The chemical reactions of metabolism are reversible, and they, too, would reach equilibrium if they occurred in the isolation of a test tube.  Because systems at equilibrium are at a minimum of G and can do no work, a cell that has reached metabolic equilibrium is dead.

A cell in our body is not in equilibrium. The constant flow of materials in and out of the cell keeps the metabolic pathways from ever reaching equilibrium, and the cell continues to do work throughout its life. This principle is illustrated by the open (and more realistic) hydroelectric system.

A Closed Hydroelectric System: Water flowing downhill turns a turbine that drives a generator providing electricity to a light bulb, but only until the system reaches equilibrium.

An Open Hydroelectric System: Flowing water keeps driving the generator because the intake and outflow of water keep the system from reaching equilibrium.

A Multistep Open Hydroelectric System: Cellular respiration is analogous to this system: Glucose is broken down in a series of exergonic reactions that power the work of the cell.  The product of each reaction becomes the reactant for the next, so no reaction reaches equilibrium.

ATP Powers Cellular Work by Coupling Exergonic Reactions to Endergonic Reactions:

Mechanical Work, such as the beating of cilia, the contraction of muscle cells, and the movement of chromosomes during cellular respiration.

Transport Work, the pumping of substances across membranes against the direction of spontaneous movement.

Chemical Work, the pushing of endergonic reactions, which would not occur spontaneously, such as the synthesis of polymers from monomers.

Energy Coupling: The use of an exergonic process to drive an endergonic one.  ATP is responsible for mediating most energy coupling in cells, and in most cases, it acts as the immediate source of energy that powers cellular work.

The Structure and Hydrolysis of ATP:

ATP contains the sugar ribose, with the nitrogenous base adenine and a chain of three phosphate groups bonded to it.  It can be broken by hydrolysis.  When the terminated phosphate bond is broken, a molecule of inorganic phosphate leaves the ATP, which becomes adenosine diphosphate, or ADP.

If the DG of an endergonic reach is less than the amount of energy released by ATP hydrolysis, then the two reactions can be coupled so that, overall, the coupled reactions are exergonic.  Because their hydrolysis releases energy, the phosphate bonds of ATP are sometimes referred to as high-energy phosphate bonds. 

The phosphate bonds of ATP are no usually strong bonds, as “high-energy” may imply; rather, the molecule itself has high energy in relation to that of the products (ATP and P). ATP releases on hydrolyzing a phosphate group are somewhat greater than the energy most other molecules could deliver. The triphosphate tail of ATP is the chemical equivalent of a compressed spring.

How ATP Performs Work:

When ATP is hydrolyzed in a test tube, the release of free energy merely heats the surrounding water. Shivering uses ATP hydrolysis during muscle contraction to generate heat and warm the body. With the help of specific enzymes, the cell is able to couple the energy of ATP hydrolysis directly to endergonic processes by transferring a phosphate group from ATP to some other molecule, such as the reactant. 

The recipient of the phosphate group is then said to be phosphorylated.  The key to coupling exergonic and endergonic reactions is the formation of this phosphorylated intermediate, which is more reactive (less stable) than the original unphosphorylated molecule.

The Regeneration of ATP:

An organism at work uses ATP continuously, but ATP is a renewable resource that can be regenerated by the addition of phosphate to ADP. Shuttling of inorganic phosphate and energy is called the ATP cycle, and it couples the cell’s energy-yielding (exergonic) processes to the energy-consuming (endergonic ones).

Because ATP formation for ADP and P is not spontaneous, free energy must be spent to make it occur.  Catabolic (exergonic) pathways, especially cellular respiration, provide the energy for the endergonic process of making ATP.

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