Cellular response to environmental stimuli

Single-celled organisms like as Bacteria can face enormous changes in their external environment over very short periods of time.  If we consider Escherichia coli and its life cycle we can see how significant these changes are.  In its normal habitat of the colon the E. coli cell is kept at a more or less constant temperature of 37^?C is surrounded through other microbes and nutrients in the form of partly or completely digested food and is in a low-oxygen environment. After excretion and before the cell re-enters the digestive system of the similar or a different host E. coli is suddenly thrust into a colder well oxygenated more aqueous and nutrient-poor situation. To be able to cope with all these changes in lifestyle the cell must quickly turn off some genes and turn on others. At the gene level the switches are repressor and activator proteins while at the protein level enzymes can be switched on or off through the presence or absence of metabolites.  These cytoplasmic responses have come into effect due to changes outside the cell and the way in that an indicator in which the outside world has changed is carried from the cell wall and to the genes or proteins which might be involved in a response is called signal transduction.

Some  molecules which have  a significant effect  on  the  cell are  small  and  can  diffuse or be actively  transported. Once in the cell they can have a direct effect on their goal for example, sugars such as lactose and alcohols such as methanol. However, some useful growth components must be isolated from the cytoplasmic contents cyanide, formal- dehyde, and even oxygen can be used for growth but are also cytotoxic or are unable to pass easily through the cell wall and membrane large molecules such as polydextrans. Other changes on the outside of the cell must be transformed into a signal which enzymes can recognize. Change in pH ultraviolet light or heat are pertinent examples. Even the enzymes of extremophiles can only tolerate relatively small ranges of heat and pH since the cell as a whole can survive a greater range of temperature or acidity. Rather than let environmental changes into the cell directly at their external concentrations the exterior of the cell has developed systems to sense change and then relay which change to a target. Frequently this relay involves the transfer of phosphate groups from a sensor protein onto one or more relay proteins and finally to the effector protein. For this reason it is sometimes called a phosphorelay pathway. When we examine cell response pathways it is convenient to think of them as linear pathways in which a sensor relays a series of phosphate molecules via a defined set of proteins to an effector. In the cytoplasm, the translocation of a signal from sensor to effector is certainly many more branched with overlapping sets of relay proteins accepting phosphates from many different sensors. For this reason our understanding of the entire process of cellular sensing has become intertwined with computational techniques such as neural networking. To gain access to the entire branched cellular response we must first understand simple examples such as two-component regulatory systems and then develop it to look at a more global response such as the effect of oxygen on the cell or chemotaxis.