KEY TAKEAWAYS •
Cell signaling can involve cell to cell connections, paracrine, autocrine, or endocrine signaling.
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Ligands can be extracellular or intracellular, depending on where they bind. The most hydrophobic ligands will be intracellular.
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Signaling pathways can turn on genes, turn off genes, affect cellular metabolism, or affect transcription.
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Calcium, phosphatidyl inositol, and cyclic AMP are common intracellular second messengers.
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G protein-coupled receptors are a large family of receptors that involve many different activities in eukaryotic cells only.
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