Chapter 24, Problem 24.47EP

(a)

Interpretation Introduction

Interpretation: To indicate whether ${\text{CO}}_2$ is associated with (1) glycolysis, (2) pyruvate oxidation, (3) lactate fermentation, or (4) ethanol fermentation.

Concept introduction: In the glycolysis metabolic pathway, a glucose molecule breaks down into two pyruvate molecules. Two ATP molecules and NADH reduced coenzymes are produced in the glycolysis pathway.

Pyruvate is the end product in the glycolysis. The production of the fate of pyruvate varies with the nature of the organism and the cellular conditions. The three common fates of pyruvate are as follows:

Answer to Problem 24.47EP

Carbon dioxide $\left({\text{CO}}_2\right)$ is produced in the $\text{acetyl CoA}$ formation and ethanol fermentation. Therefore, ${\text{CO}}_2$ is associated with (2) pyruvate oxidation and (4) ethanol fermentation.

Explanation of Solution

Reason for correct option:

Under aerobic conditions, pyruvate is converted to $\text{acetyl CoA}$ by the involvement of pyruvate dehydrogenase complex enzymes in the human body. This conversion involves oxidation as well as decarboxylation reaction. The chemical reaction for the formation of $\text{acetyl CoA}$ is as follows:

The overall reaction equation for the conversion of pyruvate to $\text{acetyl CoA}$ is as follows:

$\text{Pyruvate}+{\text{NAD}}^{+}+\text{CoA}-\text{SH}\to \text{Acetyl}-\text{CoA}+\text{NADH}+{\text{CO}}_2$

The process of ethanol fermentation takes place in two steps. In step 1, the pyruvate molecule is converted to acetaldehyde by pyruvate decarboxylase enzymes. Carbon dioxide molecule is produced in this step. The chemical reaction is as follows:

In step 2, acetaldehyde is reduced to ethanol by alcohol dehydrogenase enzymes. The chemical reaction is as follows:

The overall ethanol fermentation equation is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{2H}}^{+}\stackrel{\text{Two steps}}{\to }\text{Ethanol}+{\text{CO}}_2+{\text{NAD}}^{+}$

Therefore, ${\text{CO}}_2$ is produced the pyruvate oxidation and ethanol fermentation.

Reason for incorrect option:

The net overall equation for the glycolysis process is as follows:

$\text{Glucose}+2\text{ADP}+2{\text{P}}_i+2{\text{NAD}}^{+}\to 2\text{pyruvate}+2\text{NADH}+2\text{ATP}+2{\text{H}}^{+}+2{\text{H}}_2\text{O}$

The overall reaction equation for lactate fermentation is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{H}}^{+}\to \text{lactate}+{\text{NAD}}^{+}$

${\text{CO}}_2$ is neither consumed nor formed in the glycolysis and lactate fermentation. Therefore, ${\text{CO}}_2$ is not associated with glycolysis and lactate fermentation.

(b)

Interpretation Introduction

Interpretation: To indicate whether $\text{acetyl CoA}$ is associated with (1) glycolysis, (2) pyruvate oxidation, (3) lactate fermentation, or (4) ethanol fermentation.

Concept introduction: In the glycolysis metabolic pathway, a glucose molecule breaks down into two pyruvate molecules. Two ATP molecules and NADH reduced coenzymes are produced in the glycolysis pathway.

Pyruvate is the end product in the glycolysis. The production of the fate of pyruvate varies with the nature of the organism and the cellular conditions. The three common fates of pyruvate are as follows:

Answer to Problem 24.47EP

Pyruvate is oxidized to $\text{acetyl CoA}$ under the aerobic conditions in the human body. Therefore, $\text{acetyl CoA}$ is associated with (2) pyruvate oxidation.

Explanation of Solution

Reason for correct option:

Under aerobic conditions, pyruvate is converted to $\text{acetyl CoA}$ by the involvement of pyruvate dehydrogenase complex enzymes in the human body. This conversion involves oxidation as well as decarboxylation reaction. The overall reaction equation for the conversion of pyruvate to $\text{acetyl CoA}$ is as follows:

$\text{Pyruvate}+{\text{NAD}}^{+}+\text{CoA}-\text{SH}\to \text{Acetyl}-\text{CoA}+\text{NADH}+{\text{CO}}_2$

$\text{Acetyl CoA}$ is formed when pyruvate is oxidized. Therefore, $\text{acetyl CoA}$ is associated with (2) pyruvate oxidation.

Reason for incorrect option:

The net overall equation for the glycolysis process is as follows:

$\text{Glucose}+2\text{ADP}+2{\text{P}}_i+2{\text{NAD}}^{+}\to 2\text{pyruvate}+2\text{NADH}+2\text{ATP}+2{\text{H}}^{+}+2{\text{H}}_2\text{O}$

The overall ethanol fermentation equation is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{2H}}^{+}\stackrel{\text{Two steps}}{\to }\text{Ethanol}+{\text{CO}}_2+{\text{NAD}}^{+}$

The overall reaction equation for lactate fermentation is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{H}}^{+}\to \text{lactate}+{\text{NAD}}^{+}$

$\text{Acetyl CoA}$ is neither consumed nor formed in the glycolysis, ethanol fermentation, and lactate fermentation. Therefore, $\text{acetyl CoA}$ is not associated with glycolysis, ethanol fermentation, and lactate fermentation.

(c)

Interpretation Introduction

Interpretation: To indicate whether ATP is associated with (1) glycolysis, (2) pyruvate oxidation, (3) lactate fermentation, or (4) ethanol fermentation.

Concept introduction: In the glycolysis metabolic pathway, a glucose molecule breaks down into two pyruvate molecules. Two ATP molecules and NADH reduced coenzymes are produced in the glycolysis pathway.

Pyruvate is the end product in the glycolysis. The production of the fate of pyruvate varies with the nature of the organism and the cellular conditions. The three common fates of pyruvate are as follows:

Answer to Problem 24.47EP

A net gain of two ATP molecules occurs when one glucose molecule is converted to two pyruvate molecules through the glycolysis pathway. Therefore, ATP is associated with (1) glycolysis.

Explanation of Solution

Reason for correct choice:

The block diagram to represent an overview of glycolysis is as follows:

The net overall equation for the glycolysis process is as follows:

$\text{Glucose}+2\text{ADP}+2{\text{P}}_i+2{\text{NAD}}^{+}\to 2\text{pyruvate}+2\text{NADH}+2\text{ATP}+2{\text{H}}^{+}+2{\text{H}}_2\text{O}$

In step 1 and step 3, one ATP molecule is consumed in each step. In step 7 and step 10, two ATP molecules are produced in each step. Hence, in the glycolysis pathway, two ATP molecules are consumed and four ATP molecules are produced. Therefore, a net gain of two molecules of ATP occurs in the glycolysis pathway.

Reason for incorrect choice:

The overall reaction equation for the oxidation of pyruvate is as follows:

$\text{Pyruvate}+{\text{NAD}}^{+}+\text{CoA}-\text{SH}\to \text{Acetyl}-\text{CoA}+\text{NADH}+{\text{CO}}_2$

The overall ethanol fermentation equation is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{2H}}^{+}\stackrel{\text{Two steps}}{\to }\text{Ethanol}+{\text{CO}}_2+{\text{NAD}}^{+}$

The overall reaction equation for lactate fermentation is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{H}}^{+}\to \text{lactate}+{\text{NAD}}^{+}$

ATP is neither consumed nor formed in the pyruvate oxidation, ethanol fermentation, and lactate fermentation. Therefore, ATP is not associated with pyruvate oxidation, ethanol fermentation, and lactate fermentation.

(d)

Interpretation Introduction

Interpretation: To indicate whether NADH is associated with (1) glycolysis, (2) pyruvate oxidation, (3) lactate fermentation, or (4) ethanol fermentation.

Concept introduction: In the glycolysis metabolic pathway, a glucose molecule breaks down into two pyruvate molecules. Two ATP molecules and NADH reduced coenzymes are produced in the glycolysis pathway.

Pyruvate is the end product in the glycolysis. The production of the fate of pyruvate varies with the nature of the organism and the cellular conditions. The three common fates of pyruvate are as follows:

Answer to Problem 24.47EP

In the glycolysis process and oxidation of pyruvate, NADH is formed as a product while it is encountered as a reactant in lactate fermentation and ethanol fermentation. Therefore, NADH is associated with (1) glycolysis, (2) pyruvate oxidation, (3) lactate fermentation, and (4) ethanol fermentation.

Explanation of Solution

The net overall equation for the glycolysis process is as follows:

$\text{Glucose}+2\text{ADP}+2{\text{P}}_i+2{\text{NAD}}^{+}\to 2\text{pyruvate}+2\text{NADH}+2\text{ATP}+2{\text{H}}^{+}+2{\text{H}}_2\text{O}$

The overall reaction equation for the conversion of pyruvate to $\text{acetyl CoA}$ is as follows:

$\text{Pyruvate}+{\text{NAD}}^{+}+\text{CoA}-\text{SH}\to \text{Acetyl}-\text{CoA}+\text{NADH}+{\text{CO}}_2$

The overall ethanol fermentation equation is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{2H}}^{+}\stackrel{\text{Two steps}}{\to }\text{Ethanol}+{\text{CO}}_2+{\text{NAD}}^{+}$

In the absence of oxygen, pyruvate is converted to lactate by lactate dehydrogenase enzymes in the human body. The overall reaction equation for the conversion of pyruvate to lactate is as follows:

$\text{Pyruvate}+\text{NADH}+{\text{H}}^{+}\to \text{lactate}+{\text{NAD}}^{+}$

NADH is produced in the glycolysis process and pyruvate oxidation while it is consumed in ethanol fermentation and lactate fermentation. Therefore, NADH is associated with all four processes.