Notes Cell Communication & Cell Signaling!

Cell Signaling Introduction Cells can respond to many signals if they have a specific receptor for that signal. A signal transduction pathway is a sequence of molecular events and chemical reactions that lead to a cellular response, following the receptor’s activation by a signal.

Cells are exposed to many signals and may have different responses: Autocrine signals affect the same cells that release them. Paracrine signals diffuse to and affect nearby cells. Hormones travel to distant cells.

Only cells with the necessary receptors can respond to a signal—the target cell must be able to sense it and respond to it. A signal transduction pathway involves a signal, a receptor, and a response.

Example

A signal molecule, or ligand, fits into a three-dimensional site on the receptor protein. Binding of the ligand causes the receptor to change its three-dimensional shape. The change in shape initiates a cellular response.

An inhibitor, or antagonist, can bind in place of the normal ligand. Ligands are generally not metabolized further, but their binding may expose an active site on the receptor. Binding is reversible and the ligand can be released, to end stimulation. An inhibitor, or antagonist, can bind in place of the normal ligand. See Figure 5.6

Receptors are classified by their activity: Ion channel receptors Protein kinase receptors G protein–linked receptors VIDEO 5.5 Cell Visualization: Signals and calcium

Ion channel receptors, or gated ion channels, change their three-dimensional shape when a ligand binds. Example: The acetylcholine receptor, a ligand-gated sodium channel, binds acetylcholine to open the channel and allow Na+ to diffuse into the cell. See Figure 5.4 LINK Nerve cells communicate with muscle cells at neuromuscular junctions, which are described in Concept 36.1

Protein kinase receptors change their shape when a ligand binds. The new shape exposes or activates a cytoplasmic domain that has catalytic (protein kinase) activity.

Ligands binding to G protein–linked receptors expose a site that can bind to a membrane protein, a G protein. The G protein is partially inserted in the lipid bilayer, and partially exposed on the cytoplasmic surface. The activated G protein activates the effector protein, leading to signal amplification.

Figure 5.14 A G Protein–Linked Receptor Video Clip 9 min