If a mitochondrion ran out of molecular oxygen [tex]$\left( O _2\right)$[/tex], predict which of the following situations would occur in its electron transport chain.
A. Cyt [tex]$a_3$[/tex] prosthetic groups would remain oxidized and Cyt a would have nowhere to donate electrons.
B. FMN prosthetic groups would remain reduced and NADH would have nowhere to donate electrons.
C. Electrons would start to flow in the opposite direction, converting water into [tex]$O _2$[/tex] and [tex]$H ^{+}$[/tex].
D. [tex]$H _2 O$[/tex] would be made instead of [tex]$O _2$[/tex] at the end of the chain.
F. FADH2 would be the main electron donor to the electron transport chain instead of NADH.
Answer (2)
Oxygen is the final electron acceptor in the electron transport chain.
Without oxygen, electrons cannot be passed to the final acceptor, Cyt a 3 .
Cyt a 3 remains oxidized, and Cyt a has nowhere to donate electrons.
The electron transport chain is blocked, leading to a halt in ATP production.
The correct answer is: Cyt a 3 prosthetic groups would remain oxidized and Cyt a would have nowhere to donate electrons. C y t a 3 p ros t h e t i c g ro u p s w o u l d re main o x i d i ze d an d C y t a w o u l d ha v e n o w h ere t o d o na t e e l ec t ro n s .
Explanation
Understanding the Role of Oxygen When a mitochondrion runs out of molecular oxygen ( O 2 ), the electron transport chain (ETC) is directly affected because oxygen acts as the final electron acceptor in the chain. Without oxygen to accept electrons, the flow of electrons through the ETC is halted. This causes a backup of electrons, leading to specific consequences for the electron carriers in the chain.
Analyzing the Electron Flow In the electron transport chain, electrons are passed from one carrier to another until they reach the final carrier, cytochrome a 3 (Cyt a 3 ). Cyt a 3 then passes these electrons to oxygen, which combines with hydrogen ions to form water. If oxygen is absent, Cyt a 3 cannot donate the electrons it receives. Consequently, Cyt a 3 remains in its reduced state (i.e., it holds onto the electrons).
Determining the State of Cytochrome a Since Cyt a 3 cannot pass electrons to oxygen, the preceding carrier, cytochrome a (Cyt a), also cannot donate its electrons to Cyt a 3 . As a result, Cyt a remains oxidized because it has nowhere to donate its electrons.
Evaluating Other Options Considering the other options:
FMN prosthetic groups would remain reduced and NADH would have nowhere to donate electrons: While the ETC is indeed blocked, the more immediate effect is at the end of the chain where oxygen is needed.
Electrons would start to flow in the opposite direction, converting water into O 2 and H + : This is incorrect. The ETC does not reverse its flow.
H 2 O would be made instead of O 2 at the end of the chain: H 2 O cannot be made without oxygen.
FADH2 would be the main electron donor to the electron transport chain instead of NADH: The absence of oxygen does not change which molecules donate electrons; it only stops the chain's function.
Conclusion Therefore, the correct prediction is that Cyt a 3 prosthetic groups would remain oxidized, and Cyt a would have nowhere to donate electrons.
Examples
Consider a scenario where a muscle cell is undergoing intense exercise and the oxygen supply becomes limited. The electron transport chain in the mitochondria of the muscle cell would be affected. The lack of oxygen would prevent the final electron transfer, leading to a buildup of reduced electron carriers and a decrease in ATP production. This situation is similar to what happens in certain types of cellular hypoxia or ischemia, where oxygen deprivation disrupts the normal function of the electron transport chain, impacting energy production and cellular function.
The correct prediction is that Cyt a₃ prosthetic groups would remain oxidized and Cyt a would have nowhere to donate electrons due to the lack of oxygen. This disrupts the electron flow in the electron transport chain, halting ATP production. Hence, the situation leads to the blockage of the electron transport chain.
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