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Cell to cell communication is essential for multi-cellular organisms, such as human beings and oak trees, in order to coordinate their activities enabling the organism to develop. Cell communication is essential for unicellular organisms as well. The networking of these cells is very complicated. Chemical signaling is the most common mode of communication between cells. Studies suggest that identification of cells is done by chemical signaling in the cells of yeast, Saccharomyces cerevisiae. These cells are of two types a and a and secrete factor a and factor a respectively. These factors attract each other resulting in the fusion of two opposite cells. All the genes of both the original cells are transferred into the new cell. A signal on a cell’s surface is transformed into a particular cellular response in a sequence of responses referred to as a signal transduction pathway (Recce, 2005). The molecular details of signal transduction in yeast and mammals are very similar. These and other resemblances between signaling systems in bacteria and plants propose that premature versions of the cell-signaling mechanisms advanced well before earth saw its first multi-cellular organism. Scientists believe that they first evolved into ancient prokaryotes and eukaryotes and were later used for new purposes by their multi-cellular descendants.
Communication between animal cells may be through direct contact between cell molecules between surface-bound cells. This process is known as cell-cell recognition. There is another type of local signaling known as synaptic signaling which occurs in the animal nervous system. In this, an electric signal along a nerve cell triggers the secretion of neurotransmitter molecules. They diffuse across the synaptic gap to the target cell and the process continues. Both animals and plants use hormones for long-distance communication, in animals, it is known as endocrine signaling while in plants it is known as growth regulators. In animals, hormone molecules travel to target cells via vessels of the circulatory system, into which they are secreted by specialized cells. Plant hormones often reach their targets by moving through cells or diffusion through the air as gas but sometimes also travel in the vessel. Another example of long-term signaling is the transmission of the signal through the nervous system (Recce, 2005). After the signal is recognized by the receptor molecule, it is renewed into another form so the cell can respond to it.
The early work of Earl W. Sutherland suggested that the process going on at the receiving end of the cellular conversation can be dissected into three stages: reception, transduction, and response. The detection of signal molecules coming in from another cell is known as Reception. The conversion of the message into a form that can elicit a particular cellular response is known as Transduction. In the last stage, a Response is triggered by the transduced signal. This response may include any cellular activity.
Although epinephrine encounters many types of cells in the blood, only the target cells detect and react to it. The nature of a signal molecule is like that of a Ligand, which attaches to the site receptor. A change in shape is often caused by the Ligand binding (Recce, 2005). Most of the receptors are activated by this change and are permitted to interact with other cellular molecules. Messenger cells have to pass through the plasma membrane of the target cell in order to interact with intercellular molecules. A number of signaling molecules are able to cross the phospholipid interior of the membrane because of their hydrophobic nature or small size. Steroid hormones and thyroid hormones of animals are two examples of a hydrophobic chemical messenger. Many of the intracellular receptor proteins are structurally similar, suggesting an evolutionary affiliation.
When signal receptors are plasma membrane proteins, the transduction stage of cell signaling is usual1y a multistep pathway. Multistep pathways are able to trigger large cellular responses. It is a chain reaction in which an already activated receptor activates another protein which in turn activates another molecule and the chain continues. A unifying theme of all cell regulations is protein interaction. Many of the relay molecules in signal transduction pathways are protein kinases, and they often act on other protein kinases in the pathway. Protein phosphatases in the phosphorylation cascade are also very important as they can quickly eliminate phosphate groups from protein (Recce, 2005). This process is called dephosphorylation (Recce, 2005).
Many signaling pathways include tiny, non-protein, water-soluble molecules or ions known as second messengers (Recce, 2005). Research has revealed that epinephrine is only one of many hormones and other signal molecules that trigger the formation of cyclic adenosine monophosphate (cAMP), a cytosolic concentrated compound. The immediate effect of cAMP is usually the activation of a serine/threonine kinase caned protein, kinase A. Calcium is more widely used than cAMP as a second messenger in both in both G-protein and receptor tyrosine kinase pathways, because its concentration in the cytosol is normally much lower than the concentration outside the cell. In response to a signal relayed by a signal transduction pathway, the cytosolic calcium level may rise. Other second messengers, such as inositol trisphosphate (IP3) and diacylglycerol (DAG), are also involved in the process of calcium release. Because IP3 acts before calcium in these pathways, calcium may be considered a third messenger. The term second messenger is given to all small, non-protein components of signal transduction pathways.
The response to the signal transduction pathway may occur in the cytoplasm or may involve some action in the nucleus. The synthesis of enzymes and other proteins is regulated by many signaling pathways. This is done simply by turning a particular gene on or off (Recce, 2005). All the different kinds of signal receptors and relay molecules participate in various gene-regulating and other pathways. Malfunctioning growth factor pathways can lead to the development of cancer.
Different kinds of proteins are present in different cells (Recce, 2005). Studies suggest that the effectiveness of signal transduction may be improved by the existence of scaffolding proteins, large relay proteins to which numerous other relay proteins are concurrently attached (Recce, 2005). Scaffolding proteins have also been found in the brain cells that permanently hold together networks of signaling-pathway proteins at synapses, resulting in a better speed and accuracy of signal transfer between cells (Recce, 2005). In the multi-cellular organism, each molecular change in signaling pathways must last only for a short period of time for a cell to stay alert (Recce, 2005). The reversibility of the changes that signals produce is essential for continuing receptiveness of a cell (Recce, 2005). Thus, when signal molecules leave the receptor, the receptor reverts to its inactive form, then, by a variety of means, the relay molecules return to their inactive forms and so forth. As a result, the cell is soon ready to respond to a fresh signal.
Reference
Recce, N. A. (2005). Biology. San Francisco: Pearson Education, 1388.
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