dehydration
Summary
Dehydration (a condensation reaction) joins two molecules by removing the elements of water, so H₂O is released. It forms peptide, glycosidic and ester bonds. Linking n monomers into a linear chain releases n − 1 waters, and the reverse reaction is hydrolysis.
Detail
Meaning: A dehydration (condensation) reaction joins two molecules by removing the elements of water. One partner loses an –OH, the other loses an H, and H₂O is released. It forms peptide bonds, glycosidic bonds, esters and phosphoanhydride bonds. Building polymers this way is usually endergonic, so cells couple it to ATP or another energy source. The reverse is hydrolysis, which adds water to break the bond.
Equation: A–OH + H–B → A–B + H₂O, where A and B are the groups being joined and H₂O is the water released. No SI unit applies to the equation. The uncoupled ΔG (J/mol) is usually positive.
On the MCAT: Passages use polypeptides or polysaccharides. You may be asked to name the bond formed or to count water: linking n monomers into a linear chain releases n − 1 water molecules. Organic chemistry uses the same word for acid-catalyzed loss of water from an alcohol to give an alkene.
Common confusion: Dehydration here does not mean drying out. Water is a product of the reaction, not a solvent being removed. Students also swap the two terms. Dehydration synthesis builds a bond and releases water. Hydrolysis breaks a bond and consumes water.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.