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2.3 Plant Physiology: Photosynthesis
1. The Process and Importance
Definition:
Conversion of light energy into chemical energy (glucose) from CO₂ and H₂O, releasing O₂.
Equation:
6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂
Importance:
Primary energy source, oxygen production, CO₂ regulation, fossil fuel formation.
2. Site of Photosynthesis: The Chloroplast
Location:
Primarily in mesophyll cells of leaves.
Structure:
Double Membrane:
Outer and Inner.
Stroma:
Fluid-filled space; site of
dark reactions (biosynthetic phase)
.
Thylakoids:
Interconnected sacs; site of
light reactions (photochemical phase)
.
Grana:
Stacks of thylakoids.
Chlorophyll:
Pigment embedded in thylakoid membranes.
3. Phases of Photosynthesis
3.1. Light-Dependent Reactions (Photochemical Phase):
Site:
Thylakoid membranes.
Process:
Light energy activates chlorophyll, splits water (photolysis), transports electrons, produces ATP (photophosphorylation) and NADPH.
Summary:
Light energy → Chemical energy (ATP, NADPH) + O₂.
3.2. Light-Independent Reactions (Biosynthetic Phase / Calvin Cycle)
Site:
Stroma.
Process:
Uses ATP and NADPH from light reactions.
Carbon Fixation:
CO₂ combines with RuBP (catalyzed by RuBisCO) to form PGA.
Reduction:
PGA converted to G3P using ATP and NADPH.
Regeneration:
RuBP regenerated from G3P using ATP.
Summary:
CO₂ → Glucose (using ATP, NADPH).
4. Adaptations in Plants for Photosynthesis
Leaf Structure:
Large surface area, thinness, cuticle, transparent epidermis, palisade/spongy mesophyll.
Stomata:
Regulate CO₂ entry and water vapor release (opening/closing via K⁺ ion exchange).
Chloroplasts:
Orient to maximize light absorption.
Vascular Bundles:
Efficient transport of water and sugars.
Root System:
Absorbs water and minerals.
5. Experiments to Show Necessity of Factors
Destarching:
Keep plant in darkness to use up stored starch.
Starch Test (Iodine Test):
Blue-black indicates starch.
Necessity of Light:
Covered part of leaf (no light) shows no starch.
Necessity of CO₂:
Plant in CO₂-free environment shows no starch.
Necessity of Chlorophyll:
Non-green parts of variegated leaf show no starch.
Release of O₂:
Aquatic plant in light releases bubbles that rekindle a glowing splint.
6. The Carbon Cycle
Definition:
Biogeochemical cycle of carbon exchange among Earth's reservoirs.
Photosynthesis's Role:
Removes atmospheric CO₂ and converts it into organic compounds, forming the base of food chains.
Key Processes:
Photosynthesis, Consumption, Respiration, Decomposition, Fossil Fuel Formation, Combustion, Oceanic Carbon.