201. Isolated mitochondria are supplied separately with NADH or succinate. A treatment blocks complex III but leaves complexes I and II capable of accepting electrons. Oxygen consumption falls sharply with both substrates. The strongest inference is:
ⓐ. NADH and succinate donate electrons to the same entry complex
ⓑ. oxygen accepts electrons directly from complexes I and II
ⓒ. both routes converge at ubiquinone and then require complex III
ⓓ. complex III functions only during oxidation of cytoplasmic NADH
Correct Answer: both routes converge at ubiquinone and then require complex III
Explanation: NADH and succinate-linked reducing equivalents enter the respiratory system through different complexes. NADH enters through complex I, whereas electrons associated with succinate oxidation enter through complex II. Both routes transfer electrons to ubiquinone, which carries them to complex III. Blocking complex III prevents efficient downstream flow from either donor, explaining the fall in oxygen consumption in both treatments. The result does not mean that the donors share the same entry complex; their convergence occurs later at ubiquinone. Oxygen is also not reduced directly by complexes I or II. The experiment identifies complex III as part of the common downstream sequence used after donor-specific entry routes have merged. Shared use of oxygen does not make the two donor classes energetically identical, since they join the chain at different complexes. Failure of both donor treatments after a complex-III block shows that their separate entry routes merge before the inhibited complex and cannot bypass it.
202. Cytochrome c is selectively removed from otherwise intact mitochondria. Complex III can still reduce its normal acceptor site, and complex IV remains structurally present. The most likely outcome is:
ⓐ. electron transfer between complexes III and IV is interrupted
ⓑ. complex II begins transferring electrons directly to oxygen
ⓒ. ubiquinone permanently replaces oxygen as terminal acceptor
ⓓ. pyruvate oxidation becomes a substrate-level phosphorylation step
Correct Answer: electron transfer between complexes III and IV is interrupted
Explanation: Cytochrome c is the mobile carrier that connects complex III with complex IV. Removing it leaves the two membrane complexes physically present but eliminates the normal shuttle that carries electrons between them. Electrons can reach complex III through ubiquinol, yet they cannot be delivered efficiently to complex IV. Upstream carriers become increasingly reduced, while oxygen consumption and oxidative ATP formation decline. Complex II cannot bypass the missing carrier by transferring electrons directly to oxygen, since its electrons normally pass through ubiquinone, complex III, cytochrome c and complex IV. The predicted failure arises from loss of connectivity between two functional complexes rather than destruction of the complexes themselves. Loss of the mobile carrier also causes electrons to accumulate upstream even though complex IV itself remains structurally intact.
203. Use the mitochondrial arrangement described below. Structure P is a small mobile protein located on the intermembrane-space-facing surface of the inner membrane. It receives electrons from complex III and delivers them to complex IV. Structure P is:
ⓐ. ubiquinone
ⓑ. cytochrome c
ⓒ. oxaloacetate
ⓓ. ATP synthase
Correct Answer: cytochrome c
Explanation: Cytochrome c is a small mobile protein associated with the outer, intermembrane-space-facing surface of the inner mitochondrial membrane. It accepts electrons from complex III and moves them to complex IV. Ubiquinone is also mobile, but it moves within the lipid portion of the membrane and connects complexes I or II with complex III. Oxaloacetate is a soluble matrix intermediate of the TCA cycle, not an electron carrier on the membrane surface. ATP synthase is a large membrane-associated enzyme complex that uses proton flow to form ATP. The stated position and the connection between complexes III and IV uniquely identify cytochrome c and distinguish it from the other mobile carrier in the respiratory chain. Its position between complexes III and IV, together with its mobility on the membrane surface, uniquely identifies cytochrome c.
204. Evaluate the following statements about cytochrome c.
I. It is mobile rather than a permanently fixed part of complex III.
II. It transfers electrons between complexes III and IV.
III. It accepts electrons directly from both complex I and complex II.
IV. Its loss can restrict oxygen reduction despite the presence of complex IV.
ⓐ. I and III only
ⓑ. II and III only
ⓒ. I, III and IV only
ⓓ. I, II and IV only
Correct Answer: I, II and IV only
Explanation: Cytochrome c is a mobile carrier on the surface of the inner mitochondrial membrane. It receives electrons from complex III and transfers them to complex IV. It does not accept electrons directly from complexes I and II; those entry routes first converge at ubiquinone and then pass through complex III. Loss of cytochrome c interrupts delivery to complex IV, so oxygen cannot receive electrons normally even when the terminal complex remains structurally intact. The valid combination distinguishes the two mobile carriers of the pathway. Ubiquinone links the entry complexes to complex III within the membrane, while cytochrome c connects complex III with complex IV along the membrane surface. The valid set follows the actual position of cytochrome c: downstream of complex III, upstream of complex IV and essential for terminal electron delivery.
205. Four mitochondrial preparations are examined.
| Preparation | Complex-III activity | Electron transfer by endogenous cytochrome c | Complex-IV activity after reduced cytochrome c is supplied |
|---|
| P | Normal | Absent | Normal |
| Q | Absent | Low | Normal |
| R | Normal | Normal | Absent |
| S | Normal | Normal | Normal |
A selective loss of endogenous cytochrome c is best represented by:
ⓐ. Preparation P
ⓑ. Preparation Q
ⓒ. Preparation R
ⓓ. Preparation S
Correct Answer: Preparation P
Explanation: Preparation P retains normal complex-III activity, so electrons can reach the point at which cytochrome c should accept them. Electron transfer by the endogenous mobile carrier is absent, indicating that the connecting component is missing or non-functional. Complex IV responds normally when reduced cytochrome c is supplied externally, showing that the terminal complex itself remains active. This combination localises the defect to cytochrome c rather than to either neighbouring complex. Preparation Q represents a complex-III defect, while Preparation R points to failure of complex IV. The three measurements provide a functional separation of donor complex, mobile carrier and terminal complex, allowing the missing link to be identified without relying on a single observation. Normal activity after external reduced cytochrome c is supplied isolates the defect to availability of the endogenous mobile carrier.
206. Electrons donated by matrix \(\mathrm{NADH+H^+}\) reach oxygen through which order?
ⓐ. Complex I \(\rightarrow\) complex III \(\rightarrow\) ubiquinone \(\rightarrow\) complex IV \(\rightarrow\) cytochrome c \(\rightarrow\) oxygen
ⓑ. Complex II \(\rightarrow\) ubiquinone \(\rightarrow\) complex III \(\rightarrow\) cytochrome c \(\rightarrow\) complex IV \(\rightarrow\) oxygen
ⓒ. Complex I \(\rightarrow\) ubiquinone \(\rightarrow\) complex III \(\rightarrow\) cytochrome c \(\rightarrow\) complex IV \(\rightarrow\) oxygen
ⓓ. Complex I \(\rightarrow\) cytochrome c \(\rightarrow\) complex III \(\rightarrow\) ubiquinone \(\rightarrow\) complex IV \(\rightarrow\) oxygen
Correct Answer: Complex I \(\rightarrow\) ubiquinone \(\rightarrow\) complex III \(\rightarrow\) cytochrome c \(\rightarrow\) complex IV \(\rightarrow\) oxygen
Explanation: Matrix NADH transfers electrons first to complex I. Complex I passes them to ubiquinone, which becomes reduced and carries the electrons within the membrane to complex III. Complex III then transfers them to cytochrome c. The mobile protein delivers the electrons to complex IV, where oxygen acts as the terminal acceptor. Complex II is the entry point for succinate-linked \(\mathrm{FADH_2}\), not matrix NADH. Ubiquinone must appear before complex III, while cytochrome c must appear after it. The complete order combines the donor-specific entry stage with the common downstream sequence and places oxygen only at the terminal end.
207. Assertion: Complex IV receives electrons directly from ubiquinone.
Reason: Complex IV receives electrons from cytochrome c and transfers them to oxygen.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Assertion is false, but Reason is true
Explanation: Ubiquinone carries electrons from complexes I and II to complex III, not directly to complex IV. Complex III then passes those electrons to cytochrome c. Cytochrome c is the immediate donor to complex IV, which transfers the electrons to oxygen. The assertion skips two required pathway components and is false. The reason accurately describes both the donor to complex IV and the terminal destination of its electrons. This distinction is important when predicting the location of a block. Accumulation of reduced cytochrome c points toward failure at complex IV, while accumulation of ubiquinol with little reduced cytochrome c points toward failure at complex III. The correct sequence places ubiquinone upstream of complex III and cytochrome c immediately upstream of complex IV.
208. Complex IV is inhibited in isolated mitochondria supplied with NADH, oxygen and ADP. Reduced cytochrome c accumulates, while oxygen consumption and ATP production decline. The observations show that complex IV normally:
ⓐ. transfers electrons from ubiquinone to complex III
ⓑ. passes electrons from cytochrome c to oxygen
ⓒ. converts succinate into fumarate in the matrix
ⓓ. phosphorylates glucose during glycolysis
Correct Answer: passes electrons from cytochrome c to oxygen
Explanation: Reduced cytochrome c accumulates when it can no longer donate electrons to complex IV. The simultaneous decline in oxygen consumption shows that terminal electron transfer to oxygen has been interrupted. ATP production falls as electron flow can no longer sustain the energy-conserving conditions required for oxidative phosphorylation. These three observations identify complex IV as the connection between reduced cytochrome c and oxygen. Ubiquinone transfers electrons earlier, between the entry complexes and complex III. Succinate oxidation is associated with complex II, while glucose phosphorylation occurs in the cytoplasm during glycolysis. The experiment links carrier accumulation, terminal oxygen use and ATP production to one blocked membrane complex. Adding more oxygen would not bypass an inhibited complex IV; the terminal complex itself must transfer the electrons.
209. Arrange the electron-transfer route followed by reducing equivalents associated with succinate oxidation.
P. Complex II
Q. Ubiquinone
R. Complex III
S. Cytochrome c
T. Complex IV
U. Oxygen
ⓐ. \(P\rightarrow Q\rightarrow R\rightarrow S\rightarrow T\rightarrow U\)
ⓑ. \(P\rightarrow R\rightarrow Q\rightarrow T\rightarrow S\rightarrow U\)
ⓒ. \(Q\rightarrow P\rightarrow S\rightarrow R\rightarrow T\rightarrow U\)
ⓓ. \(P\rightarrow Q\rightarrow S\rightarrow R\rightarrow U\rightarrow T\)
Correct Answer: \(P\rightarrow Q\rightarrow R\rightarrow S\rightarrow T\rightarrow U\)
Explanation: Succinate-linked electrons enter the respiratory chain through complex II. They are transferred to ubiquinone, which carries them to complex III. Complex III passes the electrons to cytochrome c, and cytochrome c delivers them to complex IV. Oxygen receives them only after the terminal complex. This route bypasses complex I but shares every component from ubiquinone onward with the NADH route. The correct order must preserve both the special entry point and the common downstream pathway. Placing cytochrome c before complex III or oxygen before complex IV would reverse essential donor-acceptor relationships. The sequence also provides the pathway basis for the lower stated ATP yield assigned to \(\mathrm{FADH_2}\) compared with NADH. Because succinate-linked electrons enter at complex II, they reach ubiquinone without first passing through complex I and then follow the common downstream route.
210. A mutation prevents complex IV from transferring electrons to oxygen but leaves oxygen concentration and all upstream carriers initially normal. What change is expected as respiration continues?
ⓐ. Upstream carriers become increasingly oxidised and ATP formation rises
ⓑ. Oxygen is reduced normally through complex II instead
ⓒ. Cytochrome c remains oxidised while ubiquinone disappears
ⓓ. Upstream carriers become reduced and oxidative ATP formation falls
Correct Answer: Upstream carriers become reduced and oxidative ATP formation falls
Explanation: Complex IV provides the final route through which electrons leave the carrier chain and reach oxygen. When this transfer is blocked, reduced cytochrome c cannot unload its electrons. The reduced state then spreads backward as complex III, ubiquinone and the entry complexes lose access to oxidised downstream acceptors. NADH and FADH\(_2\) are reoxidised less effectively, and oxygen consumption decreases despite oxygen remaining physically present. Electron-transfer energy can no longer maintain the conditions needed for normal oxidative phosphorylation, so ATP formation falls. Complex II cannot substitute as a terminal oxidase. The changed condition demonstrates that oxygen availability alone is insufficient when the machinery connecting the chain to oxygen is non-functional.
211. Consider the following statements about complex IV.
I. It is also called cytochrome c oxidase.
II. It contains cytochromes \(a\) and \(a_3\) together with copper centres.
III. It donates electrons to complex III.
IV. It transfers electrons to the terminal oxygen acceptor.
ⓐ. I and III only
ⓑ. II and III only
ⓒ. I, II and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Complex IV is cytochrome c oxidase, the terminal electron-transfer complex of the mitochondrial chain. Its described composition includes cytochromes \(a\) and \(a_3\) and copper-containing centres that participate in transfer of electrons toward oxygen. It receives electrons from reduced cytochrome c rather than donating them backward to complex III. Oxygen is reduced at this terminal complex and contributes to water formation. The valid statements identify the complex by name, key components and direction of transfer. The incorrect statement reverses the sequence between complexes III and IV. Electron flow in the functioning chain is directional, moving from lower-numbered entry complexes through mobile carriers toward the terminal oxygen acceptor. Cytochrome c links complexes III and IV, whereas ubiquinone connects complexes I and II with complex III.
212. Carrier states are recorded after four treatments.
| Record | Ubiquinol | Reduced cytochrome c | Oxygen consumption |
|---|
| P | High | Low | Low |
| Q | High | High | Very low |
| R | Low | Low | High |
| S | Moderate | Low | Moderate |
A block at complex IV is most consistent with:
ⓐ. Record P
ⓑ. Record Q
ⓒ. Record R
ⓓ. Record S
Correct Answer: Record Q
Explanation: Complex IV normally accepts electrons from reduced cytochrome c and transfers them to oxygen. Blocking it causes reduced cytochrome c to accumulate. As downstream electron acceptance fails, the reduced state also builds up in earlier carriers, including ubiquinol. Oxygen consumption becomes very low since electrons cannot reach the terminal acceptor efficiently. Record Q combines high levels of both reduced carriers with minimal oxygen use. Record P is more consistent with a block at complex III, where ubiquinol accumulates but cytochrome c receives few electrons. The table distinguishes terminal failure from an earlier interruption by examining the reduction state of a carrier immediately before complex IV together with an upstream carrier. Simultaneous accumulation of ubiquinol and reduced cytochrome c places the interruption beyond both carriers. For the supplied conditions, oxygen consumption reports terminal electron transfer, while ATP output additionally requires membrane coupling and ATP synthase.
213. A terminal membrane complex receives electrons from cytochrome c. At that site, the net terminal relation is represented by \(\mathrm{\frac{1}{2}O_2+2H^++2e^-\rightarrow H_2O}\). This relation identifies:
ⓐ. oxygen reduction and water formation at complex IV
ⓑ. carbon-dioxide release during the link reaction
ⓒ. lactate formation during anaerobic glycolysis
ⓓ. direct ATP formation from phosphoenolpyruvate
Correct Answer: oxygen reduction and water formation at complex IV
Explanation: The relation shows molecular oxygen accepting electrons and hydrogen ions to form water. This terminal reduction occurs at complex IV after electrons have travelled through cytochrome c. Oxygen is not releasing carbon or accepting a phosphate group; it is completing the respiratory electron-transfer sequence by receiving reducing equivalents. The process allows upstream carriers to return to their oxidised forms and supports continued aerobic metabolism. Link-reaction carbon dioxide arises through decarboxylation of pyruvate, while lactate formation uses pyruvate as an organic electron acceptor. Direct ATP formation from phosphoenolpyruvate is a cytoplasmic substrate-level phosphorylation. The stated donor, acceptor and product collectively identify terminal mitochondrial oxygen reduction.
214. Assertion: Oxygen is not a direct reactant in the individual reactions of the TCA cycle but is required for sustained aerobic cycle activity.
Reason: Oxygen directly oxidises citrate into isocitrate in the mitochondrial matrix.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Assertion is true, but Reason is false
Explanation: The assertion is true. Oxygen acts at the end of the electron-transport chain rather than appearing as a direct reactant in each TCA reaction. Its terminal acceptance of electrons permits NADH and FADH\(_2\) to be reoxidised, restoring \(\mathrm{NAD^+}\) and FAD for matrix dehydrogenase reactions. Without this recycling, the cycle slows as oxidised coenzymes become depleted. The reason is false since citrate is rearranged into isocitrate without direct oxidation by molecular oxygen. Oxygen dependence of the TCA cycle is indirect but essential, arising through redox coupling between the matrix pathway and the inner-membrane electron-transport system. Sustained cycle turnover resumes only when oxidised coenzymes are replenished through functional downstream electron transfer.
215. Oxygen is abruptly removed from isolated mitochondria supplied with pyruvate and ADP. Within a short period, NADH rises, oxygen consumption ceases and ATP formation falls. Which conclusion is best supported?
ⓐ. Oxygen is required directly for glycolytic cleavage of glucose
ⓑ. Pyruvate can no longer enter the mitochondrial matrix
ⓒ. Oxygen normally supplies carbon for the TCA cycle
ⓓ. without terminal oxygen, upstream carriers remain reduced
Correct Answer: without terminal oxygen, upstream carriers remain reduced
Explanation: Oxygen removal eliminates the final electron acceptor of the respiratory chain. Electrons can no longer leave complex IV efficiently, so the carrier system becomes increasingly reduced. NADH accumulates as its oxidation slows, while the supply of \(\mathrm{NAD^+}\) required for pyruvate oxidation and the TCA cycle declines. Loss of electron flow also reduces formation of the proton-motive conditions used for oxidative ATP synthesis. The observations do not show that oxygen supplies carbon or controls pyruvate transport directly. They demonstrate a terminal action with broad upstream consequences. A molecule used only at the final step can remain essential to the entire aerobic sequence when every earlier oxidation depends on continued removal of electrons.
216. Oxygen is restored after a brief, fully reversible oxygen-free interval in isolated mitochondria. The electron carriers and membrane complexes have not been damaged. The most likely response is:
ⓐ. NADH continues accumulating while oxygen consumption remains absent
ⓑ. electron flow resumes, NADH is oxidised and ATP formation recovers
ⓒ. complex IV begins transferring electrons backward to complex I
ⓓ. the TCA cycle permanently changes into alcoholic fermentation
Correct Answer: electron flow resumes, NADH is oxidised and ATP formation recovers
Explanation: Reintroducing oxygen restores the terminal electron acceptor required by complex IV. Electrons can again move through the downstream chain, allowing reduced cytochrome c, ubiquinol and the entry complexes to return toward their oxidised states. NADH oxidation resumes and replenishes the \(\mathrm{NAD^+}\) pool used in pyruvate oxidation and the TCA cycle. Renewed electron flow also re-establishes the energy-conserving membrane conditions that support oxidative phosphorylation, so ATP formation recovers. The brief interval leaves the carrier system undamaged, making recovery rather than persistent failure the expected result. Electron flow remains directed toward oxygen and does not reverse through the respiratory complexes.
217. Mitochondrial measurements are made before and after oxygen removal.
| Condition | NADH level | Reduced cytochrome c | ATP formation |
|---|
| Oxygen present | Moderate | Moderate | High |
| Oxygen absent | High | High | Low |
The pattern most strongly indicates that oxygen:
ⓐ. phosphorylates ADP directly without electron transport
ⓑ. is required only for conversion of glucose into pyruvate
ⓒ. terminal electron acceptance reoxidising upstream carriers
ⓓ. converts reduced cytochrome c directly into acetyl CoA
Correct Answer: terminal electron acceptance reoxidising upstream carriers
Explanation: In the oxygen-free condition, both NADH and cytochrome c accumulate in reduced form. This indicates that electrons are entering or remaining within the carrier system but cannot be removed efficiently at the terminal end. ATP formation also falls, linking continued electron flow with oxidative phosphorylation. Oxygen does not directly phosphorylate ADP; its role is to receive electrons at complex IV and permit the chain to continue cycling between reduced and oxidised states. Glycolysis can form pyruvate without oxygen, and acetyl CoA formation is a separate matrix reaction. The table demonstrates how terminal oxygen use controls upstream redox balance and energy capture throughout aerobic respiration. The respiratory chain is not the direct machinery for substrate-level ATP formation, allowing that process to persist transiently without oxygen. The simultaneous accumulation of NADH and reduced cytochrome c shows that removing oxygen prevents terminal electron discharge and leaves the upstream chain reduced.
218. Evaluate the following statements about the role of oxygen in aerobic respiration.
I. It receives electrons at the terminal end of the mitochondrial carrier chain.
II. Its reduction contributes to formation of water.
III. It is consumed directly during every reaction of glycolysis and the TCA cycle.
IV. Its absence causes upstream electron carriers to remain increasingly reduced.
ⓐ. I, II and IV only
ⓑ. I and III only
ⓒ. II, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Oxygen acts as the terminal electron acceptor at complex IV. It receives electrons and hydrogen equivalents and is reduced to water. This final removal of electrons permits the upstream respiratory carriers to be reoxidised and reused. When oxygen is absent, reduced cytochrome c, ubiquinol, NADH and other carriers accumulate, eventually limiting pyruvate oxidation and TCA-cycle activity. Oxygen is not a direct reactant in every glycolytic or TCA reaction. Glycolysis does not directly require it, and the matrix pathway depends on it indirectly through coenzyme regeneration. The valid statements connect the terminal chemical event with its system-wide effect on the earlier stages of aerobic respiration. The terminal role of oxygen therefore explains why its absence affects earlier matrix oxidation indirectly even though oxygen is not consumed in each matrix reaction.
219. Two identical mitochondrial preparations contain reduced cytochrome c, active complex IV and suitable hydrogen ions. Preparation P receives oxygen, while Preparation Q is kept oxygen-free. P oxidises cytochrome c and forms water; Q shows neither change. The strongest inference is:
ⓐ. Cytochrome c is converted into water during electron transfer
ⓑ. Oxygen is the terminal acceptor for cytochrome-c oxidation
ⓒ. Complex IV produces oxygen from reduced cytochrome c
ⓓ. Water formation occurs before electrons reach complex IV
Correct Answer: Oxygen is the terminal acceptor for cytochrome-c oxidation
Explanation: Both preparations contain the same reduced donor and terminal complex, while oxygen is the only stated difference. Oxidation of cytochrome c and formation of water occur only when oxygen is supplied. This supports the conclusion that oxygen accepts the electrons transferred through complex IV and becomes reduced during water formation. Cytochrome c itself is not converted into water; it donates electrons and returns to its oxidised state. Complex IV does not manufacture oxygen, and water formation cannot precede delivery of electrons to the terminal complex. The controlled comparison establishes oxygen's role through simultaneous changes in the electron donor and terminal product rather than through oxygen consumption alone. Because oxygen was the only altered variable, the paired oxidation of cytochrome c and appearance of water specifically demonstrate terminal electron acceptance by oxygen.
220. Assertion: Oxidative phosphorylation is the direct transfer of phosphate from phosphoenolpyruvate to ADP.
Reason: Oxidative phosphorylation uses energy from respiratory electron transfer to create proton-motive conditions that drive ATP synthesis.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Assertion is false, but Reason is true
Explanation: Transfer of phosphate directly from phosphoenolpyruvate to ADP is substrate-level phosphorylation, one of the ATP-forming reactions of glycolysis. Oxidative phosphorylation operates through a different mechanism at the inner mitochondrial membrane. Energy released as electrons move through the respiratory carriers is used to establish a proton-motive gradient. Return movement of protons through ATP synthase then supports phosphorylation of ADP. The assertion incorrectly assigns a direct cytoplasmic phosphate-transfer reaction to oxidative phosphorylation. The reason accurately states the redox and chemiosmotic basis of mitochondrial ATP formation. Distinguishing the two mechanisms requires attention to the source of energy, the presence or absence of an electron-transport chain and the cellular location of ATP synthesis. The reason correctly identifies the electron-transfer and proton-gradient mechanism, while the assertion describes a different, substrate-level route.
