Class 11 Biology Notes Chapter 13 (Photosynthesis in higher plants) – Biology Book

Detailed Notes with MCQs of Chapter 13, 'Photosynthesis in Higher Plants'. This is a crucial chapter, not just for your Class 11 understanding, but also forms the basis for many questions in competitive government exams. Pay close attention to the processes, locations, and key molecules involved.

Photosynthesis in Higher Plants: Detailed Notes

1. Introduction

• Definition: Photosynthesis is an anabolic, endergonic process by which green plants (and some other organisms) use sunlight energy to synthesize food (glucose) from carbon dioxide and water. Oxygen is released as a byproduct.
• Importance:
  • Primary source of all food on Earth.
  • Responsible for the release of oxygen into the atmosphere, essential for aerobic respiration.
• Overall Equation:
  6CO₂ + 12H₂O ---(Light Energy, Chlorophyll)--> C₆H₁₂O₆ + 6H₂O + 6O₂

2. Early Experiments (Key Discoveries)

• Joseph Priestley (1770s): Discovered oxygen. Showed that plants restore air that breathing animals or burning candles remove (using bell jar, candle, mouse, mint plant).
• Jan Ingenhousz (1779): Showed that sunlight is essential for plant purification of air. Only green parts of plants release O₂. Demonstrated this using an aquatic plant (Hydrilla).
• Julius von Sachs (1854): Provided evidence that glucose is produced during photosynthesis and is usually stored as starch. Identified chloroplasts as the site of glucose formation.
• T.W. Engelmann (1883): Described the first action spectrum of photosynthesis using green alga Cladophora, aerobic bacteria, and a prism to split light. Bacteria accumulated mainly in blue and red light regions.
• Cornelius van Niel (mid-20th century): Based on studies with purple and green sulfur bacteria, demonstrated that O₂ evolved by green plants comes from H₂O, not CO₂. He proposed the general equation:
  2H₂A + CO₂ ---(Light)--> 2A + CH₂O + H₂O
  (Where H₂A is the hydrogen donor; H₂O in green plants, H₂S in sulfur bacteria).

3. Site of Photosynthesis: The Chloroplast

• Occurs in green parts of plants, mainly leaves, specifically in the mesophyll cells.
• Chloroplast Structure:
  • Double-membraned organelle.
  • Stroma: The fluid-filled inner space containing enzymes for the dark reaction (CO₂ fixation), ribosomes, circular DNA, and starch granules.
  • Grana: Stacks of flattened membrane sacs called thylakoids. Thylakoids contain chlorophyll pigments and are the site of the light reaction.
  • Stroma Lamellae: Membranous tubules connecting grana, also contain pigments and participate in light reaction (mainly cyclic photophosphorylation).
• Division of Labour:
  • Light-dependent reactions (Light Reaction): Occur in the thylakoid membranes (grana and stroma lamellae). Trap light energy, synthesize ATP and NADPH.
  • Light-independent reactions (Dark Reaction / Biosynthetic Phase): Occur in the stroma. Use ATP and NADPH to reduce CO₂ and synthesize sugars.

4. Pigments Involved in Photosynthesis

• Location: Thylakoid membranes.
• Types:
  • Chlorophyll a (Chl a): The main or primary photosynthetic pigment (blue-green). Acts as the reaction center. Absorbs maximally in blue and red regions.
  • Chlorophyll b (Chl b): Accessory pigment (yellow-green). Absorbs light and transfers energy to Chl a. Broadens the absorption spectrum.
  • Xanthophylls: Accessory pigment (yellow).
  • Carotenoids: Accessory pigment (yellow to yellow-orange). Protect Chl a from photo-oxidation (excessive light damage) and transfer energy.
• Absorption Spectrum: A graph showing the amount of light absorbed by a pigment at different wavelengths.
• Action Spectrum: A graph showing the rate of photosynthesis at different wavelengths of light. The action spectrum closely follows the absorption spectrum of chlorophylls, indicating their role in photosynthesis.
• Photosystems: Pigments are organized into two discrete photochemical Light Harvesting Complexes (LHC) within Photosystem I (PS I) and Photosystem II (PS II).
  • Each photosystem has a reaction center (a specific Chl a molecule) and hundreds of accessory pigment molecules (antenna molecules) bound to proteins.
  • PS I Reaction Center: Chlorophyll a molecule absorbing maximally at 700 nm (P700).
  • PS II Reaction Center: Chlorophyll a molecule absorbing maximally at 680 nm (P680).

5. The Light Reaction (Photochemical Phase)

• Location: Thylakoid membranes.
• Events:
  1. Light Absorption: Pigments absorb light energy.
  2. Water Splitting (Photolysis): Occurs associated with PS II on the inner side of the thylakoid membrane.
     2H₂O ---> 4H⁺ + O₂ + 4e⁻
     • Source of electrons for PS II.
     • Source of protons (H⁺) contributing to the proton gradient.
     • Releases O₂ as a byproduct.
  3. Oxygen Evolution: Released from water splitting.
  4. Formation of High-Energy Intermediates: ATP and NADPH.
• Electron Transport System (ETS) - The Z Scheme (Non-Cyclic Photophosphorylation):
  • Involves both PS II and PS I.
  • Light excites P680 (PS II), electrons move through an ETS (cytochrome b6f complex) to P700 (PS I).
  • During this electron transport, protons are pumped into the thylakoid lumen, creating a proton gradient for ATP synthesis.
  • Light excites P700 (PS I), electrons move to another ETS and finally reduce NADP⁺ to NADPH + H⁺ (in the stroma).
  • Electrons lost by P680 are replaced by electrons from water splitting. Electrons lost by P700 are replaced by electrons coming from PS II via the ETS.
  • Products: ATP, NADPH, O₂.
• Cyclic Photophosphorylation:
  • Involves only PS I.
  • Occurs when only ATP is needed, or when NADP⁺ is not available (e.g., in stroma lamellae which lack PS II and NADP reductase enzyme).
  • Excited electrons from P700 cycle back to PS I through the ETS (cytochrome complex).
  • Generates a proton gradient, leading to ATP synthesis only.
  • No water splitting, no O₂ release, no NADPH formation.
• Chemiosmotic Hypothesis (ATP Synthesis):
  • Proposed by Peter Mitchell.
  • ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane.
  • Proton Accumulation in Lumen: Due to (i) water splitting, (ii) H⁺ pumping by cytochrome complex during ETS, (iii) H⁺ removal from stroma during NADP⁺ reduction.
  • Proton Gradient Breakdown: The gradient is broken down when H⁺ moves from the lumen to the stroma through the transmembrane channel of the ATP synthase enzyme (CF₀-CF₁ particle).
  • ATP Synthase: Has two parts: CF₀ (transmembrane channel) and CF₁ (headpiece on stromal side). Energy released from proton movement facilitates conformational changes in CF₁ activating ATP synthesis (ADP + Pi --> ATP).

6. The Dark Reaction (Biosynthetic Phase / Calvin Cycle)

• Location: Stroma of the chloroplast.
• Nature: Light-independent, but dependent on the products of the light reaction (ATP and NADPH). Uses these to fix CO₂ into sugars.
• The Calvin Cycle (C3 Pathway): Discovered by Melvin Calvin using radioactive ¹⁴C in algal photosynthesis.
  • Primary CO₂ Acceptor: Ribulose-1,5-bisphosphate (RuBP) - a 5-carbon ketose sugar.
  • Key Enzyme: RuBisCO (Ribulose Bisphosphate Carboxylase-Oxygenase). Most abundant enzyme on Earth.
  • Steps:
    1. Carboxylation: CO₂ combines with RuBP (5C) to form an unstable 6C intermediate, which immediately splits into two molecules of 3-phosphoglycerate (3-PGA) (3C). This is the first stable product of the C3 cycle. Enzyme: RuBisCO.
    2. Reduction: 3-PGA is phosphorylated by ATP and then reduced by NADPH to form glyceraldehyde-3-phosphate (G3P) (Triose Phosphate). (Series of reactions).
    3. Regeneration: Most of the G3P molecules are used to regenerate RuBP (using ATP). Some G3P molecules are used to synthesize glucose, sucrose, starch etc.
  • Stoichiometry: For every 1 CO₂ molecule fixed: 3 ATP and 2 NADPH are required.
  • For 1 Glucose (6C) molecule: 6 turns of the cycle are needed, requiring 6 CO₂, 18 ATP, and 12 NADPH.

7. The C4 Pathway (Hatch and Slack Pathway)

• Adaptation: Found in plants adapted to dry tropical regions (e.g., Maize, Sugarcane, Sorghum, Amaranthus). They are more efficient in high temperature and light intensity.
• Special Feature: Kranz Anatomy.
  • Large Bundle Sheath Cells surrounding vascular bundles. These have thick walls impervious to gas exchange, no intercellular spaces, and contain large numbers of chloroplasts (agranal or with reduced grana).
  • Mesophyll Cells: Packed around bundle sheath cells. Contain chloroplasts with well-developed grana.
• Mechanism: A mechanism to concentrate CO₂ at the site of RuBisCO enzyme in bundle sheath cells, minimizing photorespiration.
  • In Mesophyll Cells:
    • Primary CO₂ acceptor: Phosphoenolpyruvate (PEP) - a 3-carbon molecule.
    • Enzyme: PEP carboxylase (PEPcase). Has high affinity for CO₂, no oxygenase activity.
    • CO₂ + PEP ---> Oxaloacetic Acid (OAA) - a 4-carbon acid (First stable product).
    • OAA is converted to other 4C acids like Malic acid or Aspartic acid.
  • Transport: These 4C acids are transported to Bundle Sheath Cells.
  • In Bundle Sheath Cells:
    • The 4C acid is decarboxylated (breaks down) to release CO₂ and a 3C molecule (Pyruvic acid).
    • The released CO₂ enters the Calvin Cycle (C3 pathway) which occurs here. RuBisCO functions efficiently due to high CO₂ concentration.
    • The 3C molecule (Pyruvate) is transported back to the mesophyll cells.
  • Regeneration: Pyruvate is converted back to PEP in mesophyll cells, requiring ATP.
• Significance: C4 plants are more efficient photosynthetically than C3 plants in conditions of high light, high temperature, and limited water, mainly because they avoid photorespiration. They require more ATP (30 ATP for 1 glucose vs 18 ATP in C3).

8. Photorespiration (C2 Cycle)

• Occurrence: In C3 plants (e.g., Rice, Wheat, Soybean) under conditions of high O₂, low CO₂, high temperature, and high light intensity.
• Mechanism: RuBisCO acts as an oxygenase instead of carboxylase. It binds O₂ to RuBP.
  RuBP + O₂ ---(RuBisCO)---> 1 molecule of 3-PGA + 1 molecule of Phosphoglycolate (2C).
• Process: Phosphoglycolate is metabolized in a pathway involving Chloroplasts, Peroxisomes, and Mitochondria. It results in the release of CO₂ (in mitochondria) and consumption of ATP.
• Wasteful Process:
  • Does not produce ATP or NADPH.
  • Releases already fixed CO₂, reducing photosynthetic output (by up to 25%).
  • Uses ATP.
• Why C4 plants lack photorespiration: They have a mechanism (PEPcase and Kranz anatomy) that increases the CO₂ concentration at the RuBisCO site (in bundle sheath cells), ensuring RuBisCO acts primarily as a carboxylase.

9. Factors Affecting Photosynthesis

• Both internal (plant) and external (environmental) factors influence the rate.
• Internal Factors: Number, size, age, and orientation of leaves; mesophyll cells and chloroplasts; internal CO₂ concentration; amount of chlorophyll. Plant growth stage and genetics.
• External Factors:
  • Light:
    • Quality: PAR (Photosynthetically Active Radiation, 400-700 nm) is utilized. Maximum photosynthesis in blue and red light.
    • Intensity: Rate increases with intensity up to a light saturation point (usually 10% of full sunlight for shade plants, higher for sun plants). Beyond saturation, rate may decrease due to photoinhibition/photooxidation.
    • Duration: Longer duration generally leads to more photosynthesis, assuming other factors are optimal.
  • Carbon Dioxide Concentration:
    • Major limiting factor in nature (atmospheric conc. ~0.03-0.04%).
    • Rate increases with CO₂ conc. up to a saturation point. C3 plants saturate around 450 µlL⁻¹ (0.045%), while C4 plants saturate around 360 µlL⁻¹ (0.036%) due to higher efficiency.
    • Very high concentrations can become damaging over long periods. Greenhouse crops (tomato, bell pepper) benefit from CO₂ enrichment.
  • Temperature:
    • Dark reactions are enzymatic and temperature-controlled. Light reactions are less sensitive.
    • Optimum temperature varies. C3 plants have a lower optimum (20-25°C) than C4 plants (30-40°C).
    • High temperatures can denature enzymes, reducing the rate.
  • Water:
    • Water stress causes stomata to close, reducing CO₂ availability.
    • Also affects leaf turgidity and surface area, reducing metabolic activity. Direct effect on photolysis is rarely limiting.
• Blackman's Law of Limiting Factors (1905): "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 rate if its quantity is changed." Essentially, the rate is limited by the slowest step or scarcest resource.

Multiple Choice Questions (MCQs)

1. In the overall equation of photosynthesis, the O₂ released comes from:
   a) CO₂
   b) H₂O
   c) C₆H₁₂O₆
   d) Sunlight

2. Which scientist used a prism, Cladophora, and aerobic bacteria to plot the first action spectrum of photosynthesis?
   a) Joseph Priestley
   b) Jan Ingenhousz
   c) T.W. Engelmann
   d) Julius von Sachs

3. The light-dependent reactions of photosynthesis occur primarily in the:
   a) Stroma
   b) Outer membrane of chloroplast
   c) Thylakoid membranes (Grana)
   d) Cytoplasm

4. Which of the following is the primary photosynthetic pigment?
   a) Chlorophyll b
   b) Xanthophyll
   c) Carotenoid
   d) Chlorophyll a

5. During non-cyclic photophosphorylation (Z-scheme), electrons lost by PS II (P680) are replaced by electrons from:
   a) PS I (P700)
   b) ATP
   c) The splitting of water
   d) NADPH

6. Cyclic photophosphorylation results in the formation of:
   a) ATP only
   b) NADPH only
   c) ATP and NADPH
   d) ATP, NADPH and O₂

7. In the Calvin cycle (C3 pathway), the primary CO₂ acceptor is _______ and the key enzyme is _______.
   a) PEP, PEPcase
   b) RuBP, RuBisCO
   c) 3-PGA, RuBisCO
   d) OAA, PEPcase

8. Kranz anatomy and the presence of PEPcase enzyme in mesophyll cells are characteristic features of:
   a) C3 plants
   b) C4 plants
   c) CAM plants
   d) All photosynthetic plants

9. Photorespiration occurs when RuBisCO binds with _______ instead of CO₂ and is considered wasteful because it _______.
   a) H₂O, produces extra sugar
   b) O₂, consumes ATP and releases CO₂
   c) N₂, consumes NADPH
   d) O₂, produces ATP and NADPH

10. According to Blackman's Law of Limiting Factors, if light intensity is low, increasing the temperature or CO₂ concentration will:
    a) Significantly increase the rate of photosynthesis
    b) Significantly decrease the rate of photosynthesis
    c) Have little to no effect on the rate of photosynthesis
    d) Stop photosynthesis completely

Answer Key:

1. b) H₂O
2. c) T.W. Engelmann
3. c) Thylakoid membranes (Grana)
4. d) Chlorophyll a
5. c) The splitting of water
6. a) ATP only
7. b) RuBP, RuBisCO
8. b) C4 plants
9. b) O₂, consumes ATP and releases CO₂
10. c) Have little to no effect on the rate of photosynthesis (as light is the limiting factor)

Study these notes thoroughly. Remember the locations, inputs, outputs, and key differences between the pathways. Good luck with your preparation!