Photosynthesis is the fundamental process by which green plants prepare their own food using sunlight, carbon dioxide, and water. It is the primary source of energy for all living organisms on Earth.
This process not only produces glucose but also releases oxygen, which is essential for respiration.
What is Photosynthesis?
Photosynthesis is the process of converting light energy into chemical energy stored in glucose.
General Equation of Photosynthesis
6CO2+12H2Olight, chlorophyllC6H12O6+6O2+6H2O
Site of Photosynthesis
Photosynthesis occurs in chloroplasts, mainly in the mesophyll cells of leaves.
Structure of Chloroplast
- Outer membrane
- Inner membrane
- Stroma (fluid matrix)
- Thylakoids (flattened sacs)
- Grana (stacks of thylakoids)
Photosynthetic Pigments
Pigments are substances that absorb light energy.
Types of Pigments
1. Chlorophyll a
- Primary pigment
- Reaction center pigment
2. Chlorophyll b
- Accessory pigment
- Transfers energy to chlorophyll a
3. Carotenoids
- Include carotenes and xanthophylls
- Protect against photooxidation
Absorption Spectrum vs Action Spectrum
- Absorption spectrum: Light absorbed by pigments
- Action spectrum: Effectiveness of light in photosynthesis
Maximum photosynthesis occurs in:
- Blue light
- Red light
Photosystems
Two photosystems are involved:
1. Photosystem II (PS II)
- First to function
- Reaction center: P680
2. Photosystem I (PS I)
- Works after PS II
- Reaction center: P700
Light Reactions (Photochemical Phase)
- Occur in thylakoid membranes
- Require light
Main Events
- Photolysis of water
- Release of oxygen
- Formation of ATP and NADPH
Photolysis of Water
2H2O→4H++4e−+O2
Electron Transport System (ETS)
- Electrons move from PS II → PS I
- Energy released is used to synthesize ATP
Types of Photophosphorylation
1. Non-Cyclic Photophosphorylation
- Both PS II and PS I involved
- Produces ATP, NADPH, and O₂
2. Cyclic Photophosphorylation
- Only PS I involved
- Produces ATP only
- No oxygen or NADPH formed
Dark Reactions (Biosynthetic Phase)
- Occur in stroma
- Do not require light directly
- Use ATP and NADPH
Calvin Cycle (C3 Cycle)
Occurs in most plants.
Steps:
1. Carboxylation
- CO₂ combines with RuBP
- Enzyme: RuBisCO
- Forms 3-PGA
2. Reduction
- 3-PGA → G3P using ATP and NADPH
3. Regeneration
- RuBP is regenerated
- Allows cycle to continue
C4 Pathway (Hatch and Slack Cycle)
Found in tropical plants.
Examples:
- Maize
- Sugarcane
Key Features
- Initial fixation by PEP carboxylase
- Forms oxaloacetic acid (OAA)
- Occurs in mesophyll and bundle sheath cells
Advantages
- No photorespiration
- High efficiency in high temperature
CAM Pathway
- Found in xerophytic plants (desert plants)
Features:
- Stomata open at night
- CO₂ fixed at night
- Calvin cycle during day
Photorespiration
- Occurs when RuBisCO acts as oxygenase
- Consumes O₂ and releases CO₂
- Reduces efficiency of photosynthesis
Factors Affecting Photosynthesis
1. Light
- Intensity, quality, duration
- Red and blue light most effective
2. Carbon Dioxide
- Increased CO₂ increases rate up to a limit
3. Temperature
- Affects enzyme activity
4. Water
- Essential for photolysis
- Deficiency reduces photosynthesis
Blackman’s Law of Limiting Factors
- Rate of photosynthesis is limited by the slowest factor
Differences Between C3 and C4 Plants
| Feature | C3 Plants | C4 Plants |
|---|---|---|
| First product | 3-PGA | OAA |
| Photorespiration | High | Absent |
| Efficiency | Low | High |
| Example | Wheat | Maize |
Importance of Photosynthesis
- Produces oxygen
- Forms food base of ecosystem
- Maintains CO₂ balance
- Source of fossil fuels
Important NEET & CBSE Points
- Primary pigment: Chlorophyll a
- CO₂ fixation enzyme: RuBisCO
- Light reaction site: Thylakoid
- Dark reaction site: Stroma
- Photolysis occurs in PS II
- C4 plants show no photorespiration
Quick Revision Notes
- Light reaction → ATP + NADPH
- Dark reaction → Glucose
- C3 = Calvin cycle
- C4 = Hatch-Slack pathway
- CAM = Night CO₂ fixation
Frequently Asked Questions (FAQs)
Q1. What is the site of photosynthesis?
Chloroplast.
Q2. What is RuBisCO?
Enzyme responsible for CO₂ fixation.
Q3. What is photorespiration?
Wasteful process where O₂ is consumed instead of CO₂.
Q4. Which plants are most efficient?
C4 plants.
Conclusion
Photosynthesis in higher plants is a highly coordinated process involving light capture, energy conversion, and carbon fixation. It is essential for life on Earth and forms a crucial topic for CBSE board exams and NEET preparation.