Notes Class 11 Chapter 12 Biology Maharashtra Board

Photosynthesis

Introduction

Photosynthesis is a vital biological process through which green plants, algae, and some bacteria convert solar energy into chemical energy, producing carbohydrates (glucose) from carbon dioxide (CO₂) and water (H₂O). It is the primary source of energy for Earth’s ecosystems, often described as “bottled solar energy” because it captures sunlight and stores it in chemical bonds. Oxygen (O₂) is released as a byproduct, essential for aerobic respiration and the formation of the ozone layer.

Key Equation:

6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O (in the presence of light and chlorophyll)

Significance:

Converts inorganic substances (CO₂ and H₂O) into organic food.
Transforms solar energy into chemical energy.
Releases O₂, supporting life and protecting Earth via the ozone layer.
Contributes to fossil fuel formation (coal, petroleum, natural gas).

12.1 Chloroplast: The Site of Photosynthesis

Location: Chloroplasts are primarily found in the mesophyll cells of leaves. CO₂ enters through stomata, and water is supplied via veins.
Structure:
- Shape: Discoid or lens-shaped in higher plants.
- Membranes: Bounded by a double membrane (outer and inner).
- Stroma: Ground substance inside the membranes, containing enzymes for the dark reaction.
- Grana: Stacks of chlorophyll-bearing, double-membrane sacs (thylakoids) within the stroma.
- Thylakoids: Individual sacs in grana, containing pigments in their membranes.
- Lamellae: Connect grana, forming stroma lamellae.
- Other Components: DNA, ribosomes, plastoglobules (lipid droplets).

Pigments in Thylakoid Membranes:

Chlorophylls:
- Chlorophyll a: Contains a methyl group (-CH₃), absorbs blue, violet, and red light.
- Chlorophyll b: Contains an aldehyde group (-CHO), complements chlorophyll a.
- Structure: Porphyrin ring (head, tetrapyrrole) and phytol tail (hydrocarbon).
Carotenoids: Lipid-based, red, orange, yellow, or brown pigments.
- Carotenes: Orange-red (C₄₀H₅₆).
- Xanthophylls: Contain oxygen.
- Functions: Absorb light, transfer energy to chlorophyll, protect chlorophyll from photo-oxidation.

CO₂ Availability:

Atmosphere: 0.03% CO₂ (2200 billion tons).
Oceans: ~50 times more CO₂ (dissolved gas or carbonates).
Annual Fixation: ~70 billion tons of carbon fixed by green plants.

12.2 Nature of Light

Light as Energy: Light is electromagnetic radiation, traveling as:
- Photons (quantum theory): Discrete particles carrying a quantum of energy.
- Waves (wave theory): Varying wavelengths producing different colors.
Visible Spectrum: 390-730 nm, between ultraviolet and infrared.
Absorption Spectrum: Graph showing light absorption by pigments at different wavelengths.
- Chlorophyll a and b absorb maximally in blue, violet, and red regions.
Action Spectrum: Graph showing the rate of photosynthesis at different wavelengths.
- Closely matches the absorption spectrum of chlorophyll a and b, confirming their role in photosynthesis.

Activities:

Chlorophyll Fluorescence: Grind spinach leaves in acetone, filter, and observe the filtrate under a torch in a dark room. The solution appears red due to chlorophyll fluorescence (re-emission of absorbed light).
Paper Chromatography: Separate chloroplast pigments using a chromatography strip with petroleum ether and acetone (9:1). Pigments form distinct green (chlorophyll) and yellow (carotenoid) bands.

Why Chlorophyll Appears Green/Red?

Reflected Light: Chlorophyll absorbs blue and red light, reflecting green, making leaves appear green.
Transmitted Light: In thin solutions, transmitted light appears red due to fluorescence.

Pigments in Other Plants:

Tomatoes, carrots, and chilies contain lycopene (red pigment).

12.3 Mechanism of Photosynthesis

Photosynthesis involves two main phases:

Light Reaction (Photochemical): Occurs in thylakoid membranes, requires light.
Dark Reaction (Biochemical): Occurs in stroma, light-independent.

Historical Insights:

Van Niel (1931): Studied photosynthetic bacteria using H₂S instead of H₂O, producing sulfur instead of O₂. Proposed that O₂ in green plants comes from H₂O, not CO₂.
6CO₂ + 12H₂S → C₆H₁₂O₆ + 6H₂O + 12S↓
Ruben (1941): Used H₂¹⁸O (heavy oxygen isotope) in Chlorella, confirming O₂ evolved contains ¹⁸O, proving water as the O₂ source.
Hill (1937): Demonstrated that isolated chloroplasts in a CO₂-free medium with a ferric compound produce O₂ upon illumination (Hill Reaction).
- Proved: O₂ comes from H₂O photolysis, and electrons reduce CO₂.
Ruben and Kamen: Used hemoglobin, which turns red (oxyhemoglobin) with O₂, confirming water as the O₂ source.

General Equation:

6CO₂ + 12H₂¹⁸O → C₆H₁₂O₆ + 6H₂O + 6¹⁸O₂

12.4 Light Reaction

Location: Thylakoid membranes.
Process: Solar energy is trapped by chlorophyll, converted into chemical energy (ATP) and reducing power (NADPH). O₂ is evolved via water photolysis.
Photoexcitation:
- Chlorophyll absorbs a photon, boosting an electron to a higher energy level (excited state).
- When light is absent, the electron returns to its ground state.
Reaction Centers:
- P680 and P700: Special chlorophyll molecules absorbing light at 680 nm (Photosystem II, PS-II) and 700 nm (Photosystem I, PS-I).
- Antenna Molecules: Accessory pigments and other chlorophylls that harvest and transfer light energy to reaction centers.
Photosystems:
- PS-II: Initiates photolysis of water, releases O₂, and transfers electrons to PS-I.
- 4H₂O → 4H⁺ + 4OH⁻ 4OH⁻ → 4(OH) + 4e⁻ 4(OH) → 2H₂O + O₂↑
- - - Protons (H⁺) create a gradient for ATP synthesis.
  - PS-I: Receives electrons from PS-II, reduces NADP⁺ to NADPH.
- Electron Transport Chain (ETC):
  - Electrons from PS-II pass through plastoquinone (PQ), cytochrome b6-f, plastocyanin (PC), and ferredoxin (FeS) to PS-I.
  - PS-I electrons reduce NADP⁺ via ferredoxin-NADP reductase.
12.5 Photophosphorylation
Formation of ATP in chloroplasts using light energy.
1. Cyclic Photophosphorylation:
  - Involves only PS-I.
  - Electrons cycle back to PS-I via the ETC, producing ATP but no NADPH or O₂.
  - Occurs when NADPH is sufficient.
2. Non-Cyclic Photophosphorylation:
  - Involves both PS-I and PS-II.
  - Electrons from PS-II reduce NADP⁺ via PS-I, producing ATP, NADPH, and O₂.
  - Water photolysis replenishes PS-II electrons.
Products of Light Reaction:
- ATP: Energy carrier.
- NADPH: Reducing power.
- O₂: Byproduct.
12.6 Dark Reaction (Calvin Cycle/C3 Pathway)
- Location: Stroma.
- Process: CO₂ is fixed into carbohydrates using ATP and NADPH from the light reaction.
- Key Scientist: Melvin Calvin (Nobel Prize, 1961) traced the pathway using ¹⁴CO₂ in Chlorella.
- Steps:
  1. Carboxylation:
    - Ribulose-1,5-bisphosphate (RuBP, 5C) reacts with CO₂, catalyzed by RuBisCO (RuBP carboxylase).
    - Forms an unstable 6C intermediate, splitting into two 3-phosphoglyceric acid (3-PGA, 3C) molecules.
  2. Reduction (Glycolytic Reversal):
    - 3-PGA is phosphorylated by ATP to form 1,3-diphosphoglyceric acid.
    - Reduced by NADPH to glyceraldehyde-3-phosphate (3-PGAL).
    - 2 of 12 3-PGAL molecules form one glucose (C₆H₁₂O₆).
  3. Regeneration of RuBP:
    - 10 of 12 3-PGAL molecules regenerate 6 RuBP molecules via complex reactions, consuming 6 ATP.
    - Ensures continuous CO₂ fixation.
- Requirements for 1 Glucose:
  - 6 CO₂, 18 ATP, 12 NADPH.
- Cycle Summary:
- 6CO₂ + 18ATP + 12NADPH → C₆H₁₂O₆ + 6H₂O + 18ADP + 18Pi + 12NADP⁺

12.7 Photorespiration

Conditions: High temperature, bright light, high O₂, low CO₂.
Process: A wasteful process in C3 plants where RuBisCO acts as an oxygenase, not carboxylase.
Steps (PCO Cycle):
- RuBP + O₂ → 3-PGA (3C) + Phosphoglycolate (2C).
- Phosphoglycolate → Glycolate (in chloroplast) → Glyoxylate (in peroxisome) → Glycine (via transamination).
- In mitochondria, 2 Glycine → Serine + CO₂ + NH₄⁺.
- Serine → Glycerate (in peroxisome) → 3-PGA (in chloroplast).
Loss: ~25% of fixed carbon is lost as CO₂.
Organelles Involved: Chloroplast, peroxisome, mitochondrion.
Significance: Reduces photosynthetic efficiency in C3 plants.

12.8 C4 Pathway (Hatch-Slack Pathway)

Discoverers: M.D. Hatch and C.R. Slack (sugarcane studies).
Plants: Sugarcane, maize, sorghum, some dicots.
First Product: Oxaloacetic acid (4C), hence C4 plants.
Anatomy: Kranz Anatomy (wreath-like):
- Bundle sheath cells: Large chloroplasts, minimal/no grana, Calvin cycle enzymes.
- Mesophyll cells: Small chloroplasts, well-developed grana, initial CO₂ fixation.
Process:
- Mesophyll Cells:
  - CO₂ + Phosphoenolpyruvate (PEP, 3C) → Oxaloacetic acid (4C), catalyzed by PEP carboxylase.
  - Oxaloacetic acid → Malic acid (4C).
- Bundle Sheath Cells:
  - Malic acid → Pyruvic acid (3C) + CO₂ (via malic enzyme).
  - CO₂ enters Calvin cycle (C3 pathway).
- Pyruvic Acid: Returns to mesophyll, regenerates PEP (consumes 2 ATP, forming AMP).
ATP Requirement:
- C3 pathway: 18 ATP/glucose.
- C4 pathway: 30 ATP/glucose (12 additional for PEP regeneration).
Advantages:
- No photorespiration due to high CO₂ concentration in bundle sheath cells.
- Efficient CO₂ fixation at low atmospheric CO₂ levels.
- Higher productivity in tropical/subtropical regions.

12.9 CAM Pathway (Crassulacean Acid Metabolism)

Plants: Desert plants (e.g., Kalanchoe, Opuntia, Aloe).
Adaptation: Stomata open at night (scotoactive) to reduce water loss.
Process:
- Night (Phase I – Acidification):
  - CO₂ + PEP → Oxaloacetic acid → Malic acid (stored in vacuoles).
- Day (Phase II – Deacidification):
  - Malic acid → Pyruvic acid + CO₂.
  - CO₂ enters Calvin cycle in mesophyll cells (no Kranz anatomy).
Enzymes: PEP carboxylase (night), RuBisCO (day).
Advantages:
- Conserves water by closing stomata during the day.
- Efficient CO₂ fixation in arid conditions.

12.10 Factors Affecting Photosynthesis

A. External Factors:

Light:
- Supplies energy for photosynthesis.
- Quality: Red light maximizes photosynthesis, followed by blue.
- Intensity: Optimal in bright, diffused sunlight; decreases in very low or high intensity.
- Continuous photosynthesis is sustainable without plant damage.
Carbon Dioxide:
- Atmospheric CO₂ (0.03%) is the primary source.
- Limiting factor under normal conditions.
- Rate increases with CO₂ up to 1%; higher concentrations inhibit.
Temperature:
- Optimal: 25-30°C.
- Maximum: Up to 55°C in some plants (e.g., Opuntia).
- Rate increases with temperature until optimal, then declines.
Water:
- Raw material for photosynthesis.
- Indirectly affects rate by maintaining cell turgidity and protoplasm hydration.

B. Internal Factors:

Chlorophyll Quantity: Acts as a biocatalyst; small amounts are sufficient.
Sugar Accumulation: Slows photosynthesis.
Leaf Anatomy: Cuticle thickness, epidermis, intercellular spaces, stomata distribution, and chlorenchyma development influence the rate.

Blackman’s Law of Limiting Factors:

The rate of a process (e.g., photosynthesis) is limited by the slowest factor present in the minimum amount.
Example: At fixed light intensity, increasing CO₂ increases photosynthesis until light becomes limiting. Further rate increase requires higher light intensity.

Significance of Photosynthesis

Produces food for all life forms.
Converts solar energy into chemical energy.
Releases O₂ for respiration and ozone formation.
Forms fossil fuels, supporting energy needs.