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General ---------Biology----------------------------------------------------- ------------------Online-----------------------------wilcox@sunline.net Ch. 6  Metabolism of carbohydrates, fats, proteins, nucleic acids

"Free energy," like "hydrogen bond," is a bit of a misnomer.  The energy is "free" only in that it is free from demand of entropy.  Calling this energy free, is like a dictator calling the money you have after paying tax "free money."

Spontaneous transformations, ones that result from ordered systems falling into disorder, and releasing their energy, are called exergonic.  The breakdown of food molecules, releasing their chemical energy,  is an example of an exergonic reaction.  The reaction would likely happen spontaneously, given enough time, and cells use enzymes to speed up the process.

Cells, tissues and organisms must receive fresh inputs of food energy and drain away waste heat and waste molecules; as the bottom portion of the figure illustrates.  The system never reaches equilibrium, and the light of life shines continuously...a disequilibrium.   Death come when either energy inputs are stopped or removal of waste heat or molecules are prevented.

"Metabolic Disequilibrium"

The metabolic pathways of living systems run continuously.  Equilibrium is avoided because as fast as the "product" from a reaction is generated, the product is used as the "substrate" for the next step in the pathway.

Try and visualize what is happening in the interrelated chemical pathways shown below. The chemicals are being shuttled around from enzyme (dot) to enzyme (dot).  Each enzyme has plenty of substrate ready to enter it's active site, and the enzyme has no problem releasing its product, because the products are removed as fast as they are produced.  Products are continuously removed so they do not build up and stop the reaction.

a.  a large fishing net
b.  a transportation system of roads, bridges and parking lots in a major city
c.  the continuous flow of automobile traffic in a major city, with intersections and stop lights
d.  traffic gridlock in a major city with all incoming highways closed, all parking lots full and all stop lights on red

As long as the cell has a steady supply of food molecules, and is able to expel the carbon dioxide and other waste to the surroundings, the chemical reactions of the cell do not reach equilibrium.  They continue on, and the properties of life emerge.  The continuous flow of energy and molecules in a major biochemical pathway is analogous to flowing traffic in a major city.

Most chemical reactions are not "alive."  Most quickly reach equilibrium and stop. 

Living chemical reactions share two fundamental features:

• they are anabolic, or building order in a universe that is otherwise falling into disorder (entropy)
• they are maintained at a metabolic disequilibrium, with new substrates added, and waste products removed

Living reactions are often reversible, and would normally reach equilibrium if they occurred in isolation from other reactions.  One of the functions of cells and organelles is to "organize" and "partition" these living chemical reactions into separate pathways so each pathway can maintain its disequilibrium.

An illustration from Chapter 2 shows a reversible reaction where products build up and react to form substrate, reversing the reaction.  This reaction eventually reaches an equilibrium.

In Chapter 7, A Tour of the Cell, we look at how the various organelles inside cells organize and partition different pathways, while creating intersections where pathways flow together when appropriate.

For example:  a young pig eats 100 calories of corn.  Exergonic reactions break the corn down, giving off energy.  Most of the energy given off is lost as heat, but some of it is stored in endergonic reactions producing new molecules of pig tissue.  The pig has coupled the energy from the breakdown of food with the metabolic reactions required to build more pig tissue.

At the organism level, this coupling is easy to see.  Assume you build your child a dog house.  You eat a breakfast and then use that energy to cut boards and nail them together in a dog house, according to the plans you got from the pet store.  The saw and the hammer allow you to efficiently couple the energy released from breakfast molecules into an ordered arrangement of boards called a dog house. 

Now back to the pig.  At the cellular level in the pig, the coupling is accomplished by a molecule called "ATP."  ATP stores the energy given off from exergonic reactions (e.g.. the breaking down of corn molecules) until it can be used in endergonic reactions (e.g. the build up of new pig tissue ), according to the plan carried in the pig's DNA. 

Some people think of ATP as a  rechargeable battery because it can be recharged for use again and again.

ATP picks up the energy from the exergonic reactions of digestion and carries this energy to the reactions involved in building new pig tissue ( the endergonic reactions).  ATP transfers energy to a substrate molecule or an enzyme by transferring a phosphate functional group.  Then the spent ATP molecule returns to the site of the exergonic reactions to get recharged with another phosphate functional group.