Covalent modification refers to the process where a chemical group is added or removed from a molecule through covalent bonds. This modification can alter the molecule's structure and function, impacting biological processes. Common examples include the addition of phosphate groups in protein phosphorylation, which can activate or deactivate enzymes. This type of modification is crucial in regulating various cellular activities, such as metabolism and signal transduction. Enzymes like kinases and phosphatases play key roles in these processes by adding or removing chemical groups, respectively, thereby influencing the activity of target molecules and cellular responses.