This MCQ module is based on: Chemiosmosis Calvin Cycle
TOPIC 48 OF 90
Chemiosmosis Calvin Cycle
🎓 Class 11
Biology
CBSE
Theory
Ch 11 – Photosynthesis in Higher Plants
⏱ ~14 min
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Chemiosmosis and the Calvin Cycle
Part 2 left us with electrons flowing and a vague promise that ATP appears somewhere along the way. That is not an explanation. We now examine how ATP is actually synthesised in the chloroplast, and then follow the ATP and NADPH into the reactions that build sugar.
11.6.3 Chemiosmotic Hypothesis
The chemiosmotic hypothesis has been put forward to explain the mechanism of ATP synthesis in the chloroplast. Like in respiration, in photosynthesis too ATP synthesis is linked to the development of a proton gradient across a membrane. This time these are the membranes of the thylakoid.
One crucial difference from respiration. Here the proton accumulation is towards the inside of the membrane, i.e., in the lumen. In respiration, protons accumulate in the intermembrane space of the mitochondria when electrons move through the ETS (Chapter 12). The machinery is analogous; the compartment that fills up is different.
What causes the proton gradient? Three contributions
To find the steps that cause a proton gradient to develop, we must look again at the activation of electrons and their transport.
(a) Splitting of water dumps protons into the lumen. Since splitting of the water molecule takes place on the inner side of the membrane, the protons or hydrogen ions that are produced by the splitting of water accumulate within the lumen of the thylakoids.
(b) Electron transport carries protons across the membrane. As electrons move through the photosystems, protons are transported across the membrane. This happens because the primary acceptor of electron, which is located towards the outer side of the membrane, transfers its electron not to an electron carrier but to an H carrier. Hence this molecule removes a proton from the stroma while transporting an electron. When this molecule passes on its electron to the electron carrier on the inner side of the membrane, the proton is released into the inner side or lumen side of the membrane.
(c) NADP reduction takes protons out of the stroma. The NADP reductase enzyme is located on the stroma side of the membrane. Along with electrons that come from the acceptor of electrons of PS I, protons are necessary for the reduction of NADP⁺ to NADPH + H⁺. These protons are also removed from the stroma.
Add the three together and the accounting is unambiguous: within the chloroplast, protons in the stroma decrease in number, while in the lumen there is accumulation of protons. This creates a proton gradient across the thylakoid membrane as well as a measurable decrease in pH in the lumen.
Why are we so interested in the proton gradient?
Because it is the breakdown of this gradient that leads to the synthesis of ATP. The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the CF₀ of the ATP synthase.
The ATP synthase enzyme consists of two parts:
- CF₀ — embedded in the thylakoid membrane, forming a transmembrane channel that carries out facilitated diffusion of protons across the membrane.
- CF₁ — protrudes on the outer surface of the thylakoid membrane, on the side that faces the stroma.
The breakdown of the gradient provides enough energy to cause a conformational change in the CF₁ particle of the ATP synthase, which makes the enzyme synthesise several molecules of energy-packed ATP.
The four requirements of chemiosmosis. Chemiosmosis requires (1) a membrane, (2) a proton pump, (3) a proton gradient and (4) ATP synthase. Energy is used to pump protons across a membrane, to create a gradient or a high concentration of protons within the thylakoid lumen. ATP synthase has a channel that allows diffusion of protons back across the membrane; this releases enough energy to activate the ATP synthase enzyme that catalyses the formation of ATP.
Along with the NADPH produced by the movement of electrons, the ATP will be used immediately in the biosynthetic reaction taking place in the stroma, responsible for fixing CO₂ and synthesis of sugars.
🎯 Interactive: Where do the protons come from and go?
Effect on the lumen: Protons are added to the lumen
The water splitting complex sits on the inner side of the thylakoid membrane, so the 2H+ released when water is split accumulate inside the lumen. This is the first and most direct contribution to the gradient.
11.7 Where are the ATP and NADPH Used?
The products of the light reaction are ATP, NADPH and O₂. Of these, O₂ diffuses out of the chloroplast, while ATP and NADPH are used to drive the processes leading to the synthesis of food — more accurately, sugars. This is the biosynthetic phase of photosynthesis.
This process does not directly depend on the presence of light but is dependent on the products of the light reaction, i.e., ATP and NADPH, besides CO₂ and H₂O.
In-text question — how could this be verified? It is simple: immediately after light becomes unavailable, the biosynthetic process continues for some time, and then stops. If light is then made available, the synthesis starts again. The brief continuation shows the reactions are not themselves photochemical; the eventual stop shows they cannot run without the ATP and NADPH that only light can make.
In-text question — is calling the biosynthetic phase the “dark reaction” a misnomer? Discuss. Yes, it is misleading, and for two distinct reasons. First, these reactions do not require darkness — in a leaf in sunlight they run all day, in fact fastest then, because that is when ATP and NADPH are abundant. Second, they are not independent of light; they merely are not directly driven by photons. The experiment above proves the dependence. Some enzymes of the pathway are additionally light-activated. Accurate alternatives: biosynthetic phase, carbon fixation reactions, or light-independent reactions — the last being acceptable only if “independent” is understood as “not directly photochemical”.
The search for the first product of CO₂ fixation
We saw earlier that CO₂ is combined with H₂O to produce (CH₂O)n or sugars. It was of interest to scientists to find out what was the first product formed when CO₂ is taken into a reaction, or fixed.
Just after World War II, among the several efforts to put radioisotopes to beneficial use, the work of Melvin Calvin is exemplary. His use of radioactive ¹⁴C in algal photosynthesis studies led to the discovery that the first CO₂ fixation product was a 3-carbon organic acid. He also contributed to working out the complete biosynthetic pathway; hence it was called the Calvin cycle after him. The first product identified was 3-phosphoglyceric acid, or in short PGA.
In-text question — how many carbon atoms does PGA have? Three. The name says so: 3-phosphoglyceric acid, a 3-carbon organic acid. This is precisely why the pathway is called the C₃ pathway.
Scientists also tried to know whether all plants have PGA as the first product of CO₂ fixation. Experiments conducted over a wide range of plants led to the discovery of another group of plants where the first stable product of CO₂ fixation was again an organic acid, but one which had 4 carbon atoms. This acid was identified to be oxaloacetic acid, or OAA.
Since then, CO₂ assimilation during photosynthesis was said to be of two main types:
- plants in which the first product of CO₂ fixation is a C₃ acid (PGA) — the C₃ pathway;
- plants in which the first product is a C₄ acid (OAA) — the C₄ pathway.
11.7.1 The Primary Acceptor of CO₂
Here is the question that puzzled the scientists studying the ‘dark reaction’: how many carbon atoms would a molecule have which, after accepting (fixing) CO₂, would have 3 carbons (of PGA)?
The obvious arithmetic suggests two. And that is exactly the trap the researchers fell into: they believed that since the first product was a C₃ acid, the primary acceptor would be a 2-carbon compound, and they spent many years trying to identify a 2-carbon compound.
The unexpected answer. The studies very unexpectedly showed that the acceptor molecule was a 5-carbon ketose sugar — ribulose bisphosphate (RuBP). The arithmetic works because 5 + 1 = 6 carbons, and that 6-carbon intermediate at once splits into two molecules of 3-PGA, not one. Read the reaction as a split, not an addition, and the puzzle dissolves.
11.7.2 The Calvin Cycle
Calvin and his co-workers then worked out the whole pathway and showed that the pathway operated in a cyclic manner — the RuBP was regenerated.
A statement to hold on to. The Calvin pathway occurs in all photosynthetic plants; it does not matter whether they have C₃ or C₄ (or any other) pathways. The C₄ pathway, which we meet in Part 4, is an addition to the Calvin cycle, never a replacement for it.
For ease of understanding, the Calvin cycle can be described under three stages: carboxylation, reduction and regeneration.
Stage 1 — Carboxylation
Carboxylation is the fixation of CO₂ into a stable organic intermediate. It is the most crucial step of the Calvin cycle, where CO₂ is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase, which results in the formation of two molecules of 3-PGA.
Why the enzyme is called RuBisCO. Since this enzyme also has an oxygenation activity, it would be more correct to call it RuBP carboxylase-oxygenase, or RuBisCO. That second activity is not a detail — it is the whole basis of photorespiration, which we take up in Part 4.
Stage 2 — Reduction
These are a series of reactions that lead to the formation of glucose. The steps involve the utilisation of 2 molecules of ATP for phosphorylation and two of NADPH for reduction, per CO₂ molecule fixed. The fixation of six molecules of CO₂ and 6 turns of the cycle are required for the formation of one molecule of glucose from the pathway.
Stage 3 — Regeneration
Regeneration of the CO₂ acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. The regeneration steps require one ATP for phosphorylation to form RuBP.
The budget: what goes in, what comes out
Hence for every CO₂ molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required — two ATP in the reduction stage plus one ATP in the regeneration stage.
Why cyclic photophosphorylation exists. Look at the ratio: 3 ATP to 2 NADPH, not 1:1. It is probably to meet this difference in the number of ATP and NADPH used in the dark reaction that the cyclic photophosphorylation of Part 2 takes place — a route that makes ATP alone, topping up exactly the molecule that is needed in excess.
In-text question — how many ATP and NADPH are required to make one molecule of glucose? To make one molecule of glucose, 6 turns of the cycle are required, because glucose has six carbons and each turn fixes one CO₂. So multiply the per-CO₂ cost by six: ATP = 3 × 6 = 18 and NADPH = 2 × 6 = 12.
| In | Out |
|---|---|
| Six CO₂ | One glucose |
| 18 ATP | 18 ADP |
| 12 NADPH | 12 NADP |
📐 Activity 11.3 — Audit the carbon in one turn of the cycle
What to do. On paper, write the number of carbon atoms present at each named point in a single turn of the Calvin cycle: RuBP before carboxylation; the CO₂ that joins it; the total immediately after carboxylation; each PGA molecule; both PGA molecules together. Then answer: where did the extra carbon go, and why does the cycle need six turns rather than two to build a 6-carbon glucose?
The count. RuBP = 5 C. Incoming CO₂ = 1 C. Total = 6 C. Each 3-PGA = 3 C; two of them together = 6 C. Carbon balances exactly — nothing is lost, the 6-carbon intermediate simply splits into two equal halves.
Why six turns, not two. It is tempting to think that two turns give two 3-PGA pairs and therefore a 6-carbon sugar. But most of the fixed carbon must be spent rebuilding RuBP: the acceptor is a 5-carbon molecule and it has to be regenerated every turn, or the cycle stops. Only a small fraction of the triose phosphate leaves the cycle towards sucrose and starch. NCERT states the requirement plainly — the fixation of six molecules of CO₂ and 6 turns of the cycle are required for the formation of one molecule of glucose, at a cost of 18 ATP and 12 NADPH.
🎯 Competency-Based Questions
Scenario: Isolated intact chloroplasts are illuminated in a buffered medium and the pH of the thylakoid lumen is measured continuously. At time T1 an uncoupler is added that makes the thylakoid membrane freely permeable to protons, without damaging any protein complex. Electron transport and water splitting are monitored separately.
Q1. What happens to the lumen pH after T1, and what happens to ATP synthesis? L3 Apply
Before T1 the lumen shows a measurable decrease in pH as protons accumulate. After T1 the protons leak freely across the membrane, so the lumen pH rises back towards that of the stroma and the gradient collapses. Since it is the breakdown of the gradient through the CF₀ channel that provides the energy for the conformational change in CF₁, ATP synthesis stops — even though electron transport and water splitting may continue for a while.
Q2. Fill in the blanks: The ATP synthase of the chloroplast has two parts — ______ is embedded in the thylakoid membrane and forms a ______ that carries out ______ of protons, while ______ protrudes on the surface facing the ______. L1 Remember
CF₀; transmembrane channel; facilitated diffusion; CF₁; stroma.
Q3. A student claims that because the Calvin cycle uses 3 ATP and 2 NADPH per CO₂, non-cyclic photophosphorylation alone should be able to supply the leaf. Analyse the flaw. L4 Analyse
The flaw is the assumed ratio. Non-cyclic flow generates ATP and NADPH together, in step with electron flow, so it cannot easily produce ATP in excess of NADPH. The Calvin cycle, however, demands them in the ratio 3 : 2. NCERT notes that it is probably to meet this difference in the number of ATP and NADPH used in the dark reaction that cyclic photophosphorylation takes place — a pathway yielding ATP but no NADPH, which is exactly the correction required.
Q4. Early researchers hunted for years for a 2-carbon primary acceptor of CO₂. Why was that a reasonable guess, and what single structural fact made it wrong? L4 Analyse
It was reasonable because the first detected product was a 3-carbon acid, PGA, and simple arithmetic (2 C + 1 C from CO₂ = 3 C) points to a 2-carbon acceptor. The fact that made it wrong is that two molecules of PGA are formed, not one. The acceptor is the 5-carbon ketose sugar RuBP; 5 C + 1 C gives a 6-carbon intermediate that immediately splits into two 3-carbon PGA molecules. Reading the step as a split rather than a simple addition resolves the arithmetic and is the reason the discovery took so long.
Q5. Two students argue. One says respiration and photosynthesis use “the same” chemiosmotic mechanism; the other says they are fundamentally different. Evaluate both positions. L5 Evaluate
The first student is closer to the truth. Both processes satisfy the same four requirements — a membrane, a proton pump, a proton gradient and ATP synthase — and in both, ATP synthesis is linked to the development of a proton gradient across a membrane, broken down through a channel in the enzyme. The second student is right about one specific point, which NCERT itself flags: the compartment that accumulates protons differs. In photosynthesis protons accumulate towards the inside of the thylakoid, in the lumen; in respiration they accumulate in the intermembrane space of the mitochondria. The fair verdict: the mechanism is the same and is one of the great unifying ideas of bioenergetics; the geometry and the energy source driving the pump differ. Calling them “fundamentally different” overstates a difference of compartment.
🧠 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): The pH of the thylakoid lumen falls during the light reaction.
Reason (R): Water splitting occurs on the inner side of the thylakoid membrane and electron carriers release protons into the lumen, while NADP reduction removes protons from the stroma.
Answer: A. All three contributions push protons into the lumen or draw them out of the stroma, producing both the gradient and a measurable decrease in lumen pH.
Assertion (A): The Calvin cycle operates only in C₃ plants.
Reason (R): The first product of CO₂ fixation in C₃ plants is 3-phosphoglyceric acid.
Answer: D. The assertion is false — the Calvin pathway occurs in all photosynthetic plants, whether they have the C₃ or the C₄ (or any other) pathway. The reason is true but does not rescue the assertion.
Assertion (A): Carboxylation is described as the most crucial step of the Calvin cycle.
Reason (R): It is the step in which CO₂ is fixed into a stable organic intermediate by RuBisCO, giving two molecules of 3-PGA.
Answer: A. Until carboxylation occurs, no atmospheric carbon has entered the plant at all; the reduction and regeneration stages merely process and recycle what carboxylation captured.
Coming next. Part 4 takes up Sections 11.8 to 11.10 — the C₄ or Hatch and Slack pathway with Kranz anatomy, photorespiration and why RuBisCO behaves as an oxygenase, the complete C₃ versus C₄ comparison table, and the factors affecting the rate of photosynthesis including Blackman's Law of Limiting Factors.
Frequently Asked Questions - Chemiosmosis and the Calvin Cycle
What is the chemiosmotic hypothesis in photosynthesis?
It explains how ATP is made in the chloroplast: ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane. Protons accumulate in the thylakoid lumen, and their diffusion back to the stroma through the CF0 channel of ATP synthase releases enough energy to make ATP.
How is the proton gradient created across the thylakoid membrane?
Three ways. Splitting of water on the inner side of the membrane releases protons into the lumen; as electrons move through the photosystems, an H carrier takes a proton from the stroma and releases it into the lumen; and NADP reductase on the stroma side removes protons from the stroma to reduce NADP+ to NADPH.
What are CF0 and CF1 in ATP synthase?
CF0 is embedded in the thylakoid membrane and forms a transmembrane channel that carries out facilitated diffusion of protons. CF1 protrudes on the outer surface of the thylakoid membrane on the side facing the stroma, and the energy released by proton flow causes a conformational change in CF1 that makes it synthesise ATP.
Why is the term dark reaction a misnomer?
Because these reactions neither require darkness nor are independent of light. They are not directly driven by photons, but they depend on the ATP and NADPH made by the light reaction, which is why the biosynthetic process continues only for a short time after light is withdrawn and then stops. Better names are the biosynthetic phase or the carbon fixation reactions.
What is the primary acceptor of CO2 in the Calvin cycle?
Ribulose bisphosphate or RuBP, a 5-carbon ketose sugar. Scientists searched for years for a 2-carbon acceptor because the first product PGA has three carbons, before discovering that the 5-carbon RuBP plus CO2 splits into two molecules of 3-PGA.
What are the three stages of the Calvin cycle?
Carboxylation, in which RuBisCO fixes CO2 onto RuBP to give two molecules of 3-PGA; reduction, in which 2 ATP and 2 NADPH per CO2 are used to form carbohydrate; and regeneration, in which the acceptor RuBP is re-formed using one more ATP so the cycle can continue.
How many ATP and NADPH molecules are needed to make one glucose in the Calvin cycle?
Three ATP and two NADPH are needed per CO2 fixed, and six turns of the cycle are needed for one glucose. That gives 18 ATP and 12 NADPH for six CO2 and one molecule of glucose.
Does the Calvin cycle occur in C4 plants too?
Yes. The Calvin pathway occurs in all photosynthetic plants regardless of whether they use the C3 or C4 pathway. In C4 plants it simply takes place in the bundle sheath cells rather than the mesophyll cells.
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