All organisms make use of a molecule called glucose and a process called glycolysis to meet some or all of their energy needs. For single-celled prokaryotic organisms, such as bacteria, this is the only process available for generating ATP (adenosine triphosphate, the "energy currency" of cells).
Eukaryotic organisms (animals, plants and fungi) have more sophisticated cellular machinery and can get a lot more out of a molecule of glucose – over fifteen times as much ATP, in fact. This is because these cells employ cellular respiration, which in its entirety is glycolysis plus aerobic respiration.
A reaction involving oxidative decarboxylation in cellular respiration called the bridge reaction serves as a processing center between the strictly anaerobic reactions of glycolysis and the two steps of aerobic respiration that occur in the mitochondria. This bridge stage, more formally called pyruvate oxidation, is thus essential.
Approaching the Bridge: Glycolysis
In glycolysis, a series of ten reactions in the cell cytoplasm converts the six-carbon sugar molecule glucose into two molecules of pyruvate, a three-carbon compound, while producing a total of two ATP molecules. In the first part of glycolysis, called the investment phase, two ATP are actually needed to move the reactions along, while in the second part, the return phase, this is more than compensated for by the synthesis of four ATP molecules.
Investment phase: Glucose has a phosphate group attached and then is rearranged into a fructose molecule. This molecule in turn has a phosphate group added, and the result is a doubly phosphorylated fructose molecule. This molecule is then split and becomes two identical three-carbon molecules, each with its own phosphate group.
Return phase: Each of the two three-carbon molecules has the same fate: It has another phosphate group attached, and each of these is used to make ATP from ADP (adenosine diphosphate) while being rearranged into a pyruvate molecule. This phase also generates a molecule of NADH from a molecule of NAD+.
The net energy yield is thus 2 ATP per glucose.
The Bridge Reaction
The bridge reaction, also called the transition reaction, consists of two steps. The first is the decarboxylation of pyruvate, and the second is the attaching of what is left to a molecule called coenzyme A.
The end of the pyruvate molecule is a carbon double-bonded to an oxygen atom and single-bonded to a hydroxyl (-OH) group. In practice, the H atom in the hydroxyl group is dissociated from the O atom, so this portion of pyruvate can be thought of as having one C atom and two O atoms. In decarboxylation, this is removed as CO2, or carbon dioxide.
Then, the remnant of the pyruvate molecule, called an acetyl group and having the formula CH3C(=O), becomes joined to coenzyme A at the spot previously occupied by the carboxyl group of pyruvate. In the process, NAD+ is reduced to NADH. Per molecule of glucose, the bridge reaction is:
2 CH3C(=O)C(O)O- + 2 CoA + 2 NAD+ → 2 CH3C(=O)CoA + 2 NADH
After the Bridge: Aerobic Respiration
Krebs Cycle: The Krebs cycle location is in the mitochondrial matrix (the material inside the membranes). Here, acetyl CoA combines with a four-carbon molecule called oxaloacetate to create a six-carbon molecule, citrate. This molecule is pared back down to oxaloacetate in a series of steps, starting the cycle anew.
The result is 2 ATP along with 8 NADH and 2 FADH2 (electron carriers) for the next step.
Electron Transport Chain: These reactions occur along the inner mitochondrial membrane, in which four specialized coenzyme groups, named Complex I through IV, are embedded. These use the energy in the electrons on NADH and FADH2 to drive ATP synthesis, with oxygen being the final electron acceptor.
The result is 32 to 34 ATP, putting the overall energy yield of cellular respiration at 36 to 38 ATP per molecule of glucose.
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