All life depends on the availability of usable energy. This energy is released when
(1) organisms convert solar energy into the chemical energy found in food molecules(2) respiration occurs in the cells of producers and high-energy molecules enter the atmosphere(3) cells carry out the process of respiration(4) animal cells synthesize starch and carbon dioxide

Respiration is the biochemical process in which the cells of an organism obtain energy by breaking down organic molecules in presence or absence of oxygen (aerobic or anaerobic) resulting in the release of Carbondioxide (CO2), water and Adenosine triphosphate (ATP).

Food molecules (containing stored energy in their chemical bonds) absorbed after digestion are broken down and the energy within their molecules are freed. This freed energy in form of ATP, is used to power the organism's movement and physiological functions.

Note that, ATP is an energy carrying molecule and a usable form of energy by cells. This is so because ATP releases energy quickly. Energy is released from ATP when the end phosphate (Pi) is removed to become ADP (adenosine diphosphate), which is a low energy molecule.

Aerobic cellular respiration consists of Glycolysis, Kreb's cycle and Oxidative phosphorylation. A total of 38 ATP molecules is produced in the cytosol of prokaryotes while a total of 36 ATP molecules is produced in the mitochondria of eukaryotic cells.