Cell – Cell Communication in Biology
Communication between cells is common in nature. Cell signaling occurs in all multicellular organisms, providing an indispensable mechanism for cells to influence one another. The cells of molecular organisms use a variety of molecules as signals, including not only peptides, but also large proteins, individual amino acids, nucleotides, steroids and other lipids.
Even dissolved gases are used as signals. Nitric oxide (NO) plays a wide variety of roles. Some of the molecules are attached to the surface of the signaling cell; others are secreted through the plasma membrane or released by exocytosis.
Cell Surface Receptors
Any given cell of a multicellular organism is exposed to a constant stream of signals. At any time, hundreds of different chemical signals may be in the environment surrounding the cell. However, each cell responds only to a certain signals and ignores the rest, like a person following the conversation of one or two individuals in a noisy, crowded room. How does a cell “choose” which signals to respond to? Located on or within the cell are receptor proteins, each with a three-dimensional shape that fits the shape of a specific signal molecule. When a signal molecule approaches a receptor protein of the right shape, the two can bind. This binding induces a change in the receptor protein’s shape, ultimately producing a response in the cell. Hence, a given cell responds to the signal molecules that fit the particular set of receptor proteins it possesses and ignores those for which it lacks receptors.
Types of Cell Signaling: Cells can communicate through any of four basic mechanisms, depending primarily on the distance between the signaling and responding cells. In addition to using these four basic mechanisms, some cells actually send signals to themselves, secreting signals that bind to specific receptors on their own plasma membranes. This process, called autocrine signaling, is thought to play an important role in reinforcing developmental changes.
Direct Contact: The surface of a eukaryotic cell is richly populated with proteins, carbohydrates and lipids attached to and extending outward from the plasma membrane. When cells are very close to one another, some of the molecules on the plasma membrane of an adjacent cell. Many of the important interactions between cells in early development occur by means of direct contact between cell surfaces.
Paracrine Signaling: Signal molecules released by cells can diffuse through the extracellular fluid to other cells. If those molecules are taken up by neighboring cells, destroyed by extracellular enzymes, or quickly removed from the etracellular fluid in some other way, their influence is restricted to cells in the immediate vicinity of the releasing cell. Signals with such short-lived, local effects are called paracrine signals. Like direct contact, paracrine signaling plays an important role in the early development, coordinating the activities of clusters of neighboring cells.
Endocrine signaling: if a released signal molecule remains in the extracellular fluid, it may enter the organism’s circulatory system and travel widely throughout the body these longer lived signal molecules, which may affect cells very distant from the releasing cell, are called hormones, and this type of intercellular communication is known as endocrine signaling.
Synaptic Signaling: In animals, the cells of the nervous system provide rapid communication with distant cells. Their signal molecules, neurotransmitters, do not travel to the distant cells through the circulatory systems as hormones do. Rather the long, fiberlike extensions of nerve cells release neurotransmitters from their tips very close to the target cells. The association of a neuron and its target cell is called a chemical synapse, and this type of intercellular communication is called synaptic signaling. While paracrine signals move through the fluid between cells, neurotransmitters cross the synaptic gap and persist only briefly.
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