Fad

Succinic acid

Succinyl-CoA

a-Ketoglutaric acid

a-Ketoglutaric acid

Succinic acid

Figure

For each turn of the citric acid cycle (two "turns" or citric acids per glucose), one ATP is produced directly, eight hydrogens with high-energy electrons are released, and two CO2 molecules are produced.

Citric Acid Cycle

The citric acid cycle begins when a 2-carbon acetyl CoA molecule combines with a 4-carbon oxaloacetic acid molecule to form the 6-carbon citric acid (see fig. 4.10). As citric acid is formed, CoA is released and can be used again to form acetyl CoA from pyruvic acid. The citric acid is changed through a series of reactions back into oxaloacetic acid. The cycle repeats as long as oxygen and pyruvic acid are supplied to the mitochondrion.

The citric acid cycle has three important consequences:

1. One ATP is produced for each citric acid molecule that goes through the cycle.

2. For each citric acid molecule, eight hydrogen atoms with high-energy electrons are transferred to the hydrogen carriers NAD+ and the related FAD (flavine adenine dinucleotide):

3. As the 6-carbon citric acid reacts to form the 4-carbon oxaloacetic acid, two carbon dioxide molecules are produced.

The carbon dioxide produced by the formation of acetyl CoA and in the citric acid cycle dissolves in the cytoplasm, diffuses from the cell, and enters the bloodstream. Eventually, the respiratory system excretes the carbon dioxide.

Electron Transport Chain

The hydrogen and high-energy electron carriers (NADH and FADH2) generated by glycolysis and the citric acid cycle now hold most of the energy contained in the original glucose molecule. In order to couple this energy to ATP synthesis, the high-energy electrons are handed off

ATP Synthase

Energy

Electron transport chain

Energy

Electron transport chain

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment