Dehydration synthesis and hydrolysis are paired chemical processes that build and break many biological molecules. Dehydration synthesis joins smaller molecules, often called monomers, into larger molecules by removing water. Hydrolysis does the reverse by using water to split larger molecules into smaller parts.
These reactions matter because they explain how cells make proteins, carbohydrates, lipids, and nucleic acids, and how digestion breaks food into usable nutrients.
In dehydration synthesis, an OH group from one molecule and an H atom from another combine to form H2O, while a new covalent bond forms between the remaining parts. In hydrolysis, a water molecule is added across a bond, with H attaching to one fragment and OH attaching to the other. Enzymes usually control these reactions in living systems so they happen quickly and specifically.
Together, the two processes are like opposite directions of molecular construction and molecular breakdown.
Key Facts
- Dehydration synthesis: monomer + monomer -> dimer + H2O
- Hydrolysis: polymer + H2O -> smaller molecules
- Dehydration synthesis forms covalent bonds by removing water.
- Hydrolysis breaks covalent bonds by adding water.
- For n monomers joined into one linear polymer, water molecules released = n - 1.
- Common examples include amino acids forming proteins, sugars forming polysaccharides, and nucleotides forming nucleic acids.
Vocabulary
- Monomer
- A monomer is a small molecular unit that can bond with other similar units to form a larger molecule.
- Polymer
- A polymer is a large molecule made of many repeating or related monomer units bonded together.
- Dehydration synthesis
- Dehydration synthesis is a reaction that joins molecules by forming a bond and releasing a water molecule.
- Hydrolysis
- Hydrolysis is a reaction that breaks a chemical bond by adding water.
- Covalent bond
- A covalent bond is a chemical bond in which atoms share electrons.
Common Mistakes to Avoid
- Thinking dehydration synthesis means a molecule simply dries out. It specifically means a covalent bond forms while H2O is produced from atoms removed from the reacting molecules.
- Thinking hydrolysis releases water. Hydrolysis uses water as a reactant to break a bond, so H2O appears on the left side of the reaction.
- Forgetting that one water molecule is released for each bond formed in a simple linear polymer. Joining 10 monomers into one chain forms 9 bonds, not 10.
- Assuming dehydration synthesis and hydrolysis happen without energy or enzymes in cells. In living systems, these reactions are usually controlled by specific enzymes and are tied to energy changes.
Practice Questions
- 1 A cell joins 6 glucose monomers into one straight-chain carbohydrate. How many water molecules are released during dehydration synthesis?
- 2 A protein fragment has 12 amino acids in one linear chain. How many peptide bonds must be broken by hydrolysis to separate it into 12 individual amino acids, and how many water molecules are used?
- 3 Explain why dehydration synthesis and hydrolysis are considered opposite processes, using the roles of water and chemical bonds in your answer.