Living organisms, like humans and other mammals, constantly need to expend energy to perform physical work, to maintain body temperature in the presence of heat exchange to the environment and to produce, transport, and replace molecules that are their constituents. This energy is provided by oxidation of organic substances: Carbohydrates, fats, and amino acids, primarily introduced to the organism by feeding. Unlike conventional heat engines where the chemical energy is first converted into thermal energy and then into mechanical work, living organisms are able to convert part of nutrients’ chemical energy directly into work. This is possible because the oxidation of nutrients inside the organisms, known as metabolism, proceeds through many steps allowing the capture of some of the energy in an intermediate chemical from (adenosine triphosphate or ATP) which is used almost exclusively by living things  for direct conversion into mechanical energy as well as for promoting many biological reactions.
Updated on March 17, 2020