Class 11 Biology Notes Chapter 13 (Chapter 13) – Examplar Problems (English) Book

Detailed Notes with MCQs of Chapter 13: Photosynthesis in Higher Plants. This is a fundamentally important chapter, not just for your Class 11 understanding, but also because questions frequently appear from this section in various government exams where Biology is a component. Pay close attention as we break it down.

Chapter 13: Photosynthesis in Higher Plants - Detailed Notes

1. Introduction to Photosynthesis

• Definition: Photosynthesis is an anabolic, endergonic process by which green plants, algae, and some bacteria synthesize their own food (organic compounds, primarily glucose) using light energy, carbon dioxide, and water. Oxygen is released as a byproduct.
• Overall Equation:
  6CO₂ + 12H₂O --- (Light Energy, Chlorophyll) ---> C₆H₁₂O₆ + 6H₂O + 6O₂
• Significance:
  • Primary source of all food on Earth.
  • Responsible for the release of oxygen into the atmosphere, essential for aerobic respiration.

2. Early Experiments & Historical Perspective

• Joseph Priestley (1770s): Demonstrated the essential role of air in the growth of green plants. His bell jar experiments with a candle, mouse, and mint plant showed that plants restore air that breathing animals or burning candles remove (later identified as O₂ production and CO₂ consumption).
• Jan Ingenhousz (1779): Showed that sunlight is essential for the plant process that purifies the air (photosynthesis). He also demonstrated that only the green parts of the plants release oxygen.
• Julius von Sachs (1854): Provided evidence that glucose is produced when plants grow and is usually stored as starch. He identified chlorophyll's location within special bodies (chloroplasts).
• T.W. Engelmann (1883): Using Cladophora (a green alga) and aerobic bacteria, he plotted the first action spectrum of photosynthesis. He observed bacteria accumulated mainly in the regions illuminated with blue and red light, corresponding to the absorption maxima of chlorophyll.
• Cornelius van Niel (mid-20th century): Based on studies with purple and green sulfur bacteria, he demonstrated that oxygen evolved during photosynthesis comes from water (H₂O), not carbon dioxide (CO₂). This was later confirmed using radioisotope techniques (¹⁸O). The general equation he proposed was: 2H₂A + CO₂ --- (Light) ---> 2A + CH₂O + H₂O (where H₂A is an oxidizable compound like H₂O or H₂S).

3. Site of Photosynthesis: The Chloroplast

• Photosynthesis occurs primarily in the green parts of plants, mainly leaves, within specialized organelles called chloroplasts.
• Chloroplast Structure: Double-membraned organelle containing:
  • Grana (Singular: Granum): Stacks of flattened sacs called thylakoids. The thylakoid membranes contain chlorophyll pigments and are the site of the light-dependent reactions.
  • Stroma: The fluid-filled space enclosed by the inner membrane. It contains enzymes for the light-independent reactions (Calvin cycle), chloroplast DNA, and ribosomes.
  • Stroma Lamellae: Membranous tubules connecting the thylakoids of different grana.
• Division of Labour: There's a clear separation:
  • Light Reactions: Occur in the grana/thylakoid membranes. Trap light energy, synthesize ATP and NADPH.
  • Dark Reactions (Biosynthetic Phase): Occur in the stroma. Use ATP and NADPH to reduce CO₂ and synthesize sugars.

4. Pigments Involved in Photosynthesis

• Chlorophyll a: (Bright or blue-green) The primary photosynthetic pigment. It absorbs light mainly in the blue-violet and red regions and directly participates in converting light energy to chemical energy. Reaction center pigment.
• Chlorophyll b: (Yellow-green) An accessory pigment. Absorbs light and transfers energy to Chlorophyll a. Broadens the spectrum of light that can be used.
• Xanthophylls: (Yellow) Accessory pigments.
• Carotenoids: (Yellow to yellow-orange) Accessory pigments.
• Role of Accessory Pigments:
  • Absorb light at different wavelengths and transfer energy to Chlorophyll a (widening the absorption spectrum).
  • Protect Chlorophyll a from photo-oxidation (damage by excess light).
• Absorption Spectrum: A graph plotting light absorption versus wavelength for a pigment.
• Action Spectrum: A graph plotting the rate of photosynthesis versus wavelength of light. The action spectrum closely follows the absorption spectrum of chlorophylls a and b, indicating their primary role.

5. The Light-Dependent Reactions (Photochemical Phase)

• Location: Thylakoid membranes.
• Events:
  1. Light Absorption: Pigments absorb photons of light.
  2. Water Splitting (Photolysis): Occurs on the inner side of the thylakoid membrane, associated with Photosystem II (PS II).
     2H₂O → 4H⁺ + O₂ + 4e⁻
     • Source of O₂ released.
     • Source of electrons for the electron transport chain.
     • Source of protons (H⁺) contributing to the proton gradient.
  3. Oxygen Release: Byproduct of water splitting.
  4. Formation of ATP and NADPH: High-energy intermediates used in the dark reactions.
• Photosystems: Complexes of pigments and proteins.
  • Photosystem II (PS II): Reaction center chlorophyll a absorbs maximally at 680 nm (P680). Associated with water splitting. Located primarily in the appressed regions of grana thylakoids.
  • Photosystem I (PS I): Reaction center chlorophyll a absorbs maximally at 700 nm (P700). Involved in both cyclic and non-cyclic photophosphorylation. Located primarily in the non-appressed regions of grana thylakoids and stroma lamellae.
  • Light Harvesting Complex (LHC): Contains hundreds of accessory pigment molecules bound to proteins, surrounding the reaction center. They capture light energy and funnel it to the reaction center.
• Electron Transport Chain (ETC) - The Z-Scheme:
  • Electrons released from P680 (PS II) after light absorption pass through a series of electron carriers: Plastoquinone (PQ) → Cytochrome b₆f complex → Plastocyanin (PC).
  • During this transport (especially at Cytochrome b₆f), protons (H⁺) are pumped from the stroma into the thylakoid lumen, creating a proton gradient.
  • Electrons reach PS I (P700). After absorbing more light energy, P700 releases excited electrons.
  • These electrons pass through another ETC (involving Ferredoxin) and ultimately reduce NADP⁺ to NADPH in the stroma, catalyzed by NADP reductase.
  • The electron "hole" in PS II (P680) is filled by electrons from water splitting. The electron "hole" in PS I (P700) is filled by electrons coming from PS II via the ETC. This overall flow resembles the letter 'Z'.
• Photophosphorylation: Synthesis of ATP from ADP and inorganic phosphate (Pi) using light energy.
  • Non-Cyclic Photophosphorylation: The standard Z-scheme involving both PS II and PS I. Produces ATP, NADPH, and O₂. Electrons flow from water to NADP⁺.
  • Cyclic Photophosphorylation: Involves only PS I. Occurs when only ATP is needed or when NADP⁺ is not available. Electrons cycle back from Ferredoxin to the Cytochrome b₆f complex and then back to PS I. Produces only ATP (no NADPH, no O₂ release). Often occurs in stroma lamellae.
• Chemiosmotic Hypothesis (Peter Mitchell): Explains ATP synthesis.
  • Proton Gradient: Accumulation of H⁺ inside the thylakoid lumen due to (i) water splitting, (ii) H⁺ pumping by Cytochrome b₆f complex during ETC, and (iii) consumption of H⁺ in the stroma during NADPH formation.
  • ATP Synthase: An enzyme complex spanning the thylakoid membrane with two parts: F₀ (transmembrane channel for proton passage) and F₁ (headpiece on the stromal side where ATP synthesis occurs).
  • Mechanism: The breakdown of the proton gradient (protons flowing down their concentration gradient from lumen to stroma through the F₀ channel) provides the energy for the F₁ particle to synthesize ATP.

6. The Light-Independent Reactions / Biosynthetic Phase (Calvin Cycle)

• Location: Stroma of the chloroplast.
• Purpose: Uses the ATP and NADPH generated during the light reactions to fix atmospheric CO₂ into carbohydrates (sugars).
• The Calvin Cycle (C3 Pathway - Melvin Calvin): Occurs in all photosynthetic plants (C3, C4, CAM).
  • Step 1: Carboxylation:
    • CO₂ combines with a 5-carbon acceptor molecule, Ribulose-1,5-bisphosphate (RuBP).
    • Catalyzed by the enzyme RuBisCO (Ribulose bisphosphate carboxylase-oxygenase).
    • Forms an unstable 6-carbon intermediate which immediately splits into two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound. (This is why it's called the C3 pathway).
  • Step 2: Reduction:
    • 3-PGA is phosphorylated by ATP (forming 1,3-bisphosphoglycerate) and then reduced by NADPH (forming Glyceraldehyde-3-phosphate, G3P or triose phosphate).
    • Requires 2 ATP and 2 NADPH per CO₂ molecule fixed.
    • G3P is the primary sugar produced. Some G3P is used to synthesize glucose/sucrose, while most is used for regeneration.
  • Step 3: Regeneration:
    • Most of the G3P molecules are used to regenerate the initial CO₂ acceptor, RuBP.
    • This complex series of reactions requires ATP. (1 ATP per CO₂ fixed).
• Inputs & Outputs (per CO₂ fixed):
  • Inputs: 1 CO₂, 3 ATP, 2 NADPH
  • Outputs: 1/6 Glucose (indirectly, via G3P), 3 ADP, 2 NADP⁺
• To make one molecule of Glucose (C₆): The cycle must turn 6 times.
  • Inputs: 6 CO₂, 18 ATP, 12 NADPH
  • Outputs: 1 Glucose, 18 ADP, 12 NADP⁺

7. The C4 Pathway (Hatch and Slack Pathway)

• Adaptation: Found in plants adapted to dry tropical regions (e.g., maize, sugarcane, sorghum). They are more efficient at CO₂ fixation in high light and temperature, and low CO₂ conditions. They lack photorespiration.
• Kranz Anatomy: Characteristic leaf anatomy:
  • Bundle Sheath Cells: Large cells surrounding the vascular bundles, having thick walls impervious to gas exchange, no intercellular spaces, and a large number of chloroplasts. Site of the Calvin Cycle.
  • Mesophyll Cells: Clustered around bundle sheath cells. Site of initial CO₂ fixation.
• Mechanism: Spatial separation of initial CO₂ fixation and the Calvin Cycle.
  1. In Mesophyll Cells:
     • Primary CO₂ acceptor is a 3-carbon molecule, phosphoenolpyruvate (PEP).
     • Enzyme: PEP carboxylase (PEPCase). Has high affinity for CO₂, no oxygenase activity.
     • CO₂ + PEP → Oxaloacetic acid (OAA), a 4-carbon compound (hence C4 pathway).
     • OAA is converted to other 4-carbon acids (like malic acid or aspartic acid).
  2. Transport: These 4-carbon acids are transported to the bundle sheath cells.
  3. In Bundle Sheath Cells:
     • The 4-carbon acid is decarboxylated (releases CO₂).
     • This concentrates CO₂ around RuBisCO, ensuring it functions primarily as a carboxylase, minimizing photorespiration.
     • The released CO₂ enters the Calvin Cycle (C3 pathway) using RuBisCO.
     • The remaining 3-carbon molecule is transported back to the mesophyll cells to regenerate PEP (requires ATP).
• Significance: Increases CO₂ concentration at the RuBisCO site, minimizing photorespiration and increasing efficiency in hot/dry conditions.

8. Photorespiration

• Definition: A light-dependent process where RuBisCO acts as an oxygenase, binding O₂ instead of CO₂. Occurs in C3 plants under conditions of high temperature, high light intensity, high O₂ concentration, and low CO₂ concentration.
• Mechanism:
  • RuBP + O₂ --- (RuBisCO) ---> One molecule of PGA (3C) + One molecule of Phosphoglycolate (2C).
  • Phosphoglycolate is metabolized through a complex pathway involving chloroplasts, peroxisomes, and mitochondria.
  • CO₂ is released during this pathway (in mitochondria).
• Consequences:
  • Considered a wasteful process as it consumes fixed carbon (releases CO₂).
  • Uses ATP but produces no sugar or NADPH.
  • Reduces the efficiency of photosynthesis in C3 plants under certain conditions.
• Absence in C4 Plants: C4 plants avoid photorespiration because the initial fixation by PEPCase and the subsequent concentration of CO₂ in bundle sheath cells ensures RuBisCO primarily encounters CO₂.

9. Factors Affecting Photosynthesis

• The rate of photosynthesis is influenced by several external (environmental) and internal (plant-related) factors.
• 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 process if its quantity is changed." Essentially, the rate is limited by the factor that is in shortest supply.
• External Factors:
  • Light:
    • Intensity: Rate increases with intensity up to a saturation point (usually 10% of full sunlight). Beyond saturation, the rate may decrease due to photo-oxidation.
    • Quality (Wavelength): Maximum photosynthesis occurs in blue and red light.
    • Duration: Longer duration generally leads to more photosynthesis, assuming other factors are optimal.
  • Carbon Dioxide Concentration: Major limiting factor. Rate increases with CO₂ concentration up to a saturation point (around 0.05%). Current atmospheric concentration (~0.03-0.04%) is often limiting for C3 plants. C4 plants show saturation at higher levels.
  • Temperature: Photosynthesis involves enzymes, so it's temperature-sensitive. Has an optimal temperature range (differs for C3 and C4 plants; C4 plants generally have a higher optimum). Very high temperatures denature enzymes.
  • Water: Water stress causes stomata to close (reducing CO₂ availability) and can affect metabolic activity, thus reducing photosynthesis. Direct effect is less pronounced than indirect effects.
• Internal Factors:
  • Number, size, age, and orientation of leaves.
  • Number of mesophyll cells and chloroplasts.
  • Internal CO₂ concentration.
  • Amount of chlorophyll.
  • Genetic makeup of the plant.

Multiple Choice Questions (MCQs)

1. In the overall equation for photosynthesis, the oxygen (O₂) released comes from:
   a) CO₂
   b) H₂O
   c) C₆H₁₂O₆
   d) ATP

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

3. The light-dependent reactions of photosynthesis occur in the _______, while the light-independent reactions occur in the _______.
   a) Stroma; Grana
   b) Grana; Stroma
   c) Cytoplasm; Stroma
   d) Stroma; Cytoplasm

4. Which pigment acts as the reaction center in both Photosystem I and Photosystem II?
   a) Chlorophyll b
   b) Carotenoid
   c) Xanthophyll
   d) Chlorophyll a

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

6. In the C4 pathway, the primary CO₂ acceptor is _______ and it is found in the _______ cells.
   a) RuBP; Bundle sheath
   b) PEP; Mesophyll
   c) PGA; Mesophyll
   d) OAA; Bundle sheath

7. Photorespiration is favoured by:
   a) Low O₂ and high CO₂ concentration
   b) High O₂ and low CO₂ concentration
   c) Low temperature and high light intensity
   d) High humidity and low light intensity

8. Kranz anatomy is a characteristic feature of the leaves of:
   a) C3 plants
   b) C4 plants
   c) CAM plants
   d) All photosynthetic plants

9. The enzyme responsible for the primary carboxylation step in the Calvin cycle (C3) is:
   a) PEP carboxylase
   b) RuBisCO
   c) NADP reductase
   d) ATP synthase

10. According to Blackman's Law of Limiting Factors, if light intensity, CO₂ concentration, and temperature are suboptimal, the rate of photosynthesis will be limited by:
    a) The factor which is furthest from its optimum
    b) The factor which is closest to its optimum
    c) The factor which is present in the lowest amount relative to its requirement
    d) Always the CO₂ concentration

Answer Key:

1. b) H₂O
2. c) T.W. Engelmann
3. b) Grana; Stroma
4. d) Chlorophyll a
5. b) Only ATP
6. b) PEP; Mesophyll
7. b) High O₂ and low CO₂ concentration
8. b) C4 plants
9. b) RuBisCO
10. c) The factor which is present in the lowest amount relative to its requirement

Remember to thoroughly understand the mechanisms, especially the Z-scheme, Calvin cycle, C4 pathway, and the conditions favouring photorespiration. Comparing C3 and C4 plants is also a very common exam topic. Good luck with your preparation!