How is cellular respiration useful to our world?

How can we view Human or living beings in thermodynamic view of the world?

• Thermodynamics discusses much about transfer of energy, from one state to another, in equilibrium state. We humans, or life in general, how can we think of us as a system in this thermodynamic world. I mean life also consists of many simpler systems, like digestion, circulation, respiration etc. and how they accommodate with thermodynamic description of things. Is heat produced in our body is an end result or we are  an example of http://en.wikipedia.org/wiki/Carnot_heat_engine, and heat produced is the loss incurred by system ?

• Answer:

  Physically we're definitely thermodynamic engines, but not heat engines.  Rather we run on chemical potential in the food we eat. As mammals we generate an enormous amount of heat relative to any mechanical work we do, but much of it is heat specifically for the sake of being warm rather than just waste from some other process. A http://en.wikipedia.org/wiki/Muscle considered in isolation as a generator of work has an efficiency of 18-26%, which is quite decent. As far as I know that's simply as good as evolution has managed to achieve rather than a fundamental limit. Chemical energy from food is low-entropy and can in principle be turned into work with arbitrarily high efficiency, as long as you can avoid processes that are effectively thermal.