11Class 11 Biology
Photosynthesis in Higher Plants short notes
Short notes on the light reactions, the Calvin cycle, C4 plants and photorespiration. Questions often ask for numbers and locations, so note where each step happens.
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Basics
- The overall reaction is 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O.
- Light reactions happen in thylakoids, and the dark reactions in the stroma.
- Oxygen comes from water, not carbon dioxide.
- Chlorophyll a is the main pigment, with reaction centres PS I (P700) and PS II (P680).
Light reactions
- Non-cyclic flow uses both photosystems and makes ATP and NADPH. Water splits at PS II.
- Cyclic flow uses PS I only and makes only ATP.
- ATP forms by chemiosmosis, as protons flow through ATP synthase from the lumen to the stroma.
Calvin cycle (C3)
- The steps are carboxylation, reduction and regeneration of RuBP.
- RuBisCO fixes CO₂ to RuBP, giving the first stable product 3-PGA, which has 3 carbons.
- One glucose needs 6 turns, using 18 ATP and 12 NADPH.
C4 plants and photorespiration
- C4 plants (maize, sorghum) have Kranz anatomy, with bundle sheath cells around the veins.
- Mesophyll PEP carboxylase fixes CO₂ into oxaloacetate, which has 4 carbons, and RuBisCO then works in the bundle sheath.
- Photorespiration happens when RuBisCO binds O₂, wasting energy with no sugar or ATP made. C4 plants mostly avoid it.
- Factors: light, CO₂, temperature and water. Blackman's law says the slowest factor limits the rate.
Where students slip up
- Saying oxygen comes from CO₂.
- Thinking dark reactions need darkness. They just don't directly need light.
- Mixing up the first products: 3-PGA for C3, and oxaloacetate for C4.
Try it on your weakest chapter.
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