Question

An organism uses only the glycerophosphate shunt pathway to transport cytosolic NADH to mitochondria. For every two electrons transported, complex I, complex III, and complex IV of the electron transport chain in this organism transport 2.5, 1.5, and 2.0 protons (H$^+$), respectively. The H$^+$ to ATP ratio of F$_0$F$_1$-ATPase of this organism is 4.0. Terminal electron acceptor is oxygen.
The number of ATP molecules synthesized by oxidizing NADH from glycolysis is _________ (rounded off to two decimal places).}

Hint:

Glycerophosphate shunt electrons enter at complex II (as FADH$_2$). So {exclude} complex I when counting pumped protons; then divide by the given H$^+$/ATP to get ATP yield.

Answer 1

Step 1: Identify entry point of electrons.
Glycerophosphate (glycerol-3-phosphate) shunt passes electrons from cytosolic NADH to mitochondrial FAD $⇒$ enters the ETC as FADH$_2$, bypassing complex I.
Step 2: Protons pumped per cytosolic NADH via this shunt.
Only complexes III and IV pump: $1.5 + 2.0 = 3.5$ H$^+$ per 2e$^-$.
Step 3: ATP yield from those protons.
Given H$^+$/ATP $=4.0 ⇒$ ATP per cytosolic NADH $= \dfrac{3.5}{4.0}=0.875$.
Step 4: Glycolysis produces 2 NADH per glucose.
Total ATP from the two cytosolic NADH $= 2 \times 0.875 = 1.75$.
Therefore, the ATP synthesized by oxidizing glycolytic NADH via the glycerophosphate shunt is 1.75.