NCERT Class 11 Biology: Chapter 11 — Photosynthesis in Higher Plants

11.1 Historical Experiments: Priestley (Bell jar), Ingenhousz (Sunlight), Sachs (Starch), Engelmann (Action spectrum) #

All life on Earth ultimately depends on sunlight for energy. Photosynthesis is the vital physicochemical process by which green plants harvest solar radiant energy to drive the synthesis of organic food compounds from inorganic carbon dioxide and water, releasing oxygen as a byproduct.

Photosynthesis is responsible for the release of oxygen into the Earth’s atmosphere and forms the primary basis of all food chains on the planet. Key historical milestones detailed in NCERT include:

• Joseph Priestley (1770): Using a bell jar, candle, and mouse, Priestley discovered that a burning candle or breathing mouse foul the air, but introducing a sprig of mint plant restores the air, allowing both the mouse to survive and the candle to burn. In 1774, Priestley discovered Oxygen.
• Jan Ingenhousz (1779): Showed that sunlight is essential for this plant purification process. By using an aquatic plant (Hydrilla), he demonstrated that in bright sunlight, tiny bubbles formed around the green parts of the plant, but not in the dark. He identified these bubbles as oxygen.
• Julius von Sachs (1854): Provided evidence that the green substance in plants (chlorophyll) is located in specialized bodies (chloroplasts) and that the product of photosynthesis is glucose, which is usually stored as starch.
• T.W. Engelmann (1843–1909): Used a glass prism to split white light into its spectral components to illuminate a green alga, Cladophora, suspended in an aerobic bacterial suspension. Bacteria accumulated primarily in the blue and red light regions of the split spectrum, producing the first action spectrum of photosynthesis.
• Cornelius van Niel (1897–1985): Based on studies of purple and green sulfur bacteria, van Niel demonstrated that photosynthesis is essentially a light-dependent reaction in which hydrogen from an oxidizable compound reduces carbon dioxide to carbohydrates. In green sulfur bacteria, H₂S is the hydrogen donor, yielding sulfur instead of oxygen. He correctly inferred that the O₂ evolved by green plants comes from H₂O, not from CO₂ (later confirmed using radioisotope ¹⁸O).

11.2 Photosynthetic Pigments: Chlorophyll a (Reaction center), Chlorophyll b, Carotenoids & Xanthophylls #

In higher plants, photosynthesis occurs within green leaves inside mesophyll cells, which contain a high concentration of chloroplasts. Chloroplasts align themselves along the walls of the mesophyll cells to receive optimal incident light.

Inside the chloroplast, there is a clear division of physiological labor between two structural compartments:

• Membranous System (Grana & Stroma Lamellae): Consists of disc-shaped thylakoids stacked into grana connected by flat membranous tubes called stroma lamellae. The membrane network contains light-harvesting pigments, electron transport chains, and ATP synthase. It is responsible for trapping light energy and synthesizing ATP and NADPH (the Photochemical Phase or Light Reactions).
• Fluid Stroma: The enzymatic matrix surrounding the thylakoids. It contains the soluble enzymes responsible for carbon dioxide fixation and carbohydrate synthesis (the Biosynthetic Phase or Dark Reactions). Crucial NCERT fact: Dark reactions do not require darkness; they simply do not directly require light photons, but depend strictly on the products of the light reaction (ATP and NADPH).

11.3 Light Reaction (Photochemical Phase): Photosystem II (P680) & Photosystem I (P700) #

A chromatographic separation of leaf pigments reveals that the green color of leaves is not due to a single pigment, but to four distinct photosynthetic pigments:

An Absorption Spectrum shows the wavelengths of light absorbed by individual pigments (Chlorophyll a absorbs maximally in the blue and red regions). An Action Spectrum shows the rate of photosynthesis at different wavelengths. The close overlap between the absorption spectrum of chlorophyll a and the action spectrum of photosynthesis proves that chlorophyll a is the chief pigment driving photosynthesis.

11.4 Photolysis of Water (Oxygen-Evolving Complex) & The Z-Scheme Electron Flow #

The Light Reactions (Photochemical phase) encompass light absorption, water splitting, oxygen release, and the synthesis of high-energy chemical intermediates: ATP and NADPH.

The pigments are organized into two distinct photochemical Light Harvesting Complexes (LHC) within Photosystem I (PS I) and Photosystem II (PS II). Each photosystem consists of hundreds of pigment molecules bound to proteins, forming an antenna complex that channels photon energy toward a single special chlorophyll a molecule known as the Reaction Center:

• In PS I, the reaction center chlorophyll a has an absorption peak at 700 nm, hence it is called P700.
• In PS II, the reaction center chlorophyll a has an absorption peak at 680 nm, hence it is called P680.

11.5 Cyclic vs Non-Cyclic Photophosphorylation #

The synthesis of ATP from ADP and inorganic phosphate in the presence of light is called Photophosphorylation. Electron transport occurs in two modes:

• Non-Cyclic Photophosphorylation (The Z-Scheme):

  Both PS II and PS I work in series connected by an electron transport chain. When PS II absorbs 680 nm red light, electrons in P680 are excited and transferred to a primary electron acceptor. The electrons pass down an electron transport system of cytochromes to PS I. This downhill transport powers ATP synthesis.

  Simultaneously, electrons in P700 of PS I are excited by 700 nm light and transferred to another acceptor, finally reducing NADP⁺ to NADPH + H⁺ via ferredoxin-NADP⁺ reductase. Because this flow of electrons from water to PS II, through the ETS to PS I, and finally to NADP⁺ forms a zigzag pattern on a redox potential scale, it is termed the Z-Scheme.

  Photolysis of Water (Splitting of Water): The electrons extracted from PS II are continuously replaced by the photolytic splitting of water catalyzed by the Oxygen-Evolving Complex (OEC) on the inner thylakoid lumen side: 2H2O → 4H+ + O2 + 4e−. The protons accumulate in the lumen, while oxygen gas is released into the atmosphere. The water splitting complex requires Manganese (Mn²⁺) and Chlorine (Cl⁻).

• Cyclic Photophosphorylation: Occurs when only PS I is functional (found in stroma lamellae, which lack PS II and NADP reductase enzyme), or when light of wavelength beyond 680 nm is available. The excited electron does not pass to NADP⁺; instead, it is cycled back to PS I through the electron transport chain, generating ATP only (no NADPH or O₂ is formed).

11.6 Chemiosmotic Hypothesis of ATP Synthesis (Peter Mitchell) #

How does electron flow in the thylakoid membrane drive the synthesis of ATP? In 1961, Peter Mitchell proposed the Chemiosmotic Hypothesis, which explains that ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane.

Protons accumulate in high concentration inside the thylakoid lumen, creating a steep proton gradient and lowering the lumen pH (~5.0) relative to the stroma (~8.0). Three biological events create this proton gradient:

1. Water Splitting: Occurs on the inner surface of the thylakoid membrane, releasing protons directly into the lumen.
2. Plastoquinone Proton Pumping: When electrons move through the photosystems, the primary electron acceptor transfers electrons to an H-carrier (Plastoquinone, PQ). PQ picks up protons from the stroma; when it passes electrons to cytochrome b₆f on the inner side, it releases protons into the lumen.
3. NADP⁺ Reductase Reaction: Located on the stroma side of the membrane; removes protons from the stroma to reduce NADP⁺ to NADPH.

This high proton concentration in the lumen creates a Proton Motive Force. The gradient is broken as protons diffuse back across the membrane into the stroma through the transmembrane channel of the ATP Synthase complex (CF₀-CF₁). The breakdown of the gradient induces conformational changes in the catalytic CF₁ headpiece, driving the condensation of ADP and Pi into ATP.

11.7 Dark Reaction (Biosynthetic Phase): The Calvin C3 Cycle (Carboxylation, Reduction, Regeneration) #

The ATP and NADPH generated during the light reactions are utilized in the stroma to assimilate atmospheric CO₂ into carbohydrates (glucose). This biosynthetic pathway was discovered by Melvin Calvin using radioactive ¹⁴C in algal photosynthetic studies, and is called the Calvin Cycle (C₃ Pathway).

The first stable product of CO₂ fixation in C₃ plants is a 3-carbon organic acid: 3-phosphoglyceric acid (3-PGA). The primary CO₂ acceptor molecule is a 5-carbon ketose sugar called Ribulose-1,5-bisphosphate (RuBP). The Calvin cycle operates in three sequential stages:

1. Carboxylation: The most crucial step of the cycle. CO₂ is fixed onto RuBP catalyzed by the enzyme RuBisCO (RuBP carboxylase-oxygenase) to produce two molecules of 3-PGA.
2. Reduction: A series of reactions leading to the formation of triose phosphates (G3P) and eventually glucose. For every CO₂ molecule fixed, 2 ATP (for phosphorylation) and 2 NADPH (for reduction) are consumed.
3. Regeneration: RuBP must be regenerated to sustain the cycle. Regeneration of one RuBP molecule consumes 1 ATP.

11.8 The C4 Pathway (Hatch & Slack): Kranz Anatomy, Bundle Sheath Cells & PEPCase Adaptation #

Plants adapted to dry tropical environments have evolved a specialized photosynthetic mechanism called the C₄ Pathway (Hatch-Slack Pathway) to overcome a wasteful process called photorespiration. Examples: Maize, Sugarcane, Sorghum.

• Kranz Anatomy: The leaves of C₄ plants possess large cells arranged in a wreath-like ring around the vascular bundles, called bundle sheath cells (Kranz is German for wreath). Bundle sheath cells have several layers of thick walls impervious to gaseous exchange, no intercellular spaces, and contain large numbers of agranal chloroplasts.
• Mechanism of the C₄ Pathway:
  1. In mesophyll cells, atmospheric CO₂ is fixed by a 3-carbon acceptor, Phosphoenolpyruvate (PEP), catalyzed by the enzyme PEP Carboxylase (PEPcase). Mesophyll cells lack RuBisCO completely!
  2. The first stable product is a 4-carbon organic acid: Oxaloacetic acid (OAA). OAA is converted into malic acid or aspartic acid and transported to the bundle sheath cells.
  3. In bundle sheath cells, malic acid undergoes decarboxylation to release CO₂ and a 3-carbon pyruvate. The released CO₂ raises the local CO₂ concentration, where RuBisCO fixes it into the standard Calvin C₃ cycle! The pyruvate returns to mesophyll to regenerate PEP (consuming 2 ATP).
• Photorespiration (The C₂ Cycle): RuBisCO has an active site that can bind both CO₂ and O₂. In C₃ plants under high temperature, intense light, and low CO₂, RuBisCO binds O₂ instead of CO₂, forming one molecule of 3-PGA and one molecule of 2-carbon phosphoglycolate. Photorespiration produces neither ATP nor sugars; instead, it consumes ATP and releases CO₂—a highly wasteful process. In C₄ plants, photorespiration is completely absent because bundle sheath cells maintain high internal CO₂ concentrations.
• Blackman’s Law of Limiting Factors (1905): If a chemical process is affected by more than one factor, its rate is governed by the factor which is nearest to its minimal value—the factor which directly affects the process if its quantity is changed. Primary factors affecting photosynthesis are light intensity, carbon dioxide concentration (major limiting factor in C₃ plants), temperature, and water.

11.9 Photorespiration (C2 Cycle) & Factors Affecting Photosynthesis (Blackman’s Law of Limiting Factors) #

Photorespiration (C2 Cycle) & Factors Affecting Photosynthesis (Blackman’s Law of Limiting Factors) is a core concept in this chapter. Understanding this requires analyzing its structural organization, biological mechanisms, and regulatory feedback pathways.

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