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PHOTOSYNTHESIS IN HIGHER PLANTS — Study Material, Revision Notes & PDF Guide

Comprehensive chapter-wise study notes, key formulas, conceptual summaries, and practice resources for PHOTOSYNTHESIS IN HIGHER PLANTS under the Biology syllabus of NEET UG 2027.

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Chapter 11: PHOTOSYNTHESIS IN HIGHER PLANTS – NEET UG Complete Master Notes (NCERT Biology Class 11)





1. Introduction to Photosynthesis



Photosynthesis is an anabolic, endergonic, oxidation-reduction, physico-chemical process by which green plants, algae, and photosynthetic bacteria convert radiant solar energy into chemical energy stored in organic bonds of carbohydrates (glucose and starch), utilizing atmospheric carbon dioxide and water as raw materials.





Fundamental Significance of Photosynthesis:

Photosynthesis

├── 1. Primary Energy Gateway:

│   └── The sole biological process that captures solar energy and converts it into food.

│       All living organisms on Earth (heterotrophs) directly or indirectly depend on it.

│

└── 2. Atmospheric Oxygen Sustenance:

    └── The exclusive natural biological source for the release of molecular Oxygen (O2)

        into the biosphere, sustaining aerobic cellular respiration and maintaining the ozone layer.


  • General Multi-Step Balanced Equation:


$$6\text{CO}_2 + 12\text{H}_2\text{O} \xrightarrow[\text{Chlorophyll}]{\text{Light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{H}_2\text{O} + 6\text{O}_2 \uparrow$$


  • Substrate Water Requirement: 12 molecules of $\text{H}_2\text{O}$ are required as substrates because all 6 molecules of released $\text{O}_2$ originate entirely from the photolytic splitting of water, not from carbon dioxide.
  • Melvin Calvin (Nobel Prize 1961): Utilized radioactive $^{14}\text{C}$ in algal photosynthesis (Chlorella and Scenedesmus) to elucidate the cyclic pathway of carbon assimilation (the Calvin Cycle). Proposed that plants convert light energy into chemical energy by transferring electrons through an organized array of pigment molecules.




2. Early Landmark Experiments in Photosynthesis



The modern understanding of photosynthesis evolved through a series of classic, elegant experiments:





Historical Experiments Timeline:

Scientist & Period               Experimental Design                        Key Discovery & Inference

───────────────────────────────────────────────────────────────────────────────────────────────────────────

Early Simple Tests               Variegated leaf (Croton) & Partially      Proved that CHLOROPHYLL and LIGHT

                                 covered leaf with black paper             are strictly essential for starch synthesis.

Moll's Half-Leaf Experiment      One half of leaf enclosed in test tube    Proved that CO2 IS ESSENTIAL for photosynthesis.

                                 with KOH-soaked cotton (absorbs CO2);     The enclosed part tested negative for starch,

                                 other half exposed to atmospheric air     while the exposed part tested positive.

Joseph Priestley (1770 - 1774)   Bell jar containing burning candle and    Proved that plants RESTORE TO THE AIR whatever

                                 mouse; candle extinguished, mouse died.   breathing animals and burning candles remove.

                                 Added a Mint plant (Mentha) to bell jar;  Discovered Oxygen gas (O2) in 1774.

                                 candle kept burning, mouse survived.

Jan Ingenhousz (1779)            Placed Priestley's bell jar setup in      Showed that SUNLIGHT is indispensable for plants

                                 dark vs sunlight; used aquatic plant      to purify foul air. Proved that only the GREEN

                                 (Hydrilla) under bright illumination      PARTS of plants release bubbles of OXYGEN.

Julius von Sachs (1854)          Hydroponic and cytological plant studies  Proved that green parts produce GLUCOSE, which

                                                                           is stored as STARCH. Recognized that green

                                                                           pigment is confined to bodies (chloroplasts).

T.W. Engelmann (1882)            Used prism to split light into spectral   Described the FIRST ACTION SPECTRUM of

                                 components; illuminated green alga        photosynthesis. Maximum O2 evolution occurred

                                 (Cladophora) in aerobic bacterial broth   in the BLUE and RED regions of the spectrum.

Cornelius van Niel (1930s)       Comparative studies on Purple and Green   Demonstrated photosynthesis is a light-dependent

                                 Sulfur Bacteria (used H2S as H-donor);    redox reaction. Inferred that O2 evolved by green

                                 byproduct was elemental sulfur, not O2    plants comes from H2O and NOT from CO2.

Ruben, Kamen et al. (1941)       Used heavy non-radioactive oxygen isotope Proved van Niel's hypothesis: All evolved O2

                                 (O-18) in green algal suspension          originates from photolysis of H2O.


  • Van Niel's Generalized Formulation:


$$2\text{H}_2\text{A} + \text{CO}_2 \xrightarrow{\text{Light}} 2\text{A} + [\text{CH}_2\text{O}] + \text{H}_2\text{O}$$


  • For sulfur bacteria: $\text{H}_2\text{S}$ is the hydrogen donor; byproduct is Sulfur (\text{S}).
  • For green plants: $\text{H}_2\text{O}$ is the hydrogen donor; byproduct is Oxygen (\text{O}_2).




3. Site of Photosynthesis: The Chloroplast



In higher plants, photosynthesis takes place predominantly in the leaves, specifically within the chloroplasts of mesophyll cells.





Chloroplast Structural Architecture & Division of Labour:

Chloroplast

├── 1. Membranous System (Grana & Stroma Lamellae):

│   ├── Site of the LIGHT REACTION (Photochemical Phase).

│   ├── Functions: Trapping solar photons, photolysis of water, release of O2,

│   │   and synthesis of assimilatory powers (ATP and NADPH).

│   ├── Grana Thylakoids: Possess BOTH Photosystem I (PS I) and Photosystem II (PS II).

│   └── Stroma Lamellae (Frets): Possess ONLY Photosystem I (PS I); LACK PS II and NADP reductase.

│

└── 2. Stroma (Aqueous Matrix):

    ├── Site of the DARK REACTION (Biosynthetic / Carbon Phase).

    ├── Contains soluble enzymes (e.g., RuBisCO, Calvin cycle enzymes), circular dsDNA,

    │   70S ribosomes, and starch granules.

    └── Functions: Fixation of atmospheric CO2 and synthesis of sugars (glucose, starch).


  • Chloroplast Alignment in Mesophyll Cells:
  • Under low/moderate light intensity: Chloroplasts align with their broad flat surfaces parallel to the cell walls to capture maximum incident light.
  • Under extremely high light intensity: Chloroplasts align perpendicular (edge-on) to the cell walls or along lateral walls to avoid photo-oxidation and photodamage (parastrophe).




4. Photosynthetic Pigments & Spectra



The coloration of green leaves is not caused by a single pigment, but by a coordinated consortium of four distinct pigments that can be resolved via paper chromatography:





Photosynthetic Pigment Profile:

Pigment Name         Chromatogram Color        Chemical Nature & Role

───────────────────────────────────────────────────────────────────────────────────────────

Chlorophyll a        Bright or Blue-Green      CHIEF photosynthetic pigment; forms the core

                                               reaction centres (P680 and P700).

Chlorophyll b        Yellow-Green              Accessory antenna pigment; absorbs light and

                                               transfers energy to Chlorophyll a.

Xanthophylls         Yellow                    Oxygenated carotenoids (e.g., Lutein); accessory

                                               pigment; protects against photo-oxidation.

Carotenoids          Yellow to Yellow-Orange   Hydrocarbon carotenes (e.g., beta-carotene);

                                               accessory antenna pigment; shields chlorophyll.






Roles of Accessory Pigments (Chlorophyll b, Xanthophylls, Carotenoids):

Accessory Pigments

├── 1. Spectral Expansion:

│   └── Absorb radiant light across wavelengths where Chlorophyll a absorbs poorly,

│       funneling excitation energy to the reaction centre.

│

└── 2. Photoprotection (Shielding Mechanism):

    └── Quench excess excitation energy, preventing PHOTO-OXIDATION (solarization)

        and oxidative destruction of Chlorophyll a in bright sunlight.


  • Absorption Spectrum vs Action Spectrum:
  • Absorption Spectrum: A graph showing the percentage of light energy absorbed by a pure pigment at different wavelengths of the visible spectrum (400 - 700\ \text{nm}). Chlorophyll a exhibits sharp absorption peaks in the Blue (430 - 470\ \text{nm}) and Red (660 - 680\ \text{nm}) regions.
  • Action Spectrum: A graph depicting the relative rate of photosynthesis [measured as O2 evolved or CO2 consumed] across different wavelengths of visible light.
  • Superimposition Insight: The action spectrum of photosynthesis roughly matches the absorption spectrum of Chlorophyll a, demonstrating that Chlorophyll a is the primary pigment driving photosynthesis, while the slight differences reflect the contribution of accessory pigments.




5. Light Reaction (Photochemical Phase)



The light reaction encompasses light absorption, water splitting, oxygen release, and the generation of high-energy chemical intermediates: ATP and NADPH (termed assimilatory power).





Organization of Light Harvesting Complexes (LHC):

Photosystem (LHC)

├── Antenna Complex:

│   └── Hundreds of pigment molecules (Chl b, carotenoids, xanthophylls) bound to proteins.

│       Harvest light photons like a parabolic satellite dish and funnel excitation energy.

│

└── Reaction Centre:

    └── A single specialized Chlorophyll a molecule that releases energized electrons.

        ├── In Photosystem I (PS I): Reaction centre is P700 (absorption maximum at 700 nm).

        └── In Photosystem II (PS II): Reaction centre is P680 (absorption maximum at 680 nm).




6. Electron Transport & The Z-Scheme



When light falls on the photosynthetic apparatus, electrons flow from water through PS II and PS I to reduce $\text{NADP}^+$ to $\text{NADPH} + \text{H}^+$.





The Z-Scheme Electron Flow Pathway:

                  [Primary Electron Acceptor (Pheophytin)]

                                ▲

                                │ (Uphill excitation)

                       [P680* (PS II)]

                                ▲

                                │

[Photolysis of H2O] ──────────► P680 ──► [Plastoquinone (PQ)]

2H2O -> 4H+ + O2 + 4e-                     │

                                           ▼ (Downhill via Cytochrome b6f complex -> ATP generated)

                                         [Plastocyanin (PC)]

                                           │

                                           ▼

                                         [P700 (PS I)] ──► [P700*]

                                                               │ (Uphill excitation)

                                                               ▼

                                                  [Primary Acceptor (Fe-S)]

                                                               │

                                                               ▼ (Downhill via Ferredoxin)

                                                  [NADP+ Reductase] ──► NADPH + H+


  1. Excitation of PS II: P680 absorbs $680\ \text{nm}$ red light, propelling an electron to an outer orbit.
  2. Primary Acceptor & Downhill Transport: The excited electron is captured by a primary electron acceptor (pheophytin) and transferred downhill along an electron transport chain:


$$\text{Pheophytin} \rightarrow \text{Plastoquinone (PQ)} \rightarrow \text{Cytochrome } b_6f \text{ complex} \rightarrow \text{Plastocyanin (PC)} \rightarrow \text{PS I (P700)}$$


  1. Excitation of PS I: Simultaneously, P700 absorbs $700\ \text{nm}$ red light, promoting its electron to another primary electron acceptor (an iron-sulfur protein) with a more negative redox potential.
  2. Reduction of $\text{NADP}^+$: The electron moves downhill through Ferredoxin (Fd) to the enzyme $\text{NADP}^+$ Reductase, which catalyzes the formation of $\text{NADPH} + \text{H}^+$ on the stroma side of the thylakoid membrane.
  3. The Z-Scheme Terminology: Coined because when all participating redox carriers are placed sequentially according to their standard redox potential (E_0'), the pathway traces a characteristic Z-shape.




6.1 Photolysis of Water (Water Splitting Complex)



Continuous non-cyclic electron transport requires a constant supply of replacement electrons for oxidized $\text{P680}^+$.



  • Oxygen-Evolving Complex (OEC):
  • Physically situated on the inner (lumen) side of the thylakoid membrane, tightly associated with PS II.
  • Requires essential catalytic mineral cofactors: Manganese (\text{Mn}^{2+}) and Chloride (\text{Cl}^-) ions.
  • Stoichiometry of Water Splitting:


$$2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + \text{O}_2 \uparrow + 4e^-$$


  • Fate of Products:
  • Electrons (4e^-): Transferred directly to oxidized $\text{P680}^+$ reaction centres.
  • Protons (4\text{H}^+): Released and accumulated inside the thylakoid lumen, contributing to the transmembrane proton gradient.
  • Oxygen (\text{O}_2): Diffuses freely out of the leaf through stomata into the atmosphere.




7. Cyclic vs Non-Cyclic Photophosphorylation



Phosphorylation is the synthesis of $\text{ATP}$ from $\text{ADP}$ and inorganic phosphate (\text{Pi}) utilizing energy. When driven by light energy in chloroplasts, it is termed photophosphorylation.





Photophosphorylation Modalities:

Photophosphorylation

├── 1. Non-Cyclic Photophosphorylation:

│   ├── Both PS II and PS I operate in series (Z-Scheme).

│   ├── Electron flow is UNIDIRECTIONAL (Non-cyclic: H2O -> PS II -> PS I -> NADP+).

│   ├── Photolysis of water occurs; O2 is evolved.

│   ├── Products: BOTH ATP and NADPH + H+ are formed.

│   └── Location: Appressed regions of GRANA THYLAKOID membranes.

│

└── 2. Cyclic Photophosphorylation:

    ├── Only PS I (P700) is functional.

    ├── Electron expelled from P700 cycles back via:

    │   P700 -> Primary Acceptor -> Ferredoxin -> Cytochrome b6f -> Plastocyanin -> P700.

    ├── Photolysis of water is ABSENT; NO O2 is evolved.

    ├── Products: ONLY ATP is formed (NO NADPH is synthesized).

    └── Locations & Conditions:

        ├── STROMA LAMELLAE membranes (naturally lack PS II and NADP reductase).

        └── Triggered when incident light has wavelengths beyond 680 nm (far-red light).






Detailed Comparison Matrix:

Feature                     Non-Cyclic Photophosphorylation             Cyclic Photophosphorylation

───────────────────────────────────────────────────────────────────────────────────────────────────────────

Photosystems Involved       Both PS I (P700) and PS II (P680)           Only PS I (P700)

Electron Donor              External donor: Water (H2O)                 Internal donor: P700 itself

Electron Fate               Transferred to NADP+; does not return       Cycles back to P700 reaction centre

Photolysis of Water         Present (OEC active)                        Completely absent

Oxygen (O2) Evolution       Present                                     Absent

Assimilatory Products       ATP + NADPH + H+                            Only ATP (No NADPH)

Thylakoid Site              Grana lamellae membranes                    Stroma lamellae & non-appressed grana

Wavelength Sensitivity      Operates at wavelengths <= 680 nm           Operates even at wavelengths > 680 nm

Biological Role             Provides ATP and NADPH for Calvin cycle     Supplies additional ATP required for

                                                                        Calvin cycle and C4/CAM pathways




8. Chemiosmotic Hypothesis of ATP Synthesis



Proposed by Nobel laureate Peter Mitchell (1961) to explain the mechanism of light-driven $\text{ATP}$ synthesis. Like oxidative phosphorylation in mitochondria, photosynthetic phosphorylation is coupled to the establishment of an electrochemical proton gradient (\Delta\text{pH}) across a biological membrane.





Key Difference between Photosynthetic and Mitochondrial Chemiosmosis:

System                      Proton Accumulation Site               Membrane Involved

───────────────────────────────────────────────────────────────────────────────────────────

Chloroplast (Photosynthesis) THYLAKOID LUMEN (Inside)               Thylakoid membrane

Mitochondria (Respiration)   INTERMEMBRANE SPACE (Outside matrix)   Inner mitochondrial membrane






Three Causes of the Proton Gradient Across the Thylakoid Membrane:

Proton Gradient Genesis

├── 1. Photolysis of Water on the Lumen Side:

│   └── Splitting of H2O occurs on the INNER surface of the thylakoid membrane,

│       directly dumping H+ protons into the thylakoid lumen.

│

├── 2. Proton Pumping by Plastoquinone (PQ):

│   └── The primary electron acceptor of PS II is located on the stroma-facing outer surface.

│       It transfers electrons not to an electron carrier, but to an H-carrier (Plastoquinone).

│       PQ picks up a proton from the STROMA, and upon transferring its electron to

│       Cytochrome b6f on the lumen side, it dumps the proton into the LUMEN.

│

└── 3. Proton Consumption by NADP+ Reductase in the Stroma:

    └── NADP+ Reductase is positioned on the STROMA side of the thylakoid membrane.

        To reduce NADP+ to NADPH + H+, it continuously removes protons from the STROMA.


  • Net Outcome:
  • High proton concentration ([\text{H}^+]) and low $\text{pH}$ (\approx 5.0) inside the thylakoid lumen.
  • Low proton concentration ([\text{H}^+]) and high $\text{pH}$ (\approx 8.0) in the stroma.
  • This generates a substantial proton motive force (PMF).




ATP Synthase (CF0 - CF1 Complex) Architecture:

Enzyme Complex

├── CF0 Complex (Transmembrane Channel):

│   ├── Hydrophobic protein assembly embedded across the lipid bilayer of thylakoid membrane.

│   └── Provides a selective hydrophilic channel for the FACILITATED DIFFUSION of protons

│       down the electrochemical gradient from the lumen back to the stroma.

│

└── CF1 Complex (Catalytic Headpiece):

    ├── Hydrophilic protein complex protruding on the STROMA-FACING outer surface.

    └── The passage of protons through CF0 releases free energy that triggers a conformational

        change in CF1, activating its catalytic site to synthesize ATP:

        ADP + Pi ──► ATP


  • Four Core Prerequisites for Chemiosmosis:


  1. A selectively impermeable membrane (thylakoid membrane).
  2. A proton pump (plastoquinone/cytochrome system).
  3. A proton gradient across the membrane.
  4. The $\text{CF}_0-\text{CF}_1$ ATP Synthase enzyme.




9. Biosynthetic Phase: The Calvin Cycle (C3 Pathway)



The products of the light reaction—$\text{ATP}$ and $\text{NADPH}$—together with $\text{CO}_2$ and $\text{H}_2\text{O}$ are utilized in the stroma to synthesize sugars in the biosynthetic phase (traditionally termed the dark reaction).



[!NOTE] Why Calling the Biosynthetic Phase the "Dark Reaction" is a Misnomer: The biosynthetic phase does not require photons directly, but it is strictly dependent on the continuous supply of products from the light reaction [ATP and NADPH]. When light is switched off, sugar synthesis continues briefly and then halts because $\text{ATP}$ and $\text{NADPH}$ pools become exhausted. Furthermore, key enzymes including RuBisCO are light-activated.


  • Primary $\text{CO}_2$ Acceptor: Ribulose-1,5-bisphosphate (RuBP), a 5-carbon phosphorylated ketose sugar.
  • First Stable Reaction Product: 3-Phosphoglyceric acid (3-PGA), a 3-carbon organic acid.
  • Universal Occurrence: The Calvin cycle operates in all photosynthetic plants, regardless of whether they exhibit C3, C4, or CAM adaptations.




The Three Stages of the Calvin Cycle:

Calvin Cycle

├── 1. Carboxylation (Most Crucial Stage):

│   ├── Fixation of atmospheric CO2 onto the primary acceptor RuBP (5C).

│   ├── Catalyzed by the bifunctional enzyme RuBisCO:

│   │   RuBP (5C) + CO2 + H2O ──► 2 Molecules of 3-PGA (3C each)

│   └── Requires NO ATP and NO NADPH.

│

├── 2. Reduction:

│   ├── Sequence of reactions converting 3-PGA into Triose Phosphate (G3P / DHAP).

│   ├── Per CO2 fixed: Consumes 2 molecules of ATP (phosphorylation)

│   │   and 2 molecules of NADPH (reduction).

│   └── 6 turns of cycle fix 6 CO2, generating 12 triose phosphates (2 form 1 Glucose).

│

└── 3. Regeneration:

    ├── Regeneration of the CO2 acceptor RuBP to sustain continuous cyclic operation.

    └── Consumes 1 molecule of ATP for phosphorylation per CO2 fixed.






Quantitative Material and Energy Balance of the Calvin Cycle:

Per Turn of Calvin Cycle (1 CO2 fixed):

├── Input:  1 CO2 + 3 ATP + 2 NADPH

└── Output: 1/6 Glucose + 3 ADP + 2 NADP+ + 3 Pi



For Synthesis of 1 Net Molecule of Glucose (C6H12O6; 6 Turns of Cycle):

┌───────────────────────────────┬───────────────────────────────┐

│          WHAT GOES IN         │         WHAT COMES OUT        │

├───────────────────────────────┼───────────────────────────────┤

│ 6 Molecules of CO2            │ 1 Molecule of Glucose (C6H12O6)│

│ 18 Molecules of ATP           │ 18 Molecules of ADP           │

│ 12 Molecules of NADPH         │ 12 Molecules of NADP+         │

└───────────────────────────────┴───────────────────────────────┘




10. The C4 Pathway (Hatch & Slack Pathway)



Plants adapted to arid, high-temperature tropical environments (e.g., Maize, Sugarcane, Sorghum, Amaranthus) have evolved a specialized C4 dicarboxylic acid pathway that eliminates photorespiration and maximizes biomass productivity.





Hallmark Adaptations of C4 Plants:

C4 Evolutionary Superiority

├── 1. Kranz Anatomy in Leaves (Specialized cellular dimorphism).

├── 2. High Temperature Tolerance (Optimum 30°C - 45°C).

├── 3. Responsiveness to High Light Intensities (No photo-inhibition).

├── 4. Complete Absence of Photorespiration (Enables high CO2 fixation).

└── 5. Elevated Water Use Efficiency & High Biomass Yields.


10.1 Kranz Anatomy



  • Kranz is a German word meaning wreath or ring, describing the radial arrangement of cells around vascular bundles.
  • Two Structurally and Functionally Distinct Photosynthetic Cell Types:




Kranz Anatomical Cell Dimorphism:

Feature                     Mesophyll Cells                             Bundle Sheath Cells

───────────────────────────────────────────────────────────────────────────────────────────────────────────

Position & Arrangement      Loosely arranged with intercellular spaces  Large, concentric wreath-like layers

                                                                        tightly encircling vascular bundles

Chloroplast Morphology      GRANAL chloroplasts (thylakoids stacked)   AGRANAL chloroplasts (large, lack grana)

Cell Walls                  Thin, permeable to gases                    Thick, IMPERVIOUS to gaseous exchange;

                                                                        completely lack intercellular spaces

Key Enzyme Present          PEP Carboxylase (PEPcase)                   RuBisCO (Ribulose bisphosphate carboxylase)

Key Enzyme Absent           RuBisCO is ABSENT                           PEPcase is ABSENT

Primary Reaction            Initial CO2 fixation into C4 acid (OAA)     Decarboxylation & Calvin Cycle




10.2 Steps of the Hatch and Slack Pathway





Hatch and Slack Pathway Intercellular Flow:

[MESOPHYLL CELL CYTOPLASM]

Atmospheric CO2 ──► HCO3-

       │

       ▼ + Phosphoenolpyruvate (PEP, 3C)  [Catalyzed by PEPcase]

Oxaloacetic Acid (OAA, 4C)  <--- First stable C4 product

       │

       ▼ Reduction (NADPH -> NADP+)

Malic Acid / Aspartic Acid (4C)

       │

       ▼ Transport via Plasmodesmata

[BUNDLE SHEATH CELL]

Malic Acid (4C)

       │

       ▼ Decarboxylation [Catalyzed by Malic Enzyme]

Pyruvic Acid (3C)  +  CO2 (Released at high local concentration)

       │                      │

       │                      ▼

       │               Enters standard CALVIN CYCLE (C3)

       │               via RuBisCO -> Sugars synthesized

       ▼

Transport back via Plasmodesmata

[MESOPHYLL CELL]

Pyruvic Acid (3C) ──► Phosphoenolpyruvate (PEP, 3C)

       ▲

       └── Consumes 2 ATP equivalents (ATP -> AMP + PPi) [Catalyzed by Pyruvate Pi Dikinase]






Energy Tally for Glucose Synthesis in C4 Plants:

Operation                   ATP Consumed per CO2       Total ATP for 1 Glucose (6 CO2)

───────────────────────────────────────────────────────────────────────────────────────────

Calvin Cycle (in BS cells)  3 ATP                      18 ATP  (+ 12 NADPH)

C4 Regeneration of PEP      2 ATP                      12 ATP

Total Energy Cost for C4    5 ATP + 2 NADPH            30 ATP + 12 NADPH


[!IMPORTANT] To synthesize one molecule of glucose, a C3 plant requires 18 ATP, whereas a C4 plant requires 30 ATP. Despite this higher energetic cost, C4 plants produce significantly more net biomass because they completely suppress wasteful photorespiration.




11. Photorespiration (C2 Cycle / Photosynthetic Carbon Oxidation)



Photorespiration is a light-dependent wasteful process occurring in C3 plants under conditions of high temperature, high $\text{O}_2$ concentration, and low $\text{CO}_2$ concentration.



  • Bifunctional Nature of RuBisCO:
  • RuBisCO active site binds competitively to both $\text{CO}_2$ (acting as a carboxylase) and $\text{O}_2$ (acting as an oxygenase).
  • When the $[\text{CO}_2] : [\text{O}_2]$ ratio is high, carboxylation predominates.
  • When temperatures rise or stomata partially close, $[\text{O}_2]$ increases relative to $[\text{CO}_2]$, favoring oxygenation.
  • The Oxygenase Reaction:


$$\text{RuBP (5C)} + \text{O}_2 \xrightarrow{\text{RuBisCO Oxygenase}} \text{3-PGA (3C)} + \text{2-Phosphoglycolate (2C)}$$


  • Organellar Triad: Photorespiration involves the cooperation of three distinct organelles in strict sequence:


$$\text{Chloroplast} \rightarrow \text{Peroxisome} \rightarrow \text{Mitochondrion}$$




Why Photorespiration is a Wasteful Process:

Photorespiration Consequences

├── 1. Zero Sugar Synthesis: No glucose, starch, or carbohydrates are produced.

├── 2. Zero Energy Yield: Neither ATP nor NADPH is synthesized.

├── 3. Net Carbon Loss: Releases 25% of previously fixed organic carbon as CO2 in mitochondria.

└── 4. Direct Energy Consumption: Consumes ATP during conversion steps.


  • Why Photorespiration is Absent in C4 Plants:


C4 plants possess an internal $\text{CO}_2$-concentrating pump. Decarboxylation of C4 malate inside bundle sheath cells continuously releases high levels of $\text{CO}_2$ around RuBisCO. This high local $\text{CO}_2$ saturation ensures that RuBisCO functions exclusively as a carboxylase, completely suppressing its oxygenase activity.





12. Master Comparison: C3 vs C4 Plants





Complete Master Matrix (NCERT Table 11.1 Resolved):

Diagnostic Characteristic               C3 Plants                               C4 Plants

───────────────────────────────────────────────────────────────────────────────────────────────────────────

Primary CO2 Acceptor                    RuBP (Ribulose-1,5-bisphosphate, 5C)   PEP (Phosphoenolpyruvate, 3C)

Primary Acceptor Carbon Number          5 Carbons                               3 Carbons

Primary CO2 Fixation Enzyme             RuBisCO (in mesophyll)                  PEP Carboxylase (in mesophyll)

First Stable Fixation Product           3-PGA (3-Phosphoglyceric acid, 3C)     OAA (Oxaloacetic acid, 4C)

First Product Carbon Number             3 Carbons                               4 Carbons

Kranz Anatomy                           ABSENT                                  PRESENT

Leaf Chloroplasts                       Monomorphic (Granal only)               Dimorphic (Granal in mesophyll;

                                                                                Agranal in bundle sheath)

Cell Type Performing Calvin Cycle       Mesophyll cells only                    Bundle sheath cells only

Cell Type for Initial CO2 Fixation      Mesophyll cells                         Mesophyll cells

Number of Cell Types Fixing CO2         One (Mesophyll)                         Two (Mesophyll + Bundle sheath)

Presence of RuBisCO                     Present in mesophyll cells              Present in bundle sheath only

Presence of PEPcase                     Absent / Insignificant                  Present in mesophyll cells

ATP Required per Glucose Molecule       18 ATP (+ 12 NADPH)                     30 ATP (+ 12 NADPH)

Photorespiration at High Light/Temp     High and wasteful (Lost ~25% carbon)    COMPLETELY ABSENT

CO2 Compensation Point                  High (25 to 100 ppm)                    Very low (0 to 10 ppm)

CO2 Saturation Point                    High: Beyond 450 µL/L (ppm)             Low: At ~ 360 µL/L (ppm)

Light Saturation Intensity              At lower intensities (~10% sunlight)    Does not saturate even at full sunlight

Temperature Optimum                     20°C to 25°C                            30°C to 45°C

Biomass Yield & Productivity            Medium / Lower                          High / Superior

Representative Plant Examples           Wheat, Rice, Barley, Tomato, Potato     Maize, Sugarcane, Sorghum, Amaranthus




13. Factors Affecting Photosynthesis



Photosynthesis is governed by an interplay of internal (plant) and external (environmental) factors.



  • Internal (Plant) Factors: Number, size, age, and orientation of leaves; mesophyll cells and chloroplast count; internal $\text{CO}_2$ concentration; chlorophyll content (governed by genetics and plant growth status).
  • External Factors: Sunlight intensity, light quality, duration, temperature, atmospheric $\text{CO}_2$ concentration, and soil water availability.


13.1 Blackman's Law of Limiting Factors (1905)



Formulated by British plant physiologist F.F. Blackman:



"If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed."




Blackman's Law Analogy:

Process Rate = Paced by the slowest factor!

Example: Even if a green leaf is supplied with optimal light and excess CO2,

photosynthesis will remain severely suppressed if ambient temperature is near freezing.

Elevating the temperature toward its optimum immediately accelerates the rate of photosynthesis.




13.2 Individual External Factors



1. Light



Three light parameters affect photosynthesis: Light Intensity, Light Quality (Wavelength), and Duration of Exposure.



  • Light Intensity Dynamics:
  • At low light intensities, there is a linear relationship between incident light and $\text{CO}_2$ fixation rates.
  • At higher light intensities, the rate plateaus and no further increase occurs because other factors [e.g., CO2, temperature] become limiting.
  • Light Saturation: Occurs at approximately 10% of full sunlight.
  • Hence, except for plants growing under dense forest canopies or in deep shade, light is rarely a limiting factor in nature.
  • Solarization (Photo-oxidation): Extremely high light intensities beyond the saturation threshold cause the photo-oxidative destruction of chlorophyll molecules and inactivate reaction centres, causing a steep drop in photosynthesis.


2. Carbon Dioxide Concentration



CO2 is the major limiting factor for photosynthesis in nature because its atmospheric concentration is extremely low [0.03% to 0.04%, or 300 – 400 ppm].



  • Increasing $\text{CO}_2$ concentration up to $0.05\%$ (500\ \text{ppm}) enhances $\text{CO}_2$ fixation rates; beyond this, it causes stomatal closure and toxicity over prolonged periods.
  • Differential Response of C3 and C4 Plants:
  • At low light intensity, neither group responds to elevated $\text{CO}_2$.
  • At high light intensity, both show increased photosynthetic rates.
  • C4 Plants: Reach saturation at a lower threshold of


$$\approx 360\ \mu\text{L}\cdot\text{L}^{-1}$$


(360\ \text{ppm}).



  • C3 Plants: Continue responding to higher $\text{CO}_2$ levels, reaching saturation only beyond


$$450\ \mu\text{L}\cdot\text{L}^{-1}$$


(450\ \text{ppm}).



  • Greenhouse Crop Applications [CO2 Fertilization]:


Because current atmospheric $\text{CO}_2$ levels are sub-optimal for C3 plants, greenhouse crops such as Tomatoes and Bell Peppers are cultivated in $\text{CO}_2$-enriched atmospheres, resulting in accelerated growth and substantially higher agricultural fruit yields.



3. Temperature



  • The light reactions, being photochemical, are temperature-insensitive or influenced to a minimal degree.
  • The dark reactions (biosynthetic phase) are catalyzed by soluble enzymes in the stroma and are therefore highly temperature-sensitive.
  • Temperature Optima:
  • C4 plants have a high temperature optimum of $30^\circ\text{C} - 45^\circ\text{C}$.
  • C3 plants have a lower temperature optimum of $20^\circ\text{C} - 25^\circ\text{C}$.
  • Tropical species possess higher temperature thresholds than temperate species.


4. Water



Although water is a direct substrate in the light reaction (photolysis), water deficiency primarily impacts photosynthesis indirectly through general physiological stress:



  1. Stomatal Closure: Water stress triggers abscisic acid (\text{ABA}) accumulation, closing stomatal apertures and sharply restricting atmospheric $\text{CO}_2$ entry into leaves.
  2. Leaf Wilting: Loss of cell turgor causes leaves to wilt and droop, dramatically reducing the exposed surface area available for capturing sunlight.
  3. Metabolic Reduction: Cellular dehydration slows enzymatic activity across mesophyll cells.




14. NEET High-Yield Exam Traps & Misconceptions





NEET High-Yield Trap Matrix:

Exam Concept                  Common Trap / Misconception                   Correct NCERT Fact

──────────────────────────────────────────────────────────────────────────────────────────────────────────

Source of Evolved Oxygen      Assuming O2 comes from CO2 reduction          All released O2 comes exclusively from the

                                                                            photolysis of WATER (H2O), proved via O-18.

Water Splitting Complex Site  Thinking OEC is on the outer stroma face      The water-splitting complex is located on the

                                                                            INNER (LUMEN) side of the thylakoid membrane!

Stroma Lamellae Pigments      Assuming stroma lamellae have both PS I & II  Stroma lamellae contain ONLY PS I; they

                                                                            completely LACK PS II and NADP reductase!

Cyclic Photophosphorylation   Thinking cyclic pathway produces NADPH        Cyclic photophosphorylation generates ONLY ATP;

                                                                            it produces zero NADPH and evolves zero O2!

Proton Gradient Direction     Confusing chloroplast with mitochondria       In chloroplasts, protons accumulate in the

                                                                            THYLAKOID LUMEN (Lumen pH is acidic, ~5.0).

Primary CO2 Acceptor in C3    Assuming primary acceptor is a 2C compound    The primary acceptor in C3 is a 5-CARBON

                                                                            ketose sugar: RuBP (Ribulose-1,5-bisphosphate).

Calvin Cycle Occurrence       Believing C4 plants do not use Calvin cycle   The Calvin cycle operates in ALL photosynthetic

                                                                            plants; C4 plants run it in BUNDLE SHEATH cells.

RuBisCO Localization in C4    Thinking RuBisCO is in mesophyll of C4        RuBisCO is EXCLUSIVE to Bundle Sheath cells in C4;

                                                                            Mesophyll cells have PEPcase and LACK RuBisCO!

ATP Cost for 1 Glucose        Equating ATP cost in C3 and C4                Synthesis of 1 Glucose requires:

                                                                            18 ATP in C3 plants vs 30 ATP in C4 plants!

Light as Limiting Factor      Assuming light is the main limiting factor    Light saturation occurs at 10% of full sunlight;

                                                                            CO2 is the chief limiting factor in nature!

Photorespiration Organelles   Misordering photorespiratory organelles       The correct organelle sequence is:

                                                                            Chloroplast ──► Peroxisome ──► Mitochondrion.