Electron Transport Chain and Oxidative Phosphorylation
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MCAT Biology › Electron Transport Chain and Oxidative Phosphorylation
Most scientists subscribe to the theory of endosymbiosis to explain the presence of mitochondria in eukaryotic cells. According to the theory of endosymbiosis, early pre-eukaryotic cells phagocytosed free living prokaryotes, but failed to digest them. As a result, these prokaryotes remained in residence in the pre-eukaryotes, and continued to generate energy. The host cells were able to use this energy to gain a selective advantage over their competitors, and eventually the energy-producing prokaryotes became mitochondria.
In many ways, mitochondria are different from other cellular organelles, and these differences puzzled scientists for many years. The theory of endosymbiosis concisely explains a number of these observations about mitochondria. Perhaps most of all, the theory explains why aerobic metabolism is entirely limited to this one organelle, while other kinds of metabolism are more distributed in the cellular cytosol.
The primary purpose of the electron transport chain of mitochondria described in the passage is __________.
the generation of energy to sequester protons in the intermembrane space
to directly phosphorylate ADP
to directly phosphorylate AMP
to synthesize ATP synthase
to carry ADP into the mitochondrial matrix
Explanation
The electron transport chain serves to pump protons into the intermembrane space. The result is the buildup of the electrochemical gradient, and the passage of protons through ATP synthase. Essentially, the electron transport chain establishes the conditions for oxidative phosphorylation to occur.
Cyanide is very toxic in high enough doses because it binds irreversibly to cytochrome C. Which of the following is not an effect of cyanide's inhibition of cytochrome C?
Increase in the rate of the citric acid cycle's activity
Build up of NADH
Increased pH in the mitochondrial intermembrane space
Increase in fermentation activity
Increased ratio of ADP:ATP
Explanation
The inhibition of cytochrome C means that the electron transport chain is no longer able to shuttle electrons from complex III to complex IV, which means it is no longer able to accept electrons from electron carriers. As a result, the citric acid cycle would slow down since there would be a build-up of NADH, which allosterically inhibits several enzymes in the citric acid cycle.
Since the electron transport chain no longer functions properly, there wouldn't be as many 
A person is born with a mutation that causes their cells to not have the ability to produce the NADH dehydrogenase complex, the complex that allows the electron transport chain to make ATP from NADH. Will this patient be able to produce any enery at all from the ETC?
Yes—FADH2 can still enter the ETC.
No—there are no other molecules the ETC uses.
No—NADH is necessary for the ETC to use other molecules to make ATP.
Yes—NAD+ can still enter the ETC.
No—FAD can still enter the ETC.
Explanation
FADH2 enters the ETC at the succinate-Q oxidoreductase complex. While this doesn't generate as much energy as NADH will because the electrons travel a shorter distance, there are still 2 ATP molecules made for each FADH2.
Most scientists subscribe to the theory of endosymbiosis to explain the presence of mitochondria in eukaryotic cells. According to the theory of endosymbiosis, early pre-eukaryotic cells phagocytosed free living prokaryotes, but failed to digest them. As a result, these prokaryotes remained in residence in the pre-eukaryotes, and continued to generate energy. The host cells were able to use this energy to gain a selective advantage over their competitors, and eventually the energy-producing prokaryotes became mitochondria.
In many ways, mitochondria are different from other cellular organelles, and these differences puzzled scientists for many years. The theory of endosymbiosis concisely explains a number of these observations about mitochondria. Perhaps most of all, the theory explains why aerobic metabolism is entirely limited to this one organelle, while other kinds of metabolism are more distributed in the cellular cytosol.
A scientist is studying typical mitochondria as described in the passage. In the course of his study, he measures the generation of NADH and FADH2. What is the normal destination of NADH and FADH2?
Electron transport chain proteins
ATP synthase
Pyruvate dehydrogenase
The intermembrane space
The mitochondrial matrix
Explanation
NADH and FADH2 are electron carriers. They bring electrons from their production point (glycolysis or the Kreb's cycle) to the electron transport chain proteins. The electrons are then passed down the electron transport chain to generate energy.
When a certain bacterium undergoes aerobic respiration, which area would have the lowest pH?
Extracellular
Nucleus
Cytoplasm
Mitochondrial matrix
Golgi body
Explanation
Bacteria are prokaryotes. Since prokaryotes do not have any membrane bound organelles, during respiration, protons are pumped from the cytoplasm to the extracellular region between the plasma membrane and the cell wall. This results in a gradient between those two regions, thus extracellular would have a lower pH.
The drug, DNP, destroys the H+ gradient that forms in the electron transport chain. What is the most likely consequence?
The cells will be forced to perform fermentation.
Glycolysis will stop.
ATP production will increase.
Oxygen consumption will increase.
No effect will occur.
Explanation
If the proton gradient of the electron transport chain were to be destroyed, the cell would need to perform cellular respiration without an electron transport chain. The only option would be to move to anaerobic respiration, which requires fermentation.
Which of the following areas of the mitochondria has the lowest pH?
The intermembrane space
The cytosol
The mitochondrial matrix
The mitochondrial christae
Explanation
ATP synthase, which is located on the inner mitochondrial membrane, requires a proton gradient in order to create ATP. This means that the protons need to be pumped across the inner mitochondrial membrane into the intermembrane space. This results in the intermembrane space having the lowest pH in the mitochondria, due to the high proton concentration.
The mitochondrial matrix is the interior of the inner mitochondrial membrane, while the cytosol is not a part of the mitochondria. Neither of these have particularly low pH values. Christae are the folds of the inner mitochondrial membrane that increase its surface area for the electron transport chain processes; though structurally useful in facilitating respiration, the pH of christae is roughly the same as that of the mitochondrial matrix.
Given a healthy individual with a normal metabolic rate, which of the following compounds is the most energy rich?
NADH
ATP
GTP
FADH2
Explanation
This question is asking about ATP production during cellular respiration. During oxidative phosphorylation (the electron transport chain), each 1 ATP is produced for each GTP, 2 ATP are produced for each FADH2, and 3 ATP are produced for each NADH.
During aerobic respiration, which of the following pathways correctly orders the process of cellular metabolism after glycolysis in eukaryotic cells?
Citric acid cycle 
Pyruvate decarboxylation 
Pyruvate decarboxylation 
Citric acid cycle 
Explanation
After glycolysis is complete, we have generated pyruvate from glucose. We would then expect pyruvate decarboxylation to be the first step after glycolysis in aerobic respiration. When pyruvate is decarboxylated, we generate acetyl CoA, which fuels the Krebs cycle (aka TCA, and citric acid cycle). We would expect the next step after decarboxylation to be the citric acid cycle. In the citric acid cycle we generate FADH2 and NADH, which release free energy in oxidative phosphorylation to generate the proton gradient across the mitochondrial membrane to fuel ATP synthase.
Imagine that a toxin is introduced to the body and inhibits the establishment of the proton gradient in the intermembrane space. What would you predict would be the result?
ATP synthase would be unable to produce ATP
Pyruvate would be unable to enter the Krebs cycle
NADH would be oxidized
Substrate-level phosphorylation would be inhibited
Fermentation could not occur
Explanation
ATP synthase is dependent on a proton gradient in the intermembrane space in order to produce ATP. As a result, the toxin will make it inactive. Oxidative phosphorylation would be inhibited in this case, as opposed to substrate-level phosphorylation.
Pyruvate is a product of glycolysis, and would not be affected by the toxin. NADH is key in the establishment of the proton gradient, so we would expect that it would be unable to be oxidized due to the toxin. Protons produced in the conversion of NADH to NAD+ (+H+) establish the proton gradient. If the gradient is absent, NADH is likely not be oxidized.