How does the cell use energy? The short answer is to do work. A cell does three types of work: mechanical (e.g., contracting muscle cells), transport (e.g., moving substances across the cell membrane), and chemical (e.g., non-spontaneous reactions between molecules). A major source of chemical energy for this work is adenosine triphosphate (ATP ), which is illustrated in this figure.
ATP is a 5-carbon sugar (ribose) attached to a nitrogenous base (adenine; recall our discussion of the nucleotides DNA and RNA) and a group of three phosphates. The three phosphates are the triphosphate component of adenosine triphosphate, and they are very unstable. This instability is due to the three negative charges that create an intramolecular strain in one area of the molecule. Most reactions that involve ATP depend on the hydrolysis of the third phosphate to liberate the potential energy that can be used to do work. In addition to free energy that can do work, ADP (adenosine diphosphate) and inorganic phosphate (PO4 ; abbreviated Pi or a circled P) are released during hydrolysis. The overall reaction is summarized in the figure below. The amount of energy that is made available to do work during this process is variable (depending on a number of factors, including temperature).
Energy is made available from the hydrolysis of ATP to ADP + Pi. This energy provides energy for endergonic reactions. The many mechanisms that different reactions use to obtain energy from ATP are remarkable. Generally they all involve tapping into high-energy electrons found associated with the terminal phosphate group. The use of an exergonic (energy-releasing) process to drive an endergonic (energy-requiring) process is called energy coupling . There are many ways that this can be done. Examine how energy coupling can be used to allow an energetically unfavorable (non-spontaneous ) reaction to take place.
Glutamine (an amino acid) synthesis is a thermodynamically unfavorable reaction: Glu + NH3 ----> Glu-NH2
The energy required for this reaction to run forward (+ΔG) is +3.4 kcal/mol, therefore, it is a non-spontaneous reaction (it cannot run without an input of energy). However, this reaction does occur with the help of ATP. This is accomplished by coupling the removal of the terminal phosphate of ATP and the addition of ammonia to glutamic acid. In the first step, glutamic acid becomes an unstable phosphorylated intermediate when ATP is broken down. In the second step, ammonia displaces the phosphate group and glutamine is formed. This figure summarizes how ATP can be coupled to a reaction, such that an unfavorable reaction becomes exergonic.
The hydrolysis of ATP to ADP and Pi releases energy(-ΔG = -7.3 kcal/mol) in excess of the energy required (+3.4 kcal/mol) for the synthesis of glutamine.
This type of energy coupling procedure is extremely important, and it allows many endergonic anabolic reactions to occur in the cell.
Watch this short video on ATP (from Ricochet Science):
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