All living things need energy to survive, so cells spend a good deal of effort converting energy into a form that can be packaged and used. As animals have evolved, so has the complexity of the energy production systems. The respiratory system, digestive system, circulatory system and lymphatic system are all parts of the body in humans that are necessary just to capture energy in a single molecule that can sustain life.
The intent of cellular respiration is to generate ATP, or adenosine triphosphate, the main energy storage and transfer molecule of the cell. ATP is a complex molecule that includes a few different molecular ring structures, but the important thing about it to remember is the three attached phosphate groups, from which the name triphosphate derives. ATP is used to sustain such processes as muscle contraction, upon which time it becomes a molecule of ADP, or adenosine diphosphate. In cellular respiration, ADP is fitted with phosphate to become ATP once more.
Chemical energy is generated by the breakdown of molecules. There are many ways to describe this reaction, but the term catabolism is used to describe the exact breakdown of molecules for energy in the cell. In the case of ATP, the removal of a phosphate group releases a large amount of free energy, allowing the molecule to be used for work. The chemical reaction occurs through hydrolysis, in which the decomposition of a substance is accompanied by the insertion of a water molecule between two bonds. Once the third phosphate group has been removed, ADP has only two left. This is why a single phosphate group will restore ADP to ATP.
Energy enters the process of cellular respiration in the form of glucose, a simple sugar which is digested and broken down from complex carbohydrates, then brought into the cells through the body’s circulatory systems. Glucose is a common and easily modified molecule. Plants synthesize glucose as an end product in the process known as photosynthesis by converting light energy into chemical energy.
Animals, on the other hand, extract energy from glucose with the intention of producing ATP as an end result. Unlike photosynthesis, the energy remains in the same form. It begins as chemical energy and ends as chemical energy. This is accomplished by modifying glucose in a very complex and involved pathway in order to extract the necessary energy to reset ADP to ATP. In other words, if the breakdown of ATP supplies work for the body, then an outside source of energy will need to be broken down to restore ATP, because the energy cannot be recaptured and is only flowing one way through the system.
From a single molecule of glucose a cell can generate a net yield of 36 molecules of ATP. However, most of the energy within an organism is lost as heat, so the process is highly inefficient. In fact, all energy releasing reactions liberate heat. The energy that remains behind can be put to work keeping an organism active and alive.