What type of reaction combines two molecules while removing a small molecule, usually water?

A condensation reaction is characterized by the joining of two molecules to form a larger molecule, accompanied by the elimination of a small molecule such as water. This process is fundamental in the synthesis of macromolecules, such as proteins and nucleic acids, where amino acids or nucleotides link together, respectively, while releasing water.

In biochemistry, condensation reactions are essential for building polymers from monomers. For example, during protein synthesis, peptide bonds are formed between amino acids through a condensation reaction, resulting in the release of water. Similarly, nucleotides are linked through phosphodiester bonds in nucleic acid synthesis via condensation.

The other types of reactions listed do not fit this description. Catabolic reactions typically involve the breakdown of larger molecules into smaller ones, releasing energy, while redox reactions involve the transfer of electrons between molecules, and endergonic reactions require an input of energy to proceed, rather than simply combining two molecules and releasing water.