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Nutrient sensing is a cell's ability to recognize and respond to fuel substrates such as glucose. Each type of fuel used by the cell requires an alternate pathway of utilization and accessory molecules. In order to conserve resources a cell will only produce molecules that it needs at the time. The level and type of fuel that is available to a cell will determine the type of enzymes it needs to express from its genome for utilization. Receptors on the cell membrane's surface designed to be activated in the presence of specific fuel molecules communicate to the cell nucleus via a means of cascading interactions. In this way the cell is aware of the available nutrients and is able to produce only the molecules specific to that nutrient type.
A rapid and efficient response to disturbances in nutrient levels is crucial for the survival of organisms from bacteria to humans. Cells have therefore evolved a host of molecular pathways that can sense nutrient concentrations and quickly regulate gene expression and protein modification to respond to any changes.
Cell growth is regulated by coordination of both extracellular nutrients and intracellular metabolite concentrations. AMP-activated kinase and mammalian target of rapamycin complex 1 serve as key molecules that sense cellular energy and nutrients levels, respectively.
Living cells use ATP as the most important direct energy source. Hydrolysis of ATP to ADP and phosphate (or AMP and pyrophosphate) provides energy for most biological processes. The ratio of ATP to ADP and AMP is a barometer of cellular energy status and is therefore tightly monitored by the cell. In eukaryotic cells, AMP-activated protein kinase (AMPK) serves as a key cellular energy sensor and a master regulator of metabolism to maintain energy homeostasis.
Nutrient is a key regulator of tissue growth. The main mediator of cellular nutrient sensing is the protein kinase TOR (target of rapamycin). TOR receives information from levels of cellular amino acids and energy, and it regulates the activity of processes involved in cell growth, such as protein synthesis and autophagy. Insulin-like signaling is the main mechanism of systemic nutrient sensing and mediates its growth-regulatory functions largely through the protein kinase pathway. Other nutrition-regulated hormonal mechanisms contribute to growth control of modulating the activity of insulin-like signaling.
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