Expert: Florence M Rollwagen - 10/13/2008

Question
I have to complete a writing assignment for my biology class. My professor wants us to answer this question, "Define the Two Laws of Thermodynamics and what these laws mean to living cells."  I know what the laws are, and I am comfortable with their concepts.  I am just not sure on how to relate the laws  to cellular biology.  I am sure that this is pretty elementary but I am having trouble writing anything that makes a whole lot of sense.  If you could maybe point me in the right direction, I would be greatful.

Answer
Hi James:  Thanks for your question.  The laws of thermodynamics describe the characteristics of matter/energy.  The first, that matter/energy cannot be created or destroyed, only changed is particularly relevant to biology.  One of the most common examples of this law in action are respiration and photosynthesis.  In these biological systems, say respiration, glucose is broken down into CO2, and high energy electrons are generated.  So instead of glucose, think of cellulose (a polymer of glucose) as in paper or wood.  When this material burns in the fireplace it releases CO2 and heat (increased molecular motion).  In both systems, glucose is turned into CO2 and energy.  But the cell has found a way to harness this energy in a controlled way so that metabolism (anabolism and catabolism) can occur.

The second law of thermodynamics, states the energy of a closed system degrades.  The entropy (disorder) increases.  Some people use this law to say that evolution cannot occur, since evolution is an increase in order (structure).  But we don't live in a closed system, we live in a system where energy is constantly entering.  What is the source of this energy? (hint:  look up).

In photosynthesis, the second law applies, since the photonic energy from the sun is harvested and used to synthesize glucose.  Photons strike chlorophyll molecules and knock off an electron.  This high energy electron (sound familiar?) is then used to generate ATP and NADPH which is used to synthesize sugar (glucose).

You can take a look at these systems in more detail by going to www.discoverbiology.com and clicking on "Chapter 8" in the upper left corner.  There you will find tutorials and animations illustrating this concept.  This is the textbook we are using in our Biology 101 class.

Hope this helps!  Please write back if you have more questions.

FM Rollwagen, PhD