Chapter Summary

• Cells receive many signals from the environment and from other cells. Chemical signals are often at very low concentrations. Autocrine signals affect the cells that make them; juxtacrine signals affect adjacent cells; paracrine signals diffuse to and affect nearby cells; and hormones are carried through the circulatory systems of animals or the vascular systems of plants. Review Figure 7.1, Activity 7.1

• A signal transduction pathway involves the interaction of a signal molecule with a receptor; the transduction of the signal via a series of steps within the cell; and effects on the function of the cell. Review Figure 7.2

• Signal transduction pathways involve regulation of enzymes and transcription factors. A great deal of crosstalk occurs between pathways.

• Cells respond to signals only if they have specific receptor proteins that can recognize those signals.

• Binding of a signal ligand to its receptor obeys the chemical law of mass action. A key measurement of the strength of binding is the dissociation constant (K_D).

• Depending on the nature of its signal or ligand, a receptor may be located in the cell membrane or inside the target cell. Review Figure 7.4

• Receptors located in the cell membrane include ion channels, protein kinases, and G protein-coupled receptors.

• Ion channel receptors are “gated.” The gate “opens” when the three-dimensional structure of the channel protein is altered by ligand binding. Review Figure 7.5

• Protein kinase receptors catalyze the phosphorylation of themselves or other proteins. Review Figure 7.6

• A G protein has three important binding sites, which bind a G protein-coupled receptor, GDP or GTP, and an effector protein. A G protein can either activate or inhibit an effector protein. Review Figure 7.7, Animation 7.1

• Intracellular receptors include certain photoreceptors in plants and steroid hormone receptors in animals. A lipid-soluble ligand such as a steroid hormone may enter the cytoplasm or the nucleus before binding. Many intracellular receptors are transcription factors. Review Figure 7.8

• A protein kinase cascade amplifies the response to receptor binding. Review Focus: Key Figure 7.10, Animation 7.2

• Second messengers include cyclic AMP (cAMP), inositol trisphosphate (IP₃), diacylglycerol (DAG), and calcium ions. IP₃ and DAG are derived from the phospholipid phosphatidyl inositol-bisphosphate (PIP₂). Review Figures 7.11, 7.12

• The gas nitric oxide (NO) is involved in signal transduction in human smooth muscle cells. Review Figure 7.13

• Signal transduction can be regulated in several ways. The balance between activating and inactivating the molecules involved determines the ultimate cellular response to a signal. Review Figure 7.14

• The cellular responses to signals may include the opening of ion channels, the alteration of enzyme activities, or changes in gene expression.

• Activated enzymes may activate other enzymes in a signal transduction pathway, leading to impressive amplification of a signal. Review Figure 7.15, Activity 7.2

• Protein kinases covalently add phosphate groups to target proteins; cAMP binds target proteins noncovalently. Both kinds of binding change the target protein’s shape to expose or hide a region involved in function.

• Many adjacent animal cells can communicate with one another directly through small pores in their cell membranes called gap junctions. Protein structures called connexons form thin channels between two adjacent cells through which small signal molecules and ions can pass. Review Figure 7.16A

• Plant cells are connected by somewhat larger pores called plasmodesmata, which traverse both cell membranes and cell walls. The desmotubule narrows the opening of the plasmodesma. Review Figure 7.16B

• The evolution of cell communication and tissue formation can be inferred from existing organisms, such as certain green algae. Review Figure 7.17