It is the addition of phosphate to an organic substance in a biological process.
Proteins must be phosphorylated in order to function.
This is because it causes diverse actions of modification in a variety of enzymes, which governs their functioning.
This reaction is very important in biology since it is required for various biological processes like inflammation, apoptosis, metabolic control, proliferation, and subcellular transport.
Phosphorylation also specifically refers to the addition of inorganic phosphate to adenosine diphosphate in order to synthesize adenosine triphosphate that serves as the energy currency in the cell.
The reaction is catalyzed in the inner mitochondrial membrane.