Photosynthesis
🟢 Lite — Quick Review (1h–1d)
Rapid summary for last-minute revision before your exam.
Photosynthesis — Key Facts for UPTET
The process by which green plants make food using sunlight, carbon dioxide, and water, producing glucose and oxygen.
Overall Equation: $$\text{6CO}_2 + \text{6H}_2\text{O} \xrightarrow{\text{chlorophyll}} \text{C}6\text{H}{12}\text{O}_6 + \text{6O}_2$$
Requirements:
- Sunlight: Light energy is captured by chlorophyll
- Carbon dioxide: Enters through stomata on leaves
- Water: Absorbed by roots from soil
- Chlorophyll: Green pigment in chloroplasts
⚡ UPTET Exam Tip: Remember — oxygen produced during photosynthesis comes from water (not CO₂), proven by isotope experiments. This is a common UPTET question.
Key Terms:
- Stomata: Microscopic pores on leaf surface for gas exchange
- Guard cells: Control opening/closing of stomata
- Chloroplast: Site of photosynthesis
- ATP: Energy currency of cells
🟡 Standard — Regular Study (2d–2mo)
Standard content for students with a few days to months.
Photosynthesis — Detailed Study Guide
Two Stages of Photosynthesis
Stage 1: Light Reactions (in thylakoid membranes)
- Light energy captured by chlorophyll
- Water split → oxygen released
- ATP and NADPH produced
- Occurs only in presence of light
Stage 2: Dark Reactions / Calvin Cycle (in stroma)
- CO₂ is fixed into organic compounds
- Does not require light directly
- Uses ATP and NADPH from light reactions
- Glucose (C₆H₁₂O₆) is produced
Factors Affecting Photosynthesis
| Factor | Effect |
|---|---|
| Light intensity | Increases rate up to saturation point |
| CO₂ concentration | Increases rate up to optimum |
| Temperature | Optimal at 25–35°C |
| Water | Deficiency reduces photosynthesis |
| Chlorophyll | More chlorophyll = more absorption |
Experiments to Remember
- Jan Ingenhousz’s experiment: Prove that sunlight is necessary for photosynthesis
- Joseph Priestley’s experiment: Prove that plants produce oxygen
- Ingenhousz’s setup: Aquatic plant in water + funnel + inverted test tube — bubbles of O₂ collect
⚡ UPTET Shortcut: The process requiring light directly = Light reactions (thylakoids). The process that builds glucose = Calvin cycle (stroma). Keep these straight!
Structure of a Leaf (Cross-section)
- Upper epidermis: Waxy cuticle, protects
- Palisade mesophyll: Primary site of photosynthesis (closely packed cells)
- Spongy mesophyll: Air spaces for gas exchange
- Lower epidermis: Contains stomata
- Veins: Xylem (water transport) + Phloem (food transport)
🔴 Extended — Deep Study (3mo+)
Comprehensive coverage for students on a longer study timeline.
Photosynthesis — Complete Biology Notes
Detailed Calvin Cycle (C3 Cycle)
- Carbon fixation: CO₂ + RuBP → 2 molecules of 3-PGA (catalyzed by RuBisCO)
- Reduction: 3-PGA + ATP + NADPH → 2 molecules of G3P (glyceraldehyde-3-phosphate)
- Regeneration: G3P + ATP → RuBP
Output: 1 glucose from 6 CO₂ molecules (requires 6 turns, 18 ATP, 12 NADPH)
C4 Pathway (for reference)
- More efficient in hot, dry conditions
- First product: Oxaloacetic acid (4-carbon compound)
- Examples: Maize, sugarcane
- Reduces photorespiration
Significance of Photosynthesis
- Food source: Basis of all food chains — producers synthesize glucose
- Oxygen production: Generates atmospheric O₂ for respiration
- Carbon fixation: Removes CO₂ from atmosphere
- Fossil fuels: Ancient plants → coal, petroleum over millions of years
Human Applications
- Agriculture: Understanding photosynthesis improves crop yields
- Greenhouse effect: Less photosynthesis = more CO₂ retained
- Forests as carbon sinks: Amazon rainforest = “lungs of Earth”
⚡ UPTET PYQ: “Which gas is released during photosynthesis?” → (a) CO₂ (b) O₂ (c) N₂ (d) H₂ → Answer: (b) O₂
Comparison Table
| Feature | Light Reactions | Dark Reactions |
|---|---|---|
| Location | Thylakoid membrane | Stroma |
| Light required | Yes | No |
| Main product | ATP, NADPH | Glucose |
| By-product | O₂ | H₂O |
| Enzyme | Photophylase | RuBisCO |
⚡ Common Mistake: Students confuse dark reactions with occurring only at night. Dark reactions occur continuously during day — they just don’t use light directly. They need the ATP/NADPH produced by light reactions!
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