Phosphorylation is the addition of PO₃ ^2- to the OH group of a molecule. For proteins in eukaryotes, such as animals, usually serine, threonine, and tyrosine are the amino acids that are phosphorylated. All living things phosphorylate their proteins, but in prokaryotes like bacteria, phosphorylation mostly occurs on histidine and aspartate amino acids .

Phosphorylation generally occurs when an enzyme called a kinase adds a phosphate group to a molecule. In many cases this molecule is a protein in which case the protein can be activated, deactivated, or in some way perform another function. The negative charge caused by the addition of a phosphoryl group can cause a protein to take on a different shape which in turn alters its functions. Phosphorylation is especially important in cell signaling in which a signal is often sent from the cell membrane to the nucleus so the cell can respond to its environment.

Glucose is also phosphorylated when it enters a cell, since phosphorylated glucose can exit a cell. Over half of eukaryotic proteins appear to be phosphorylated, demonstrating the importance of this process. As far as the explicit process by which phosphorylation occurs, it requires the “collision” of a kinase and a protein with the necessary energy. This process is highly orchestrated so that only the correct amino acids are modified. Enzymes can often be highly specific to only modifying a single amino acid on a single protein.

The underlying mechanic of phosporylation in animals is the same as that of any other living thing: adenosine triphosphate (ATP) gives up a phosphate ion to become adenosine diphosphate (ADP), and imparting energy into whatever it is that the ATP molecule is phosphorylating. Often this involves an enzyme plucking the phosphate off and, in the process, changing shape in a way that forces another chemical reaction to occur. Exactly how this works depends on the enzyme, and there are many, MANY enzymes in animals that do this.

Check out the wikipedia article on Phosphorylation: here

The first paragraph is especially interesting - so many proteins and other molecules get phosphorylated!

Basically, there are enzymes - protein molecules that speed up some chemical reactions - and the enzymes can transfer a phosphate group from the high-energy molecule, ATP [adenosine tri-phosphate], onto some other molecule. The ATP becomes ADP [adenosine di-phosphate]; i.e., there are 3 phosphates in ATP and 2 phosphates in ADP.

Phosphorylation is a very important chemical mechanism that allows many reactions to take place in the body. Often times, reactions that are necessary for life are not energetically favorable, meaning they will not occur spontaneously. Catalysts can be used in these scenarios to change the reaction mechanism and create a new reaction that ends with the desired product. In simpler terms: animals will use the same starting chemicals, but find a different way to create their product.

In all organisms, the best alternative pathway is through the use of enzymes, which are biological catalysts. One mechanism for catalyzing a reaction in biology is phosphorylation, which takes place by adding a phosphate group (PO₃^2-) to a molecule. When a phosphate group is added, that part of the molecule becomes much more highly reactive, which then makes later parts of the reaction much easier to perform. This transferring of phosphate groups typically takes place inside of enzymes. In most scenarios, the phosphate group used comes from ATP (adenosine triphosphate). A phosphate is taken from ATP and it becomes the much less reactive ADP. When people say that ATP is like an energy source in the body, this is what they are referring to.

Organisms can use ATP to catalyze reactions, essentially spending the energy that was stored inside ATP on these unfavorable reactions. The actual mechanism for this transfer is complicated, but it follows these rules very closely. Thank you for your question!