Even single-cell organisms must respond to environmental challenges, which necessitated the development of signal-transduction mechanisms. The evolution of multicellular organisms required the evolution of cell-to-cell communication within the organism to coordinate between cells activities ranging from the induction of embryonic development to the integration of physiological responses.
All cells receive and process information. External signals such as odorants, metabolites, ions, hormones, growth factors, and neurotransmitters can all serve as chemical messengers linking neighboring or distant cells. Even external signals that are not considered chemical in nature (e.g., light and mechanical or thermal stimuli) may ultimately be transduced into a chemical messenger. Most chemical messengers interact with specific cell-surface receptors and trigger a cascade of secondary events, including the activation of intracellular second-messenger systems that mediate the cell's response to that stimulus. However, hydrophobic messengers, such as steroid hormones and some vitamins, can diffuse across the plasma membrane and interact with cytosolic or nuclear receptors. Indeed, cells use a number of intersecting intracellular signaling pathways to ensure that the cell's response to a stimulus is tightly controlled.