Cellular Respiration:
Cellular respiration is the process that releases energy from molecules such as glucose and makes it available for cellular use. The chemical reactions of cellular respiration must occur in a particular sequence, each one controlled by a different enzyme. Some of these enzymes are in the cell's cytosol, whereas others are in the mitochondria. Such precision of activity suggests that the enzymes are physically positioned in the exact sequence as that of the reactions they control. Indeed, the enzymes responsible for some of the reactions of cellular respiration are located in tiny, stalked particles on the membranes (cristae) within the mitochondria.

Cellular respiration occurs in three distinct, yet inter-connected, series of reactions: glycolysis, citric acid cycle, and the electron transport chain (oxidative phosphorylation). The products of these reactions include CO2, water, and energy. Although most of the energy is lost as heat, almost half is captured in a form that the cell can use through the synthesis of ATP adenosine triphosphate), an energy-rich molecule.

Cellular respiration includes aerobic reactions (require oxygen) and anaerobic reactions (do not require oxygen). For each glucose molecule that is decomposed completely by cellular respiration, up to thirty-eight molecules of ATP can be produced. All but two ATP molecules are formed by the aerobic reactions.

ATP molecules:
Each ATP molecule consists of three main parts-an adenine, a ribose, and three phosphates in a chain. The third phosphate of ATP is attached by a high-energy bond, and the chemical energy stored in that bond may be quickly transferred to another molecule in a metabolic process. When such an energy transfer occurs, the terminal, high-energy bond of the ATP molecule breaks, releasing its energy. Energy from the breakdown of ATP powers cellular work such as skeletal muscle contraction, active transport across cell membranes, or secretion.

An ATP molecule that loses its terminal phosphate becomes an ADP (adenosine diphosphate) molecule, which has only two phosphates. However, ATP can be resynthesized from an ADP by using energy released from cellular respiration to reattach a phosphate, a process known as phosphorylation. Thus, ATP and ADP molecules shuttle back and forth between the energy-releasing reactions of cellular respiration and the energy-utilizing reactions of the cell.

ATP is not the only kind of energy-carrying molecule within a cell, but it is the primary one. Without enough ATP, cells quickly die.

Both aerobic and anaerobic pathways begin with glycolysis. Literally "the breaking of glucose," glycolysis is a series of ten enzyme-catalyzed reactions that break down the 6-carbon glucose molecule into two 3-carbon pyruvic acid molecules. Glycolysis occurs in the cytosol, and because it does not itself require oxygen, it is sometimes referred t as the anaerobic phase of cellular respiration.

Aerobic Respiration:
If enough oxygen is available, the pyruvic acid generated by glycolysis can continue through the aerobic pathways. These reactions include the synthesis of acetyl coenzyme A or acetyl CoA, the citric acid cycle, and the electron transport chain. In addition to carbon dioxide and water, the aerobic reactions themselves yield up to thirty-six ATP molecules per glucose.

Aerobic respiration: is a sequence of reactions that begins with pyruvic acid produced by glycolysis moving from the cytosol into the mitochondrion. From each pyruvic acid, enzymes inside the mitochondria remove two hydrogen atoms, a carbon atom, and two oxygen atoms generating NADH and a CO2 and leaving a 2-carbon acetic acid. The acetic acid then combines with a molecule of coenzyme A derived from the vitamin pantothenic acid) to form acetyl CoA. CoA "carries" the acetic acid into the citric acid cycle.

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. 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 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 to the electron transport chain, which is a series of enzyme complexes that carry and pass electrons along from one to another. These complexes dot the folds of the inner mitochondrial membranes, which, if stretched out, may be 45 times as long as the cell membrane in some cells. The electron transport chain passes each electron along, gradually lowering the electron's energy level and transferring that energy to ATP synthase, an enzyme complex that uses this energy to phosphorylate ADP to form ATP.

Neither glycolysis not the citric acid cycle uses oxygen directly although they are part of the aerobic metabolism of glucose. Instead, the final enzyme of the electron transport chain gives up a pair of electrons that combine with two hydrogen ions (provided by the hydrogen carriers) and an atom of oxygen to form a water molecule:

2e- + 2H+ + 1/2 O2 = H2O

Thus, oxygen is the final electron "carrier". In the absence of oxygen, electrons cannot continue to pass through the electron transport chain, and aerobic respiration grinds to a halt.

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