GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with the Krebs Cycle

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

varsity tutors app store varsity tutors android store

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

1 Diagnostic Test 201 Practice Tests Question of the Day Flashcards Learn by Concept

Example Questions

Example Question #1 : Help With The Krebs Cycle

Which of the following is not created in the Kreb's cycle? 

Possible Answers:

Acetyl CoA

FADH2

NADH

ATP

Correct answer:

Acetyl CoA

Explanation:

The Krebs cycle is responsible for creating three molecules of NADH, and one molecule of both ATP and NADH. Acetyl CoA is the 2-carbon molecule that enters the Krebs cycle following pyruvate decarboxylation.

Example Question #2 : Help With The Krebs Cycle

Each of the following enzymes is utilized in the citric acid cycle, except __________.

Possible Answers:

Fumarase

Aconitase 

Citrate synthase

Phosphoglycerate kinase

Malate dehydrogenase

Correct answer:

Phosphoglycerate kinase

Explanation:

Each of the enzymes is involved in the citric acid cycle except for phosphoglycerate kinase, which is an enzyme that is utilized in glycolysis and gluconeogenesis, and catalyzes the reversible transfer or a phosphate group from 1,3-bisphosphoglycerate to ADP, producing and/or consuming 3-phosphoglycerate and ATP.

Example Question #3 : Help With The Krebs Cycle

In what organ does gluconeogenesis primarily occur in vertebrates? 

Possible Answers:

Lungs

Skin

Brain

Heart

Liver

Correct answer:

Liver

Explanation:

Gluconeogenesis is the process in which glucose can be generated from carbon structures that are not the canonical carbohydrate inputs. This is required for many organisms to maintain appropriate blood glucose levels, and for vertebrates gluconeogenesis primarily occurs in the liver, although it has been found to occur in the kidneys as well. 

Example Question #4 : Help With The Krebs Cycle

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

Possible Answers:

Acetyl-CoA

Carbon dioxide

Glucose

Isocitrate

Acetate

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 #5 : Help With The Krebs Cycle

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

Possible Answers:

5

10

15

20

16

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 #6 : 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 . . . the same

twice . . . three times the

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 #7 : Help With The Krebs Cycle

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:

III only

II only

II and III

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 #8 : Help With The Krebs Cycle

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

Possible Answers:

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

Two-carbon acetyl-CoA and four-carbon oxaloacetate combine 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 #9 : Help With The Krebs Cycle

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

Possible Answers:

Succinate

Alpha-ketoglutarate

Oxaloacetate

Citrate

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 #10 : Help With The Krebs Cycle

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

Possible Answers:

NAD+

NADH

ATP

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.

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

1 Diagnostic Test 201 Practice Tests Question of the Day Flashcards Learn by Concept
Learning Tools by Varsity Tutors

Incompatible Browser

Please upgrade or download one of the following browsers to use Instant Tutoring: