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PCAT Chemical Processes – Thermodynamics Law

  • by Sam
  • Oct 25, 2017
  • PCAT Question of the Day

The human body takes biological monomers in the form of amino acids and sugars and builds highly complex physiological structures from them. As such, the human body’s relationship with the second law of thermodynamics can best be described by which of the following:


A) The human body is an example of how biological systems are not part of the domain of the second law of thermodynamics, since they decrease entropy through anabolic processes.

B) There is an equilibrium between anabolism and catabolism in the human body, thus it falls under the second law of thermodynamics by not increasing entropy.

C) The human body follows the second law of thermodynamics by creating decreasing entropy within the system by increasing the entropy of the environment.

D) It is impossible to relate thermodynamic laws to complex physiological systems.


Living things don’t get a free pass when it comes to thermodynamics. The human body gets its energy by taking highly organized molecules (imagine the rings of a sugar molecule) and turning them into disorganized things (breathing out CO2 as waste). Thus, the decrease in entropy that is seen in anabolic processes in the human body comes at the expense of increasing disorder in the rest of the universe, making (C) the right answer.


A, D: These are both false. Living systems are governed by the same rules of chemistry as every other chemical system.

B: While (B) may occasionally be true, it need not be. Most notably in a growing child or adolescent, there will be more anabolism than catabolism as the body grows.

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