Cellular respiration is the process by which a living cell produces adenosine triphosphate (ATP), carbon dioxide, and water from oxygen and organic fuel. It is a catabolic pathway that involves the release of stored energy from the break down of complex molecules to more simple ones. No single chemical reaction covers the entire process of cellular respiration. Instead it is the cumulative function of glycolysis, the Krebs cycle and electron transport. In eukaryotes, the mitochondria is the primary organelle that contains the enzymes that drive cellular respiration.
Nearly all eukaryotic cells contain some mitochondria. While there may be as few as one mitochondria in a cell, often there are hundreds or thousands. The number typically depends on the metabolic activity of the cell. The mitochondria is enclosed in a two membrane envelope in which a variety of proteins are embedded. Inside these membranes is the mitochondrial matrix which contains some of the enzymes that function in cellular respiration. Other enzymes including the one that makes ATP are attached to the inner membrane. This configuration provides an efficient way for cellular respiration to occur.
Although the mitochondria contains most of the enzymes related to cellular respiration, the process actually begins in the cytosol. This reaction, known as glycolysis, involves the breakdown of glucose into two molecules of a three carbon sugar called pyruvate. During this process, two molecules of ATP are consumed while four molecules of ATP are produced, resulting in a net gain of two ATP molecules. While this energy is beneficial to the cell, it pales in comparison to the amount produced by the later stages of cellular respiration.
After glycolysis, the pyruvate is transported across the mitochondrial membranes into the matrix. Here, it goes through a series of reactions called the Krebs cycle (also known as the citric acid cycle). First it is converted to acetyl CoA. It is then slowly oxidized into carbon dioxide and water. In the process, energy is transferred to storage molecules including three NADH and one FADH2. Two molecules of ATP are also formed during this stage.
The final step in cellular respiration is the electron transport reactions. These reactions complete the oxidation of glucose and generate the greatest amount of energy. During this stage, each of the storage molecules transfers electrons to a series of coenzymes which then drive the production of ATP molecules. The actual production of ATP is the result of an enzyme called ATP synthase. This enzyme produces ATP from ADP by a process called oxidative phosphorylation. This phase of cellular respiration results in about 34 molecules of ATP.
In addition to glucose, many other compounds are used by the cell as a source of fuel. These include proteins, carbohydrates and fats. All of these complex molecules can be broken down to simpler ones which can then enter glycolysis or the Kreb's cycle at various points. For example, starch is hydrolyzed in the digestive tract producing a molecule that can be broken down by glycolysis. Similarly, glycogen can be hydrolyzed. Proteins are used as fuel, but only after they are reduced to their constituent amino acids and their amino groups are removed. Fats are the highest energy containing molecules. They are reduced to either glycerol or acetyl CoA before entering the cellular respiration reactions.