Cells are constantly receiving input from each other and their environment and even from the target cell itself. Cells receive information from direct contact with neighboring cells through the binding of membrane glycoproteins and glycolipids. They also can receive information from their immediate environment through the binding of ions or molecules to proteins on their membrane surface. Additionally, some small, nonpolar molecules are able to cross the plasma membrane and bind to receptors inside the cell. In multicellular organisms, cells can receive information from distant cells and organs within the organism in the form of traveling molecules such as hormones and cytokines (small signaling proteins).
Cell communication is important to cells in a variety of ways, by informing such activities as division, immune response, tissue formation, and cell growth. For example, the recognition of similar cells plays a critical role in yeast reproduction. There are two mating types of yeast. The cells of each type can only bind to the receptors on the cells of the other mating type. The binding acts as a signal that causes the pair of cells to grow toward each other and fuse together. The resulting combined cell carries the genomes of both of the original yeast cells. Yeast cell-cell recognition prevents yeast cells from mating with cells of other species. The recognition of cells that do not belong is also key for protection against pathogenic intruders. In addition, neighboring cells communicate to coordinate activities as tissue layers develop in a growing multicellular organism. Communication between neighboring cells can also prevent an individual cell from growing too quickly or too much.Each cell must receive information from other regions of the organism so it can properly do its job as part of a whole organism. The coordinated functioning of the organism as a whole depends on cell communication to maintain balanced internal conditions, called homeostasis. If an important change happens somewhere in an organism, a cell can send a message to distant cells to trigger the appropriate responses. For example, when human blood sugar increases, the pancreas releases a hormone called insulin. This message travels through the blood and tells other cells to increase glucose uptake, which results in the lowering of blood glucose levels. A cell needs the correct receptor on its cell surface to be able to receive a specific message. Cell-surface transmembrane proteins receive many chemical messengers from other cells. Most of these cell-surface receptors are classified as G-protein-coupled receptors (GPCR). Examples of these include those that bind epinephrine molecules in the kidneys and one of the receptors that malfunctions following infection by human immunodeficiency virus (HIV). Similar to how enzymes must structurally match the substrate that they bind, specific receptors are necessary to bind particular signaling molecules. When the signal is received, the receptor transmits the signal from outside the cell to inside the cell, through a conformational change, which triggers a series of events within the cell. Another type of receptor is called an ion-channel receptor. These are embedded within the cell membrane and act as a gate to allow ions into and out of the cell. A third type of receptor is called a receptor tyrosine kinase (RTK). These receptors use enzymes to catalyze the transfer of phosphate groups from ATP to other proteins, leading to a cellular response.