Learning Objectives
- Understand the light reactions and dark reactions of photosynthesis
- Learn about photosystems, electron transport chain, and chemiosmosis
- Study the Calvin cycle (C3 pathway) and photorespiration
- Compare C3 and C4 pathways and understand CAM photosynthesis
- Learn about factors affecting photosynthesis
Key Concepts
Overview of Photosynthesis
Overall equation: 6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2. Oxygen released comes from water (proven by Cornelius van Niel using photosynthetic bacteria and confirmed by Ruben and Kamen using heavy oxygen isotope O18). Two phases: Light reactions (thylakoid membrane) and Dark reactions/Calvin cycle (stroma).
Photosynthetic Pigments
Chlorophyll a (blue-green, primary pigment, reaction centre pigment), Chlorophyll b (yellow-green, accessory), Carotenoids (yellow/orange, accessory, also photoprotective), Xanthophylls (yellow). Accessory pigments absorb different wavelengths and transfer energy to chlorophyll a. Absorption spectrum vs Action spectrum: action spectrum of photosynthesis closely matches absorption spectrum of chlorophyll a.
Light Reactions (Thylakoid Membrane)
Photosystem II (PS II): Reaction centre is P680. Site of photolysis of water: 2H2O → 4H+ + 4e- + O2 (requires Mn, Cl). Electrons from water replace those lost by P680. Photosystem I (PS I): Reaction centre is P700. Electrons passed to ferredoxin, then to NADP+ reductase.
Non-cyclic Photophosphorylation: PS II → Plastoquinone → Cytochrome b6f → Plastocyanin → PS I → Ferredoxin → NADP+ reductase. Produces ATP, NADPH, and O2. Both PS I and PS II involved.
Cyclic Photophosphorylation: Only PS I involved. Electrons cycle back: PS I → Ferredoxin → Cytochrome b6f → Plastocyanin → PS I. Produces only ATP (no NADPH, no O2). Occurs in stroma lamellae.
Chemiosmosis: H+ gradient across thylakoid membrane drives ATP synthesis through ATP synthase (CF0-CF1 complex). H+ accumulates in thylakoid lumen due to water splitting, PQ shuttle, and NADP+ reduction in stroma.
Calvin Cycle (C3 Pathway / Dark Reactions)
Discovered by Melvin Calvin using Chlorella and radioactive C14. Occurs in stroma. Three stages:
(1) Carboxylation: CO2 + RuBP (5C) → 2 molecules of 3-PGA (3C). Enzyme: RuBisCO (most abundant enzyme on Earth). (2) Reduction: 3-PGA → G3P (glyceraldehyde-3-phosphate) using ATP and NADPH. (3) Regeneration: G3P → RuBP using ATP. For 1 glucose: 6CO2, 18 ATP, 12 NADPH required.
Photorespiration (C2 Pathway)
RuBisCO has dual function — carboxylase and oxygenase. In high O2/low CO2, RuBisCO fixes O2 instead of CO2 (oxygenase activity), producing phosphoglycolate (2C). This is photorespiration — wasteful process, no ATP produced, occurs in chloroplast → peroxisome → mitochondria. Occurs only in C3 plants.
C4 Pathway (Hatch-Slack Pathway)
Found in tropical plants: sugarcane, maize, sorghum. Kranz anatomy: Bundle sheath cells (thick-walled, no grana, with chloroplasts) surround vascular bundles. Initial fixation in mesophyll cells: CO2 + PEP (3C) → OAA (4C) by PEP carboxylase (no oxygenase activity). OAA → Malate, transported to bundle sheath cells. Malate releases CO2 for Calvin cycle. C4 plants have higher photosynthetic efficiency, no photorespiration, lower CO2 compensation point.
Factors Affecting Photosynthesis
Blackman's Law of Limiting Factors: Rate is determined by the factor nearest to its minimum value. Factors: light intensity, CO2 concentration, temperature, water availability.
Summary
Photosynthesis occurs in two phases: light reactions in thylakoids produce ATP and NADPH, while the Calvin cycle in stroma fixes CO2 into sugars. C4 plants avoid photorespiration through Kranz anatomy and PEP carboxylase. Chemiosmosis drives ATP synthesis via proton gradient. RuBisCO is the key enzyme with dual carboxylase/oxygenase activity.
Important Terms
- RuBisCO: Ribulose bisphosphate carboxylase/oxygenase — most abundant enzyme on Earth
- Photolysis: Splitting of water molecules in PS II releasing O2
- Chemiosmosis: ATP synthesis driven by proton gradient across membrane
- Kranz anatomy: Special leaf anatomy in C4 plants with prominent bundle sheath cells
- Photorespiration: Wasteful fixation of O2 by RuBisCO in C3 plants
- PEP carboxylase: Enzyme in C4 mesophyll cells that fixes CO2 without oxygenase activity
- Action spectrum: Graph showing rate of photosynthesis at different wavelengths
- Compensation point: Light intensity at which photosynthesis equals respiration
Quick Revision
- O2 in photosynthesis comes from H2O (not CO2)
- PS II (P680) → PS I (P700) in non-cyclic; only PS I in cyclic
- Non-cyclic: ATP + NADPH + O2; Cyclic: only ATP
- Calvin cycle: Carboxylation → Reduction → Regeneration
- 1 glucose needs 6CO2, 18 ATP, 12 NADPH
- C4 plants: Kranz anatomy, PEP carboxylase, no photorespiration
- RuBisCO: carboxylase + oxygenase dual activity
- Blackman's Law of Limiting Factors governs photosynthesis rate