Overview
Photosynthesis is the process by which plants convert carbon dioxide and water into glucose (food) and oxygen using sunlight energy in the presence of chlorophyll and enzymes.
Chemical Equation and Definition
- Basic equation: CO₂ + H₂O + light energy → C₆H₁₂O₆ + O₂
- Reactants (left side): carbon dioxide, water, radiant energy, chlorophyll, enzymes
- Products (right side): glucose (stored as starch) and oxygen (released to atmosphere)
- Process produces carbohydrates for plant energy; oxygen forms as by-product
- Occurs only in green plants containing chlorophyll pigment
Chloroplast Structure
- Photosynthesis takes place in the chloroplast organelle within plant cells
- Double membrane structure: outer membrane and inner membrane
- Stroma: fluid-filled area of chloroplast
- Thylakoids: disk-shaped structures stacked together
- Granum (singular) or grana (plural): stacks of thylakoids
- Chlorophyll located inside grana; essential for photosynthesis
- Additional structures include ribosomes, mitochondria, and starch granules
Two Phases of Photosynthesis
| Phase | Location | Light Requirement | Key Process | Products |
|---|
| Light Phase (Light-dependent) | Grana of chloroplast | Requires light | Photolysis (water splitting); ATP formation | Energy-rich hydrogen atoms; oxygen; ATP |
| Dark Phase (Calvin Cycle / Light-independent) | Stroma of chloroplast | Does not require light (occurs day or night) | CO₂ combines with hydrogen using ATP energy | Glucose; excess stored as starch |
Light Phase Details
- Occurs in grana where chlorophyll molecules are present
- Chlorophyll absorbs radiant energy and converts it to chemical energy
- Chemical energy used for photolysis: splitting water into hydrogen and oxygen
- Oxygen released as by-product through stomata into atmosphere
- ATP (adenosine triphosphate) formed as energy carrier
- NADP enzyme acts as taxi to transport energy-rich hydrogen atoms to stroma
Dark Phase (Calvin Cycle) Details
- Takes place in stroma of chloroplast
- Carbon dioxide from atmosphere enters the cycle
- Energy-rich hydrogen atoms combine with CO₂ using ATP energy
- Glucose is produced as final product
- Excess glucose stored as starch
- ATP becomes ADP (adenosine diphosphate) after releasing energy
- ADP and NADP return to light phase to continue cycle
ATP Energy Carrier
- ATP stands for adenosine triphosphate (three phosphates)
- Energy stored in bond between second and third phosphate
- When third phosphate breaks off, energy is released
- Released energy used to combine hydrogen with carbon dioxide
- ADP (adenosine diphosphate) returns to light phase for recharging
- Energy from light phase reattaches third phosphate to form ATP again
Importance of Photosynthesis
- Maintains constant oxygen concentration in atmosphere and water
- Maintains constant carbon dioxide concentration in atmosphere and water
- Provides food for all heterotrophic organisms (cannot produce own food)
- Makes chemical energy available for cellular functions
- Essential for survival of life on Earth
Factors Affecting Rate of Photosynthesis
| Factor | Optimal Level | Effect of Increase | Limiting Factor |
|---|
| Light Intensity | Moderate to high | Increases rate until plateau; too high closes stomata | CO₂ becomes limiting when stomata close |
| Temperature | 25°C | Below: enzymes inactive; Above 30°C: enzymes denature; stomata close | CO₂ becomes limiting when stomata close |
| Carbon Dioxide | Adequate supply | Increases rate until plateau; too much reduces efficiency | Low CO₂ directly limits photosynthesis |
Light Intensity
- Increasing light intensity increases photosynthesis rate up to optimal point
- Excessive light causes stomata to close, preventing CO₂ entry
- Graph shows rate increases then plateaus or decreases at very high intensity
Temperature
- Plants photosynthesize best at approximately 25°C
- Lower temperatures make enzymes (like NADP) inactive
- Higher temperatures (30°C+) cause enzyme denaturation
- Excessive heat closes stomata, limiting CO₂ availability
Carbon Dioxide Concentration
- CO₂ is requirement for photosynthesis; more CO₂ increases rate
- As concentration increases, rate increases until plateau
- Too much CO₂ eventually reduces efficiency
- When CO₂ is limiting factor, photosynthesis decreases
Greenhouses and Optimal Growing Conditions
- Greenhouses enable year-round plant growth in regulated environments
- Provide optimal conditions for maximum photosynthesis and growth
Light Management
- Transparent structure allows maximum light penetration
- Photometers used to measure light intensity coming in
- Accelerates photosynthesis and overall plant growth
Temperature Control
- Maintained at optimal 25°C for plant growth
- Flaps or openings regulate temperature by releasing hot air
- Thermometers monitor internal temperature continuously
- Advanced systems use air conditioning and computer-controlled sensors
- Warm air escapes through top vents; cooler air enters below
Carbon Dioxide Regulation
- Closed system requires active CO₂ management
- Plants consume CO₂ during day; can become limiting factor
- CO₂ tanks pump gas directly into greenhouse
- Organic materials with decomposition bacteria provide natural CO₂ release
- Monitoring prevents depletion during active photosynthesis periods