Biochemistry : Glycolysis Energetics

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Glycolysis Energetics

For each mol of glucose oxidized via cellular respiration, how many total moles of ATP are generated through substrate-level phorphorylation?

Possible Answers:

Correct answer:

Explanation:

Cellular respiration is a long process, and so it is easiest to break it into the following steps:

Step 1: Glycolysis

Step 2: Pyruvate decarboxylation

Step 3: Krebs cycle

Step 4: Oxidative phosphorylation

In the above steps, ATP is only produced by substrate-level phosphorylation in glycolysis and during the Krebs cycle.

In glycolysis, two molecules of pyruvate are produced for every molecule of glucose oxidized. During this process, two ATP molecules are consumed, but four are produced via substrate-level phosphorylation.

In the Krebs cycle, each pass of pyruvate through the cycle generates one molecule of GTP, which is subsequently used to generate a molecule of ATP via substrate-level phosphorylation. Thus, one molecule of ATP is produced via substrate-level phosphorylation per molecule of pyruvate oxidized. But remember that glycolysis produces two molecules of pyruvate for each molecule of glucose oxidized. Hence, the Krebs cycle will contribute a total of two molecules of ATP per glucose molecule oxidized.

Since we have a total of four moles ATP from glycolysis and two moles of ATP from the Krebs cycle (one per pyruvate), we have a cumulative production of six moles of ATP generated by substrate-level phosphorylation per mole of glucose oxidized.

Example Question #2 : Glycolysis Energetics

Energy is __________ during glycolysis.

Possible Answers:

only consumed

only produced

neither produced nor consumed

produced and consumed

Correct answer:

produced and consumed

Explanation:

The first and third steps of glycolysis involve energy consumption in the form of ATP. A phosphate group is added to glucose, and fructose-6-phosphate. In the seventh and tenth steps of glycolysis, ADP is phosphorylated at the level of the substrate into ATP. Since this is after glucose had been split into two three-carbon molecules, each molecule of glucose results in four ATP produced. However, since two were consumed early in glycolysis, the net ATP production is 2.

Example Question #3 : Glycolysis Energetics

Which steps in glycolysis convert ATP to ADP?

Possible Answers:

The second and third step

The second and fourth step

The third and fourth step

The first and third step

The first and second step

Correct answer:

The first and third step

Explanation:

The first step of glycolysis is the addition of a phosphate group to glucose to form glucose-6-phosphate. The third step of glycolysis is the addition of another phosphate group to fructose-6-phosphate to form fructose-1,6-bisphosphate. The conversion of ATP to ADP is needed to supply the phosphate group in both of these reactions. These are the only two reactions in glycolysis where ATP is used to to add phosphate groups.

Example Question #2 : Glycolysis Energetics

What is the net ATP yield of glycolysis?

Possible Answers:

1 ATP

2 ATP

4 ATP

3 ATP

Correct answer:

2 ATP

Explanation:

Glycolysis produces 4 ATP molecules. However, 2 ATP molecules are required to initiate glycolysis. Subtracting these two numbers gives the net ATP yield from glycolysis--2 ATP molecules.

Example Question #3 : Glycolysis Energetics

Why might glycolysis not proceed for an organism even when it is given glucose, , , and water?

Possible Answers:

Glycolysis requires that there be no water in the nearby environment to begin

ADP will negatively feedback on glycolysis and stop it from proceeding

Glycolysis can not proceed without NADH present

Glycolysis requires an initial input of 2 ATP to begin

 will negatively feedback on glycolysis and stop it from proceeding

Correct answer:

Glycolysis requires an initial input of 2 ATP to begin

Explanation:

Although glycolysis will ultimately produce 4 ATP, there is an initial requirement of 2 ATP for it to begin. The conversion of glucose to glucose-6-phosphate and the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate both require ATP.  

Example Question #4 : Glycolysis Energetics

During the energy investment phase of glycolysis, how many ATP are required to continue with the reactions per glucose molecule?

Possible Answers:

One

Four

Three

Zero

Two

Correct answer:

Two

Explanation:

The first and third steps of glycolysis are both energetically unfavorable. This means they will require an input of energy in order to continue forward. Per glucose molecule, 1 ATP is required for each of these steps. Therefore, a total of 2 ATP is needed during the energy investment phase of glycolysis.  

Example Question #1 : Glycolysis Energetics

While glycolysis results in the production of 4 ATP molecules, 2 must be used in the process. This results in a net production of only 2 ATP molecules per glucose.

What is the purpose of the 2 ATP molecules used in glycolysis?

Possible Answers:

To provide energy for glycolytic enzymes to combine intermediates.

To phosphorylate intermediates in the process.

To phosphorylate the final products of glycolysis.

To assist in getting glucose into the cell.

Correct answer:

To phosphorylate intermediates in the process.

Explanation:

In the glycolytic pathway, 2 molecules of ATP must be used. The purpose of these molecules is to phosphorylate 2 intermediates in the pathway:

1. Glucose must be phosphorylated to glucose-6-phosphate.

2. Fructose-6-phosphate must be phosphorylated to fructose-1,6-bisphosphate.

Example Question #6 : Glycolysis Energetics

Which of the following is characteristic of hexokinase (as opposed to glucokinase)?

Possible Answers:

It is inducible by insulin

It is not inhibited by glucose 6 phosphate

It is only specific to glucose

It is not inducible by insulin

It is found only in the liver and pancreatic beta cells

Correct answer:

It is not inducible by insulin

Explanation:

Hexokinase and glucokinase are two enzymes that serve similar roles but have different characteristics. Hexokinase is found in all tissues, is inhibited by glucose 6 phosphate, and is not induced by insulin. It has a physiologic role of providing cells with a basal level of glucose 6 phosphate necessary for energy production.

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