Energy and Metabolism

ID #2172

When a dehydration or hydrolysis reaction occurs, a molecule of water is either taken away or added. What specifically is it added to? Is it added to the macromolecule? Or is saying that a molecule of H20 is added just an easier way of referring to the H and OH added to the separate monomers?

In dehydration reactions, a covalent bond is formed between two monomers by removing water. To answer your question about where does the water go in a dehydration reaction, its important to remember that the cell is mostly made up of water. As biologists, we assume there is always water surrounding the chemistry that occurs in a living cell. So, the water that is produced in the dehydration reaction simply gets added to the aqueous part of the cell. 

In hydrolysis reactions, a covalent bond joining a polymer is broken by adding water. The water molecule is broken into H + OH, and each of those are added to the two monomers that was just broken apart. Since there are many types and examples of hydrolysis reactions, it is difficult to answer your question directly about what atom does the water attach to once it's added. In the example given in lecture 3, an oligosaccharide chain is broken apart by the addition of water resulting in the release of two products (a monosachharide and a shorter oligosaccharide), both of which gained either a H or an OH (called hydroxyl). 

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