Question

The electron transport chain (ETC), located in the inner membrane of the mitochondria, Powers cellular respiration. As a final step in the process, Cytochrome C oxidase transfers electrons to oxygen to form water. Propose the outcome of an experiment where isolated mitochondria are treated with cyanide, an inhibitor of cytochrome C Oxidase.
A) The electron would reverse flow, moving towards the start of ETC.
B) Electron carriers would bypass cytochrome C Oxidase and deliver electrons to a downstream acceptor.
C) Shutting down the ETC would not affect the Krebs cycle.
D) Electron carriers would remain reduced, cell damage and death would occur.

Answer 1

Hint: The ETC mainly happen in the cell in order to release stored energy from the $NADH+H^{+}$ and $FAD{H}_2$and this can only happen when they passed through ETS (present in the inner membrane of mitochondria), which ultimately results in the formation of $H_{2}O$.

Complete answer: Cyanide poisoning is a fatal situation, it poisons mitochondrial activity and hence the body is unable to derive energy from oxygen. After cyanide poisoning, ETC cannot pump electrons in the intermembrane space of mitochondria, which decreases the pH gradient and stop the ATP formation. These poisons mainly bind with the ferric ion of Complex IV (a3 portion) of cytochrome oxidase and prevent the cell from deriving energy from oxygen which causes rapid death.
What to do in case of cyanide poisoning? In this case, one has to take steps immediately without the consent of laboratories, administration of an antidote of cyanide can save one’s life. Hydroxocobalamin and sodium nitrite are the available cyanide antidotes that are given intravenously.
So the correct answer is option (D).

Additional information:
There are 4 types of complex (protein) involved in ETS and these are:
1) Complex I: It pumps out four hydrogens from the matrix to intermembrane space and hence creates the hydrogen gradient.
2) Complex II: It receives FADH2 which has been bypassed from Complex I.
3) Complex III: It transports the electron to cytochrome C so that it could transport it to Complex IV.
4) Complex IV: It picks two Hydrogen and finally reduces Oxygen to form water.

Note: This process is also called oxidative – phosphorylation, it is because oxidative reactions are involved in the phosphorylation of ADP to ATP. While in photosynthesis, it is called photophosphorylation because the same process requires light.