GRE Subject Test: Biology : Understanding Fermentation

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding Fermentation

Which of the following products is not formed during fermentation in humans?

Possible Answers:

All of these are products of fermentation

NAD+

Pyruvate

Lactic acid

Correct answer:

Pyruvate

Explanation:

When oxygen is not readily available in the blood, cells use fermentation as a means of anaerobic respiration. The process is used to generate NAD+, which can be used as a reactant in glycolysis to produce small amounts of ATP. Glycolysis still occurs in this environment, breaking glucose into pyruvate and producing two ATP per cycle.

Fermentation in humans converts pyruvate to lactate (lactic acid) and NADH to NAD+.

Example Question #1 : Understanding Fermentation

What is the primary purpose of fermentation in cells?

Possible Answers:

To restore glucose

To create ATP in the absence of oxygen

To create ethanol

To restore NAD+

Correct answer:

To restore NAD+

Explanation:

Under anaerobic conditions, fermentation follows the process of glycolysis. While glycolysis is responsible for creating ATP, fermentation allows the body to regenerate the NAD+ that is reduced during glycolytic processes. This key step allows glycolysis to continue, and more ATP to be made.

Example Question #3 : Understanding Fermentation

What is the purpose of lactic acid and ethanol in the fermentation process?

Possible Answers:

They catalyze the formation of water from hydrogen and oxygen

They are electron donors to continue the cycle

They are electron acceptors to continue the cycle 

They are intermediates between the krebs cycle and the electron transport chain

They store energy from glucose

Correct answer:

They are electron acceptors to continue the cycle 

Explanation:

Fermentation is a form of anaerobic respiration where there is no oxygen available as the final electron acceptor to the electron transport chain. As such, pyruvate is reduced, yielding lactic acid and, in the presence of lactate decarboxylase, ethanol. However, the products of fermentation do not undergo the Krebs cycle nor electron transport chain. The purpose of fermentation is to regenerate NAD+ so that glycolysis can continue. The cell does this by transferring the electron from NADH to pyruvate. Fermentation is less efficient per glucose than is aerobic oxidation, generating a fraction of the ATP. Lactic acid and ethanol are actually quite toxic to the cell (in humans they are immediately sent to the liver to be detoxified).

Example Question #2 : Understanding Fermentation

In anaerobic conditions, pyruvate will undergo fermentation. Which of the following molecules are end products of fermentation?

I. Ethanol

II. Butanol

III. Aspartic acid

IV. Lactic acid

Possible Answers:

I only

IV only

I and IV

II and III

I and III

Correct answer:

I and IV

Explanation:

Either ethanol or lactic acid can be produced through fermentation depending on the organism. Ethanol fermentation only occurs in bacteria and yeast. Lactic acid fermentation has been found to occur in multiple kingdoms. 

Fermentation allows for the regeneration of  which can allow for glycolysis to continue. Butanol is a four carbon alcohol and aspartic acid is an amino acid found in proteins. 

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