A ligand is a molecule that binds to a central atom, often a metal, to form a complex. This binding can influence the properties and reactivity of the central atom. Ligands can be small molecules, ions, or even larger biological molecules like proteins. They play a crucial role in various chemical processes, including catalysis and biological functions. In the context of biochemistry, ligands are essential for processes like enzyme activity and signal transduction. For example, when a ligand binds to a receptor on a cell, it can trigger a response that affects how the cell behaves. Understanding ligands is vital for developing new drugs and therapies.