Why high energy molecular bonds are weak bonds
All energy on Earth comes from its sun. But, what is energy?
Physics defines energy as an ability to do work. Chemistry simply maintains that molecules possess energy due to their motion. Physiology is a specialized form of chemistry so it is easiest to stick with the definition from chemistry.
It is atomic motion, defined as kinetic energy, which humans and other life forms organize into special functions called physiology. Atoms have moving parts and they themselves are in motion.
Why atoms and molecules move, how they got started in motion, and why they do not stop are all theoretical questions without final answers. That molecules DO possess a predictable amount of motion (kinetic energy) at any given temperature is based on scientific observation.
Constant molecular motion
The inherent property of molecules to move is not emphasized in physiology textbooks. Rather molecules are drawn in one of several static formats. One such format is displayed in the picture to the left. Such shorthand is useful but misleading.
Motion (kinetic energy) at the molecular level in humans is not noticeable. It is difficult to imagine, because our bodies appear to be relatively solid objects. The thought of all that random motion under the surface is a bit alien.
Molecules possess kinetic energy because the atoms which form them are in constant motion. Atoms link together, that is they form bonds, spontaneously if it is possible for their electrons to mingle. For more about which atoms can mingle their electrons, see my post titled “Water’s Chemistry Governs Physiology”.
Molecular bond formation
Bond formation between atoms to make molecules is a matter of the right partners with sufficient kinetic energy coming together in the correct orientation in space. The more kinetic energy an atom has, the faster it moves and the more likely it will meet a possible partner in the correct orientation.
Within molecules there are both high energy and low energy bonds. The low energy description comes from the fact that bound atoms have lower kinetic energy, or freedom of motion, than before the bond was formed. That is atoms linked together move more slowly than they did as separate entities. The energy that they lose is then available to other molecules in the environment.
The phrase strong bond is used interchangeably with the description “low energy bond”. In contrast, a “high energy bond” is one where the participant atoms retain a lot of the kinetic energy they had before the bond was formed. The retained kinetic energy of atoms in such bonds weakens the bond. Therefore, the phrase weak bond is used interchangeably with the description “high energy bond”.
Energy extraction from food
Key to understanding how people extract energy from food molecule rests with the fact that the molecular bonds in sugar are much weaker than the final bonds in the products of sugar metabolism, carbon dioxide and water. It takes less energy for the body to break the bonds in sugar than is released to the local environment by the formation of carbon dioxide and water.
The net increase in kinetic energy available to other molecules in the local environment as a result of sugar metabolism increases the likelihood that two atoms able to make a bond will find each other.
In my book “Physiology: Custom-Designed Chemistry” there is a more in-depth discussion of what energy means in physiology.
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Margaret Thompson Reece PhD, physiologist, former Senior Scientist and Laboratory Director at academic medical centers in California, New York and Massachusetts and CSO at Serometrix LLC is now CEO at Reece Biomedical Consulting LLC.
Dr. Reece is passionate about helping students, online and in person, pursue careers in life sciences. Her books “Physiology: Custom-Designed Chemistry” (2012), “Inside the Closed World of the Brain” (2015) and the workbook (2017) companion to her online course “30-Day Challenge: Craft Your Plan for Learning Physiology” are written for those new to life science. More about her books can be found at amazon/author/margaretreece.
Dr. Reece offers a free 30 minute “how-to-get-started” phone conference to students struggling with human anatomy and physiology. Schedule an appointment by email at DrReece@MedicalScienceNavigator.com.by