Q.95 Which of the following is TRUE about Kreb’s cycle?
(A) Kreb’s cycle generates NADPH
(B) The enzymes of Kreb’s cycle reside in the inter-membrane space of a mitochondria
(C) It produces ATP, the energy currency of a cell
(D) None of the above
None of the above statements about Kreb’s cycle are true. All options contain inaccuracies regarding its products, location, and energy output.
Options Explained
Kreb’s Cycle Generates NADPH (A)
False: Kreb’s cycle produces NADH (3 per cycle) and FADH2 (1 per cycle) for electron transport chain, not NADPH. NADPH comes from pentose phosphate pathway or malic enzyme for biosynthesis.
Enzymes Reside in Inter-Membrane Space (B)
False: All Kreb’s cycle enzymes operate in the mitochondrial matrix, including citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase complex. Intermembrane space hosts electron transport chain complexes.
Produces ATP, Energy Currency (C)
False: Kreb’s cycle generates 1 GTP (equivalent to ATP via substrate-level phosphorylation at succinyl-CoA synthetase), but this is minor compared to ~28-30 ATP from NADH/FADH2 oxidation via oxidative phosphorylation. ATP is not the primary product.
None of the Above (D)
Correct: All A, B, C contain factual errors about Kreb’s cycle biochemistry, location, or energetics.
Kreb’s cycle generates no NADPH, resides in mitochondrial matrix, produces minimal ATP—making “none of the above” correct for NEET biochemistry mastery.
Kreb’s Cycle Actual Products (Per Acetyl-CoA)
| Product | Quantity | Destination |
|---|---|---|
| NADH | 3 | ETC Complex I → ~7.5 ATP |
| FADH2 | 1 | ETC Complex II → ~1.5 ATP |
| GTP(=ATP) | 1 | Substrate phosphorylation |
| CO2 | 2 | Exhaled |
| Total ATP | ~10 ATP | Mostly oxidative phosphorylation |
Kreb’s Cycle Location & Regulation
-
Matrix enzymes: Citrate synthase, aconitase, 3 dehydrogenase complexes
-
Rate-limiting: Isocitrate dehydrogenase (NAD+ dependent)
-
Inhibitors: High NADH/NAD+, ATP/ADP ratios
Why Option (D) is Correct
Standard biochemistry confirms Kreb’s cycle yields NADH/FADH2/GTP/CO2 in matrix—no NADPH, no intermembrane enzymes, no significant direct ATP. All options fail factually.
