Cellular respiration is the process that animals use to disseminate energy. Animals are consumers, meaning that they do not create their own energy. Instead, animals get their energy from eating producers that create energy or from eating other consumers. The purpose of all this activity is to digest and break down the molecule known as glucose.
Glucose is a simple carbohydrate and the primary molecular input that initially enters the process of cellular respiration. Its purpose, in this respect, is to produce the molecule ATP, the main energy storage and transfer unit of a cell. Cells need a constant flow of glucose in order to remain healthy and active.
Most glucose is apprehended by the body through the digestion of complex carbohydrates. It is carried through the body by blood (which is the origin of the term "blood sugar level") and arrives at the cell so that it can enter the process of cellular respiration.
In order to galvanize the production of ATP, glucose must be constantly rearranged or augmented with different atoms. The goal is to turn the glucose molecule into an appropriate package that can eventually donate atoms and particles later in the process. The constant influx of glucose creates a condition in which the process can be sustained for the life of the organism.
The first major step of cellular respiration is the anaerobic (meaning that it doesn't require oxygen) process of glycolysis. Here the glucose is modified into a molecule of pyruvate, which after further modification ends up donating particles in order to fuel the production of ATP out of ADP and phosphate. At the end of the process oxygen accepts any loose particles and becomes water.
The main purpose of ATP is to facilitate processes such as muscle contraction in the body. But when the ATP is used, it becomes a benign molecule of ADP that can no longer facilitate any processes. It returns to the metabolic process and becomes ATP once more when it is fitted with phosphate.