What is signal transduction?

A process of signal transfer throughout an organism, mainly across or through a cell is called signal transduction. There are proteins present in the cell called as receptors which receive the signal which can be in chemical, physical or electric form. Chemical signals are called as ligands and can be produced by cells to control the metabolic activities. Regardless the type of signal, it has be transferred throughout the body and across cell membranes. Receptor proteins are found attached to the cell membrane. Each type of cell gets different signals from the body as well as the environment which is specific in it's way to produce a specific and coordinated response. This is called as signal transduction. There is an other family of molecules called secondary messengers which are activated by some proteins. Secondary messengers carry the message to the nucleus and other organelles. Other proteins use ATP as a source of energy to activate enzymes which are needed to carry out metabolic reactions. Enzymes lower the activation energy required for the reaction. The different routes followed for signal transduction to carry a signal are called as signal transduction pathways.

Step-by-step explanation

A signal have different components during signal transduction:

1. A primary messenger which may be a chemical release, electrical pulse or physical stimulation.
2. A receptor protein situated in the cellular membrane must accept the signal.
3. This protein goes into a conformational change once it receives the signal. This changes its shape and makes interactions with the molecules around it.

Examples of signal transduction:

1. The signal transduction pathway of touch and vision: It works in the same way that many nerve signals do. The stimulation of the receptor protein causes an influx of ions into the cell. There is no synthesis of secondary messenger in this case. This causes the cell membrane to depolarize (less negative charge inside the cell). A normal cell membrane is polarized or has a voltage potential. This voltage potential is created by the cell actively pumps ions out of the cell. When only one receptor protein is stimulated, only a small section of the membrane depolarizes, but when you get a strong signal, such as touching a very hot or cold surface or seeing a bright light, the entire membrane of many cells is depolarized at the same time. This event triggers an action potential, which is how the signal travels down a nerve cell. This is caused by a bunch of other receptor proteins which are sensitive to the change in voltage. Upon feeling the voltage change, they also allow the ions to even out, which sends the signal to the cell. Now the signal must cross a synapse to another nerve cell. For this another signal transduction pathway comes into the picture. As the action potential reaches the end of the first cell, specialized receptor proteins receive the signal, and trigger the release of neurotransmitters (molecules used by the nervous system to transfer messages between neurons or from neurons to muscles, also called as chemical messengers of the body). These small ligands travel across the space between cells by diffusing through the fluid, and reach to receptor proteins on the next cell. These receptors upon activation cause another action potential in the next nerve, so these are gated ion channels. This way, a signal can travel from a tip of the finger or eye to brain in microseconds.
2. Hormonal signal transduction pathway: Hormones are the signals synthesized by body for regulatory activities. Generally, a hormone is released from an endocrine gland (pancreas or thyroid). The signals transmitted by hormones are typically through a ligand-receptor signal transduction pathway. Reproductive organs of the body also release hormones, which function in the reproduction. Upon getting signal to release hormone, the cells in endocrine glands will release their accumulated hormone. The hormone is spilled in the intracellular space of the body by merging the vesicles with the cell membrane. Blood capillaries or very small blood vessels run through the intracellular space. The hormone dissolves into the bloodstream where it is carried throughout the body. The hormone insulin can cause muscle cells to uptake and store glucose, whereas it causes liver cells to prevent producing glucose. This helps regulate the total amount of blood glucose level in the bloodstream. One signal transduction pathway stimulates a cellular process in the muscle cells which increases the number of glucose transporters in their cell membrane and the other signal transduction pathway in the liver inactivates an enzyme which is required to produce glucose.