Chapter 9 Summary

Core Concepts Summary

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9.1 Cells communicate primarily by sending and receiving chemical signals.

There are four essential players in communication between two cells: a signaling cell, a signaling molecule, a receptor protein, and a responding cell. page 180

The signaling molecule binds to its receptor on the responding cell, leading to receptor activation, signal transduction and amplification, a cellular response, and eventually termination of the response. page 181

9.2 Cells can communicate over long and short distances.

Endocrine signaling takes place over long distances and often relies on the circulatory system for transport of signaling molecules. page 183

Paracrine signaling takes place over short distances between neighboring cells and relies on diffusion. page 183

Autocrine signaling occurs when a cell signals itself. page 184

Some forms of cell communication depend on direct contact between two cells. page 184

9.3 Signaling molecules bind to and activate cell-surface and intracellular receptors.

A signaling molecule, or ligand, binds specifically to the ligand-binding site of the receptor. Binding causes the receptor to undergo a conformational change that activates the receptor. page 185

Receptors for polar signaling molecules, including growth factors, are located on the plasma membrane. page 186

Receptors for nonpolar signaling molecules, such as steroid hormones, are located in the cytosol or in the nucleus. page 186

There are three major types of cell-surface receptor: G protein-coupled receptors, receptor kinases, and ion channels. All act as molecular switches. page 186

G protein-coupled receptors associate with G proteins, which relay the signal to the interior of the cell. page 186

Receptor kinases phosphorylate each other and activate target proteins. page 187

Ligand-gated ion channels open in response to a signal, allowing the movement of ions across the plasma membrane. page 187

9.4 G protein-coupled receptors are a large, conserved family of receptors that often lead to short-term responses.

G protein-coupled receptors bound to signaling molecules associate with G proteins. page 187

G proteins are active when bound to GTP and inactive when bound to GDP. page 187

Some G proteins are composed of three subunits, denoted α, β, and γ. When a G protein encounters an activated receptor, the α subunit exchanges GDP for GTP, dissociates from the β and γ subunits, and becomes active. page 188

Signal transduction cascades are amplified in the cytosol. page 188

Intracellular, cytosolic signals are short-lived before they are terminated. page 189

9.5 Receptor kinases are widespread and often lead to long-term responses.

Ligand binding to receptor kinases causes them to dimerize. The paired receptor kinases phosphorylate each other’s cytoplasmic domains, leading to activation of intracellular signaling pathways. page 192

The phosphorylated receptors bind other proteins, which in turn activate other cytosolic signaling molecules, such as Ras. page 192

When Ras is activated, the GDP to which it is bound is released and is replaced by GTP. The active, GTP-bound Ras binds to and activates the first in a series of kinases. page 192

Cancer can be caused by errors in any step of the signaling pathways involved in cell division. page 192

Mutations in receptor kinases and Ras are frequently associated with human cancers. page 193

Several signaling pathways can take place simultaneously in a single cell, and the cellular response depends on the integration of these pathways. page 193

Self-Assessment

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  1. Name the four essential elements in cell communication.

    Self-Assessment 1 Answer

    The four essential elements in cell communication are a signaling cell, a signaling molecule, a receptor protein, and a responding cell.

  2. Name the steps that occur when a signal binds to a receptor on a responding cell.

    Self-Assessment 2 Answer

    When a signal binds a receptor on a responding cell, the following steps typically occur: receptor activation, signal transduction, response, and termination.

  3. Describe one way in which endocrine, paracrine, autocrine, and contact-dependent signaling pathways are similar to one another and one way in which they are different from one another.

    Self-Assessment 3 Answer

    One way in which the signaling pathways are similar is that the steps involved in the signaling cascade are the same (that is, receptor activation, signal transduction, response, and termination). One difference between the pathways is the distance between the signaling and responding cell. Another difference is whether the signaling molecule is released by the signaling cell (endocrine, paracrine, autocrine) or a transmembrane protein (signaling between neighboring cells).

  4. Explain how cells respond to external signals, even when those signals cannot enter the cell.

    Self-Assessment 4 Answer

    Cells respond to signals by means of specific transmembrane proteins called receptors that bind to the signal (ligand) by the receptor’s specific ligand-binding site. The binding of the ligand to its receptor causes a conformational change in the receptor, activating it. The activated receptor transmits the signal inside the cell, generating a response. The type of ligand and receptor determine how the cell responds to the signal.

  5. Explain how signals can specifically target only some cells, even if they are released into the bloodstream and come into contact with many cells.

    Self-Assessment 5 Answer

    A signal can be specific for a particular cell type because the signal only recognizes receptors with a binding site specific for that signal. These receptors are not present on every cell; different cell types express a different assortment of receptor types on their surface. In this way, only a subset of cells respond to any given signal, even if that signal is present throughout the organism.

  6. Describe three different responses of a cell-surface receptor on binding a signaling molecule and undergoing a conformational change.

    Self-Assessment 6 Answer

    When a signaling molecule binds to and changes the shape of a receptor, G protein-coupled receptors bind and activate G proteins, receptor kinases phosphorylate each other, and ion channels open or close.

  7. List several responses a cell might have to a signaling molecule.

    Self-Assessment 7 Answer

    Several possible types of responses a cell might have to a signaling molecule are migration of the cell, activation of a particular protein, shape change of the cell itself, cell division, and cell differentiation, to name a few.

  8. Compare and contrast receptors associated with polar and nonpolar signaling molecules.

    Self-Assessment 8 Answer

    Cell-surface receptors are usually transmembrane proteins with an extracellular domain, transmembrane domain, and cytoplasmic domain. They are specific for polar signaling molecules that cannot diffuse across the membrane. These polar ligands bind with the receptor outside the cell. The receptor then transmits the signal to the inside of the cell, leading to a cellular response. That response could be the activation of an enzyme or transcription of a particular gene, for example. Intracellular receptors are present inside the cell, either in the cytoplasm or in the nucleus. Their signaling molecules are typically nonpolar and can easily diffuse through the plasma membrane. Once inside the cell, the ligand binds with its receptor, causing the receptor to change shape. In the case of steroid hormones, the ligand receptor complex moves into the nucleus if it is not there already, to activate gene transcription.

  9. List three ways in which a signal is amplified in a G protein-coupled receptor signaling pathway.

    Self-Assessment 9 Answer

    In the example of G protein–coupled receptors, a signal is amplified in several ways. A single activated G protein coupled receptor can activate multiple G proteins. Each active G protein activates an adenylyl cyclase molecule, each of which generates a large amount of cAMP. Each cAMP molecule activates a molecule of protein kinase A (PKA). Once activated, each PKA enzyme phosphorylates and activates multiple protein targets. See Fig. 9.11.

  10. Describe three ways in which the response of a cell to a signal can be terminated.

    Self-Assessment 10 Answer

    Response of a cell to a signal can be terminated at a variety of levels. The receptor itself could change its conformation to the inactive form, the protein interacting with the intracellular portion of the receptor could become inactivated (perhaps due to dephosphorylation), or any of the downstream proteins (like cAMP or protein kinase A) could become inactivated.