21.1:
Overview of Cell Signaling
Cells use a variety of signaling mechanisms to coordinate cellular processes and communicate with other cells. A signaling pathway is initiated when a signaling molecule called a ligand binds to its target receptor.
Paracrine signaling occurs when a secreted ligand targets the neighboring cells. Whereas, autocrine signaling occurs when the ligand targets the same cell that released it.
In a juxtacrine or contact-dependent signaling pathway, a ligand remains bound to the cell surface and interacts directly with the receptors on adjacent cells. Small soluble ligands may also directly flow between cells via gap junctions in the plasma membrane.
Multicellular organisms coordinate cellular behavior over a long distances. In endocrine signaling, hormones travel to different body parts through the bloodstream.
Synaptic signaling between neurons is a specialized form of long-range signaling. At the synaptic junction, the axon terminal of a neuron secretes ligands called neurotransmitters to communicate with target cells.
Additionally, quorum sensing is carried out by some bacterial communities to react together by sensing a critical concentration of signaling ligands.
21.1:
Overview of Cell Signaling
Despite the protective membrane that separates a cell from the environment, cells need the ability to detect and respond to environmental changes. Additionally, cells often need to communicate with one another. Unicellular and multicellular organisms use a variety of cell signaling mechanisms to communicate with the environment.
Cells respond to many types of information, often through receptor proteins positioned on the membrane. For example, skin cells respond to and transmit touch information, while photoreceptors in the retina can detect light. Most cells, however, have evolved to respond to chemical signals, including hormones, neurotransmitters, and many other types of signaling molecules. Cells can even coordinate different responses elicited by the same signaling molecule.
Typically, cell signaling involves three steps: (1) reception, (2) transduction, and (3) response. In most signal reception, a membrane-impermeable molecule, or ligand, causes a change in a membrane receptor; however, some signaling molecules, such as hormones, can cross the membrane to reach their internal receptors. The membrane receptor can then send this signal to intracellular messengers, transducing the message into a cellular response. This intracellular response may include changes in transcription, translation, protein activation, and other responses.
Unicellular organisms such as bacteria can use a type of cell signaling called quorum sensing to detect their concentration in a colony and generate coordinated responses. Eukaryotic cells can release ligands that target the same cell that produced the signal (autocrine signaling) or neighboring cells (paracrine signaling). Signals can even be sent over long distances, as in the case of some hormones, and produce responses in distant cells, called endocrine signaling. Contact-dependent signaling describes physical pathways created between neighboring cells through which cytoplasmic signals can rapidly pass. Nervous system cells can generate rapid responses through a specialization of cell signaling called synaptic signaling.