A logical question to be expected from students: “How could life develop, that is, change, evolve from simple, primitive organisms into the complex forms existing today, while at the same time there is a generally observed decline and disorganization – the second law of thermodynamics?” The explanations in biology textbooks relied upon by students and instructors are incomplete. A necessary but insufficient premise is that only total entropy of a system must increase. In this article, I present background information for a lesson plan on entropy and question biology textbook presentations on the second law and how life could evolve despite it. The principal concept is that biological information in macromolecules provides fresh insight into evolution in the earth’s thermodynamic system.

By growth in size and complexity (i.e., changing from more probable to less probable states), plants and animals appear to defy the second law of thermodynamics. The usual explanation describes the input of nutrient and sunlight energy into open thermodynamic systems. However, energy input alone does not address the ability to organize and create complex structures or explain life cycles – in particular, growth regulation and dying in the presence of adequate nutrients. Understanding the roles of macromolecules such as DNA, with their apparent information-processing capability, affords opportunity to understand biological order.