an electron transport system consisting of cytochromes.
This movement of electrons is downhill, in terms of an oxidation–reduction (redox) potential scale. An important point: the electrons are not used up as they pass through the electron transport chain — they are passed on to the pigments of photosystem PS I.
Step 2 — PS I absorbs 700 nm light
Simultaneously, electrons in the reaction centre of PS I are also excited when they receive red light of wavelength 700 nm, and are transferred to another acceptor molecule that has a greater redox potential. These electrons are then moved downhill again, this time to a molecule of energy-rich NADP⁺. The addition of these electrons reduces NADP⁺ to NADPH + H⁺.
The Z scheme. This whole scheme of transfer of electrons — starting from the PS II, uphill to the acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor, and finally downhill to NADP⁺ reducing it to NADPH + H⁺ — is called the Z scheme, due to its characteristic shape. This shape is formed when all the carriers are placed in a sequence on a redox potential scale.
📐 Activity 11.2 — Trace one electron through the Z scheme
What to do. Draw a large blank redox-potential axis on a sheet of paper, with “more negative” at the top. Then place these eight labels on it in the order an electron actually meets them, and join them with arrows: NADP⁺, P680, water splitting complex, cytochromes, acceptor of PS I, P700, acceptor of PS II, NADPH.
Correct order: water splitting complex → P680 (PS II) → acceptor of PS II (uphill — needs a photon) → cytochromes (downhill — releases energy, ATP is made here) → P700 (PS I) → acceptor of PS I (uphill again — needs a second photon) → NADP⁺ → NADPH.
What the drawing proves. Two separate photons are needed per electron, one at each photosystem, because a single photon cannot lift an electron all the way from the redox level of water to that of NADP⁺. The two uphill jumps with a downhill run in between is exactly what produces the letter Z on its side. It also shows you where the energy for ATP comes from: the downhill stretch, not the uphill jumps.
11.6.1 Splitting of Water
A question follows immediately: how does PS II supply electrons continuously? The electrons that were moved away from photosystem II must be replaced. This is achieved by electrons available due to the splitting of water.
The splitting of water is associated with PS II. Water is split into 2H⁺, [O] and electrons. This creates oxygen, one of the net products of photosynthesis. And note the direction of supply carefully: the electrons needed to replace those removed from photosystem I are provided by photosystem II.
2H2O → 4H+ + O2 + 4e−
In-text question — where are the protons and O₂ released? The chapter emphasises that the water splitting complex is associated with PS II, which is itself physically located on the inner side of the membrane of the thylakoid. Since the complex faces inward, the products of the splitting are released into the lumen of the thylakoid, not on the outer (stroma) side. This single anatomical fact is the foundation of the chemiosmotic explanation of ATP synthesis that we take up in Part 3 — protons dumped into the lumen are what build the gradient. The oxygen, being a small uncharged molecule, then diffuses out of the chloroplast.
11.6.2 Cyclic and Non-cyclic Photo-phosphorylation
Living organisms have the capability of extracting energy from oxidisable substances and storing it in the form of bond energy. Special substances like ATP carry this energy in their chemical bonds. The process through which ATP is synthesised by cells (in mitochondria and chloroplasts) is named phosphorylation.
Photo-phosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.
Non-cyclic photophosphorylation
When the two photosystems work in a series — first PS II and then PS I — a process called non-cyclic photo-phosphorylation occurs. The two photosystems are connected through an electron transport chain, as seen earlier in the Z scheme. Both ATP and NADPH + H⁺ are synthesised by this kind of electron flow.
Cyclic photophosphorylation
When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to a cyclic flow of electrons.
A possible location where this could be happening is in the stroma lamellae. While the membrane or lamellae of the grana have both PS I and PS II, the stroma lamellae membranes lack PS II as well as NADP reductase enzyme. The excited electron therefore does not pass on to NADP⁺ but is cycled back to the PS I complex through the electron transport chain.
The consequence. The cyclic flow results only in the synthesis of ATP, but not of NADPH + H⁺. Cyclic photophosphorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation — because at those wavelengths P680 of PS II cannot be excited, leaving only P700 to work.
| Feature | Non-cyclic | Cyclic |
|---|---|---|
| Photosystems involved | Both PS II and PS I, working in series — PS II first | Only PS I |
| Path of the electron | Open path: water → PS II → ETS → PS I → NADP⁺ | Closed path: electron is cycled back to the PS I complex |
| Likely site | Grana lamellae (they contain both photosystems) | Stroma lamellae (they lack PS II and NADP reductase) |
| External electron donor | Water — which is split | None needed; the same electron returns |
| Products | ATP, NADPH + H⁺ and O₂ | Only ATP |
| Also occurs when | Light of both 680 nm and 700 nm is available | Only light of wavelengths beyond 680 nm is available |
Why would a plant ever need a pathway that makes ATP alone? The answer waits in Part 3, but you can already anticipate it: the carbon-fixing reactions consume ATP and NADPH in unequal amounts, and the shortfall has to be made up by a route that produces ATP without any NADPH.
🎯 Competency-Based Questions
Scenario: A researcher works with isolated thylakoid preparations. Preparation P is made from grana lamellae. Preparation Q is made from stroma lamellae only. Each preparation is supplied with ADP, inorganic phosphate, NADP⁺ and water, and then illuminated — first with light of 660 nm, then with light of 720 nm.
Q1. Which preparation will release oxygen, and why? L3 Apply
Only preparation P. Oxygen release requires the water splitting complex, which is associated with PS II. The grana lamellae contain both PS I and PS II, whereas the stroma lamellae lack PS II, so preparation Q has no water splitting complex and can release no oxygen at all.
Q2. Under 720 nm light, preparation P also stops making NADPH although it contains PS II. Explain. L4 Analyse
P680, the reaction centre of PS II, has its absorption maximum at 680 nm. Light of 720 nm carries too little energy per photon to excite P680, so PS II is effectively switched off even though it is physically present. With PS II idle there is no electron flow from water into the chain, so the series arrangement of the Z scheme breaks. Only P700 of PS I is excited, and the chapter states explicitly that cyclic photophosphorylation also occurs when only light of wavelengths beyond 680 nm is available. The result is ATP but no NADPH.
Q3. Fill in the blanks: In PS I the reaction centre chlorophyll a has an absorption peak at ______ nm and is called ______, while in PS II it has absorption maxima at ______ nm and is called ______. The photosystems are named in the sequence of their ______, not the sequence in which they ______. L1 Remember
700; P700; 680; P680; discovery; function (PS II functions first).
Q4. An inhibitor is added that blocks the cytochrome carriers between the two photosystems but leaves both reaction centres intact. Predict the effect on (a) oxygen evolution, (b) NADPH formation, (c) ATP formation. L4 Analyse
(a) Oxygen evolution stops very soon. PS II can eject its electron once, but that electron cannot leave the chain, so PS II cannot accept a replacement from water — water splitting halts. (b) NADPH formation stops in the non-cyclic route, since PS I is no longer fed by PS II. (c) ATP formation continues, but only by the cyclic route through PS I, because the cyclic path does not require the PS II–PS I connection. This is why an inhibitor at this single point can separate ATP synthesis from NADPH synthesis experimentally.
Q5. A student writes: “The Z scheme is Z-shaped because the electrons physically travel in a zig-zag path across the thylakoid membrane.” Evaluate this statement. L5 Evaluate
The statement is incorrect in its reasoning, even though it reaches the right shape. The chapter is explicit: the Z shape is formed when all the carriers are placed in a sequence on a redox potential scale. The axis of the diagram is redox potential, not physical distance. The zig-zag records two things — two energy-absorbing uphill jumps driven by photons at PS II and PS I, separated by an energy-releasing downhill run through the cytochromes. A student who thinks the diagram is a map of the membrane will be unable to explain why two photons per electron are needed, which is the real content of the figure. A correct version: the Z shape arises from plotting the carriers in order of their redox potentials, showing two light-driven rises and two falls.
🧠 Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): Photosystem II functions before Photosystem I in the light reaction.
Reason (R): The photosystems were named in the sequence of their discovery and not in the sequence in which they function.
Answer: B. Both statements are true, but the reason explains only why the names are misleading; it does not explain why PS II must act first. The actual explanation is that PS II receives electrons from the splitting of water and hands them on to PS I.
Assertion (A): Cyclic photophosphorylation produces ATP but no NADPH.
Reason (R): The stroma lamellae lack PS II as well as the NADP reductase enzyme, so the excited electron is cycled back to PS I instead of reaching NADP⁺.
Answer: A. Both are true and the reason is the correct explanation. Without NADP reductase the electron has no route to NADP⁺, so it returns to the P700 complex, and the energy released on the way is used only for ATP synthesis.
Assertion (A): Electrons are consumed as they pass along the electron transport chain between the two photosystems.
Reason (R): Water is split to replace the electrons removed from PS II.
Answer: D. The assertion is false — the chapter states that the electrons are not used up as they pass through the chain, but are passed on to the pigments of PS I. The reason is true: water splitting replaces the electrons that left PS II for PS I, which is a different matter from electrons being consumed in transit.
Coming next. Part 3 takes up Section 11.6.3 and Section 11.7 — the chemiosmotic hypothesis and how the proton gradient across the thylakoid membrane drives ATP synthase, then the biosynthetic phase: the primary acceptor of CO₂, and the three stages of the Calvin cycle with its full ATP and NADPH budget.
Frequently Asked Questions - Light Reaction and the Electron Transport
Why is PS II numbered second if it acts first in photosynthesis?
The photosystems were named in the sequence of their discovery, not in the sequence in which they function during the light reaction. PS I was identified first, so it took the number one, but in the Z scheme electrons flow from water to PS II and only then to PS I.
What exactly is P680 and P700?
Each photosystem contains one special molecule of chlorophyll a that forms the reaction centre, while all the other pigments form the surrounding antenna. In PS I this reaction centre chlorophyll a has an absorption peak at 700 nm and is called P700; in PS II it absorbs maximally at 680 nm and is called P680.
Why is the Z scheme called a Z scheme?
Because of its characteristic shape, which appears when all the electron carriers are placed in a sequence on a redox potential scale. The electron rises uphill at PS II, runs downhill through the cytochromes, rises uphill again at PS I, and finally falls to NADP+, tracing a sideways Z.
Where does the oxygen released in photosynthesis come from, and where is it released?
It comes from the splitting of water, which is associated with PS II: 2H2O gives 4H+, O2 and 4 electrons. Because the water splitting complex sits on the inner side of the thylakoid membrane, the protons and oxygen are released into the thylakoid lumen, and the oxygen then diffuses out of the chloroplast.
What is the difference between cyclic and non-cyclic photophosphorylation?
In non-cyclic photophosphorylation both photosystems work in series, PS II first and then PS I, connected by an electron transport chain, and both ATP and NADPH + H+ are produced along with oxygen. In cyclic photophosphorylation only PS I is functional, the electron is circulated back to the PS I complex, and only ATP is produced with no NADPH.
When does cyclic photophosphorylation take place in a plant?
It is thought to occur in the stroma lamellae, which lack PS II and the NADP reductase enzyme, and it also occurs whenever only light of wavelengths beyond 680 nm is available, since such light cannot excite P680 of PS II.
What is photophosphorylation?
Phosphorylation is the process by which cells synthesise ATP, in mitochondria and in chloroplasts. Photophosphorylation is specifically the synthesis of ATP from ADP and inorganic phosphate in the presence of light.
AI Tutor
Biology Class 11 – NCERT (2025-26)
Ready
🎁 Join our community and get free AI credits!