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Chapter Four: Cell Nutrition and Respiration
This section refers only to aerobic organisms; anaerobes do not perform the Krebs cycle or the electron transport chain.
The Krebs cycle is a series of reactions which occurs in the mitochondria and results in the formation of ATP and other molecules which undergo further reactions to form more ATP.
In order for the pyruvic acid molecules from glycolysis to enter the Krebs cycle, they must first undergo one additional chemical reaction called the oxidation of pyruvic acid. In this reaction, one carbon atom and two oxygen atoms are removed, forming CO2 (carbon dioxide). In addition, a molecule of the coenzyme NAD+ becomes NADH in the process. The remaining molecule has two carbons and is known as acetyl coenzyme A and has a molecular formula of CH3CO. Remember, this process happens to both of the two pyruvic acid molecules formed during glycolysis.
In general, the Krebs cycle consists of the bonding of the two carbon acetyl coenzyme A with the four carbon compound called oxaloacetic acid. The resulting compound, which has six carbon atoms, undergoes a series of chemical reactions (which are outlined below), resulting in the formation of energy (in the form of ATP, NADH, and FADH2) and oxaloacetic acid, which can then bond with another molecule of acetyl coenzyme A so that the cycle can run again.
The Krebs cycle occurs two times for each glucose molecule from glycolysis, since it occurs for each of the two molecules of acetyl coenzyme A formed by the oxidation of the two molecules of pyruvic acid. For each turn of the Krebs cycle, one molecule of ATP, three molecules of NADH, and one molecule of FADH2 (a coenzyme similar to NADH) are formed. Therefore, for one glucose molecule, the Krebs cycle results in the formation of two molecules of ATP, six of NADH, and two of FADH2. The NADH and FADH2 then undergo the electron transport chain, resulting in the production of more ATP.
The electron transport chain is the final stage of aerobic respiration. It utilizes the molecules of NADH and FADH2 formed during glycolysis and the Krebs cycle to produce great amounts of ATP. During glycolysis and the Krebs cycle, the NAD+ and FAD coenzymes were passed high energy electrons. The purpose of the electron transport chain is to pass those electrons to other carrier molecules which hold the electrons at slightly lower energy levels. As the electrons are passed from high to low energy levels, energy is released. After many steps, the electrons are finally accepted by oxygen at the lowest energy level, producing water.
At some steps in the electron transport chain, the jumps from one energy level to the next are significantly larger than in others. The energy that is released in these steps is enough to power a process which drives ADP and phosphate molecules together to produce ATP. This process, called chemiosmosis, is discussed in the next section.
For every molecule of NADH which enters the electron transport chain, three molecules of ATP are eventually formed through chemiosmosis. For an FADH2 molecule, two molecules of ATP are eventually formed.