Question

How many ATP are formed through (ETS) Electro Transport System alone from
A) Glycolysis
B) Oxidative decarboxylation of 2 molecules of pyruvic acid
C) 2 Acetyl coenzyme A molecules respectively

• A. A-4, B-6, C-22
• B. A-4, B-6, C-24
• C. A-6, B-4, C-20
• D. A-4, B-6, C-8

Hint:

Pay close attention to "ETS alone". Exclude the 2 ATP from Glycolysis (net) and 2 GTP from Krebs Cycle which are formed by substrate-level phosphorylation.

Answer 1

The Correct Option is A

Step 1: Analyze ATP yield via ETS for each stage:
The question asks specifically for ATP formed through ETS alone (excluding substrate-level phosphorylation). A) Glycolysis:

• Glycolysis produces 2 NADH molecules in the cytoplasm.
• These NADH must be shuttled into the mitochondria to enter ETS.
• If the Glycerol-3-Phosphate shuttle is used (common in skeletal muscle/brain), each NADH yields 2 ATP.
• Total ATP via ETS = $2 \times 2 = \mathbf{4 \text{ ATP}}$.
• (Note: If Malate-Aspartate shuttle is used, yield is 6 ATP, but Option A starts with 4, suggesting the G-3-P shuttle assumption).

B) Oxidative Decarboxylation (Link Reaction):

• 2 Pyruvic acid $\rightarrow$ 2 Acetyl CoA.
• Produces 2 NADH (mitochondrial matrix).
• Each mitochondrial NADH yields 3 ATP in ETS.
• Total ATP via ETS = $2 \times 3 = \mathbf{6 \text{ ATP}}$.

C) Krebs Cycle (2 Acetyl CoA molecules):

• Per Acetyl CoA: 3 NADH and 1 FADH$_2$ are produced.
• For 2 Acetyl CoA: $6 \text{ NADH}$ and $2 \text{ FADH}_2$.
• ATP from NADH: $6 \times 3 = 18 \text{ ATP}$.
• ATP from FADH$_2$: $2 \times 2 = 4 \text{ ATP}$.
• Total ATP via ETS = $18 + 4 = \mathbf{22 \text{ ATP}}$.
• (Substrate-level phosphorylation [GTP] is excluded as per the "ETS alone" condition).

Step 2: Match with Options:
Values are: A=4, B=6, C=22. This matches Option (A).