Question 1

In the following schematic drawing of the inner and outer mitochondrial membranes, in what region (A to D) would you expect to find more ATP synthase dimers?

Accepted Answer

The ATP synthase dimers tend to localize at the tip of the cristae where the membrane curvature is greatest.

Question 2

Consider the remarkable structure of the F1Fo ATP synthase shown in the following schematic drawing. Answer the following question(s) based on the structure.
 
-In Escherichia coli, the c ring of ATP synthase is composed of 12 c subunits. For one fully productive revolution of this ring, how many protons are transported across the membrane for each molecule of ATP produced? Write down your answer as a number, rounded to the nearest integer, e.g. 17.

Accepted Answer

A full revolution of the ring would transport 12 protons and would generate 3 molecules of ATP from ADP and P?. This is equivalent to 4 protons per ATP molecule.

Question 3

In actively respiring mitochondria, where in the following schematic drawing of the inner and outer mitochondrial membranes would you expect to find the lowest local pH?

Accepted Answer

The ATP synthase dimers create proton "sinks" or "traps" at the cristae tips, which helps them make efficient use of the protons that are pumped out of the matrix.

Question 4

Consider the remarkable structure of the F1Fo ATP synthase shown in the following schematic drawing. Answer the following question(s) based on the structure.
 
-In the schematic drawing of the structure of ATP synthase shown above, indicate whether each of the numbered components of the machine (from 1 to 5, respectively) is rotary (R) or stationary (S). Your answer would be a five-letter string composed of letters R and S only, e.g. RRSRS.

Accepted Answer

The rotary part of the motor is composed of the membrane-embedded ring and its attached stalk.

Question 5

Consider the remarkable structure of the F1Fo ATP synthase shown in the following schematic drawing. Answer the following question(s) based on the structure.
 
-As seen from the F? head in the matrix, the rotation of the c ring is counterclockwise in the ATP-synthesis mode of the motor and clockwise in the ATP-hydrolysis mode of the motor. As shown in the above drawing, the "a subunit" (labeled 2) is part of the stator embedded in the membrane, and it forms the entry and exit sites for the protons that are bound by the rotating c ring. How do you think the two half-channels of the a subunit are arranged as seen from the c ring? Write down A or B from the following schematic diagram as your answer.

Accepted Answer

In the ATP-synthesis mode, protons move from the cristae space into the matrix (downward in the diagram), which rotates the c ring counterclockwise as seen from the F₁ head on the matrix side of the crista membrane. Thus, in part B of the diagram shown, protons will enter the right half-channel, bind to a c subunit, rotate along with the c ring, and only be able to exit from the left half-channel after a complete turn of the ring. If the channels were arranged as in part A, the protons could pass through the membrane after only a tiny rotation of the ring, so that many more protons would need to pass through the ATP synthase for every ATP molecule that is synthesized by the F? head.

Question 6

The synthetic toxin 2,4-dinitrophenol can uncouple ATP synthesis from mitochondrial respiration by decreasing the permeability of the inner membrane to protons. What would be the effect of dinitrophenol treatment on the amount of ATP produced by mitochondria and on the rate of ATP transport across the inner membrane, respectively?

Accepted Answer

By diminishing the electrochemical gradient across the inner membrane, the toxin interferes with ATP synthesis and ATP/ADP exchange, both of which are driven by the gradient.

Question 7

For mitochondria that are active in respiration, indicate whether the movement of each of the following molecules into (I) or out of (O) the matrix is thermodynamically favorable. Your answer would be a four-letter string composed of letters I and O only, e.g. IOOI. ( ) O² ( ) H⁺ ( ) ADP ( ) CO₂

Accepted Answer

In mitochondria that are active in respiration, O² and ADP are constantly consumed to produce H₂O and ATP, respectively. This creates concentration gradients that favor the movement of these molecules into the organelle. The opposite is true for CO² which is generated inside the matrix in decarboxylation reactions of the citric acid cycle. Protons are actively pumped out of the matrix by the proteins of the electron-transport chain, and power ATP production when they move into the matrix down their electrochemical gradient.

Question 8

In addition to their respiratory function, mitochondria have other important roles in cellular metabolism. Which of the following processes is NOT carried out mainly by mitochondria?

Accepted Answer

Fatty acid biosynthesis is confined to the cytosol, where the citrate exported from mitochondria is used to produce acetyl CoA in the initial step.

Question 9

Consider the remarkable structure of the F1Fo ATP synthase shown in the following schematic drawing. Answer the following question(s) based on the structure.
 
-With about 7000 c-ring revolutions per minute, on average how many moles of these pumps should be working at each moment in an entire human body to make about 50 kilograms of ATP per day? The molecular weight of ATP is about 500.

Accepted Answer

The number of pumps is calculated as:
(50,000 g/day ÷ 500 g/mole ATP) ÷ (7000 revolution/min × 60 min/h × 24 h/day ÷ 3 ATP/revolution/pump)
= (100 mole.ATP/day) ÷ (3,360,000 ATP/pump/day)
= ~3 × 10-⁵ mole.pump

Question 10

The free-energy change for ATP hydrolysis to ADP inside the cell is about -50 kJ/mole. The complete cellular oxidation of glucose to carbon dioxide and water has a free-energy change of about -3000 kJ/mole. What is the efficiency of the cellular metabolism in harvesting this amount of free energy in the form of ATP synthesis?

Accepted Answer

With 100% efficiency, about 60 moles of ATP would be produced by oxidizing one mole of glucose. Considering that only 30 moles of ATP are produced in the cell, the yield is about 50%.

Question 11

Indicate true (T) and false (F) statements below regarding mitochondria. Your answer would be a four-letter string composed of letters T and F only, e.g. TFTT.
( )	Mitochondria are small round organelles that are often associated with actin filaments of the cytoskeleton.
( )	In sperm cells, mitochondria are seen wrapping tightly around the nucleus.
( )	Mitochondria are large enough to be seen with modern light microscopy, and can occupy as much as 20% of cytoplasmic volume.
( )	The outer mitochondrial membrane is freely permeable to ions and small molecules.

Accepted Answer

Mitochondria form dynamic networks that are often associated with microtubules. They are found in specialized cells to localize to sites of greatest ATP consumption, such as the tail in sperm cells.

Question 12

Indicate whether each of the following descriptions better matches the outer mitochondrial membrane (O), the intermembrane space (S), the crista membrane portion of the inner mitochondrial membrane (I), or the matrix (M). Your answer would be a four-letter string composed of letters O, S, I, and M only, e.g. OMIM.
( )	It is where NADH is produced.
( )	It is composed of 75% protein by weight.
( )	It is where the respiratory chain is located.
( )	It has the same electrochemical potential for H? as the cytoplasm.

Accepted Answer

The electrochemical gradient of protons is established across the inner membrane, with higher potential on the outer side. However, there is no such gradient across the outer membrane, which is freely permeable to protons. The crista membrane houses the respiratory-chain proteins and other proteins at a very high density. The citric acid cycle proteins that generate NADH, on the other hand, are located in the matrix, which is approximately 50% protein by weight.

Question 13

The free-energy change of ATP hydrolysis to ADP inside the cell is about -50 kJ/mole. The standard free-energy change for the same reaction is about -30 kJ/mole. This necessitates that [ATP] inside the cell be …

Accepted Answer

The equilibrium constant for the hydrolysis reaction (not considering water) is:
Kₑq = [ADP]ₑq × [Pᵢ]ₑq / [ATP]ₑq
ΔG will have a greater negative value than ΔG° if and only if concentrations deviate from Kₑq such that [ADP] × [Pᵢ] / [ATP] < Kₑq.

Question 14

A facultative anaerobic bacterium can survive both aerobic and anaerobic conditions. Under both conditions, the cell maintains a proton-motive force across the plasma membrane that drives many cellular processes including nutrient import. Under aerobic conditions, the proton-motive force is produced by a respiratory chain. What is the mode of action of the plasma membrane ATP synthase in this bacterium under aerobic conditions: ATP-hydrolysis mode (H) or ATP-synthesis mode (S)? Write down H or S as your answer.

Accepted Answer

Under aerobic conditions, the proton gradient established by the respiratory chain can drive ATP synthesis, whereas under anaerobic conditions, the ATP synthase pumps protons out (by ATP hydrolysis) to maintain the gradient.

Question 15

Indicate whether each of the following descriptions better applies to cytochrome c oxidase complex (O), cytochrome c reductase complex (R), or NADH dehydrogenase complex (N). Your answer would be a four-letter string composed of letters O, R, and N only, e.g. RONN.
( )	It is the largest of the three respiratory complexes.
( )	It accounts for the majority of O? uptake in most cells.
( )	It employs the Q cycle to increase proton pumping.
( )	It contains separate modules for electron transport and proton pumping.

Accepted Answer

NADH dehydrogenase complex is the largest respiratory enzyme complex embedded in the inner mitochondrial membrane and contains separate modules for electron transport (from NADH to ubiquinone) and proton pumping (across the membrane out of the matrix). The cytochrome c reductase complex reduces cytochrome c using electrons delivered by ubiquinol by the Q cycle mechanism. Finally, the cytochrome c oxidase complex accepts electrons from cytochrome c and reduces O?.

Question 16

ATP synthase molecules in mitochondria form dimers that are localized mostly to sharp cristae ridges. What should happen if subunits of the synthase that are required for dimerization are mutated in yeast?

Accepted Answer

Dimerization of ATP synthase has a key role in controlling the curvature of the inner membrane.

Question 17

A hypothetical electron-transport chain contains an electron donor (1), an electron carrier (2), and an electron acceptor (3). Which of these molecules has the highest redox potential? Write down 1, 2, or 3 as your answer.

Accepted Answer

Higher redox potential corresponds to an increased affinity for electrons. Consecutive electron carriers along the electron-transport chain have increasingly higher redox potentials.

Question 18

You have isolated mitochondria from a liver tissue sample, suspended them in a hypotonic buffer that causes them to swell and burst their outer membrane, and then added sucrose to a final concentration of about 25%. You then layer the suspension on a density gradient, ultracentrifuge, collect the different fractions, and analyze their protein content. Which of the following proteins would you expect to be highly enriched in the lower density and higher density fractions, respectively?

Accepted Answer

The low-density fractions would mostly contain outer-membrane components such as porins, while the high-density fractions correspond to the matrix and inner membrane.

Question 19

Which of the following molecules can serve as the terminal electron acceptor in bacterial electron-transport chains?

Accepted Answer

Various compounds are used in different bacteria as terminal electron acceptors.

Question 20

Sort the following molecules to reflect the order in which they carry electrons in the respiratory chain. Your answer would be a seven-letter string composed of letters A to G only, e.g. GFADCBE.
(A)	NADH dehydrogenase complex
(B)	Cytochrome c oxidase complex
(C)	Cytochrome c reductase complex
(D)	Cytochrome c
(E)	Ubiquinone
(F)	NADH
(G)	H?O

Accepted Answer

This chain transports electrons from NADH to oxygen.

Quiz 14: Energy Conversion: Mitochondria and Chloroplasts