Cells build and break large biological molecules using two opposite chemical processes: dehydration synthesis and hydrolysis. Dehydration synthesis joins small units called monomers into larger polymers by removing a water molecule. Hydrolysis breaks polymers into monomers by adding water.
These reactions matter because they help cells store energy, build structures, digest food, and recycle molecules.
Key Facts
- Dehydration synthesis joins monomers: monomer + monomer -> polymer segment + H2O.
- Hydrolysis breaks bonds using water: polymer segment + H2O -> monomer + monomer.
- In dehydration synthesis, one monomer often loses H and another loses OH, forming H2O.
- In hydrolysis, H from water attaches to one product and OH attaches to the other product.
- Proteins form by dehydration synthesis of amino acids, creating peptide bonds.
- Polysaccharides such as starch form from monosaccharides and can be hydrolyzed back into sugars.
Vocabulary
- Monomer
- A small molecular building block that can bond with other similar units to form a polymer.
- Polymer
- A large molecule made of many repeating monomer units bonded together.
- Dehydration synthesis
- A reaction that joins molecules by removing the atoms needed to form water.
- Hydrolysis
- A reaction that breaks a chemical bond by adding water across the bond.
- Peptide bond
- A covalent bond that links amino acids together in proteins.
Common Mistakes to Avoid
- Thinking dehydration synthesis adds water, which is wrong because it removes H and OH from reactants to produce H2O while forming a new bond.
- Thinking hydrolysis builds polymers, which is wrong because hydrolysis uses water to break bonds and separate monomers or smaller units.
- Forgetting to count water molecules, which is wrong because each bond formed by dehydration synthesis usually releases one H2O and each bond broken by hydrolysis usually uses one H2O.
- Assuming all biomolecules use the same bond name, which is wrong because carbohydrates, proteins, lipids, and nucleic acids have different linkage types.
Practice Questions
- 1 A cell joins 8 glucose monomers into one polysaccharide chain using dehydration synthesis. How many water molecules are released?
- 2 A protein fragment contains 12 amino acids in a single chain. How many peptide bonds must be hydrolyzed to break it completely into individual amino acids, and how many water molecules are used?
- 3 Explain why dehydration synthesis and hydrolysis are considered opposite processes, using the role of water and chemical bonds in your answer.