All the cellular activities are grouped into two categories – anabolism
(biosynthetic activities of the cell) and catabolism (breaking up processes of
the cell). The anabolic activities are endergonic (energy dependent), while
catabolic activities are usually exergonic (energy producing). The sum total of
catabolic and anabolic reactions occurring at any time in a cell is called
metabolism. Respiration is an exergonic and catabolic physic chemical process
which involves the exchange of environmental oxygen and body carbon dioxide
through a liquid medium and the oxidation of glucose inside the mitochondria to
produce energy which is partly stored in the high energy bonds of ATP molecules
as biologically useful energy.