Many proteins are modified by the addition of a phosphate chemical group. This process is called phosphorylation. Why does this occur? To function properly, cells must regulate thousands of processes to occur at the right time and place. They have this control primarily by regulating the activity of proteins. One way proteins, like channels, enzymes and intercellular signalling molecules, can be controlled is through phosphorylation. A phosphate group is added to the protein by an enzyme called a kinase, which transfers a phosphate from ATP to the target. This phosphate group can be removed by a phosphatase enzyme, making phosphorylation, a process that can be reversed. Whether or not a protein is phosphorylated depends on whether there are more kinases or phosphatases near it. These levels can change within seconds, which allows phosphoylation to rapidly change the target protein's activity. But how does phosphorylation change the activity of a protein? First, we need to understand a little about what proteins are made of and how they're structured. Proteins consist of chains of chemical subunits, called amino acids, which fold up on each other to make a 3D structure. The amino acids of a protein interact with one another to form a structure with a specific shape, and it is that shape, which then determines the function of the protein. Some of these amino acids can be modified by phosphorylation, and the addition of a negatively charged phosphate group can affect how the amino acids bind together. This can change the 3D structure of the protein, and therefore, its function within a cell. All of these changes can modulate cellular processes in many ways, sometimes speeding them up, or slowing them down, or even stopping them entirely. Cell motion, division, metabolism, and even cell death are all regulated by phosphorylation.