Cellular Respiration & Metabolism

Metabolism refers to all the chemical reactions in the body that either create energy for the body to use or that require energy in order to build the structural and functional systems. Cellular respiration is one step in the metabolic reactions that provides energy for use by the body's cells.

Catabolism

• The metabolic reactions that create energy are called catabolism. This process begins during digestion when nutrients are broken down into glucose, amino acids and fatty acids that can be absorbed into the blood. However, they are still not in a chemical form that can be used at the cellular level. They need to be converted into ATP.

ATP

• ATP is the acronym for adenosine triphosphate. ATP is the chemical form of energy used by every cell in the body. As it is used, it breaks down and is recycled to be used again for the production of more ATP.

Cellular Respiration

• Cellular respiration is the metabolic process that converts biochemical energy from glucose into ATP. This is accomplished through a series of chemical reactions called oxidation and reduction reactions. During oxidation, molecules lose hydrogen and electrons. A reduction is the opposite reaction in which another molecule gains the hydrogen and electrons. Cellular respiration is a complicated chemical process that has three phases: Glycolysis, the Krebs Cycle and the electron transport chain.

Glycolysis

• In this phase, glucose goes through a series of chemical reactions that result in converting each molecule of glucose into two molecules of pyruvate. Two molecules of ATP are also released. Glycolysis does not require oxygen to produce pyruvate, but what happens next depends on it. In the presence of oxygen, pyruvate enters a reaction in which it attaches to coenzyme A and becomes acetyl-coenzyme A. It must be in this form to enter the Krebs Cycle.

The Krebs Cycle

• Acetyl-coenzyme A enters the mitochondria of the cell and the Krebs Cycle begins. During this phase, the acetyl-coenzyme A combines with oxaloacetic acid to form citric acid, which is why the Krebs Cycle is also called the citric acid cycle. The molecules of citric acid go through a series of reactions in which the acids are oxidized (lose hydrogen) and the hydrogen is picked up by coenzymes. The hydrogen, in the form of NADH and FADH, enters the next phase.

Electron Transport System

• The electron transport system is another series of chemical reactions in which hydrogen loses electrons and they attach to proteins called cytochromes. At each step in the reaction, the cytochromes go through oxidation-reduction reactions that allow each one to give its electron to the next one in the chain. Every time an electron is transferred, chemical energy is released in the form of ATP. As the electrons reach the end of the transport system, they attach to oxygen.

Interesting Fact

• During exercise the body needs a lot of energy quickly and cellular respiration increases to keep pace. If the supply of oxygen isn't sufficient to keep up with the rate of cellular respiration, the result is pyruvate that can't move into the next phase. This excess pyruvate is converted into lactic acid. Moderate amounts of extra lactic acid are easily buffered by the body but if too much lactic acid builds up, protective mechanisms kick-in. The result is increased respiration (shortness of breath) and a build-up of lactic acid in the muscles, which leads to muscle fatigue and pain.