Chapter 24, Problem 99CP

Interpretation Introduction

Interpretation:

The amount of ATP which is generated per gram of glucose in comparison to the amount of ATP generated per gram of arachidic acid during catabolism should be determined along with whether the result supports the fact that lipids are more effective energy-storing molecules in comparison to carbohydrates.

Concept introduction:

Carbohydrates are considered to be less effective than lipids in terms of releasing per gram consumption. But still, food rich in carbohydrate contents are more preferred over fats containing food. Energy produced by food containing high content of fat is almost double the amount of energy produced by food containing high content of carbohydrates.

Answer to Problem 99CP

The amount of which is generated per gram of glucose = 0.18mole ATP.

The amount of ATP generated per gram of arachidic acid during catabolism is equal to 0.43 mole ATP.

The number of ATP produced by complete catabolism of fat (fatty acid) is much higher than that of glucose (carbohydrate). Also, the energy produced by fat (fatty acid) is almost double the amount of energy produced by glucose (carbohydrate) by the complete catabolism. These results support the fact that lipids are more effective energy-storing molecules in comparison to carbohydrates.

Explanation of Solution

Glucose (a carbohydrate) starts with the glycolysis pathway which converts glucose to pyruvate.The initiation of the glycolysis process requires energy in the form of ATP. Total $4$ ATP units produced by the complete glycolysis but during the process $2$ ATP units are consumed. So, net production is of $2$ ATP molecules.

  

Transition reaction: On oxidation, pyruvate converted to acetyl CoA.

  

Citric acid Cycle:

  

$\text{NADH}$ and ${\text{FADH}}_{\text{2}}$ are equal to $2.5\text{ATP}$ and $1.5\text{ATP}$ respectively and $\text{GTP}$ is converted into the same amount of ATP.

From all the three reactions, total $10\text{NADH}$ units, ${\text{2FADH}}_{\text{2}}$ units, $\text{2GTP}$ units,and $2\text{ATP}$ units are produced.

  $\begin{array}{l}10\times \left(2.5\text{ATP}/\text{NADH}\right)=25\text{ATP}\\ 2\times \left(1.5\text{ATP}/{\text{FADH}}_{\text{2}}\right)=3\text{ATP}\\ 2\text{GTP}=2\text{ATP}\\ \text{And}\text{,}\text{2ATP}\end{array}$

Thus, the total $32\text{ATP}$ produced from one molecule of glucose units.

To calculate the amount of $\text{ATP}$ produced by $1\text{g}$ of glucose using the following formula:

$\frac{32\text{ATP}}{\text{mol}\text{glucose}}\times \frac{\text{mol}\text{glucose}}{180\text{g}}=0.18\text{mol}\text{ATP}/\text{gram}\text{of}\text{glucose}$

As, fats (lipids) are metabolized within the body through $\text{β}$ -oxidation by breaking them into smaller fragments. The initial step in the metabolism of fatty acids absorbs energy by cleavage of phosphate bond of $\text{ATP}$ . Then, fatty acid will combine with the Coenzyme 'A' forming fatty acid $\text{CoA}$ . In $\text{β}$ -oxidation process, at every step $1$ acetyl $\text{CoA}$ , $1\text{NADH}$ and $1{\text{FADH}}_{\text{2}}$ is produced.

The first step is the investment of energy when $\text{ATP}$ is converted to $\text{ADP}$ by breaking of $2$ phosphate bonds.

  

The number of acetyl $\text{CoA}$ produced is totally dependent on the number of carbon atoms present in the fatty acid present originally. In this case, 'arachidic acid' with molecular formula ${\text{C}}_{\text{20}}{\text{H}}_{\text{40}}{\text{O}}_{\text{2}}$ have $20$ carbon atoms. Then, $10$ acetyl $\text{CoA}$ will be produced.

Each molecule $\text{NADH}$ and ${\text{FADH}}_{\text{2}}$ are produced per cycle. But the total number of cycles required for one cycle will be one less than the number of acetyl $\text{CoA}$ produced. So, in this case, $9$

$\text{NADH}$ and ${\text{FADH}}_{\text{2}}$ will be produced.

  

Acetyl $\text{CoA}$ formed in this step enters into the citric acid cycle to produce a further molecule of $\text{ATP}$.

Every Acetyl $\text{CoA}$ molecule produced in will results in $10\text{ATP}$ molecules. Similarly, $\text{NADH}$ and ${\text{FADH}}_{\text{2}}$ are equal to $2.5\text{ATP}$ and $1.5\text{ATP}$ respectively.

  $\begin{array}{l}10\text{acetyl}\text{CoA}\times 10\text{ATP}/\text{acetyl}\text{CoA}=100\text{ATP}\\ 9\text{NADH}\times 2.5\text{ATP}/\text{NADH}=22.5\text{ATP}\\ {\text{9FADH}}_2\times 1.5{\text{ATP/FADH}}_2=13.5\text{ATP}\end{array}$

Thus, the total number of ATP generated during the cycle is $136\text{ATP}$ but during the process $2\text{ATP}$ molecules are used. Then, the total amount of ATP generated per cycle will be $\left(136-2\right)=134\text{ATP}$

To calculate the amount of $\text{ATP}$ produced by $1\text{g}$ of arachidic acid using the following formula:

  $\frac{134\text{ATP}}{\text{mol}\text{arachidic}\text{acid}}\times \frac{\text{mol}\text{arachidic acid}}{312.5\text{g}}=0.43\text{mol}\text{ATP}/\text{gram}\text{of}\text{arachidic acid}$

From the calculation, it is clear that the number of ATP produced by complete catabolism of fat (fatty acid) is much higher than that of glucose (carbohydrate). Also, the energy produced by fat (fatty acid) is almost double the amount of energy produced by glucose (carbohydrate) by the complete catabolism. And these results supportthe factthat lipids (fats) store energy more effectively than carbohydrates.