Mada za sehemu hiiDevelop an advanced understanding of concepts, theories, and principles in biologyMada 8
- Describe the concept of the cell (cell theory, organelles and biological molecules)
- Explain the physiology of photosynthesis (mechanism of light reaction and dark reaction in C3 and C4 plants)
- Describe the structure of epithelial tissues in relation to its digestive role
- Describe the physiology of gaseous exchange and respiration in mammals (transportation of gases, aerobic and anaerobic respiration mechanisms)
- Explain the concept of gaseous exchange in plants (mechanism and theories of stomata opening and closing)
- Describe the physiology of coordination (mechanism of transmission of nerve impulse, seeing, hearing and body balance)
- Discribe the application or role of synthetic phytohormones
- Explain the concept of regulation in mammals (feedback mechanisms, urine formation and osmoregulation)
Photosynthesis is the process by which green plants convert light energy into chemical energy stored in carbohydrates, using carbon dioxide and water. This study note explains the physiology of photosynthesis, focusing on the light and dark reactions and the differences between C₃ and C₄ plants.
The light reaction occurs in the thylakoid membranes (grana) of chloroplasts and requires light energy. Its products—ATP and NADPH—are essential for the dark reaction.
Photosystems
Chlorophyll pigments are arranged in groups called photosystems within the thylakoids:
- Photosystem II (PSII) contains chlorophyll P680, which absorbs light at 680 nm wavelength
- Photosystem I (PSI) contains chlorophyll P700, which absorbs light at 700 nm wavelength
These photosystems work together in non-cyclic photophosphorylation.
Non-Cyclic Photophosphorylation
This is the main pathway producing ATP and NADPH:
- Light energy is absorbed by PSII, exciting electrons in chlorophyll
- Excited electrons pass through electron carriers (including cytochromes) to PSI
- As electrons move, energy is released and used by ATP synthase to phosphorylate ADP to ATP
- Water molecules undergo photolysis:
- Electrons reach PSI, where light again excites them
- Excited electrons combine with NADP⁺ and H⁺ to form NADPH
Products: ATP, NADPH, and O₂ (released as by-product)
Cyclic Photophosphorylation
In this pathway:
- Only PSI is involved—electrons are excited and pass through carriers back to PSI
- ATP is produced, but NADPH is not generated
- No photolysis of water occurs, so no O₂ is produced
Cyclic photophosphorylation supplements ATP supply when the Calvin cycle requires more ATP than non-cyclic produces.
Comparison Table
| Feature | Cyclic Photophosphorylation | Non-Cyclic Photophosphorylation |
|---|---|---|
| Photosystems involved | PSI only | PSI and PSII |
| Source of electrons | PSI | Water (photolysis) |
| Products | ATP only | ATP, NADPH, O₂ |
| Electron flow | Returns to origin | One-directional |
| Photolysis of water | Does not occur | Occurs |
The dark reaction occurs in the stroma of chloroplasts and does not require light directly. It uses ATP and NADPH from the light reaction to fix CO₂ into carbohydrates.
The Calvin Cycle (C₃ Pathway)
The Calvin cycle has three main phases:
1. Carbon Dioxide Fixation
- CO₂ from the atmosphere diffuses through stomata
- CO₂ combines with the 5-carbon acceptor RuBP (ribulose-1,5-bisphosphate)
- The enzyme RuBP carboxylase (Rubisco) catalyzes this reaction
- RuBP + CO₂ → 2 molecules of 3-phosphoglyceric acid (3-PGA), a 3-carbon compound
2. Reduction of PGA
- ATP provides energy to convert PGA to PGAL (3-phosphoglyceraldehyde)
- NADPH provides reducing power
- PGA + ATP + NADPH → PGAL + ADP + NADP⁺
- PGAL (also called glyceraldehyde phosphate) is the first carbohydrate formed
3. Regeneration of RuBP
- Most PGAL molecules are used to regenerate RuBP
- RuBP is rebuilt using ATP
- The cycle can now repeat
Note: Six turns of the Calvin cycle are needed to produce one glucose molecule, as each turn adds one carbon (from one CO₂) and produces two PGAL molecules; only two PGAL are used to form glucose.
Plants are classified based on the first product of carbon fixation.
C₃ Plants
- The first stable product after CO₂ fixation is 3-phosphoglyceric acid (3-PGA), a 3-carbon compound
- They use only the Calvin cycle
- Examples: rice, wheat, soybeans, spinach, cotton
- Found mainly in temperate, cool, and moist environments
- Problem: Under hot and dry conditions, photorespiration occurs
Photorespiration: When stomata close to conserve water, CO₂ levels drop inside the leaf while O₂ levels rise. Rubisco then acts as an oxygenase, using O₂ instead of CO₂, producing instead of carbohydrate. This reduces photosynthetic efficiency.
C₄ Plants
- The first product after CO₂ fixation is oxaloacetate (OAA), a 4-carbon compound
- They use the Hatch-Slack pathway in addition to the Calvin cycle
- Examples: maize (corn), sugarcane, sorghum, millet
- Adapted to hot, dry, and tropical climates
C₄ Pathway Mechanism
1. Initial CO₂ Fixation in Mesophyll Cells
- CO₂ combines with PEP (phosphoenolpyruvate), a 3-carbon acceptor
- Enzyme: PEP carboxylase (has high affinity for CO₂, not inhibited by O₂)
- PEP + CO₂ → oxaloacetate (OAA, 4-carbon)
- OAA is converted to malate
2. Shunting of Malate
- Malate is transported to bundle sheath cells through plasmodesmata
- In bundle sheath chloroplasts, malate releases CO₂
- Malate → CO₂ + pyruvate + H₂
- The released CO₂ enters the Calvin cycle
3. Regeneration of PEP
- Pyruvate returns to mesophyll cells
- Pyruvate + ATP → PEP + AMP
4. Calvin Cycle in Bundle Sheath Cells
- CO₂ is fixed by Rubisco into the Calvin cycle as in C₃ plants
- This occurs at high CO₂ concentrations, avoiding photorespiration
Comparison Table
| Feature | C₃ Plants | C₄ Plants |
|---|---|---|
| First CO₂ product | 3-PGA (3-carbon) | Oxaloacetate (4-carbon) |
| Primary enzyme | RuBP carboxylase | PEP carboxylase + RuBP carboxylase |
| CO₂ acceptor | RuBP | PEP and RuBP |
| Fixation cells | Bundle sheath only | Mesophyll and bundle sheath |
| Photorespiration | High in hot/dry conditions | Minimal or absent |
| Water use efficiency | Lower | Higher |
| Examples | Wheat, rice, soybeans | Maize, sugarcane, sorghum |
In Tanzania, understanding C₃ and C₄ photosynthesis helps farmers choose appropriate crops. For instance, maize (a C₄ plant) grows well in the hot, dry lowlands of Dodoma or Shinyanga because it minimizes water loss through reduced photorespiration, while rice (a C₃ plant) is better cultivated in cooler, wetter highland regions like Mbeya where water is abundant. This knowledge enables agricultural extension workers to advise farmers on which crops to plant based on local climate conditions, improving food security and farm incomes.
Swali
In which part of the chloroplast does the light-dependent reaction of photosynthesis take place?
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