Krebs cycle products: per turn or per glucose? My diagram and my MCQ answer key disagree
Ananya Reddy11th Sample
10 Sept 2026 · 1 min read
I colour-code everything (blue for NADH, orange for FADH2, green for ATP) and in my Krebs cycle diagram I wrote per turn: 3 NADH, 1 FADH2, 1 ATP, 2 CO2.
But in the summary table for "one glucose molecule" I copied 6 NADH, 2 FADH2, 2 ATP from the cycle. And one MCQ I did said NADH produced in TCA cycle per glucose = 8 and the key said that is right. I have clearly mixed something up.
My doubts:
- I get that one glucose gives two pyruvate and so the cycle runs twice. Is the pyruvate to acetyl CoA step (link reaction) part of the cycle or not? Where does the 8 come from?
- NCERT says succinyl CoA to succinic acid also makes "GTP" which gets converted to ATP. So is it ATP or GTP? Is this the same as ATP made in ETS?
- I can't remember where the two CO2 are released and which steps give NADH. Is there a way besides cramming the whole cycle?
I tried writing the carbon numbers (6, 5, 4, ...) but then forgot the middle part. Help 🥺
7 answers & replies
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[+][–]Nisha Verma12thSample· 1mo ago
Make a three-column table and the 8 problem disappears.
Per turn of the cycle: 3 NADH, 1 FADH2, 1 ATP (via GTP), 2 CO2. Per glucose, cycle runs twice: 6 NADH, 2 FADH2, 2 ATP, 4 CO2. Link reaction per glucose (2 pyruvates, outside the cycle): 2 NADH, 2 CO2.
So 6 + 2 = 8 NADH if the question says "from pyruvate/glucose through TCA including the link step". Your MCQ key just lumped the link reaction in. Always check what the question is counting from.
[+][–]Meera Patel11thSample· 1mo ago
For the cycle itself, I would write it with carbon numbers, exactly like you started. I'll continue from where you forgot:
Acetyl CoA (2C) + oxaloacetic acid (4C) gives citric acid (6C). Citric acid goes to isocitric acid and then to alpha-ketoglutaric acid (5C): CO2 released, NADH formed. Alpha-ketoglutaric acid goes to succinyl CoA (4C): CO2 released, NADH formed. Succinyl CoA goes to succinic acid: GTP (then ATP) formed. This is substrate-level phosphorylation. Succinic acid goes to fumaric acid: FADH2 formed. Fumaric acid goes to malic acid (water added), then to OAA: NADH formed.
So the 6C goes to 5C goes to 4C. Both CO2 are lost in the 6C to 5C to 4C stretch, which also explains why the carbon count drops from 6 to 4 before OAA regenerates.
[+][–]Diya Menon11thSample· 1mo ago
omg ananya I remember it as 2 ATP per turn since there's GTP and then ATP made, like first GTP and then it gets converted and that's the 2? so 2 ATP, 3 NADH, 1 FADH2 per turn. that's what my notes say, and 4 ATP per glucose then
[+][–]Siddharth RaoDropperSample· 1mo ago
Diya, no, that's the mistake. Only ONE substrate-level phosphorylation in the cycle, and it is that succinyl CoA step. The GTP and the ATP are the same single event, GTP just hands its phosphate over to ADP. So 1 ATP per turn and 2 per glucose. The "2" you remember is per glucose.
[+][–]Ananya Reddy11thSample· 1mo agoAuthor
that clears it, I was literally about to colour two green dots in my diagram. fixing now
[+][–]Rahul DubeyDropperSample· 1mo ago
Exam habit I picked up after three attempts: before touching the options, underline what the question counts ("per turn", "per glucose", "from pyruvate", "from glucose including glycolysis"). Half of the Krebs mistakes I saw in test series were counting mistakes, not knowledge mistakes.
And to your second doubt: the ATP made at that succinyl CoA step is by substrate-level phosphorylation, directly. The ATP from ETS is oxidative phosphorylation, which uses the NADH and FADH2 these cycles produce. In the cycle only that one step makes ATP directly, everything else is collecting electrons carriers for later.
[+][–]Farhan ShaikhDropperSample· 1mo ago
cheap hack, draw the cycle once and write per turn numbers inside the circle and "x2" outside it with an arrow. whenever a question says glucose you just double what is inside. Also succinate dehydrogenase is the only enzyme of the cycle sitting in the inner mitochondrial membrane, the rest are in the matrix, which sometimes comes as a one-liner question.
