Many biological molecules are assembled by forming covalent bonds between smaller building blocks and disassembled by cleaving those bonds. Two recurring net reaction patterns are condensation and hydrolysis.

A condensation reaction joins two molecular fragments while eliminating a small molecule, often water. Schematically,

$$\mathrm{A-OH+H-B\rightarrow A-B+H_2O}.$$

The exact atoms involved depend on the functional groups and reaction mechanism. Peptide, glycosidic and many ester linkages can be represented formally as condensation products of their building blocks.

A hydrolysis reaction cleaves a covalent bond by incorporating the components of water into the products:

$$\mathrm{A-B+H_2O\rightarrow A-OH+H-B}.$$

Hydrolysis is therefore the formal reverse connectivity change of the corresponding condensation.

Example: ester linkage

A carboxylic acid and an alcohol can be represented as forming an ester plus water:

$$\mathrm{RCOOH+R'OH\rightleftharpoons RCOOR'+H_2O}.$$

Hydrolysis of the ester restores carboxylic-acid and alcohol functionality.

This reaction pattern appears in many lipids and provides a useful structural analogy for other biomolecular linkages.

Net reaction pattern versus cellular mechanism

It is important not to imagine that cells usually build large biomolecules simply by placing two building blocks together and waiting for water to fall out. Direct condensation may proceed negligibly or far too slowly on its own. Biosynthetic pathways commonly create activated intermediates—more reactive intermediates produced by preceding reactions—and use biological catalysts called enzymes to select useful reaction paths.

Likewise, hydrolysis can be extremely slow without catalysis even when the products are favored. Enzymes can make selected hydrolysis and synthesis reactions occur rapidly under cellular conditions.

Condensation and hydrolysis are therefore best understood as recurring net bonding patterns. The detailed mechanism and energetic driving force must be analyzed separately for each biochemical process.