Photosynthesis in Higher Plants 🌿 - Biology Class 11th - Handwritten Notes 📖 [PDF]📚

Photosynthesis is the process by which green plants, algae, and certain bacteria convert light energy into chemical energy, storing it in the form of glucose.

• Occurs in chloroplasts.
• Involves pigments (chlorophyll a, chlorophyll b, carotenoids, xanthophylls).
• Equation:

$$6C{O}_2+12H_{2}O\stackrel{\text{light, chlorophyll}}{\to }{C}_6{H}_{12}{O}_6+6O_2+6H_{2}O$$

Pigments and Absorption of Light

• Chlorophyll a: primary pigment (reaction center).
• Chlorophyll b, carotenoids: accessory pigments → absorb additional wavelengths and transfer energy to chlorophyll a.
• Absorption spectrum vs Action spectrum: T. W. Engelmann’s experiment showed maximum photosynthesis in blue and red light.

Site of Photosynthesis

• Light reactions: Occur in thylakoid membranes.
• Dark reactions (Calvin cycle): Occur in stroma of chloroplast.

Light Reaction (Photochemical Phase)

Takes place in thylakoid membrane. Two photosystems:

• PS II (P680) → absorbs light at 680 nm.
• PS I (P700) → absorbs light at 700 nm.

Key steps:

1. Excitation of pigments: Light excites electrons in chlorophyll.

2. Splitting of water (Photolysis):

Occurs at PS II.

$2H_{2}O\to 4H^{+}+4e^{-}+O_2$

3. Electron transport chain (ETC): Electrons travel from PS II → plastoquinone → cytochrome b6f → plastocyanin → PS I → ferredoxin → NADP⁺ reductase.

4. ATP formation: Chemiosmosis through ATP synthase (photophosphorylation).

5. NADPH formation: PS I reduces NADP⁺ to NADPH.

Types of Photophosphorylation:

• Non-cyclic: Involves both PS II & PS I, produces ATP, NADPH, and O₂.
• Cyclic: Only PS I active, produces ATP only (no NADPH, no O₂).

Dark Reaction (Biosynthetic Phase / Calvin Cycle)

Occurs in stroma. Uses ATP + NADPH (from light reaction) to fix CO₂ into glucose.

Three Phases (C3 pathway – Calvin Cycle):

1. Carboxylation:

• CO₂ + RuBP (5C) → unstable 6C compound → 2 × 3-PGA.
• Enzyme: RuBisCO (Ribulose-1,5-bisphosphate carboxylase oxygenase).

2. Reduction:

• 3-PGA → G3P using ATP + NADPH.

3. Regeneration:

• RuBP regenerated, cycle continues.

Net reaction (for 6 CO₂):

$$6CO_2 + 18ATP + 12NADPH → C_6H_{12}O_6 + 18ADP + 12NADP^+$$

C3, C4, and CAM Pathways

1. C3 Pathway (Calvin Cycle)

• Found in most plants (wheat, rice, potato, etc.).
• First stable product: 3C compound (3-PGA).
• Problem: Photorespiration due to dual nature of RuBisCO (acts as oxygenase in high O₂).
• Less efficient in hot, dry conditions.

2. C4 Pathway (Hatch–Slack Pathway)

• Found in maize, sugarcane, sorghum.
• Adapted to minimize photorespiration.
• First stable product: 4C compound oxaloacetate (OAA).

Mechanism:

Mesophyll cells:
CO₂ + PEP (3C) → OAA (4C) → Malate/Aspartate.
Enzyme: PEP carboxylase (no affinity for O₂).

Bundle sheath cells:
Malate → CO₂ + Pyruvate.
Released CO₂ enters Calvin cycle.

Features:

• Kranz anatomy: Mesophyll and bundle sheath cells are distinct.
• Higher efficiency in high temperature and light.

3. CAM Pathway (Crassulacean Acid Metabolism)

• Found in succulents (cactus, pineapple, agave).
• Adaptation to arid/desert conditions.
• Stomata open at night (to reduce water loss).

Mechanism:

• Night: CO₂ fixed into malate (stored in vacuoles).
• Day: Malate decarboxylated → CO₂ released for Calvin cycle.

Differences Table: C3 vs C4 vs CAM

Feature	C3 Plants	C4 Plants	CAM Plants
First product	3-PGA (3C)	OAA (4C)	OAA (4C, stored as malate)
Enzyme for fixation	RuBisCO	PEP carboxylase (mesophyll) + RuBisCO (bundle sheath)	PEP carboxylase (night), RuBisCO (day)
Photorespiration	High	Very low	Negligible
Stomata	Open day	Open day	Open night
Examples	Wheat, rice	Maize, sugarcane	Cactus, pineapple

Photorespiration

• In presence of high O₂ and low CO₂, RuBisCO acts as oxygenase.
• Produces 2C phosphoglycolate, which is toxic and must be salvaged (involves chloroplast, peroxisome, mitochondria).
• Wastes energy → reduces photosynthetic efficiency.
• Absent/minimal in C4 & CAM plants.