Photosynthesis Lecture

Introduction

• Next chapter: Photosynthesis
• Comparatively longer than respiration
• Involves some reactions
• Aim: Complete the chapter in this session

Basic Concepts

• Two basic types of organisms:
  • Autotrophs: Synthesize their own food (Green plants)
  • Heterotrophs: Depend on autotrophs for food (Animals, non-green parts of plants)
• Photosynthesis: Process where light energy is converted into chemical energy
  • Reactants: Carbon dioxide (CO2), Water (H2O), Light energy
  • Products: Glucose (C6H12O6), Oxygen (O2)

Key Factors

• Involves Chlorophyll (Green pigment in chloroplasts)
• Main source of food and oxygen on Earth

Experiments

1. Variegated Leaf Experiment

• Colorful leaf, half covered
• Starch formation only in green parts

2. CO2 Experiment

• K-saturated cotton absorbs CO2
• Starch formation where CO2 is available

3. Joseph Priestley

• Experiments with jar, candle, and mouse
• Role of plants in restoring air quality

4. Jan Ingenhousz

• Aquatic plants producing bubbles in light
• Importance of sunlight

5. Julius von Sachs

• Proved glucose production
• Stored as starch in chloroplasts

6. T.W. Engelmann

• Algae and aerobic bacteria indicating oxygen release

7. Cornelius van Niel

• Role of H2O in releasing O2

Photosynthesis Location

• Occurs in mesophyll cells in leaves
• Chloroplast structure:
  • Outer and inner membranes
  • Stroma (fluid inside chloroplast)
  • Grana (stacks of thylakoids)
  • Thylakoids contain chlorophyll

Reactions

Light Reactions (Photochemical phase)

• Occur in the thylakoid membranes
• Two photosystems involved: PS I and PS II
• PS II: Light energy absorbed, water splits (producing O2, protons, and electrons)
• PS I: Produces NADPH and converts ADP to ATP
• Follows Z-scheme (Non-cyclic photophosphorylation)
• When only PS I involved (Cyclic photophosphorylation): produces ATP only

Dark Reactions (Calvin Cycle or Biosynthetic Phase)

• Occur in stroma
• Do not require light but depend on products of light reactions
• Include carbon fixation, reduction phase, and regeneration of RuBP

Pigments

• Chief pigment: Chlorophyll a
• Accessory pigments: Chlorophyll b, Carotenoids (Xanthophylls, Carotene)
• More pigments help absorb different wavelengths of light efficiently

Calvin Cycle (C3 Cycle)

• Phase 1: Carbon fixation (CO2 --> 3-phosphoglycerate, 3-PGA)
• Phase 2: Reduction (Formation of G3P using ATP, NADPH)
• Phase 3: Regeneration of RuBP (Ribulose bisphosphate)
• Involves enzyme RuBisCo

C4 Pathway

• Found in tropical plants
• First product: Oxaloacetic acid (4-carbon compound)
• Involves PEP Carboxylase (Phosphoenolpyruvate Carboxylase)
• Kranz anatomy: Special structure in C4 plants
• Advantages: Efficient in high light, temperature, minimize photorespiration

Photorespiration

• Occurs when RuBisCo binds with O2 instead of CO2
• Results in formation of phosphoglycolate and 3-PGA
• Considered wasteful as it does not produce ATP or sugar

Factors Affecting Photosynthesis

• Internal: Leaf structure, age, chlorophyll content
• External: Light intensity, CO2 concentration, temperature, water availability

Blackman's Law of Limiting Factors

• States that the rate of a photosynthetic process is limited by the factor that is nearest to its minimum value

Light and CO2 as Factors

• Light:
  • Increased light intensity increases photosynthesis up to a point
  • Beyond a certain intensity, no further effect
• CO2:
  • Major limiting factor
  • Current atmospheric levels: 0.03-0.04%
  • Increasing CO2 can boost photosynthesis, especially in C3 plants
• Water:
  • Essential for oxygen production and stomatal opening
  • Deficit results in wilting and reduced photosynthesis

Summary

• Detailed study of reactions and factors influencing photosynthesis
• Emphasized importance of both light and dark reactions
• Explained different pathways (C3, C4)
• Understood the mechanism, significance, and adaptation in plants for photosynthesis