Signal Molecules

Cells do not work as isolated systems. A multicellular organism is, by definition, groups of many cells working together to keep an organism alive, and in order to do so, cells must be able to communicate with one another. This communication occurs through transmission of a signal, which is a coded message from one cell to another or within the cell itself. Even single-celled organisms communicate with other cells in their colonies or environments. In addition, all cells must be able to detect environmental changes, which they accomplish by using signal molecules.

A signal molecule is any molecule that is involved in the response of a cell to either other cells or its environment. Signal molecules enable cells to control their functions so that not all cell functions occur at all times. During cell division, mammalian cells growing in culture will not proliferate unless the culture medium contains specific proteins called growth factors. These growth factors signal the cell that it is time to divide and proliferate. When cells receive the signal from the growth factor signal molecules, they, too, respond by dividing via mitosis. The process by which a cell translates a chemical signal to a cellular response is called signal transduction. Thus, the three basic steps of cell signaling are reception, transduction, and response. Cell signaling is a very specific process. A signaling cell is a cell that produces a signal molecule, and the signal molecule binds with high affinity to a receptor. A receptor is a protein that binds to a specific extracellular signal molecule, which then begins a cell response. Extracellular means "outside of the cell," and an extracellular signal molecule is a cue molecule, such as a hormone or growth factor, that sends information to target cells. An example of an extracellular signal molecule is the hormone estrogen, which is produced in ovaries, travels through the bloodstream, and activates various sexual characteristics, such as development of breast tissue, pubic hair, and underarm hair. A cell that responds to a specific signal molecule is a target cell, which, for a given signal molecule, has a receptor specific to that signal molecule. Cells may also transmit signals within the cell itself. This depends on an intracellular signal molecule, which is a cue molecule, such as cAMP and proteins, working as a messenger or chemical signal. cAMP is a derivative of adenosine triphosphate (ATP). An example of intracellular signaling is the release of a hormone to encourage mitosis in a fertilized egg.

If a cell is not the target cell for a particular signal molecule, the cell, lacking the necessary receptors, makes no response to the signal molecule. In addition, any given cell can act both as a signaling cell and a targeting cell, and most cells send and receive any number of signaling molecules at any given time. The cells in multicellular organisms are in constant communication. For example, signals that occur in an apple tree include responses to changes in light availability, water availability, temperature, reproduction, growth, and fruit production, as well as activity involving the roots, trunk, branches, and leaves.