Chapter 24, Problem 24.54EP

Interpretation Introduction

Interpretation: To contrast, the oxidation of glucose to ${\text{CO}}_2$ and ${\text{H}}_2\text{O}$ with the oxidation of glucose to ethanol in terms of production of ATP.

Concept introduction: Adenosine triphosphate (ATP) is a molecule that is defined as the energy currency of life and provides energy to carry out the metabolic processes in the living cells. It is converted either to adenosine monophosphate (AMP) or to adenosine diphosphate (ADP) after the consumption in the metabolic processes.

The net yield of ATP for the complete oxidation of one molecule of glucose is obtained from the assembled ATP production from the glycolysis process, aerobic oxidation of pyruvate to $\text{acetyl CoA}$, the citric acid cycle, and the electron transport chain.

Answer to Problem 24.54EP

When a glucose molecule is converted into ethanol, two ATP molecules are generated. The complete oxidation of glucose molecule produces ${\text{CO}}_2$ and ${\text{H}}_2\text{O}$. In this oxidation reaction, 30 ATP molecules are generated.

Explanation of Solution

In the glycolysis metabolic pathway, a glucose molecule breaks down and is converted into two pyruvate molecules. In this reaction, two ATP molecules are generated. 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 end product in the glycolysis is pyruvate. The process of ethanol fermentation takes place in two steps. In step 1, the pyruvate molecule is converted to acetaldehyde by pyruvate decarboxylase enzymes. In step 2, acetaldehyde is reduced to ethanol by alcohol dehydrogenase enzymes. The ethanol fermentation equation is as follows:

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

Combine the reaction for the conversion of pyruvate to ethanol with the net overall reaction for glycolysis to obtain an overall reaction for the ethanol production as follows:

$\text{Glucose}+2\text{ADP}+2{\text{P}}_i\to 2\text{Ethanol}+2{\text{CO}}_2+2\text{ATP}+2{\text{H}}_2\text{O}$

The table for the production of ATP in the glycolysis process is as follows:

$\begin{array}{ccc}\text{Reaction}& \text{Comments}& \text{Yield of ATP}\\ \text{Glucose}\to \text{Glucose 6}-\text{phosphate}& \text{consumption of 1ATP}& -\text{1}\\ \text{fructose 6}-\text{phosphate}\to \text{fructose 1,6}-\text{biphosphate}& \text{consumption of}\text{1ATP}& -\text{1}\\ \text{2}\left(\text{glyceraldehyde 3}-\text{phosphate}\to \text{1,3}-\text{biphosphoglycerate}\right)& \text{each}\text{produces 1 cytosolic NADH }& \_\\ \text{2}\left(\text{1,3-biphosphoglycerate}\to \text{3}-\text{phosphoglycerate}\right)& \text{production}\text{of 1ATP by each reaction}& +\text{2}\\ \text{2}\left(\text{phosphoenolpyruvate}\to \text{2pyruvate}\right)& \text{production}\text{of 1ATP by each reaction}& +\text{2}\end{array}$

The table for the production of ATP in the aerobic oxidation of pyruvate to $\text{acetyl CoA}$ is as follows:

$\begin{array}{ccc}\text{Reaction}& \text{Comments}& \text{Yield of ATP}\\ \text{2}\left(\text{pyruvate}\to \text{acetyl CoA}+{\text{CO}}_{\text{2}}\right)& \text{production of 1NADH by}\text{each reaction}& \_\end{array}$

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 table for the production of ATP in the citric acid cycle is as follows:

$\begin{array}{ccc}\text{Reaction}& \text{Comments}& \text{Yield of ATP}\\ 2\left(\text{Isocitrate}\to \alpha -\text{ketoglutarate}+{\text{CO}}_2\right)& \text{production}\text{of 1NADH by each reaction}& \_\\ 2\left(\alpha -\text{ketoglutarate}\to \text{succinyl CoA}+{\text{CO}}_2\right)& \text{production}\text{of 1NADH by each reaction}& \_\\ 2\left(\text{succinyl CoA}\to \text{succinate}\right)& \text{production}\text{of 1GTP by each reaction}& +2\\ 2\left(\text{succinate}\to \text{fumarate}\right)& \text{production}{\text{of 1FADH}}_2\text{ by each reaction}& \_\\ 2\left(\text{malate}\to \text{oxaloacetate}\right)& \text{production}\text{of 1NADH by each reaction}& \_\end{array}$

The net overall equation for the Citric acid cycle is as follows:

$\begin{array}{c}\text{Acetyl CoA}+3{\text{NAD}}^{+}+\text{FAD}+\text{GDP}+{\text{HPO}}_4^{2-}\left({\text{P}}_i\right)+2{\text{H}}_2\text{O}\to {\text{2CO}}_2+\text{CoA}-\text{SH}+\\ 3\text{NADH}+\text{GTP}+3{\text{H}}^{+}+{\text{FADH}}_2\end{array}$

The table for the production of ATP in the electron transport chain and oxidative phosphorylation is as follows:

$\begin{array}{ccc}\text{Reaction}& \text{Comments}& \text{Yield of ATP}\\ \text{Formation of 2cytosolic NADH in the glycolysis}& \text{each produces 1}\text{.5 ATP}& +3\\ \text{Formation of 2 NADH in the oxidation of pyruvate}& \text{each produces 2}\text{.5 ATP}& +5\\ {\text{Formation of 2 FADH}}_2\text{ in the citric acid cycle}& \text{each produces 1}\text{.5 ATP}& +3\\ \text{Formation of 6 NADH in the citric acid cycle}& \text{each produces 2}\text{.5 ATP}& +15\end{array}$

NADH, ${\text{NAD}}^{+}$ and ${\text{FADH}}_2$ canceled out, thus, the net overall equation for the complete oxidation of glucose is as follows:

$\text{Glucose}+6{\text{O}}_2+30\text{ADP}+30{\text{P}}_i\to 6{\text{CO}}_2+30\text{ATP}+6{\text{H}}_2\text{O}$

The net yield of ATP is 30. 30 molecules of ATP are produced by the complete oxidation of one molecule of glucose.

Conclusion

An overall reaction for the production of ethanol is obtained by the combination of the conversion reaction of pyruvate to ethanol with the net overall reaction for glycolysis. Therefore, a net gain of two ATP molecules occurs when one glucose molecule is converted to ethanol.

A net gain of two ATP molecules occurs when one glucose molecule is converted to two pyruvate molecules through the glycolysis pathway. In the citric acid cycle, a net gain of two ATP molecules occurs when two $\text{succinyl CoA}$ molecules are converted to two succinate molecules. A net gain of 26 ATP molecules occurs in the electron transport chain and oxidative phosphorylation process. Complete oxidation of glucose molecule produces ${\text{CO}}_2$ and ${\text{H}}_2\text{O}$ along with 30 ATP molecules.