This blog post is a part of STEMTalksNC’s ever-expanding General Biology Series. In the next series of posts, we will discuss metabolism, which will help you understand how matter and energy flow during life’s processes and how that flow is regulated. This is the third post of this series on metabolism, discussing ATP’s role in Cellular Work.
ATP Powers Cellular Work by Coupling Exergonic Reactions to Endergonic Reactions
Cellular Work consists of non-spontaneous processes such as chemical work (the synthesis of polymers from monomers), transport work (active transport), and mechanical work (a contraction of muscle cells). Energy Coupling, which is where an exergonic process drives an endergonic process, is a key feature in the way that cells manage their energy resources to do work. ATP is responsible for mediating most energy coupling cells, and in most cases it acts as immediate source of energy that powers cellular work.
Structure and Hydrolysis of ATP
ATP is a nucleotide tri-phosphate consisting of sugar ribose, nitrogenous base adenine, and a chain of three phosphate groups bonded to it. The bonds between the phosphate groups of ATP can be broken by hydrolysis, which is an exergonic reaction that releases energy.. When the terminal phosphate bond is broken, ATP becomes ADP (adenosine diphosphate).
Sometimes, the phosphate bonds of ATP are referred to as high-energy phosphate bonds, but the term is misleading. The release of energy during the hydrolysis of ATP comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves.
How ATP Performs Work
When ATP is hydrolyzed in a test tube, the release of free energy merely heats the surrounding water. This process is considered inefficient. With the help of specific enzymes, the cell is able to the couple the energy of ATP hydrolysis directly to endergonic processes by transferring a phosphate group to some other molecule. The molecule that is the recipient of the phosphate group is said to be phosphorylated. This is the key to coupling: the formation of this phosphorylated intermediate, which is more reactive (less stable) than the original unphosphorylated molecule (the less stable, the more free energy change). However, for the coupling process to be exergonic, the free energy change of the endergonic reaction has to be less than the amount of energy released by ATP hydrolysis.
The Regeneration of ATP
ATP is a renewable source that can be regenerated by the addition of phosphate to ADP. However, since this addition of phosphate to ADP is endergonic, the free energy required for this process comes from the exergonic breakdown reactions in the cell.
That’s all for this post. In the next post of this series on metabolism, we will discuss how enzymes speed up metabolic reactions! Feel free to share this post to people that might benefit!
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