Key Concepts of Section 15.1

• Many cells sense and respond to light or physical stimuli such as touch or heat.

• All cells communicate through extracellular signals. In unicellular organisms, extracellular signaling molecules regulate interactions between individuals, while in multicellular organisms, they mainly regulate physiology and development.

• External signals include membrane-anchored and secreted proteins or peptides (e.g., vasopressin and insulin), small hydrophobic molecules (e.g., steroid hormones and thyroxine), small hydrophilic molecules (e.g., epinephrine), gases (e.g., O₂, nitric oxide), and physical stimuli (e.g., light).

• Hydrophobic signaling molecules interact with cytosolic receptors and mainly affect gene expression.

• Binding of a hydrophilic extracellular signaling molecule to a cell-surface receptor triggers a conformational change in the receptor, which in turn leads to activation of intracellular signal transduction pathways that ultimately modulate cellular metabolism, function, or gene expression (see Figure 15-1).

• Signals from one cell act on distant cells in endocrine signaling, on nearby cells in paracrine signaling, or on the signaling cell itself in autocrine signaling (see Figure 15-2).

• Protein phosphorylation and de-phosphorylation, catalyzed by protein kinases and phosphatases, are employed in virtually all signaling pathways. The activities of kinases and phosphatases are highly regulated by many receptors and signal transduction proteins (see Figure 15-3).

• GTP-binding proteins of the GTPase superfamily act as switches regulating many signal transduction pathways (see Figures 15-4 and 15-5).

• Some nonprotein, low-molecular-weight intracellular molecules, such as Ca²⁺ and cAMP (see Figure 15-6), act as “second messengers,” relaying and often amplifying the signal of the “first messenger”; that is, the ligand. Binding of ligand to cell-surface receptors often results in a rapid increase (or, occasionally, decrease) in the intracellular concentration of these ions or molecules.

• Signal transduction pathways allow amplification of an extracellular signal, enabling activation of a relatively small number of cell-surface receptors to trigger major changes in cell metabolism, movements, or gene expression.