This pivotal chapter illuminates the remarkable process by which plants convert light energy into chemical energy. Students explore the light-dependent and light-independent reactions in detail, examining chloroplast structure, pigment systems, electron transport chains, and the Calvin cycle. We investigate factors affecting photosynthetic rates and photorespiration, along with adaptations like C4 and CAM photosynthesis that enhance carbon fixation efficiency in challenging environments. Through understanding this fundamental process, students recognize photosynthesis as the energetic foundation supporting nearly all life on Earth.

CHAPTER 9: PHOTOSYNTHESIS IN HIGHER PLANTS

Introduction

Photosynthesis is the process by which green plants, algae, and certain bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water. This process is fundamental for life on Earth as it provides organic compounds and energy for most living organisms and releases oxygen into the atmosphere.

Basic Equation

6CO₂ + 12H₂O + Light Energy → C₆H₁₂O₆ + 6O₂ + 6H₂O

Site of Photosynthesis

Chloroplasts: Double-membrane organelles containing thylakoids arranged in stacks called grana
Structure: Outer membrane, inner membrane, intermembrane space, stroma, thylakoids, grana
Distribution: Most abundant in mesophyll cells of leaves

Pigments Involved

Chlorophylls:
- Chlorophyll a (primary pigment, blue-green)
- Chlorophyll b (accessory pigment, yellow-green)
Carotenoids:
- Carotenes (orange)
- Xanthophylls (yellow)
Phycobilins (in algae):
- Phycoerythrin (red)
- Phycocyanin (blue)

Light Reaction (Photochemical Phase)

Occurs in thylakoid membranes
Photosystem I (P700):
- Absorbs light of 700 nm wavelength
- Produces NADPH
Photosystem II (P680):
- Absorbs light of 680 nm wavelength
- Responsible for photolysis of water (splitting water molecules)
Process:
1. Light excites electrons in chlorophyll
2. Excited electrons move through electron transport chain
3. Water is split, releasing oxygen (photolysis)
4. ATP is generated (photophosphorylation)
5. NADP⁺ is reduced to NADPH

Dark Reaction (Biosynthetic Phase)

Also known as Calvin Cycle or C₃ pathway
Occurs in the stroma of chloroplasts
Does not directly require light
Steps:
1. Carbon Fixation: CO₂ combines with RuBP (5C) to form 2 molecules of 3-PGA (3C)
2. Reduction: 3-PGA is reduced to G3P (3C) using ATP and NADPH
3. Regeneration: Some G3P is used to regenerate RuBP using ATP

Photorespiration

Oxygen competes with carbon dioxide for the active site of RuBisCO enzyme
Results in reduced photosynthetic efficiency
Significant in hot, dry, bright conditions when stomata close

C₄ Pathway

Adaptation to reduce photorespiration
Initial CO₂ fixation occurs in mesophyll cells forming oxaloacetate (4C)
CO₂ is released in bundle sheath cells for the Calvin cycle
Examples: Maize, sugarcane, sorghum

CAM Pathway

Adaptation for arid conditions
Stomata open at night and close during day
CO₂ is fixed at night as organic acids
During the day, CO₂ is released internally for the Calvin cycle
Examples: Cacti, pineapple, succulents

Factors Affecting Photosynthesis

Light (intensity, duration, quality)
CO₂ concentration
Temperature (optimum range 25-35°C)
Water availability
Mineral nutrition
Leaf age and anatomy

Significance of Photosynthesis

Production of food for all heterotrophs
Oxygen release into atmosphere
Carbon dioxide utilization, reducing greenhouse effect
Forms the basis of all food chains and food webs
Source of biomass for biofuels

Complete Chapter-wise Hsslive Plus One Botany Notes

Our HSSLive Plus One Botany Notes cover all chapters with key focus areas to help you organize your study effectively:

HSSLIVE Plus One Botany Chapter 9: Photosynthesis in Higher Plants Notes