Daily Newsletter February 13, 2012

Daily Newsletter February 13, 2012

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Daily Topic: Thermodynamics and Equilibrium

Today you are asked to reflect upon the topics of thermodynamics and equilibrium as they pertain to biology. These are topics originally introduced in chemistry, so you may want to go back to your chemistry books to refresh your memory. While biochemists may use these concepts unaltered from their original meaning in chemistry, most biologists look at these two concepts from a slightly different perspective.

The Laws of Theromodynamics: (the two important ones for biology are in bold).
0. If two systems are in thermal equilibrium with a third system, they must be in thermal equilibrium with each other. (Kind of a no brainer, but it is something that comes up in biology).
1. Energy can neither be created nor destroyed, but can be changed from one form to another.
2. In all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state.
3. All processes cease as temperature approaches absolute zero. (not generally a concern to biologists).

Law 1 and 2 are the ones we most often use in biology, but how?
The first law of thermodynamics is rather simple. We convert one type of energy into another.

• Phototrophs are organisms that can convert light energy to chemical energy.
• Motile organisms (such as animals) can convert chemical energy into mechanical energy.
• Luminescent organisms can convert chemical energy into light energy.
• All organisms can convert one type of chemical energy into a different type of chemical energy.

Later in the week, we will discuss redox reactions, but for now I want to put this concept into your heads:  One of the most critical energy exchanges in biology will be the production of reducing potential.

The second law plays a role in homeostasis.  With the second law, we know that energetically, we can never break even.  Every time that we undergo a chemical reaction, we loose energy.  This loss is generally going to be as heat, which is also not good for a cell (too much heat, and the cell boils).  So we have be as efficient as possible knowing that we are always loosing energy.  How does this play out?

• Plants take in sunlight, and make glucose.
• The sunlight is high energy.
• The glucose is going to have to have less energy because we used photons to excite electrons (an energy change).
• Animals eat plants.
• We catabolize the glucose for energy (reducing potential).
• Do we get 100% of the energy in glucose?  NO.
• Animals build biomass (make proteins, lipids, etc... as needed for life).
• The energy we got from glucose is further lost when we make new biochemicals.
• We constantly need energy to keep rebuilding ourselves.
• We constantly need energy inputs to maintain homeostasis.

How does equilibrium play a role?  This come in how we experience metabolism.  In chemistry lab, to do multiple reactions, you first converted A èB, then you took B èC.  You may have moved them to different test tubes.  If nothing else, you waited until the first reaction reached equilibrium before proceeding to the next reaction.  In other words, you waited for A to convert to B, then B to C.  In living systems, there is a near seamless transition between reactions.  The product of one reaction almost instantaneously becomes the substrate for the next reaction.  A misconception is that cells never reach an equilibrium, but this is not true.  Cells are always in a state of dynamic equilibrium. 

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Daily Challenge:
Today you are tasked with describing in your own words how the laws of thermodynamics and eqiulibrium play out in living systems. Use the above a a jumping off point, but build your own discussion, examples or analogies.

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Special Blog Opportunity: You can earn up to 4 points by reflecting on Milestone Exam 1. Go over the text, and reflect on each of the questions you got wrong. To earn points, you must perform the following:
1. Explain why the keyed answer was the correct answer for any question you missed.
2. Provide a reference for the correct answer.
3. Explain why you got the question wrong.