BiologyCBSE Class 11 · Photosynthesis in Higher Plants

Calvin Cycle, C4 Pathway and Photorespiration

Updated 17 October 2026 · Fact-checked

The Calvin cycle (C3 pathway) fixes CO2 using RuBisCO into 3-phosphoglycerate, reduces it using ATP and NADPH, and regenerates RuBP; making one glucose needs 6 turns, 18 ATP and 12 NADPH. The C4 pathway fixes CO2 first into oxaloacetic acid by PEP carboxylase in mesophyll cells and releases it in bundle sheath cells, avoiding photorespiration.

Understand Calvin Cycle, C4 Pathway and Photorespiration

Calvin cycle (C3 pathway). Occurs in the stroma, in all photosynthetic plants. The first stable product is the 3-carbon 3-phosphoglyceric acid (PGA). Three stages:
1. Carboxylation: RuBP (ribulose bisphosphate, 5C) + CO₂ → 2 × PGA (3C), catalysed by RuBisCO (ribulose bisphosphate carboxylase-oxygenase), the most abundant enzyme on earth.
2. Reduction: PGA is converted to triose phosphate (sugar) using ATP and NADPH: 2 ATP and 2 NADPH per CO₂ fixed through this step.
3. Regeneration: RuBP is regenerated using 1 ATP per CO₂ fixed; the cycle can then continue.

Energy accounting. For each CO₂ fixed: 3 ATP and 2 NADPH. For one glucose (6 CO₂, 6 turns): 18 ATP and 12 NADPH. The cycle is not light-dependent directly (no light needed) but depends on products of the light reaction.

C4 pathway. In plants such as maize, sugarcane and sorghum (tropical, dry). Special Kranz anatomy: large bundle sheath cells around vascular bundles with many chloroplasts without grana (agranal). The first stable product is the 4-carbon oxaloacetic acid (OAA).

In mesophyll cells, PEP (phosphoenol pyruvate, 3C) + CO₂ → OAA, catalysed by PEP carboxylase (PEPcase), which has no affinity for O₂. OAA is changed to malic or aspartic acid, which moves to the bundle sheath.
In bundle sheath cells, these acids release CO₂ (decarboxylation) and form pyruvate; the CO₂ enters the Calvin cycle, the pyruvate returns to mesophyll to regenerate PEP (using ATP).
Advantages: high CO₂ concentration at RuBisCO, so little photorespiration; high productivity; tolerant of high temperatures (a higher temperature optimum than C3) and high light; C3 plants have an optimum 20 to 25 °C.

Photorespiration. RuBisCO can bind O₂ instead of CO₂ when O₂ is high and CO₂ low (oxygenase activity). RuBP + O₂ → one PGA + phosphoglycolate, with no ATP or NADPH synthesised; instead a pathway through chloroplast, peroxisome and mitochondrion releases CO₂ and wastes energy. No sugar is made. C4 plants avoid it by concentrating CO₂ in bundle sheath cells.

Factors affecting photosynthesis; law of limiting factors. Blackman's law (1905): when several factors affect a process, its rate is determined by the factor nearest its minimal value (the limiting factor). Factors: light (rate rises with intensity till saturation, at about 10 per cent of full sunlight for C3 plants), CO₂ concentration (major limiting factor; C4 plants respond to a rise up to about 360 µL L⁻¹ whereas C3 plants respond to higher levels), temperature (C4 higher), and water.

Flow chart: RuBP + CO₂ → PGA → (ATP, NADPH) → triose phosphate → sugar / RuBP.

Draw and label: Kranz anatomy of a C4 leaf (mesophyll, bundle sheath, vascular bundle).

Calvin Cycle, C4 Pathway and Photorespiration: key facts

Per CO2: $3 A T P + 2 N A D P H$; Calvin cycle
Per glucose: $6 C O 2∣18 A T P ∣12 N A D P H$; 6 turns
C3 first product: PGA (3C), by RuBisCO; RuBP + CO2
C4 first product: OAA (4C), by PEP carboxylase; PEP + CO2

How to answer Calvin Cycle, C4 Pathway and Photorespiration questions

Questions ask for steps, counts of ATP or NADPH, or to compare C3 and C4.

1For counts, multiply per-CO2 values by the number of CO2 fixed (6 for glucose).
2Name the enzyme and substrate for each carboxylation.
3Compare C3 and C4 on cell type, first product, enzyme and temperature.
4Link photorespiration to O2 binding by RuBisCO.
5Apply Blackman: choose the factor closest to minimum.

Calvin Cycle, C4 Pathway and Photorespiration in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Hot dry field

Suppose a maize field and a wheat field both suffer a very hot, dry spell with partly closed stomata.

Why does maize photosynthesise better, and what goes wrong in wheat?

Try it first, then show the solutionSolution

In wheat (C3) partly closed stomata lower internal CO2, so RuBisCO binds more O2 and photorespiration rises.
Maize (C4) concentrates CO2 in bundle sheath cells using PEPcase, so photorespiration is small.
Maize also has a higher temperature optimum.

Answer: C4 maize avoids photorespiration.

Memory hook: C4 keeps CO2 high.

Tricks to remember Calvin Cycle, C4 Pathway and Photorespiration

Short memory aids. Say each one aloud once, then use it on a question the same day.

3-2 rule
Per CO2: 3 ATP, 2 NADPH.
Use it when: Calvin cycle
PEP to OAA
PEP carboxylase makes OAA: 4 carbons.
Use it when: C4
Kranz = wreath
Bundle sheath wreath around the bundle.
Use it when: Anatomy

Calvin Cycle, C4 Pathway and Photorespiration: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: First products · 3 marks · Short answerBoard level

Name the first stable product, the enzyme and the CO2 acceptor in C3 and C4 plants.

Try it first, then show the solutionSolution

C3: PGA by RuBisCO with RuBP as acceptor (1 mark).
C4: OAA by PEP carboxylase with PEP as acceptor (1 mark).
C3 acceptor 5C, C4 acceptor 3C (1 mark).

Answer: PGA/RuBisCO/RuBP; OAA/PEPcase/PEP.

Example 2: Energy for glucose · 3 marks · Short answerBoard level

A leaf must build 4 molecules of glucose. How many CO2 molecules must be fixed and how much ATP and NADPH is consumed? Show the per-CO2 working.

Try it first, then show the solutionSolution

Per CO2: 3 ATP and 2 NADPH; each glucose needs 6 CO2, so 4 glucose need 24 CO2 (1 mark).
ATP = 24 x 3 = 72 (1 mark).
NADPH = 24 x 2 = 48 (1 mark).

Answer: 24 CO2; 72 ATP; 48 NADPH.

Example 3: Three CO2 · 3 marks · Short answerHarder board level

For 3 CO2 fixed, find ATP and NADPH used, and how many triose phosphates leave the cycle in the net balance of 6 turns versus 3 turns.

Try it first, then show the solutionSolution

3 CO2: ATP = 9, NADPH = 6 (1 mark).
6 turns net 1 glucose (2 triose phosphates leave) (1 mark).
3 turns net 1 triose phosphate (1 mark).

Answer: 9 ATP, 6 NADPH; 3 turns give 1 triose phosphate.

Example 4: C3 vs C4 · 4 marks · Short answerHarder board level

Compare C3 and C4 plants on leaf anatomy, chloroplasts of bundle sheath, photorespiration and temperature optimum.

Try it first, then show the solutionSolution

Leaf: normal in C3; Kranz anatomy in C4 (1 mark).
Bundle sheath chloroplasts: absent in C3; present in C4 (agranal) (1 mark).
Photorespiration: present in C3, negligible in C4 (1 mark).
Temperature optimum: about 20 to 25 C versus a higher optimum (1 mark).

Answer: Four differences.

Example 5: Photorespiration · 5 marks · Long answerHOTS / competency

Why is photorespiration wasteful? Explain how CO2 level and O2 level determine it, and how C4 plants reduce it.

Try it first, then show the solutionSolution

RuBisCO binds O2 when CO2 is low and O2 is high (1 mark).
RuBP + O2 gives one PGA and phosphoglycolate, so less carbon is fixed (1 mark).
No ATP or NADPH is made; energy is used and CO2 released (1 mark).
It occurs in chloroplast, peroxisome and mitochondrion (1 mark).
C4 plants concentrate CO2 in bundle sheath cells, so RuBisCO prefers CO2 (1 mark).

Answer: Carbon and energy loss; C4 raises CO2.

Where students lose marks in Calvin Cycle, C4 Pathway and Photorespiration

The slips examiners see most often, and what to write instead.

Mistake: Carbon dioxide acceptor
C3 and C4 swapped.
Wrong working
PEP is the acceptor in C3.
Right working
RuBP in C3; PEP in C4.
Do this: RuBP is 5C.
Usually costs: 1 mark.
Mistake: ATP per glucose 9
Per CO2 not scaled.
Wrong working
Glucose needs 9 ATP.
Right working
18 ATP (6 x 3).
Do this: Multiply.
Usually costs: 1 mark.
Mistake: Calvin cycle in thylakoid
Site wrong.
Wrong working
Calvin cycle occurs in thylakoids.
Right working
Occurs in stroma.
Do this: Stroma.
Usually costs: 1 mark.

Exam tips

Show per CO2 then multiply by 6.
Name enzymes for each carboxylation.
Describe Kranz anatomy.
Link photorespiration to O2 binding.

Practice questions on Calvin Cycle, C4 Pathway and Photorespiration

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The enzyme with no affinity for oxygen that fixes CO2 in the C4 pathway is
(a) RuBisCO (b) PEP carboxylase (c) ATP synthase (d) catalase
Hint
Mesophyll.
Answer
(b).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): RuBisCO is both a carboxylase and an oxygenase.
Reason (R): It can bind either CO2 or O2 at the same active site.
(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.
Hint
Dual activity.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
What is the role of bundle sheath cells in C4 plants?
Hint
Kranz.
Answer
They receive malic or aspartic acid, release CO2 and feed it into the Calvin cycle with RuBisCO; they lack grana.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a plant has large bundle sheath cells with many chloroplasts, makes OAA first, and grows well in hot weather.
(a) Name the pathway. (1)
(b) Name the first carboxylating enzyme. (1)
(c) Name two examples. (1)
(d) What is formed from OAA? (1)
Hint
C4.
Answer
(a) C4. (b) PEP carboxylase. (c) Maize, sugarcane or sorghum. (d) Malic or aspartic acid.
Q5 · 3 marks · Short answerHOTS / competency
Suppose a lab raises the CO2 concentration for both a C3 and a C4 plant at moderate light. Predict which responds more and explain with reference to CO2 saturation of each pathway.
Hint
C4 saturates earlier.
Answer
C3 plants respond more because RuBisCO is not saturated at ambient CO2 and photorespiration falls with more CO2. C4 plants already concentrate CO2 internally and saturate at a lower level, so extra CO2 helps them little.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.First product of Calvin cycle?Show answer
PGA.
2.Enzyme of C4 carboxylation?Show answer
PEP carboxylase.
3.Abundant enzyme on earth?Show answer
RuBisCO.
4.What is Kranz anatomy?Show answer
Bundle sheath wreath in C4 leaves.
5.ATP per CO2?Show answer
3.
6.Who proposed the law of limiting factors?Show answer
Blackman.

Calvin Cycle, C4 Pathway and Photorespiration: frequently asked questions

What is the C3 pathway?

The Calvin cycle with PGA as first product.

Why are C4 plants efficient?

They avoid photorespiration and tolerate heat.

What is photorespiration?

RuBisCO using O2 instead of CO2.

Where does the Calvin cycle occur?

In the stroma.

What is Blackman's law?

Rate is limited by the factor nearest minimum.