When one glucose is fully broken down, the cell makes 10 NADH and 2 FADH2 in total. These come from glycolysis (2 NADH), the link reaction (2 NADH) and the Krebs cycle (6 NADH + 2 FADH2). Memory hook: "2-2-6" for NADH and only Krebs gives FADH2 (2). These reduced coenzymes are not the final energy — they carry electrons to the ETS to make most of the ATP.
At a glance
Where produced
NADH: glycolysis, link reaction, Krebs cycle
FADH2: Krebs cycle only
Number per glucose
NADH: 10
FADH2: 2
Enters ETS at
NADH: Complex I
FADH2: Complex II
ATP yield (each)
NADH: about 3 ATP
FADH2: about 2 ATP
Per glucose the cell makes 10 NADH (2 glycolysis + 2 link reaction + 6 Krebs) and 2 FADH2 (only from the Krebs cycle). Use the "2-2-6" pattern for NADH and remember FADH2 comes only from Krebs.
Your doubts, answered
How many NADH and FADH2 are made from one glucose molecule?
Per glucose you get 10 NADH and 2 FADH2 in total. Break-up: glycolysis makes 2 NADH; the link reaction (pyruvate to acetyl CoA) makes 2 NADH; the Krebs cycle makes 6 NADH and 2 FADH2. Remember values are for 2 pyruvate/2 acetyl CoA because one glucose gives two 3-carbon pyruvate molecules, so per-turn Krebs numbers (3 NADH + 1 FADH2) are doubled.
Where exactly is FADH2 produced in respiration?
FADH2 is produced only in the Krebs cycle, at one step: when succinate is oxidised to fumarate by succinate dehydrogenase (Complex II). One turn makes 1 FADH2, so two turns per glucose make 2 FADH2. FADH2 is NOT made in glycolysis or the link reaction. NCERT states: one point in the cycle where FAD is reduced to FADH2.
How many NADH are formed in the Krebs cycle?
Three NADH per single turn of the Krebs cycle, at three oxidation points: isocitrate to alpha-ketoglutarate, alpha-ketoglutarate to succinyl CoA, and malate to oxaloacetate. Since one glucose gives two turns, the Krebs cycle makes 6 NADH per glucose. NCERT says there are three points where NAD+ is reduced to NADH + H+.
Is NADH produced in glycolysis too, or only in the mitochondria?
NADH is produced in both places. Glycolysis happens in the cytoplasm and makes 2 NADH (when PGAL / 3-phosphoglyceraldehyde is oxidised). The link reaction and Krebs cycle happen in the mitochondrial matrix and make 2 + 6 = 8 more NADH. So NADH is the coenzyme common to all three stages, while FADH2 is unique to Krebs.
Why does one turn of Krebs give 3 NADH but 8 NADH appear per glucose overall?
Do not mix the two counts. Per glucose the totals are 10 NADH (2 glycolysis + 2 link + 6 Krebs) and 2 FADH2. NCERT phrases part of this as 'glucose has been broken down to release eight molecules of NADH + H+ and two of FADH2 besides two ATP in TCA' up to the end of Krebs, that 8 is the mitochondrial part (link + Krebs). Add the 2 glycolytic NADH to reach 10.
⚠️ The NEET trap ✗ FADH2 is produced in glycolysis and Krebs cycle, so total FADH2 per glucose is more than 2. ✓ FADH2 is made at only one step, the Krebs cycle (succinate to fumarate). Per glucose there are exactly 2 FADH2. Glycolysis and the link reaction make NADH, never FADH2. 🧠 FADH2 has only ONE home: Krebs, one step, 2 per glucose. If a question 'adds' FADH2 anywhere else, it is wrong.
Real NEET questions
NEET 2017
Which statement is wrong for Krebs' cycle?
A · There are three points in the cycle where NAD+ is reduced to NADH + H+
B · There is one point in the cycle where FAD+ is reduced to FADH2
C · During conversion of succinyl CoA to succinic acid, a molecule of GTP is synthesised
D · The cycle starts with condensation of acetyl group (acetyl CoA) with pyruvic acid to yield citric acid ✓
Solution: D is wrong: the Krebs cycle starts with condensation of the acetyl group with oxaloacetic acid (OAA), not pyruvic acid, to form citric acid. Options A and B correctly state that one turn makes 3 NADH (three NAD+ reduced) and 1 FADH2 (one FAD reduced), and C correctly gives the substrate-level phosphorylation (GTP) step.
NEET 2018
What is the role of NAD+ in cellular respiration?
A · It is a nucleotide source for ATP synthesis.
B · It functions as an electron carrier. ✓
C · It functions as an enzyme.
D · It is the final electron acceptor for anaerobic respiration.
Solution: NAD+ accepts hydrogen atoms/electrons during glycolysis and the TCA cycle to become NADH + H+, then delivers those electrons to the ETS, so it acts as a mobile electron (hydrogen) carrier. It is a coenzyme, not an enzyme; it is not consumed to build ATP's backbone; and in respiration O2, not NAD+, is the final electron acceptor.
Solved Respiration in Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the total NADH and FADH2 produced per glucose?
10 NADH and 2 FADH2 per glucose molecule: 2 NADH from glycolysis, 2 NADH from the link reaction, and 6 NADH plus 2 FADH2 from the Krebs cycle.
Why do NADH and FADH2 give different amounts of ATP?
NADH enters the ETS at Complex I (higher energy level), so one NADH yields about 3 ATP. FADH2 enters later at Complex II, skipping one pumping site, so one FADH2 yields about 2 ATP.
Are NADH and FADH2 the actual energy currency?
No. NADH and FADH2 are reduced coenzymes (electron carriers). They store energy temporarily and hand electrons to the electron transport system, where most ATP, the real energy currency, is made by oxidative phosphorylation.
Does glycolysis produce FADH2?
No. Glycolysis produces only 2 NADH (and 2 net ATP). FADH2 is produced solely in the Krebs cycle at the succinate to fumarate step.
What is FAD reduced to and at which enzyme?
FAD is reduced to FADH2 by succinate dehydrogenase (Complex II) when succinate is oxidised to fumarate in the Krebs cycle.
