GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with the Electron Transport Chain

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Example Question #1 : Help With The Electron Transport Chain

What is the purpose of coenzyme Q10 during the electron transport chain?

Possible Answers:

Carry protons from the mitochondrial matrix into the intermembrane space

Bring oxygen to the end of the electron transport chain to accept electrons

Move electrons from complex I or II to complex III

Regulate the function of ATP synthase

Correct answer:

Move electrons from complex I or II to complex III

Explanation:

Coenzyme Q10 is a fat-soluble molecule that facilitates the transfer of electrons from complex I or II to complex III in the electron transport chain. The mobility of coenzyme Q10 in the membrane allows for this unique function. Each complex in the membrane is then able to use the donated electron to push protons into the intermembrane space, generating the gradient that will eventually be used to synthesize ATP.

Coenzyme Q10 does not directly facilitate the movement of protons. Rather, it aids in the transfer of electrons to initiate the process that allows for proton movement. Coenzyme Q10 is also not involved with the regulation of ATP synthase or with bringing oxygen to the electron transport chain.

Example Question #2 : Help With The Electron Transport Chain

At which complex in the electron transport chain is NADH oxidized?

Possible Answers:

Complex II

Complex I

Complex IV

Complex III

Correct answer:

Complex I

Explanation:

NADH is the first electron carrier to be oxidized by the electron transport chain, a process that occurs at complex I. FADH2 is oxidized further down the chain in complex II, causing it to produce less ATP on average than NADH.

Example Question #1 : Help With The Electron Transport Chain

Which of the following molecules will be most abundant surrounding the electron transport chain?

Possible Answers:

Sphingolipids

Cytosol

Glucose

Phospholipids

Correct answer:

Phospholipids

Explanation:

Electron transport chain (ETC) consists of a series of electron carriers on the inner membrane of the mitochondria. The electrons are transferred down these carriers and this movement is used to generate ATP. The question asks for the molecule most abundant surrounding these electron carriers. Since they are found on the inner membrane, the electron carriers in ETC are surrounded by phospholipids (most abundant molecule in a membrane).

Example Question #4 : Help With The Electron Transport Chain

Which of the following is true regarding the final electron acceptor in the electron transport chain?

Possible Answers:

It picks up electron and protons

It contributes to the production of energy

It only picks up electrons

It is a byproduct of anaerobic respiration

Correct answer:

It picks up electron and protons

Explanation:

The final electron acceptor in electron transport chain is oxygen. The electrons traverse along the electron transport chain and ultimately end up being taken up by an oxygen atom. Upon accepting electrons, the oxygen atom becomes negatively charged and attracts protons (hydrogen atoms). An oxygen atom binds to two hydrogen atoms and forms water.

Energy production in ETC is facilitated by the generation of the proton gradient by the proton pump. This gradient is utilized by ATP synthase to produce ATP. Oxygen does not play a role in this process. Anaerobic respiration is a type of respiration that occurs in the absence of oxygen. Examples of anaerobic respiration include glycolysis and fermentation.

Example Question #2 : Help With The Electron Transport Chain

Anaerobic respiration occurs in the __________ and aerobic respiration occurs in the __________.

Possible Answers:

mitochondria . . . mitochondria

mitochondria . . . cytosol

cytosol . . . cytosol

cytosol . . . mitochondria

Correct answer:

cytosol . . . mitochondria

Explanation:

Cellular respiration has three main processes: glycolysis, Krebs cycle, and electron transport chain. Glycolysis is an anaerobic process whereas Krebs cycle and ETC are aerobic processes. A molecule of glucose enters the cell and undergoes glycolysis in the cytosol. Some of the products of glycolysis are transported to mitochondria where they undergo Krebs cycle and then, eventually, the electron transport chain; therefore, anaerobic respiration occurs in the cytosol and aerobic respiration occurs in the mitochondria.

Anaerobic respiration produces very little ATP and the ATP produced is not sufficient to power all of the active processes in a cell. Aerobic respiration, particularly ETC, produces a lot of ATP. This is why mitochondria is called the “powerhouse” of the cell. Note that the products of glycolysis can undergo another type of anaerobic respiration called fermentation. This also produces very little ATP.

Example Question #3 : Help With The Electron Transport Chain

Which of the following is true regarding electron transport chain (ETC)?

I. Proton pump generates ATP

II. Electron affinity of  and  is lower than the carriers in ETC

III. Electrochemical gradient of sodium facilitates production of ATP

Possible Answers:

I only

II only

I and II

II and III

Correct answer:

II only

Explanation:

Electron transport chain is a series of electron carriers located on the inner membrane of the mitochondria. Electrons traverse across these electron carriers and this motion allows for the proton pump to generate a proton gradient across the inner membrane. This gradient is generated by pumping protons from the inside of the mitochondria to the periplasmic space (space between inner and outer mitochondrial membranes). The excess protons in the periplasmic space are transported back into the mitochondria and this movement facilitates the generation of ATP by the ATP synthase. Note that ATP is generated by ATP synthase, not the proton pump.

 and  are electron carriers that carry electrons from glycolysis and Krebs cycle. These carriers enter the ETC and donate their electrons to the carriers in ETC. This occurs because ETC carriers have higher affinity for electrons. Remember that each subsequent carrier in ETC has a higher affinity for electrons, so that it is able to snatch the electron from the previous carrier. As mentioned, the ATP generation is facilitated by the proton gradient, not the sodium gradient.

Example Question #7 : Help With The Electron Transport Chain

What directly drives ATP synthase to generate ATP from ADP and inorganic phosphate? 

Possible Answers:

Phosphorylation 

Proton gradient

Electron transfer

Reduction of  to 

Ubiquitination

Correct answer:

Proton gradient

Explanation:

The correct answer is proton gradient.  and  donate electrons to electron transport chain complexes and pass them along the membrane, causing protons  to move from the mitochondrial matrix to the intermembrane space. The high proton concentration gradient causes protons to travel through the inner mitochondrial transmembrane ATP synthase to equilibrate the gradient. Passing of protons through ATP synthase promotes synthesis of ATP from ADP from inorganic phosphate. 

Example Question #4 : Help With The Electron Transport Chain

Cytochrome C is a small protein localized to the inner mitochondrial membrane that has an important role in the last steps of the electron transport chain. What is cytochrome C's primary role in the electron transport chain? 

Possible Answers:

Cytochrome C accepts electrons from complex II and shuttles them to Complex III, which is required for further activation of complex IV

Cytochrome C accepts electrons from complex III in the mitochondrial membrane and transfers them to complex IV

Cytochrome C removes electrons from complex IV and transfers them to molecular oxygen in the mitochondria

Cytochrome C catalyzes the conversion of 2 ADP to 2 ATP, thus contributing to the energetic gain in the electron transport chain

Cytochrome C accepts electrons from complex IV and directly transfers these electrons to molecular oxygen

Correct answer:

Cytochrome C accepts electrons from complex III in the mitochondrial membrane and transfers them to complex IV

Explanation:

Cytochrome C is responsible for accepting the electrons generated in the bc1 complex (complex III) and transferring them over to complex IV. In other words, cytochrome C oxidizes complex III and is oxidized by complex IV. This is required to then add those electrons to molecular oxygen, which forms water and contributes to the proton gradient required for ATP production. 

Example Question #5 : Help With The Electron Transport Chain

What is the final electron acceptor in the electron transport chain?

Possible Answers:

Cytochrome C

Correct answer:

Explanation:

The final electron acceptor in the electron transport chain is oxygen gas. one oxygen gas molecule will accept 4 electrons and combine with 4 protons in order to create 2 water molecules. This reaction is the reason water is a byproduct of aerobic cellular respiration.

Example Question #10 : Help With The Electron Transport Chain

Krebs cycle occurs in the __________ and the electron transport chain occurs in the __________.

Possible Answers:

mitochondrial matrix . . . mitochondrial inner membrane

mitochondrial inner membrane . . . mitochondrial matrix

mitochondrial matrix . . . cytoplasm

cytoplasm . . . mitochondrial matrix

Correct answer:

mitochondrial matrix . . . mitochondrial inner membrane

Explanation:

Glycolysis, the first step in metabolism of carbohydrates, occurs in the cytoplasm. The products of glycolysis (pyruvate and NADH) are transported to the mitochondrial matrix. The products undergo series of reactions called the Krebs cycle. The products of Krebs cycle and NADH from glycolysis enter the inner mitochondrial membrane and go through the electron transport chain (ETC). During ETC, oxidative phosphorylation generates most of the ATP used by the cells.

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