This chapter reveals how plants release and utilize the energy stored in organic molecules through cellular respiration. Students examine glycolysis, the Krebs cycle, and electron transport chain in detail, understanding how these processes generate ATP while consuming oxygen. We compare aerobic and anaerobic pathways and explore the respiratory quotient concept. Through studying plant respiration, students gain insight into energy metabolism in all living organisms and develop appreciation for the complementary relationship between photosynthesis and respiration in maintaining the biosphere’s energy balance.

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

1. Chlorophylls:
   • Chlorophyll a (primary pigment, blue-green)
   • Chlorophyll b (accessory pigment, yellow-green)
2. Carotenoids:
   • Carotenes (orange)
   • Xanthophylls (yellow)
3. 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

1. Light (intensity, duration, quality)
2. CO₂ concentration
3. Temperature (optimum range 25-35°C)
4. Water availability
5. Mineral nutrition
6. 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

CHAPTER 10: RESPIRATION IN PLANTS

Introduction

Respiration is the biochemical process by which organic compounds (usually carbohydrates) are broken down to release energy for cellular activities. Unlike animals, plants lack specialized respiratory organs but perform cellular respiration in all living cells.

Basic Equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP + Heat)

Types of Respiration

Aerobic Respiration

• Requires oxygen
• Occurs in cytoplasm and mitochondria
• Complete oxidation of respiratory substrate
• Produces more energy (36-38 ATP per glucose molecule)

Anaerobic Respiration

• Occurs in absence of oxygen
• Takes place in cytoplasm
• Incomplete oxidation of respiratory substrate
• Produces less energy (2 ATP per glucose molecule)
• Examples: alcohol fermentation, lactic acid fermentation

Respiratory Substrate

• Carbohydrates (primarily glucose) are the preferred substrate
• Proteins, fats, and organic acids can also be used
• Respiratory Quotient (RQ) = CO₂ evolved/O₂ consumed
  • Carbohydrates: RQ = 1
  • Proteins: RQ = 0.9
  • Fats: RQ = 0.7

Steps of Aerobic Respiration

1. Glycolysis (Embden-Meyerhof-Parnas Pathway)

• Occurs in cytoplasm
• Glucose (6C) → 2 Pyruvic acid (3C)
• Net gain: 2 ATP, 2 NADH
• Does not require oxygen (anaerobic phase)

2. Link Reaction (Pyruvate Dehydrogenase Complex)

• Occurs in mitochondrial matrix
• Pyruvic acid (3C) → Acetyl-CoA (2C) + CO₂
• Produces: 2 NADH (from 2 pyruvate)

3. Krebs Cycle (Tricarboxylic Acid Cycle)

• Occurs in mitochondrial matrix
• Acetyl-CoA enters cycle by combining with oxaloacetate (4C)
• For each glucose molecule (2 acetyl-CoA):
  • 6 NADH, 2 FADH₂, 2 ATP produced
  • 4 CO₂ released

4. Electron Transport Chain (ETC)

• Located in inner mitochondrial membrane
• Electrons from NADH and FADH₂ pass through a series of carriers
• Oxygen is the final electron acceptor
• Energy released is used to pump protons across the membrane
• Creates proton gradient used by ATP synthase to produce ATP
• Yields: 26-28 ATP from the electrons of one glucose molecule

Fermentation (Anaerobic Respiration)

1. Alcoholic Fermentation:
   • Occurs in yeast and some plant tissues
   • Pyruvate → Acetaldehyde → Ethanol + CO₂
   • Net yield: 2 ATP per glucose
2. Lactic Acid Fermentation:
   • Occurs in bacteria and sometimes in plant tissues
   • Pyruvate → Lactic acid
   • Net yield: 2 ATP per glucose

Respiratory Balance Sheet

• Glycolysis: 2 ATP (net)
• Krebs Cycle: 2 ATP
• Electron Transport Chain: 32-34 ATP
• Total: 36-38 ATP per glucose molecule

Factors Affecting Respiration

1. Oxygen availability
2. Temperature (Q₁₀ value typically 2-2.5)
3. Water availability
4. Light (indirectly affects through stomatal opening)
5. Carbon dioxide concentration
6. Substrate availability
7. Age and type of tissue

Significance of Respiration

• Energy production for cellular activities
• Production of intermediates for various biosynthetic pathways
• Breakdown of storage material during germination
• Necessary for active transport, cell division, and growth
• Provides carbon skeletons for amino acid synthesis

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:

1. Chapter 1 Biological Classification Notes
2. Chapter 2 Plant Kingdom Notes
3. Chapter 3 Morphology of Flowering Plants Notes
4. Chapter 4 Anatomy of Flowering Plants Notes
5. Chapter 5 Cell: The Unit of Life Notes
6. Chapter 6 Cell Cycle and Cell Division Notes
7. Chapter 7 Transport in Plants Notes
8. Chapter 8 Mineral Nutrition Notes
9. Chapter 9 Photosynthesis in Higher Plants Notes
10. Chapter 10 Respiration in Plants Notes
11. Chapter 11 Plant Growth and Development Notes