Photosynthesis Class 11 Notes

Photosynthesis is the cornerstone of life on Earth. This vital biological process allows green plants, algae, and certain bacteria to harness solar energy and convert it into chemical energy. In the context of Class 11 NCERT Biology, the chapter on "Photosynthesis in Higher Plants" explores how autotrophic organisms sustain life and contribute to the global food chain.

Definition of Photosynthesis

Photosynthesis is the physiological process in which light energy is captured by chlorophyll and converted into chemical energy. In this process, plants convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂) using sunlight.

Chemical Equation:

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

Sites of Photosynthesis

Photosynthesis primarily occurs in chloroplasts, which are abundant in mesophyll cells of green leaves. The grana within chloroplasts host the light-dependent reactions, while the stroma is the site for light-independent (dark) reactions or the Calvin Cycle.

Pigments Involved in Photosynthesis

Chlorophyll a (primary pigment)
Chlorophyll b
Xanthophylls
Carotenoids

These pigments absorb light energy, transfer electrons, and protect chlorophyll from photo-oxidation.

Light Reaction (Photochemical Phase)

This phase occurs in the thylakoid membrane and includes:

Photon absorption by chlorophyll
Photolysis of water:
```
2H₂O → 4H⁺ + 4e⁻ + O₂
```
ATP and NADPH formation via cyclic and non-cyclic photophosphorylation

Photosystems:

PSI: Absorbs light at 700 nm
PSII: Absorbs light at 680 nm

Electron Transport Chain (ETC)

Electrons from PSII pass through an electron transport system, generating ATP through chemiosmosis. NADP⁺ is the final electron acceptor, forming NADPH.

Types of Photophosphorylation:

Non-cyclic: ATP + NADPH + O₂
Cyclic: Only ATP

Dark Reaction (Biosynthetic Phase)

Occurs in the stroma, not requiring direct light but needing ATP and NADPH. This phase involves the Calvin Cycle:

Carbon Fixation by RuBisCO
Reduction to form glucose precursors
Regeneration of RuBP

Types of Photosynthetic Pathways

1. C₃ Cycle (Calvin Cycle)

Common in temperate plants
First stable product: 3-PGA

2. C₄ Cycle (Hatch & Slack Pathway)

Adaptation in tropical grasses
First product: Oxaloacetic acid (OAA)
Specialized Kranz Anatomy

3. CAM Pathway

Occurs in succulents (e.g., cactus)
Stomata open at night
CO₂ stored as malic acid

Photorespiration

In C₃ plants, under low CO₂ and high O₂, RuBisCO binds with oxygen, leading to energy loss. C₄ and CAM plants are adapted to minimize this wasteful process.

Factors Affecting Photosynthesis

Light Intensity
CO₂ Concentration
Temperature
Water Availability
Leaf Anatomy

Significance of Photosynthesis

Oxygen generation for aerobic life
Food and energy source for heterotrophs
Carbon dioxide regulation
Source of fossil fuel formation

Important Experiments

Joseph Priestley: Oxygen discovery via mint and candle
Jan Ingenhousz: Light is essential
Engelmann: Blue and red light most effective
Blackman: Law of Limiting Factors

Conclusion

Photosynthesis is the foundation of the food web and life support system. Class 11 students must understand its intricate steps, pigment roles, and energy conversions for a complete grasp of plant physiology and ecological relevance.

Top 10 MCQs on Photosynthesis – Class 11 NCERT

The first stable product of C₃ cycle is:
a) Oxaloacetate
b) Malate
c) 3-PGA ✅
d) Pyruvate
Photosystem I absorbs light maximally at:
a) 660 nm
b) 700 nm ✅
c) 680 nm
d) 720 nm
In which part of the chloroplast does the Calvin cycle occur?
a) Grana
b) Stroma ✅
c) Thylakoid
d) Lumen
Which enzyme is responsible for CO₂ fixation in the Calvin cycle?
a) PEP carboxylase
b) RuBisCO ✅
c) ATP synthase
d) NADP⁺ reductase
Kranz anatomy is found in:
a) C₃ plants
b) C₄ plants ✅
c) CAM plants
d) None of the above
Which is the site of the light reaction?
a) Stroma
b) Outer membrane
c) Thylakoid membrane ✅
d) Cytoplasm
CAM plants open stomata:
a) During the day
b) At night ✅
c) Continuously
d) Only during rainfall
Main product of non-cyclic photophosphorylation is:
a) ATP only
b) NADPH only
c) ATP, NADPH, and O₂ ✅
d) Glucose
Photorespiration leads to:
a) Increase in glucose
b) Loss of fixed carbon ✅
c) More oxygen production
d) ATP production
In Engelmann’s experiment, the maximum oxygen release occurred in:
a) Red region
b) Blue and red regions ✅
c) Green region
d) Yellow region

UPSC CSE Mains Practice Question

Q. Explain the significance of photosynthesis in maintaining ecological balance and evaluate the adaptive mechanisms in C₄ and CAM plants that reduce photorespiration. (250 words).