Cells cannot survive without ATP. ATP is essential because it powers nearly every activity that requires energy input in the cell. There is an intimate relationship between our requirements to eat and breathe. Both are essential for the production of ATP. The next few pages of this module describe how our cells make ATP from the nutritents we consume and the air we breathe.
The illustration below shows the link between the breathing and making ATP. The athlete breathes in oxygen, which enters the bloodstream in the lungs and is distributed to all cells. ATP is generated in cellular respiration and used in muscle contraction in this example.
Cellular respiration is an example of a metabolic pathway. That means that it is not a single chemical reaction, but a series of reactions. A specific enzyme catalyzes each reaction in a metabolic pathway. Substrate A is changed into product B, which then becomes the substrate for the next reaction in the pathway, in which B is changed into C and so forth:
A ---> B ---> C ---> D
We call the molecules of the pathway intermediates because the products of 1 reaction become the substrates for the next reaction. Glucose, for instance, is a key intermediate in several metabolic pathways. In many ways, a group of metabolic pathways is similar to a detailed road map. Just as a map shows a network of roads that connect various cities and towns, metabolism can be thought of as a network of chemical reactions connecting various intermediate products.
More than two dozen reactions are involved in cellular respiration also known as aerobic respiration. We can group them into 4 main metabolic stages: glycolysis, the intermediate stage, citric acid cycle, and the electron transport system.