Chemical signaling molecules are small molecules that act to carry a message in a biological system. A cell can secrete signaling molecules and allow them to diffuse through the bloodstream. Signaling molecules can also be stuck to the surfaces of cells. It would be impossible to give a comprehensive description, so we’ll just mention a couple of examples of chemical signaling. Apoptosis, which is the intentional death of a cell, is a process that involves chemical signaling. An external signal reaches the cell, which sets off a series of reactions inside the cell, ultimately leading to its death. Calcium ions are another species often involved in cell signaling as their concentration affects the activity of many proteins and is also important for telling cells when to reproduce. Hormones are another type of chemical signals. They travel through our bodies, controlling growth of muscles and tissue, our reproductive systems, and our metabolism, among other things. These examples are just a small sample of the huge number of processes controlled by chemical signaling.