Engines need energy to move. This is true whether you are talking about the internal combustion engines that power most cars or the processes that power organic life forms. Internal combustion engines get their energy through the process of combustion, while organisms get their energy through a process called cellular respiration. The two processes are very similar in nature.
Both cellular respiration and combustion require a core fuel for the process to happen at all. This fuel is stored energy, and the entire process of combustion or respiration is to convert that energy from its stored state -- in the fuel -- to another state that the engine, either mechanical or bionic, can use to power its other operations. While fossil fuels and sugar molecules have very different structures, they both have a series of molecular bonds that the energy harvesting process will break apart.
While breaking apart the bonds to release the stored energy from the fuels -- either fossil fuels for combustion or sugars for respiration -- the bonds will not break themselves apart. In each case, a catalyst is required to start the reaction that will break the bonds apart. In the case of combustion, the catalyst is a spark. Fossil fuels are flammable, so the spark will ignite the fuel in a cylinder, breaking apart the bonds and releasing the energy. For respiration, enzymes are used to break the sugar molecule apart.
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After the bonds for the fuel are broken, the energy being released needs to be transported to the part of the "engine" where it will be used. For internal combustion engines, the force of the explosion pushes on a piston, which translates the force of the explosion into mechanical energy to run the engine. For respiration, the energy is stored by creating adenosine triphosphate (ATP). These ATP molecules are then transported to parts of the organism that require energy. Breaking a phosphate bond will create adenosine diphosphate, and the energy that was stored in one of the bonds will be used by the organism.
After cellular respiration and internal combustion have gotten what they needed from the fuels, there will be byproducts from the conversion. In the case of internal combustion, they are noxious gases like carbon monoxide. In the case of respiration, the sugar molecule is broken into two molecules of pyruvic acid. Internal combustion engines get rid of their waste products through exhaust pipes, while organisms dispose of pyruvic acid through the process of fermentation.