Mitochondria

Mitochondria are membrane-enclosed organelle [a specialized subunit within a cell that has a specific function, and is separately enclosed within its own lipid membrane].  They are about 1–10 micrometers (μm) in size and are found in most eukaryotic cells. Animals, plants, fungi, and protists are eukaryotes - organisms whose cells are organized into complex structures enclosed within membranes.

Mitochondria have a number of functions; they supply cellular energy and are involved in a range of other processes, such as signaling, cellular differentiation, cell death, as well as the control of the cell cycle and cell growth.

Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. 

Only a fraction of the original chemical energy, however, is used for work [kinetic energy or gravitational potential energy].  Most of the chemical energy is converted into heat.  In growing organisms the energy that is converted to heat serves a vital purpose, as it allows the organism tissue to be highly ordered with regard to the molecules it is built from. You see a gradually more complex ordering of molecules in the food chain, as simpler organisms [with higher energy efficiency] are eaten by more complex ones [with apparently lower energy efficiency].