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Biology · Class 11· Quick Revision

Photosynthesis — Last-Minute Revision

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Must-Remember Points

  1. Photosynthesis equation: 6CO2 + 6H2O → C6H12O6 + 6O2 (needs light + chlorophyll)
  2. Two stages: Light reactions (thylakoid) and Calvin cycle (stroma)
  3. Light reactions produce: ATP + NADPH + O2
  4. Calvin cycle produces: G3P → glucose
  5. O2 comes from water (photolysis), NOT from CO2
  6. RuBisCO fixes CO2 in Calvin cycle — most abundant enzyme on Earth
  7. PS II acts first, PS I acts second (naming is by discovery order)
  8. Non-cyclic photophosphorylation: both PS → ATP + NADPH + O2
  9. Cyclic photophosphorylation: only PS I → ATP only (no O2, no NADPH)
  10. Per glucose: 6 CO2, 18 ATP consumed, 12 NADPH consumed in Calvin cycle
  11. Chlorophyll absorbs red (680nm) and blue (700nm) light, reflects green
  12. Factors affecting rate: light intensity, CO2 concentration, temperature

Formulas & Equations

  • 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O (detailed equation)
  • 6CO2 + 6H2O → C6H12O6 + 6O2 (simplified equation)
  • H2O → 2H+ + 2e- + ½O2 (photolysis)
  • ADP + Pi → ATP (photophosphorylation)
  • NADP+ + 2H+ + 2e- → NADPH (reduction)
  • 3 CO2 + 9 ATP + 6 NADPH → 1 G3P (per 3 Calvin cycle turns)

Exam Traps to Watch For

  • Trap: 'Dark reactions happen at night.' WRONG. They happen during the day too — they just do not directly need light. They need ATP and NADPH from light reactions.

  • Trap: 'PS I acts first.' WRONG. PS II acts first despite the name. Remember: II before I in function, I before II in discovery.

  • Trap: 'O2 comes from CO2.' WRONG. O2 comes from splitting water. Proven by isotope tracing experiments (Ruben and Kamen, 1941).

  • Trap: 'One Calvin cycle turn makes one glucose.' WRONG. One turn makes one G3P (net). Need 6 turns (6 CO2) for one glucose.

  • Trap: Forgetting to mention 'in the presence of chlorophyll and sunlight' when writing the photosynthesis equation in board exams.

If You Remember Only 5 Things

  1. Equation: 6CO2 + 6H2O → C6H12O6 + 6O2
  2. Location: Chloroplast (thylakoid for light, stroma for Calvin)
  3. Light reactions → ATP + NADPH + O2; Calvin cycle → glucose
  4. RuBisCO = key enzyme for carbon fixation
  5. O2 from water, not from CO2

Related Topics

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