Photosynthesis in Higher Plants 🌿 | Class 11 Notes | NEET

Photosynthesis in Higher Plants 🌿 | Class 11 Notes | NEET | NCERT

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:

6 C O_{2} + 12 H_{2} O \overset{light, chlorophyll}{\to} C_{6} H_{12} O_{6} + 6 O_{2} + 6 H_{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.

Photosynthesis in Higher Plants 🌿 - Biology Class 11th - Handwritten Notes

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.

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