CBSE Class 11 Biology – Photosynthesis in Higher Plants | Complete Study Module & Revision Notes (NCERT Based)

Course Details

Course: CBSE Class 11 Biology
Unit: Unit IV – Plant Physiology
Chapter: Chapter 13 – Photosynthesis in Higher Plants
Prescribed Textbook: NCERT
Board: CBSE

CBSE Board Examination Context

• Chapter 13 is a core conceptual chapter of Plant Physiology
• Frequently assessed through:
  • MCQs and Assertion–Reason questions
  • Short and Long Answer Questions
  • Case-based / source-based questions
• Forms the foundation for Respiration, Plant Growth, and Productivity
• Highly relevant for Class 11 annual examinations and Class 12 Biology preparation

Introduction to Photosynthesis

Photosynthesis is the fundamental physiological process by which green plants, algae, and cyanobacteria synthesize organic food using carbon dioxide, water, and light energy in the presence of chlorophyll. It is the primary source of food and oxygen for almost all living organisms on Earth.

Photosynthesis not only sustains plant life but also:

• Maintains atmospheric oxygen–carbon dioxide balance
• Provides energy base for all food chains
• Regulates global carbon cycle

The overall reaction of photosynthesis is:

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

This chapter explains how light energy is captured, how carbon is fixed, and how environmental and internal factors regulate photosynthesis.

1. Site and Pigments of Photosynthesis (Brief Overview)

Photosynthesis occurs in chloroplasts, mainly in the mesophyll cells of leaves.

• Grana → Site of light reactions
• Stroma → Site of dark reactions

Photosynthetic Pigments

• Chlorophyll a (primary pigment)
• Chlorophyll b
• Carotenoids (carotenes and xanthophylls)

Accessory pigments help in absorbing light of different wavelengths and protect chlorophyll from photo-oxidation.

2. Light Reaction (Light-Dependent Reactions)

2.1 Definition

Light reaction refers to the photochemical phase of photosynthesis where light energy is converted into chemical energy (ATP and NADPH).

2.2 Site of Light Reaction

• Occurs in thylakoid membranes (grana) of chloroplast

2.3 Photosystems

Two photosystems operate in light reaction:

Photosystem II (PS II)

• Reaction centre: P680
• Absorbs light of wavelength 680 nm
• Responsible for photolysis of water

Photosystem I (PS I)

• Reaction centre: P700
• Absorbs light of wavelength 700 nm
• Involved in NADPH formation

2.4 Photolysis of Water

• Occurs at PS II
• Water splits into:
  • Protons (H⁺)
  • Electrons
  • Oxygen (released as by-product)

This explains the oxygen evolution during photosynthesis.

2.5 Electron Transport Chain

• Electrons move from PS II → PS I → NADP⁺
• This movement generates ATP by chemiosmosis

2.6 Photophosphorylation

A. Cyclic Photophosphorylation

• Only PS I is involved
• Produces ATP only
• Occurs when NADP⁺ is limited

B. Non-Cyclic Photophosphorylation

• Both PS II and PS I involved
• Produces ATP, NADPH, and O₂

2.7 Significance of Light Reaction

• Converts solar energy into usable chemical energy
• Produces ATP and NADPH required for dark reaction
• Releases oxygen into atmosphere

3. Dark Reaction (Biosynthetic Phase / Calvin Cycle)

3.1 Definition

Dark reaction refers to the light-independent reactions where carbon dioxide is fixed into carbohydrates using ATP and NADPH.

3.2 Site

• Occurs in stroma of chloroplast

3.3 Calvin Cycle

The Calvin cycle consists of three main steps:

1. Carboxylation

• CO₂ combines with Ribulose-1,5-bisphosphate (RuBP)
• Enzyme: RuBisCO
• Forms two molecules of 3-phosphoglycerate (3-PGA)

2. Reduction

• 3-PGA is reduced to glyceraldehyde-3-phosphate (G3P)
• Uses ATP and NADPH

3. Regeneration

• RuBP is regenerated to continue the cycle
• Requires ATP

3.4 Role of RuBisCO

• Most abundant enzyme on Earth
• Shows dual activity:
  • Carboxylase (desired)
  • Oxygenase (leads to photorespiration)

3.5 Importance of Dark Reaction

• Actual synthesis of glucose
• Converts inorganic carbon into organic compounds
• Links photosynthesis with plant growth and metabolism

4. C₃ and C₄ Pathways

4.1 C₃ Pathway

Characteristics

• First stable product: 3-PGA (3 carbon compound)
• Occurs in most plants (wheat, rice, potato)
• CO₂ fixation occurs in mesophyll cells
• Photorespiration is common

Limitations

• RuBisCO binds oxygen at high temperature
• Leads to photorespiration, reducing efficiency

4.2 C₄ Pathway (Hatch and Slack Pathway)

Characteristics

• First stable product: Oxaloacetic acid (4 carbon compound)
• Occurs in maize, sugarcane, sorghum
• CO₂ fixation in mesophyll cells
• Calvin cycle in bundle sheath cells

Key Enzyme

• PEP carboxylase (high affinity for CO₂)

4.3 Advantages of C₄ Pathway

• No photorespiration
• Higher photosynthetic efficiency
• Adapted to high light intensity and temperature

4.4 Comparison Between C₃ and C₄ Plants

5. Factors Affecting Photosynthesis

Photosynthesis is influenced by internal and external factors.

5.1 External Factors

Light

• Intensity, quality, and duration affect rate
• Red and blue light most effective

Carbon Dioxide

• Increased CO₂ increases rate up to saturation point

Temperature

• Optimum temperature: 25–35°C
• High temperature increases photorespiration in C₃ plants

Water

• Indirectly affects photosynthesis
• Water stress leads to stomatal closure

5.2 Internal Factors

Chlorophyll Content

• More chlorophyll → higher photosynthetic capacity

Leaf Anatomy

• Larger leaf area enhances photosynthesis

Stomatal Opening

• Regulates CO₂ entry

Protoplasmic Factors

• Enzyme activity and cellular health

5.3 Law of Limiting Factors (Blackman)

When several factors affect photosynthesis, the rate is determined by the factor present in minimum amount, even if others are favourable.

Revision Notes (Quick Recall)

• Light reaction occurs in grana
• Dark reaction occurs in stroma
• PS II splits water
• ATP and NADPH are assimilatory power
• RuBisCO has dual nature
• C₄ plants avoid photorespiration
• CO₂ is limiting factor under normal conditions

Important Exam-Oriented Points

• Differences between cyclic and non-cyclic photophosphorylation
• Role of RuBisCO
• Comparison of C₃ and C₄ pathways
• Factors affecting rate of photosynthesis
• Significance of light and dark reactions

Common Student Mistakes to Avoid

• Thinking dark reaction occurs at night
• Confusing PS I and PS II functions
• Mixing C₃ and C₄ pathways
• Ignoring role of temperature in photorespiration

Chapter Summary

Photosynthesis in higher plants involves two major phases: light reactions that convert solar energy into chemical energy, and dark reactions that fix carbon dioxide into carbohydrates. Plants use different pathways like C₃ and C₄ to adapt to environmental conditions. The rate of photosynthesis depends on multiple internal and external factors. This chapter forms the backbone of plant physiology and ecological productivity.

Best Suited For

• CBSE Class 11 Annual Examinations
• NCERT-based school assessments
• Conceptual revision & foundation for Class 12 Biology