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Both prokaryotic and eukaryotic cells process information from their environments. This information can be in the form of a physical stimulus, such as the light reaching your eyes as you read this book, or chemicals that bathe a cell, such as the carbohydrate lactose in the solution around bacteria. A signal may come from outside the organism, such as the scent of a female moth seeking a mate in the dark, or from a neighboring cell within the organism, such as the liver, where signals from other organs cause liver cells to regulate their uptake or release of glucose.
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Chemical signals that target cells can be classified according to signal source and mode of signal delivery.
A signal transduction pathway includes a signal, a receptor, and a response
Not all cells respond to a signal, because some lack the ability to receive the signal.
The presence of a signal does not always lead to a response; a cell does not respond to every signal, just as you are not paying close attention to every stimulus in your environment as you read this. To respond to a signal, a cell must have a specific receptor that can detect it and a way to use that information to influence cellular processes. A signal transduction pathway is a sequence of molecular events and chemical reactions that lead to a cell’s response to a signal. Signal transduction pathways vary greatly in their details, but every such pathway involves a signal, a receptor, and a response. This section will provide a brief overview of signal transduction. We’ll consider receptors in Key Concept 7.2, and other aspects of signal transduction in Key Concepts 7.3 and 7.4.