Animals intake energy in the form of starch, which needs to be broken down by the animal's digestive system. Since starch is too large to diffuse into the membrane of animal cells, digestion needs to take place so that starch can be converted to glucose. In humans, the nutrients from the products of digestion are absorbed by the villi in the small intestine. The liver is the final destination for nutrients such as glucose, until the glucose enters the blood, that is.
Cellular respiration is the process in which chemical energy stored in glucose is used to make ATP, which is produced by mitochondria in the cells.