Chapter 11, Problem 18STP

Summary Introduction

To identify and summarize:

The steps of the cycle in the given figure

Introduction:

Organisms obtain energy through a process called cellular respiration. This process occurs in two stages; glycolysis and aerobic respiration. Glycolysis is an anaerobic process. Aerobic respiration includes Krebs cycle and electron transport. Krebs cycle is a series of reactions in which pyruvate is broken down into carbon dioxide.

Answer to Problem 18STP

The cycle in the given figure is of Krebs cycle. The steps of the cycle can be summarized as follows:

• AcetylCoA combines with a 4- carbon compound to form a 6- carbon compound known as citric acid.
• Citric acid is then broken down in a series of steps releasing carbon dioxide and generates one ATP, 3 NADH and 1 FADH₂.
• Finally acetyl CoA and citric acid are regenerated and the cycle continues.

Explanation of Solution

The following figure is given:

The end products of glycolysis are two ATP molecules and two pyruvate molecules. Most of the energy from the glucose is still present in pyruvate. In the presence of oxygen, pyruvate is transported into the mitochondrial matrix, where it is finally converted to carbon dioxide. The series of reactions involved in breaking down pyruvate to carbon dioxide is called Krebs cycle or tricarboxylic acid (TCA) cycle. It is also called citric acid cycle.

Just before Krebs cycle starts, pyruvate reacts with coenzyme CoA to form a two carbon intermediate called acetyl CoA. Carbondioxide is released and NAD+ is converted to NADH. AcetylCoA then moves to mitochondrial matrix. The reaction produces two CO₂ molecules and two NADH.

The Krebs cycle starts with acetyl CoA combining with a 4- carbon compound to form a 6- carbon compound known as citric acid. Citric acid then breaks down in a series of steps releasing carbon dioxide and generating one ATP, 3 NADH and 1 FADH₂. FAD and NAD+ are electron carriers.

Finally acetyl CoA and citric acid are regenerated and the cycle continues. Two turns of Krebs cycle occurs for one glucose molecule. The net yield of Krebs cycle is 6CO₂ molecules, 2 ATP, 8NADH and 2FADH₂.