India's #1 AI Tutorformula-sheet · Biology · Chapter 11

Class 11 Biology Chapter 11 Photosynthesis in Higher Plants — Formulas & Key Points

Photosynthesis in Higher Plants is one of the most formula-intensive chapters in NCERT Class 11 Biology, requiring students to master chemical equations, pathway sequences and quantitative relationships. CBSE board exams and NEET consistently ask 2-3 mark questions on the overall equation, differences between C3 and C4 pathways, and light versus dark reactions. This revision sheet organizes every formula, definition and key point from Chapter 11 into ready-to-revise tables with proper chemical notation, units and application contexts.

Your child's private AI tutor — trained on NCERT.

3-day free trial · ₹1 to start · Cancel anytime.

Start 3-day free trial →

Key takeaways

✓The overall photosynthesis equation is 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O in the presence of light and chlorophyll.
✓Light reactions occur in thylakoid membranes producing ATP and NADPH; dark reactions occur in stroma fixing CO₂ into sugars.
✓C3 pathway (Calvin cycle) involves RuBisCO enzyme and first stable product is 3-phosphoglycerate (3-carbon compound).
✓C4 pathway first stable product is oxaloacetic acid (4-carbon); occurs in mesophyll and bundle sheath cells with minimal photorespiration.
✓Photorespiration occurs when RuBisCO binds O₂ instead of CO₂, reducing photosynthetic efficiency in C3 plants.
✓Action spectrum peaks match absorption spectrum peaks of chlorophyll a (430 nm and 662 nm) proving its primary role in photosynthesis.
✓The Emerson enhancement effect shows that red and far-red light together produce more photosynthesis than their sum individually, proving two photosystems exist.

Core Photosynthesis Equations and Stoichiometry

The overall equation of photosynthesis represents the net reaction but hides intermediate steps. Van Niel's experiments with purple and green sulphur bacteria proved that oxygen evolves from water, not CO₂. The Z-scheme shows the quantitative flow of electrons through photosystems II and I. Understanding stoichiometry helps calculate ATP and NADPH requirements for glucose synthesis. For every one glucose molecule synthesized in the Calvin cycle, 18 ATP and 12 NADPH molecules are consumed. The light reactions must therefore run multiple times to supply enough reducing power and energy currency for the dark reactions to proceed.

Light Reactions — Photosystems and Electron Transport

Light reactions convert light energy into chemical energy (ATP and NADPH) through photophosphorylation. Two types exist: non-cyclic (involves both PS II and PS I, produces ATP, NADPH and O₂) and cyclic (involves only PS I, produces only ATP). The P680 reaction centre in photosystem II absorbs 680 nm light; P700 in photosystem I absorbs 700 nm light. Photolysis of water occurs on the inner surface of thylakoid membrane, releasing electrons, protons and oxygen. The proton gradient across the thylakoid membrane drives ATP synthase (chemiosmotic hypothesis by Peter Mitchell). For non-cyclic photophosphorylation, the net equation per 2 electrons transferred is: 2H₂O + 2NADP⁺ + nADP + nPi → O₂ + 2NADPH + 2H⁺ + nATP, where n is approximately 2-3 depending on H⁺ stoichiometry.

PS II: P680 → Pheophytin → Plastoquinone → Cytochrome b₆f complex → Plastocyanin
PS I: P700 → A₀ (Chl a) → A₁ (Vitamin K) → Ferredoxin → NADP reductase → NADPH
Water photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂ (occurs at oxygen-evolving complex with Mn cluster)
ATP synthesis: The electrochemical gradient (about 3 pH units) drives conformational changes in CF₀-CF₁ ATP synthase

Dark Reactions — Calvin Cycle (C3 Pathway) Formulas

The Calvin cycle occurs in the stroma and does not directly require light but depends on ATP and NADPH from light reactions. It has three phases: carboxylation, reduction and regeneration. RuBP (ribulose-1,5-bisphosphate) is the CO₂ acceptor; RuBisCO (ribulose bisphosphate carboxylase-oxygenase) is the enzyme catalysing the first step. The first stable product is 3-phosphoglyceric acid (3-PGA), a 3-carbon compound, hence the name C3 pathway. To synthesize one glucose (6-carbon), six turns of the Calvin cycle are needed, requiring 6 CO₂, 18 ATP and 12 NADPH. The net equation for synthesis of one glucose is: 6CO₂ + 18ATP + 12NADPH + 12H₂O → C₆H₁₂O₆ + 18ADP + 18Pi + 12NADP⁺ + 6H₂O. Out of 12 molecules of 3-PGA formed, 10 are used to regenerate 6 RuBP molecules; 2 molecules condense to form glucose.

Carboxylation: 6 RuBP (5C) + 6 CO₂ → 12 3-PGA (3C) catalysed by RuBisCO
Reduction: 12 3-PGA + 12 ATP + 12 NADPH → 12 G3P (glyceraldehyde-3-phosphate) + 12 ADP + 12 NADP⁺ + 12 Pi
Regeneration: 10 G3P (3C) + 6 ATP → 6 RuBP (5C) + 6 ADP
Net sugar output: 2 G3P → 1 Glucose (via gluconeogenesis)

C4 Pathway (Hatch-Slack Pathway) — Key Equations and Anatomy

C4 plants like maize, sugarcane and sorghum have a unique leaf anatomy called Kranz anatomy with two types of photosynthetic cells: mesophyll and bundle sheath. The first stable product is oxaloacetic acid (OAA), a 4-carbon compound. CO₂ fixation occurs twice: first in mesophyll cells by PEP carboxylase (forming OAA), then in bundle sheath cells by RuBisCO (Calvin cycle). This spatial separation minimizes photorespiration because bundle sheath cells maintain high CO₂ concentration. PEP carboxylase has a higher affinity for CO₂ than RuBisCO and no oxygenase activity. The C4 pathway requires 5 ATP per CO₂ fixed (compared to 3 ATP in C3) but compensates by eliminating photorespiration losses. Mesophyll cell reaction: PEP (3C) + CO₂ + ATP → OAA (4C) + AMP + PPi. OAA is reduced to malate or converted to aspartate, transported to bundle sheath, decarboxylated to release CO₂ for the Calvin cycle.

Mesophyll: CO₂ + PEP (via PEP carboxylase) → OAA → Malate or Aspartate
Bundle sheath: Malate → Pyruvate + CO₂ (decarboxylation); CO₂ enters Calvin cycle
Pyruvate returns to mesophyll: Pyruvate + ATP + Pi → PEP + AMP + PPi (regeneration)
Net cost: 2 extra ATP per CO₂ (one for PEP carboxylase, one for PEP regeneration) beyond Calvin cycle requirement

Photorespiration and RuBisCO Dual Activity

Photorespiration is a wasteful pathway occurring when RuBisCO binds O₂ instead of CO₂, especially at high temperatures, high light intensity and low CO₂ concentration. It involves three organelles: chloroplast, peroxisome and mitochondria. When RuBP reacts with O₂, one molecule of 3-PGA and one molecule of 2-phosphoglycolate (2C) are formed. The 2-phosphoglycolate is metabolized through the photorespiratory pathway, consuming ATP and releasing CO₂ without producing ATP or NADPH, reducing net photosynthetic efficiency by 25-50% in C3 plants. The salvage pathway converts two molecules of glycolate (2C) into one molecule of 3-PGA (3C), releasing one CO₂. C4 and CAM plants have mechanisms to minimize photorespiration by maintaining high CO₂ concentration around RuBisCO.

Oxygenase reaction: RuBP + O₂ → 3-PGA (3C) + 2-Phosphoglycolate (2C)
Photorespiratory pathway: 2 Glycolate (2C each) → 1 3-PGA (3C) + 1 CO₂ + 1 NH₃
Energy cost: Consumes ATP in peroxisomes; releases fixed CO₂ (net carbon loss)
Conditions favouring photorespiration: High O₂/CO₂ ratio, high temperature (above 30°C), high light, water stress

CAM Pathway (Crassulacean Acid Metabolism) and Temporal Separation

CAM plants (succulents like Opuntia, Pineapple, Bryophyllum) separate CO₂ fixation and Calvin cycle temporally rather than spatially. Stomata open at night when transpiration loss is minimal, allowing CO₂ entry. CO₂ is fixed into oxaloacetic acid (OAA) by PEP carboxylase, then reduced to malate and stored in vacuoles. During the day, stomata close to conserve water; stored malate is decarboxylated to release CO₂ for the Calvin cycle. This adaptation allows survival in arid environments but results in slow growth due to limited CO₂ availability. The night fixation reaction is: CO₂ + PEP → OAA → Malate (stored in vacuole as malic acid). Day decarboxylation: Malate → Pyruvate + CO₂; the released CO₂ enters the Calvin cycle behind closed stomata, maintaining internal CO₂ concentration.

Night (stomata open): PEP + CO₂ → OAA → Malate → Vacuolar storage
Day (stomata closed): Malate → CO₂ + Pyruvate; CO₂ → Calvin cycle
Advantage: Minimal water loss; survival in extreme arid and saline conditions
Disadvantage: Limited CO₂ uptake capacity; slow growth rate compared to C3 and C4 plants

Photosynthetic Pigments — Absorption and Action Spectra

Chlorophyll a is the primary photosynthetic pigment with absorption peaks at 430 nm (blue) and 662 nm (red). Chlorophyll b absorbs at 453 nm and 642 nm, acting as an accessory pigment transferring energy to chlorophyll a. Carotenoids (β-carotene, xanthophylls) absorb in the blue-green region (400-550 nm) and protect against photooxidative damage. The action spectrum (rate of photosynthesis vs wavelength) closely matches the absorption spectrum of chlorophyll a, proving its central role. The red drop phenomenon shows a sharp decline in photosynthesis efficiency beyond 680 nm. Emerson's enhancement effect demonstrated that simultaneous illumination with 650 nm (red) and 700 nm (far-red) light produces more oxygen than the sum of individual rates, proving the existence of two photosystems working in series.

Chlorophyll a: C₅₅H₇₂O₅N₄Mg; absorbs 430 nm and 662 nm; present in all photosynthetic organisms
Chlorophyll b: C₅₅H₇₀O₆N₄Mg; absorbs 453 nm and 642 nm; found in higher plants and green algae
Carotenoids: Absorb 400-550 nm; quench triplet chlorophyll and singlet oxygen, preventing damage
Rf values in chromatography: Carotene (0.95) > Xanthophyll (0.70) > Chlorophyll a (0.59) > Chlorophyll b (0.42)

Important Constants, Values and Definitions for CBSE Exams

Several numerical values and technical terms from Photosynthesis in Higher Plants appear frequently in CBSE Class 11 Biology board exams and NEET. The compensation point is the light intensity at which the rate of photosynthesis equals the rate of respiration (net CO₂ exchange is zero). Light saturation point is the light intensity beyond which further increase does not increase the rate of photosynthesis. The quantum requirement (number of quanta needed to fix one CO₂) is theoretically 8-10 but practically 10-12 due to inefficiencies. Red drop occurs beyond 680 nm wavelength. RuBisCO constitutes about 50% of total leaf protein, making it the most abundant protein on Earth. A typical C3 plant leaf has 20-50 chloroplasts per mesophyll cell; each chloroplast contains 40-60 thylakoids stacked into grana.

Wavelengths: PS II absorbs 680 nm (P680); PS I absorbs 700 nm (P700); peak photosynthesis at 430 nm and 662 nm
Quantum yield: 8-10 photons theoretically needed per CO₂ fixed; actual requirement 10-12 photons
ATP/NADPH ratio: Non-cyclic photophosphorylation produces ATP:NADPH ratio close to 1.5:1; Calvin cycle needs 1.5:1 ratio
Chlorophyll content: Typically 1-2% of leaf dry weight; Chl a:Chl b ratio around 3:1 in higher plants

Memory Tricks, Mnemonics and Common Mistakes to Avoid

Students often confuse the stoichiometry of ATP and NADPH in the Calvin cycle or mix up C3 and C4 first products. Use the mnemonic 'C3 plants are COMMON' (3-PGA is the common first product, found in most plants). For C4, remember '4-MAIZE' (4-carbon OAA, found in maize). Photorespiration can be recalled as 'Photo-RESPIRATION wastes oxygen and releases CO₂ just like respiration'. The sequence of carriers in the electron transport chain can be memorized as 'People Purchase Potatoes, Carrots, Peas' (Pheophytin, Plastoquinone, Cytochrome complex, Plastocyanin) for PS II. For the Calvin cycle phases, use 'Car-Red-Regen' (Carboxylation, Reduction, Regeneration). A common error is writing the overall equation without balancing water molecules; the correct net form is 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O, showing 12 water consumed and 6 produced.

Common mistake: Writing PS I absorbs 680 nm (wrong — it is P680 in PS II); PS I has P700
Formula error: Forgetting water balance; always show 12H₂O consumed and 6H₂O released in overall equation
Pathway confusion: C3 first product is 3-PGA (not PEP); C4 first product is OAA (not malate — malate is the transport form)
Stoichiometry error: For one glucose, need 6 CO₂, 18 ATP, 12 NADPH (not 12 ATP); remember 3 ATP per CO₂ fixed

Quick Revision Tables and Last-Minute Checklist

Before your CBSE Class 11 Biology board exam or NEET, revise these comparison tables and checklists. Ensure you can draw and label the Z-scheme, write the three phases of the Calvin cycle with correct stoichiometry, and explain differences between C3, C4 and CAM pathways with examples. Practice writing balanced chemical equations for photolysis, light reactions and the overall Calvin cycle. Understand the dual role of RuBisCO and conditions favouring oxygenase versus carboxylase activity. For numerical problems, remember the ATP and NADPH requirement per glucose and per CO₂. Revise pigment absorption peaks and the significance of action spectrum. CBSETUTOR.ai offers 24×7 doubt-solving with photo upload for Chapter 11 practice questions at just ₹999 per month (covers all subjects for Classes 6-12) with a 3-day free trial — helpful when revising complex pathways like C4 and CAM at odd hours before exams.

Overall equation: 6CO₂ + 12H₂O + light → C₆H₁₂O₆ + 6O₂ + 6H₂O (in presence of chlorophyll)
Calvin cycle: 6 RuBP + 6 CO₂ → 12 3-PGA → 12 G3P → 2 G3P (net) + 10 G3P (regenerate 6 RuBP)
C3 vs C4: First product 3-PGA (3C) vs OAA (4C); enzyme RuBisCO vs PEP carboxylase; photorespiration high vs negligible
Key terms: Stroma (site of Calvin cycle), Grana (stacks of thylakoids), Photophosphorylation (light-driven ATP synthesis), Photolysis (splitting of water)

Formula and Pathway Comparison Tables

The tables below consolidate all formulas, pathway steps and comparisons in Chapter 11 for quick reference during revision. Use these to cross-check your answers in practice papers and NCERT exercise questions. These tables are aligned with the latest CBSE Class 11 Biology syllabus and the terminology used in NCERT textbooks, ensuring exam relevance. Print or screenshot these for last-minute revision sessions. When solving numerical problems on ATP or NADPH requirements, always start from the balanced equation and count molecules carefully. For conceptual questions on adaptations, compare the three pathways side-by-side using the C3 vs C4 vs CAM table. Review the pigment table before questions on chromatography or absorption spectra.

Frequently asked questions

What is the overall balanced equation for photosynthesis in higher plants?+

The net balanced equation is 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O in the presence of light and chlorophyll. This shows 12 water molecules consumed (6 in light reactions for photolysis, 6 in CO₂ hydration) and 6 water molecules released during glucose formation, with oxygen coming exclusively from water photolysis as proved by Van Niel using isotope labelling experiments.

How many ATP and NADPH molecules are needed to synthesize one glucose molecule in the Calvin cycle?+

18 ATP and 12 NADPH molecules are required to synthesize one glucose (6-carbon) molecule. Since six CO₂ molecules must be fixed, this means 3 ATP and 2 NADPH are consumed per CO₂ fixed. Out of 18 ATP, 12 are used in the reduction phase (converting 3-PGA to G3P) and 6 are used in the regeneration phase (converting G3P back to RuBP).

What is the first stable product in C3 and C4 pathways respectively?+

In C3 plants (Calvin cycle), the first stable product is 3-phosphoglyceric acid (3-PGA), a 3-carbon compound formed when RuBisCO catalyses the carboxylation of RuBP. In C4 plants, the first stable product is oxaloacetic acid (OAA), a 4-carbon compound formed when PEP carboxylase fixes CO₂ onto phosphoenolpyruvate in mesophyll cells. This distinction is a common 1-mark or 2-mark CBSE board exam question.

Why is photorespiration considered a wasteful process?+

Photorespiration is wasteful because it consumes oxygen, releases previously fixed CO₂, consumes ATP without producing NADPH or ATP, and reduces net photosynthetic productivity by 25-50% in C3 plants. It occurs when RuBisCO's oxygenase activity dominates over carboxylase activity at high temperatures and low CO₂ concentrations. The process involves three organelles (chloroplast, peroxisome, mitochondria) and salvages only part of the carbon, releasing the rest as CO₂ with ammonia as a byproduct.

What is Kranz anatomy and why is it important in C4 plants?+

Kranz anatomy refers to the wreath-like arrangement of mesophyll cells around bundle sheath cells in C4 plant leaves. Bundle sheath cells have thick walls, no intercellular spaces and numerous chloroplasts without grana (or with poorly developed grana). This anatomical specialization allows spatial separation of initial CO₂ fixation (in mesophyll via PEP carboxylase) from the Calvin cycle (in bundle sheath via RuBisCO), maintaining high CO₂ concentration around RuBisCO and minimizing photorespiration even under hot, dry conditions.

How does CAM photosynthesis differ from C4 photosynthesis?+

CAM (Crassulacean Acid Metabolism) and C4 both use PEP carboxylase for initial CO₂ fixation into 4-carbon acids, but CAM separates the processes temporally (night vs day) while C4 separates them spatially (mesophyll vs bundle sheath). CAM plants open stomata at night to fix CO₂ into malate stored in vacuoles, then close stomata during the day and decarboxylate malate to supply CO₂ for the Calvin cycle. This extreme water-conservation adaptation suits desert succulents but results in slower growth than C4 plants.

What is the significance of the Emerson enhancement effect?+

The Emerson enhancement effect shows that photosynthesis driven by simultaneous exposure to red light (around 650 nm) and far-red light (around 700 nm) is greater than the sum of photosynthesis rates under each wavelength alone. This proved that two separate photosystems (PS II absorbing shorter wavelengths and PS I absorbing longer wavelengths) work in series, not independently. It was crucial evidence for the Z-scheme model of photosynthetic electron transport.

Why does RuBisCO have both carboxylase and oxygenase activity?+

RuBisCO (ribulose bisphosphate carboxylase-oxygenase) has a dual catalytic function because CO₂ and O₂ compete for the same active site on the enzyme. Under normal atmospheric conditions (21% O₂, 0.04% CO₂), both reactions occur, though carboxylase activity dominates. High temperatures and low CO₂ concentrations favour oxygenase activity, leading to photorespiration. This dual activity is an evolutionary limitation; C4 and CAM plants evolved mechanisms to concentrate CO₂ around RuBisCO to suppress the oxygenase function.

What are the differences between cyclic and non-cyclic photophosphorylation?+

Non-cyclic photophosphorylation involves both PS II and PS I, produces ATP and NADPH, evolves oxygen from water photolysis, and follows a Z-shaped electron flow path. Cyclic photophosphorylation involves only PS I, produces only ATP (no NADPH), does not evolve oxygen, and electrons cycle back to P700 via the cytochrome b₆f complex and plastocyanin. Cyclic photophosphorylation occurs when the Calvin cycle needs extra ATP beyond what non-cyclic photophosphorylation provides, adjusting the ATP:NADPH ratio.

How many turns of the Calvin cycle are needed to produce one glucose molecule?+

Six turns of the Calvin cycle are needed to produce one glucose molecule because each turn fixes one CO₂ molecule and glucose contains six carbon atoms. In six turns, 6 CO₂ molecules are fixed to form 12 molecules of 3-PGA, which are reduced to 12 molecules of G3P (glyceraldehyde-3-phosphate). Out of these 12 G3P, 10 are recycled to regenerate 6 RuBP molecules for the next cycle, and the net gain of 2 G3P molecules condense to form one glucose via the gluconeogenesis pathway.

Related resources

Important Questions: CBSE Class 11 Biology Chapter 11 Photosynthesis in Higher Plants CBSE Class 11 Biology Chapter 11 Photosynthesis in Higher Plants Worksheet with Answers CBSE Class 11 Biology Chapter 10 Cell Cycle and Cell Division Worksheet with Answers Important Questions: CBSE Class 11 Biology Chapter 10 Cell Cycle and Cell Division AI Tutor for Class 11: The Smart Alternative to Tuition AI Tutor for Class 11 Accountancy: Learn Faster with Instant Help Online Class 9 Biology Tutor in Mumbai — AI-Powered, 24/7 CBSE Support Class 9 Mathematics Chapter 2 Polynomials — Formulas & Key Points

Keep learning — related guides

Class 11Biology

Biology Tutor in Awadhpuri Bhopal for NEET & CBSE Boards

Class 11Biology

Biology Tutor in Bairagarh Bhopal for CBSE NEET Prep

Class 11Biology

Class 11 Biology Chapter 18 Neural Control and Coordination — Formulas & Key Points

Class 11Biology

Class 11 Biology Chapter 15 Body Fluids and Circulation — Formulas & Key Points

Class 11Biology

CBSE Class 11 Biology Chapter 19 Chemical Coordination and Integration Worksheet with Answers

Class 11Biology

Class 11 Biology Chapter 19 Chemical Coordination and Integration — Formulas & Key Points

Ready to give your Class 11 child the tutor that never sleeps?

CBSETUTOR.ai covers every chapter in the Class 11 NCERT syllabus — Maths, Science, Social Science, English, Hindi and more. 24×7. Patient. Unlimited. 3-day free trial.

Start your child's 3-day free trial →