A signal is detected when the chemical signal also known as a ligand binds to a receptor protein on the surface of the cell or inside the cell. Transduction : When the signaling molecule binds the receptor it changes the receptor protein in some way.
This change initiates the process of transduction. Signal transduction is usually a pathway of several steps. Each relay molecule in the signal transduction pathway changes the next molecule in the pathway. Membrane receptors function by binding the signal molecule ligand and causing the production of a second signal also known as a second messenger that then causes a cellular response.
These type of receptors transmit information from the extracellular environment to the inside of the cell by changing shape or by joining with another protein once a specific ligand binds to it. Intracellular receptors are found inside the cell, either in the cytopolasm or in the nucleus of the target cell the cell receiving the signal.
Chemical messengers that are hydrophobic or very small steroid hormones for example can pass through the plasma membrane without assistance and bind these intracellular receptors. Once bound and activated by the signal molecule, the activated receptor can initiate a cellular response, such as a change in gene expression. Since signaling systems need to be responsive to small concentrations of chemical signals and act quickly, cells often use a multi-step pathway that transmits the signal quickly, while amplifying the signal to numerous molecules at each step.
Steps in the signal transduction pathway often involve the addition or removal of phosphate groups which results in the activation of proteins. Enzymes that transfer phosphate groups from ATP to a protein are called protein kinases. Many of the relay molecules in a signal transduction pathway are protein kinases and often act on other protein kinases in the pathway. Extracellular signals are generally ligands of transmembrane receptors and are usually derived from either other cells or the environment, while an intracellular signal consists of a series of chemical interactions in either the cellular membrane or the intracellular space, or from the cellular membrane to the intracellular space.
Most cell signals are chemical in nature. For example, prokaryotic organisms have sensors that sense nutrients and help them navigate toward food sources. In vertebrate organisms, photons, growth factors, hormones, neurotransmitters, and extracellular matrix components are some of the several types of chemical signals used by cells.
These substances can exert their effects locally, or they might travel over long distances. For instance, neurotransmitters are a class of short-range signaling molecules that move across the tiny spaces between neighboring neurons or between neurons and muscle cells. Other signaling molecules travel much farther to reach their targets. For example, follicle-stimulating hormone, travels from the mammalian brain to the ovary to trigger the egg release. Some cells also respond to mechanical stimuli.
Sensory cells in the skin respond to touch, whereas similar cells in the ear react to the movement of sound waves. In addition, specialized cells in the human vascular system detect changes in blood pressure. An environmental signal, such as a hormone, is first received by interaction with a cell-surface receptor. A few non-polar signal molecules such as steroid hormones are able to diffuse through the cell membrane, and hence enter the cell.
Once inside the cell, these molecules bind to their receptors that interact directly with DNA and modulate gene transcription. However, most extracellular chemical signals are water-soluble and membrane impermeable. These molecules act as ligands for transmembrane receptors and cause a structural change in these receptor molecules, which then transmit the signal through a sequence of molecular switches to intracellular signaling pathways. How do signal transduction pathways occur? Can signal transduction pathways talk to each other?
How were signal transduction pathways discovered? How are signal transduction pathways turned off? See all questions in Signal Transduction Pathways. Impact of this question views around the world. You can reuse this answer Creative Commons License.
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