Explain the Calvin cycle (carbon fixation)
2.7.7· Biology › Photosynthesis
Calvin Cycle HAI KYA?
Ise cycle kyun kehte hain? Kyunki wo starting molecule jo CO₂ ko pakadta hai — ek 5-carbon sugar jise RuBP (ribulose-1,5-bisphosphate) kehte hain — end mein regenerate hona chahiye, taaki cycle dobara chal sake. Agar RuBP rebuild na karo, toh factory ke paas CO₂ pakadne ke "hooks" khatam ho jayenge.
TEEN phases (KYA hota hai)
Phase 1 — Carbon Fixation
- Enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase) CO₂ ko RuBP (5C) se attach karta hai.
- Isse ek unstable 6-carbon intermediate banta hai jo turant 3-PGA ke do molecules (3-phosphoglycerate, 3C each) mein toot jaata hai.
3-PGA kyun hota hai, koi 6C molecule kyun nahi? 6C product itna unstable hota hai ki wo pal bhar mein do 3C halves mein toot jaata hai — isliye is pathway ko C₃ kehte hain (pehla stable product 3 carbons ka hota hai).
Phase 2 — Reduction
- Har 3-PGA ko ATP se ek phosphate milta hai → 1,3-bisphosphoglycerate.
- Phir NADPH electrons donate karta hai (use reduce karta hai) → G3P (glyceraldehyde-3-phosphate, 3C).
Yahan do energy molecules kyun? Ek bare acid (3-PGA) ki energy low hoti hai; ek sugar (G3P) ki energy high hoti hai. Us energy hill ko charhne ke liye ATP (energy) aur NADPH (reducing power / electrons) se pay karna padta hai.
Phase 3 — Regeneration of RuBP
- Har 6 G3P mein se, sirf 1 bahar jaata hai glucose/starch banane ke liye. Baaki 5 G3P ko ATP use karke 3 RuBP mein rearrange kiya jaata hai.
5 ko 6 mein se kyun rakhte hain? Carbon accounting: 5 × 3C = 15 carbons ko 3 × 5C = 15 carbons of RuBP banana padta hai. Cycle apna zyada tar product sirf chalta rehne ke liye reinvest karti hai — sirf surplus harvest hota hai.
Numbers KAISE kaam karte hain (bookkeeping scratch se derive karo)
Humein net 1 G3P chahiye (ek usable 3-carbon sugar). Yaad mat karo — atoms gino.
Common Mistakes (Inhe Steel-man karo)
Recall Feynman: ek 12-saal ke bachche ko samjhao
Socho ek patti ke andar ek chhoti si LEGO factory hai. Sunlight do tarah ki batteries charge karti hai (ATP aur NADPH). Factory un batteries ko use karke hawa se invisible CO₂ pakadti hai aur use ek special 5-block LEGO catcher pe snap karti hai. Har catch se tute hue pieces bante hain, batteries unhe glue aur power karke ek sugar piece banati hain, aur factory apne catchers hamesha rebuild karti hai taaki aur hawa pakad sake. Kafi grabs ke baad, uske paas itne sugar pieces ho jaate hain ki khana (glucose) ban sake. Koi sunlight nahi = dead batteries = factory ruk jaati hai.
Active Recall
Chloroplast mein Calvin cycle kahan hoti hai?
Light reactions ke kaun se do products Calvin cycle ko power karte hain?
Carbon fixation ko kaun sa enzyme catalyze karta hai?
Fixation mein CO₂ ko kaun sa molecule pakadta hai?
Fixation ka pehla stable product kya hai, aur kitne carbons hote hain?
Unstable 6C intermediate kyun matter karta hai?
Reduction phase mein NADPH kya karta hai?
1 net G3P banane ke liye kitne CO₂ chahiye?
Ek glucose ke liye cycle ke kitne turns chahiye?
Ek glucose ke liye ATP aur NADPH cost?
6 G3P mein se (per net 1 G3P system), kitne RuBP regenerate karte hain?
Calvin cycle truly ek "dark" reaction kyun nahi hai?
RuBisCO ka "oxygenase" part kya cause karta hai?
Connections
- Light Reactions — is cycle ke liye ATP aur NADPH supply karte hain.
- ATP aur NADPH — energy + electron currencies.
- RuBisCO — central enzyme; Photorespiration mein bhi key hai.
- C4 and CAM Plants — RuBisCO ke oxygenase problem se bachne ke adaptations.
- Cellular Respiration — is cycle ke banaye glucose ko use karta hai (building vs burning ka reverse logic).
- Chloroplast Structure — stroma vs thylakoid locations.