GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Cellular Respiration and Photosynthesis

Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology

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Example Questions

Example Question #1 : Help With The Krebs Cycle

Pyruvate from glycolysis must be converted to what before starting the Krebs cycle?

Possible Answers:

Acetyl-CoA

Isocitrate

Glucose

Acetate

Carbon dioxide

Correct answer:

Acetyl-CoA

Explanation:

The end product of glycolysis is two molecules of pyruvate, however the molecule used to starts the Krebs cycle is acetyl-CoA. Before going into the Krebs cycle then these 2 pyruvate molecules need to be converted into 2 acetyl-CoA molecules via pyruvate dehydrogenase complex, which occurs in the mitochondria and releases carbon dioxide.

Example Question #2 : Help With The Krebs Cycle

How many total molecules of NADH are produced from 2 glucose molecules during cellular respiration?

Possible Answers:

16

5

15

20

10

Correct answer:

16

Explanation:

During glycolysis, 2 molecules of NADH are produced per glucose. During the Krebs cycle, 3 molecules of NADH are produced per acetyl-CoA. With 2 molecules of glucose, glycolysis can run twice and produce 4 molecules of acetyl-CoA. Since 2 pyruvates are produced from glucose during glycolysis, a total of 4 are made from our 2 glucose molecules. These 4 pyruvates are then converted to 4 acetyl-CoA molecules. Each of these acteyl-CoA molecules runs through the Krebs cycle yielding a total 12 molecules of NADH. 4 from glycolysis, 12 from the TCA cycle so 16 molecules of NADH total.

Example Question #3 : Help With The Krebs Cycle

For a given molecule of glucose, Krebs cycle produces __________ the amount of NADH and __________ amount of ATP as glycolysis.

Possible Answers:

twice . . . three times the

twice . . . the same

three times . . . twice the

three times . . . the same

Correct answer:

three times . . . the same

Explanation:

Each turn of Krebs cycle produces 3 NADH and 1 ATP. Recall that each turn of Krebs cycle requires a molecule of acetyl-CoA (two-carbon molecule). Acetyl-CoA comes from pyruvate, which in turn comes from glucose. During glycolysis, a molecule of glucose (six-carbon molecule) is converted into two pyruvate molecules; therefore, a molecule of glucose will eventually lead to two acetyl-CoA molecules. This means that there are two turns of Krebs cycle for every glucose molecule and, therefore, for a given molecule of glucose Krebs cycle produces 6 NADH and 2 ATP.

Glycolysis produces a net of 2 ATP and 2 NADH; therefore, Krebs cycle produces three times the NADH and the same amount of ATP as glycolysis.

Example Question #21 : Cellular Respiration And Photosynthesis

A researcher adds an enzyme inhibitor that drastically slows down the progression of the Krebs cycle. What additional things will the researcher observe?

I. There will be a buildup of alpha-ketoglutarate

II. The molecule inhibits isocitrate dehydrogenase

III. The molecules in the Krebs cycle will be stuck in a five-carbon intermediate

Possible Answers:

II and III

III only

II only

I and III

Correct answer:

II only

Explanation:

The question states that the enzyme inhibitor slows down the Krebs cycle. This suggests that the inhibitor blocks the rate-determining step of the cycle. Recall that the rate-determining step of Krebs cycle is the isocitrate dehydrogenase step. This converts the six-carbon isocitrate to five-carbon alpha-ketoglutarate. Blocking this step leads to the build up of six-carbon isocitrate and decrease in all of the downstream molecules (including alpha-ketoglutarate).

Example Question #4 : Help With The Krebs Cycle

Which of the following is true regarding the initial reaction of the Krebs cycle?

Possible Answers:

Two-carbon acetyl-CoA and four-carbon oxaloacetate combine to form a six-carbon malate molecule

Two-carbon oxaloacetate and four-carbon acetyl-CoA to form a six-carbon malate molecule

Two-carbon oxaloacetate and four-carbon acetyl-CoA combine to form a six-carbon citrate molecule

Two-carbon acetyl-CoA and four-carbon oxaloacetate combine to form a six-carbon citrate molecule

Correct answer:

Two-carbon acetyl-CoA and four-carbon oxaloacetate combine to form a six-carbon citrate molecule

Explanation:

The first step of Krebs cycle is the formation of a six-carbon molecule from a two-carbon and a four-carbon molecule. The two-carbon molecule acetyl-CoA combines with the four-carbon molecule oxaloacetate to form a six-carbon molecule, citrate. The citrate molecule undergoes a series of reactions in the Krebs cycle that eventually leads to a five-carbon intermediate and, finally, regeneration of the four-carbon oxaloacetate (to be used for the next cycle). The two-carbon molecule, acetyl-CoA, comes from the pyruvate molecule from glycolysis (recall that pyruvate comes from glucose).

Example Question #24 : Cellular Metabolism

Which molecule is regenerated by the Krebs cycle in order to accept the next acetyl-CoA?

Possible Answers:

Alpha-ketoglutarate

Citrate

Succinate

Oxaloacetate

Correct answer:

Oxaloacetate

Explanation:

The Krebs cycle starts when oxaloacetate combines with acetyl-CoA in order to create citrate. The process is able to work in a cyclic fashion due to the cycle's ability to remake oxaloacetate at the end, so that it can combine with another acetyl-CoA and start the process again.

Example Question #166 : Gre Subject Test: Biochemistry, Cell, And Molecular Biology

Which of the following molecules stimulates the enzyme isocitrate dehydrogenase in the Krebs cycle?

Possible Answers:

NADH

ATP

NAD+

FADH2

Correct answer:

NAD+

Explanation:

The Krebs cycle is useful in not only making ATP molecules, but also for creating high-energy electron carriers, such as NADH and FADH2. As a result, the enzyme isocitrate dehydrogenase will be stimulated when these high energy molecules are depleted in the cell. NAD+, or the oxidized form of NADH, stimulates isocitrate dehydrogenase to work more efficiently. All the other options are high-energy molecules, which would slow down the cycle, as enough energy has already been produced.

Example Question #1 : Biochemistry

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

Possible Answers:

Carry protons from the mitochondrial matrix into the intermembrane space

Regulate the function of ATP synthase

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

Move electrons from complex I or II to complex III

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 #91 : Biochemistry

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

Possible Answers:

Complex I

Complex III

Complex II

Complex IV

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:

Cytosol

Glucose

Sphingolipids

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).

All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

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