Imagine a tiny merry-go-round in your cell. You toss a small 2-piece snack (acetyl-CoA) onto it. As it spins one full loop, the machine rips off energy sparks and clips them onto little battery-trucks called NADH and FADH₂. It also puffs out two bits of smoke (CO₂) — that's the air you breathe out! At the end of the loop the merry-go-round is empty again, ready for the next snack. The battery-trucks then drive off to a separate factory that turns those sparks into lots of real energy (ATP).
What is the true fuel input of the Krebs cycle?
Acetyl-CoA (2 carbons), not glucose.
What 4C molecule does acetyl-CoA combine with to start the cycle?
Oxaloacetate (forming citrate).
How many CO₂ are released per turn of the Krebs cycle?
2 CO₂.
How many NADH per turn of the Krebs cycle?
3 NADH.
How many FADH₂ per turn?
1 FADH₂.
How many ATP (substrate-level) per turn?
1 ATP (or GTP).
Per glucose, how many times does the Krebs cycle run, and why?
Twice, because glucose yields 2 pyruvate → 2 acetyl-CoA.
Total Krebs outputs per glucose?
4 CO₂, 6 NADH, 2 FADH₂, 2 ATP.
Why is the Krebs cycle called a "cycle"?
Oxaloacetate is regenerated, so it returns to its start to run again.
Is O₂ directly used in the Krebs cycle reactions?
No; O₂ is used in the ETC. The cycle stops without O₂ because NAD⁺/FAD can't be recycled.
What is the main purpose of the Krebs cycle?
To load electron carriers (NADH, FADH₂), not to make ATP directly.
Where are the 6 CO₂ per glucose released?
2 in the link reaction, 4 in the Krebs cycle (2 per turn × 2).
Dekho, Krebs cycle ka asli kaam ATP banana nahi hai — uska kaam hai electron carriers ko load karna, yaani NADH aur FADH₂ banana. Glucose seedha cycle mein nahi jaata; pehle glycolysis use 2 pyruvate mein todta hai, phir link reaction har pyruvate ko acetyl-CoA (2 carbon) bana deta hai. Yahi acetyl-CoA cycle ka real fuel input hai.
Ab simple bookkeeping karo. Acetyl-CoA ke 2 carbon andar aate hain, aur ek turn mein 2 carbon CO₂ bankar bahar nikal jaate hain — isiliye cycle apne shuruaati point (oxaloacetate) par wapas aa jaata hai, isliye ise "cycle" kehte hain. Electrons ki ginti: ek turn mein 3 NADH, 1 FADH₂, 1 ATP, aur 2 CO₂. Yaad rakhne ke liye mantra: "3-1-1-2".
Glucose ek hai, par acetyl-CoA do bante hain, isliye cycle 2 baar chalti hai. Isliye per glucose: 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP. Aur jo 6 CO₂ tum saans se bahar nikaalte ho — 2 link reaction se, 4 Krebs cycle se.
Ek important baat: O₂ seedha cycle ke andar use nahi hota. Lekin O₂ ke bina ETC ruk jaata hai, NAD⁺ recycle nahi hota, aur fir cycle bhi ruk jaati hai. Isliye exam mein likhna: Krebs cycle aerobic hai indirectly, kyunki uske carriers ko recharge karne ke liye O₂ chahiye.
