II.

Chemical Reactions and Metabolic Pathways

To understand cellular respiration, it is necessary to understand the nature of chemical reactions. Chemical reactions can occur outside of living organisms—the rusting of a car, for example, is a chemical reaction—or they can occur within organisms, where they are termed biochemical reactions. In a chemical or biochemical reaction, the bonds between atoms that hold molecules together break apart, and the atoms rearrange to form new molecules. Water molecules, for example, are composed of hydrogen and oxygen atoms, and under certain conditions, the bonds between these atoms can break and reform to yield separate molecules of hydrogen and oxygen gas. In living organisms, most biochemical reactions occur with the help of enzymes, specialized proteins designed to carry out specific reactions. All biochemical reactions release energy in the form of heat as they occur.

Cells carry out biochemical reactions to create needed molecules—such as proteins or starch—or to destroy these molecules once they are no longer needed. If certain molecules are built or destroyed in a single biochemical reaction, the reaction may release too much heat, which could incinerate the cell. To control the release of heat, cells build up and break down most molecules in a linked series of small reactions that release only a little bit of heat at a time. The series of linked biochemical reactions is called a metabolic pathway.

Cellular respiration is one of the most important metabolic pathways found in cells. This enzyme-assisted, step-by-step process not only protects the cell from lethal temperature increases but also provides the cell with a mechanism of transferring the energy of glucose to ATP in a controlled manner.