glucose + oxygen water + carbon dioxide + about 30 ATP molecules + heat
ADP + Pi + energy ADP~Pi with the ~Pi indicating a special energy-rich bond or, using standard symbols:
ADP + Pi + energy ATP
The chief purpose for the breakdown or catabolism of nutrients such as carbohydrates (sugars) and lipids (fats) is to form energy. About 60% of energy that is released during catabolism is in the form of heat with the remaining energy used to form adenosine triphosphate (ATP). As an example of the use of energy, ATP is required for the needed energy for muscle contraction, and the ATP needed must be continuously made since it can exist only for about one and a half minutes. Another purpose for the metabolic transformations is to create needed substances required for the cell to function.
Summarizing the breakdown of glucose as occurring in one step is convenient to indicate the amount of energy formed; however, if this were to occur in one step in the cell, then too much energy would be released at one location in the cell.
glucose + oxygen water + carbon dioxide + about 36 ATP molecules + heat
Rather than being modified using one step, glucose is modified in simple ways using many steps. These steps need not be memorized, but a quick overview of the initial ten steps emphasizes the point of having many simple transformations occurring. As soon as the first transformation occurs, one no longer has "glucose," but rather a new product is formed.
During two of these enzymatic transformations [at step 7 and step 10], ATP is generated. For this to occur, the energy released is not lost as heat, but rather it is used to connect a phosphate (Pi) to a molecule of adenosine diphosphate (ADP). Therefore, at these special small steps during the transformation of glucose (catabolism of glucose), a molecule of adenosine diphosphate containing two phosphates is changed to a molecule of adenosine triphosphate that is a molecule containing three phosphates. The bond holding the third phosphate is of a special type so that at the needed time; i.e., muscle contraction, the special bond can be broken to yield the needed energy.
ADP + Pi + energy ADP~Pi with the ~Pi indicating a special energy-rich bond
or, using standard symbols:
ADP + Pi + energy ATP
ATP ADP + Pi + energy
The adenosine diphosphate (ADP) and phosphate (Pi) can be re-used to generate more ATP.
Since the transformations in the cell are in an ordered arrangement, the step-wise transformations are called metabolic pathways.
In the example of glucose breakdown, the first ten steps occur without the requirement of oxygen. These steps are said to be anaerobic and occur in the general cytoplasm outside the mitochondria. The remainder of the breakdown steps of glucose requires oxygen (approximately eight steps). These latter steps are said to be aerobic. There are many more energy packets (in the form of ATP) formed during the metabolic steps that occur within the mitochondria when oxygen is available compared to the number of ATP molecules formed during metabolic changes occurring outside the mitochondria without oxygen.
When oxygen is available, the final products formed from the breakdown of glucose are carbon dioxide and water. When oxygen is not available, glucose can be only partially broken down to form lactic acid.