The answer: “exergonic chemical reaction“
Posted on Sandwalk (by Laurence A. Moran, the Blogger and lead author of Principles of Biochemistry):
My friend and colleague, Alex Palazzo, alerted me to an interview of Bill Martin published in the July 11, 2016 issue of Current Biology [Bill Martin]. I loved all his answers—Bill Martin is one of my scientific heroes—but his answer to the last question was particularly insightful. The question was, “What’s the single most important thing that you have come to realize about nature?”
His answer was ….
Life is an exergonic chemical reaction. It’s the energy releasing redox reaction at the core of metabolism that makes life run, and throughout all of life’s history it is one and the same reaction that has been running in uninterrupted continuity from life’s onset. Everything else is secondary, manifestations of what is possible when the energy is harnessed to make genes that pass the torch.
I’m a biochemist so you might think I’m a little bit biased but let me tell you why this answer is so important.
First, it emphasizes the point that “life” is just a bunch of chemical reactions. As we say in the textbooks, “Living things obey the standard laws of physics and chemistry. No “vitalistic” force is required to explain life at the molecular level.”1 This is an extremely important concept but, unfortunately, one that is ignored in most undergraduate courses. Most teachers will probably agree with the concept but they often assume their students accept it as well. Consequently, teachers don’t emphasize it in class and don’t explain the evidence that supports the concept. That’s a mistake, especially if we are teaching biochemistry.
Every single student who takes a biochemistry or molecular biology course should be taught that cells obey the laws of physics and chemistry. They should be given the evidence and they should be given the opportunity to discuss and debate the concept until they are convinced it is true. This is far more important than memorizing the structures of the nucleotides and the common amino acids.
Bill Martin’s second point is that cellular energy is derived from redox reactions. This is important for understanding how modern living cells work. It’s also important for understanding the best explanations for how life arose in the first place. (Bill Martin has some very good ideas about this [Metabolism first and the origin of life].) We do our students a great disservice if we fail to teach biochemistry from an evolutionary perspective, including the best ideas on how life began. That’s important knowledge that promotes critical thinking and a big picture understanding of biology.
My test for whether biochemistry students really understand the fundamental concepts of biochemistry is how they answer the following question on an exam; “Explain how chemoautotrophs2 get the energy to make carbohydrates, proteins, and nucleic acids.” I don’t think most of our students could answer this question correctly. I don’t think most biochemistry professors could answer the question!
I rewrote the metabolism chapters in my textbook back in 2002 because I realized we were not putting enough emphasis on oxidation-reduction (redox) reactions. We now have a thorough explanation of redox reactions and how Gibbs free energy change is related to reduction potential. For those of you who haven’t yet bought my textbook and read it from cover-to-cover , the key concept is, “The standard Gibbs free energy change of an oxidation-reduction reaction is calculated from the reduction potentials of the two half-reactions.” (p. 320)
An oxidation-reduction reaction is one where electrons are passed from one molecule to another. Here’s the description I wrote on page 164 of Principles of Biochemstry.
Oxidation–reduction reactions are central to the supply of biological energy. In an oxidation–reduction (redox) reaction, electrons from one molecule are transferred to another. The terminology here can be a bit confusing so it’s important to master the meaning of the words oxidation and reduction—they will come up repeatedly in the rest of the book. Oxidation is the loss of electrons: a substance that is oxidized will have fewer electrons when the reaction is complete. Reduction is the gain of electrons: a substance that gains electrons in a reaction is reduced. Oxidation and reduction reactions always occur together. One substrate is oxidized and the other is reduced.
Recall that Bill Martin’s most important lesson is that, “It’s the energy releasing redox reaction at the core of metabolism that makes life run …” The answer to the exam question is that chemoautotrophs couple the energy of oxidation-reduction reactions to fixation of carbon dioxide, nitrogen, phosphorus, etc., leading to the biosynthesis of the complex chemicals required for complex life. The electrons in the initial redox reaction are derived from oxidizing compounds like hydrogen, ammonia, nitrite, hydrogen sulfide, or iron. These were all present and abundant when life first arose.
Thank-you Bill Martin for emphasizing the importance of fundamental concepts in biochemistry. I only wish everyone appreciated your answer.
1. Moran, L.A., Horton, H.R., Scrimgeour, K.G., and Perry, M.D. (2012) Principles of Biochemistry 5th ed., Pearson Education Inc. page 175 [Pearson: Principles of Biochemistry 5/E] p. 4
2. Chemoautotrophs are organisms—all of them are bacteria—that don’t capture light energy by photosynthesis and don’t require any organic molecules as food sources. They are the best examples of what metabolism in the earliest life forms must have looked like.
Read the original interview here