CHAPTER SUMMARY

9.1 CELLS COMMUNICATE PRIMARILY BY SENDING AND RECEIVING CHEMICAL SIGNALS.

9.2 CELL SIGNALING CAN BE CLASSIFIED ACCORDING TO THE DISTANCE BETWEEN THE SIGNALING AND RESPONDING CELLS.

9.3 SIGNALING MOLECULES BIND TO AND ACTIVATE SPECIFIC CELL-SURFACE AND INTRACELLULAR RECEPTORS.

9.4 SIGNALS ARE TRANSMITTED ACROSS THE PLASMA MEMBRANE AND ARE OFTEN AMPLIFIED IN THE CYTOSOL, LEADING TO A CELLULAR RESPONSE FOLLOWED BY TERMINATION.

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Self-Assessment Question 1

Name the four players in cell communication.

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Model Answer:

The four players in cell communication are a signaling cell, a signaling molecule, a receptor molecule, and a responding cell.

Self-Assessment Question 2

Name the steps that are often involved in cell communication.

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Model Answer:

The steps involved in cell communication are receptor activation, signal transduction, response, and termination.

Self-Assessment Question 3

Describe one way in which endocrine-, paracrine-, autocrine-, and juxtacrine-signaling pathways are similar to one another and one way they differ from one another.

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Model Answer:

One way in which the signaling pathways are similar is that the steps involved in the signaling cascade are the same (ie receptor activation, signal transduction, response and termination). One difference between the pathways is the distance between the signaling and responding cell. Another difference is the nature of the signaling molecule and whether it’s a small molecule or transmembrane protein.

Self-Assessment Question 4

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

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Model Answer:

Cells can respond to external signals by utilizing specific transmembrane proteins called receptors that bind to the signal (ligand) via the receptor’s specific ligand-binding site. The binding of 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 will respond to the signal.

Self-Assessment Question 5

Explain how signals can be specific in targeting only some cells, even if they are released into the bloodstream and come into contact with many cells.

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Model Answer:

A signal can be specific for a particular cell type even if the signal is in the bloodstream because the signal will only recognize 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 will respond to any given signal, even if that signal is present throughout the organism.

Self-Assessment Question 6

List several possible types of responses a cell might have to a signaling molecule.

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Model Answer:

Several possible types of responses a cell might have to a signaling molecule are migration of the cell to another part of the body, activation of a particular protein, shape change of the cell itself, growth (cell division), production of defensive enzymes, self-destruction (apoptosis), and differentiation, to name a few.

Self-Assessment Question 7

Compare and contrast cell-surface receptors and intracellular receptors in terms of their location, chemical properties of their signaling molecules, and types of responses mediated.

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Model 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 in the extracellular matrix (outside the cell). The receptor then transmits the signal to the inside of the cell, leading to a cellular response which could be the activation of an enzyme, or transcription of a particular gene, for example. Intracellular receptors exist inside the cell, either in the cytoplasm or in the nucleus itself. Their signaling molecules are typically nonpolar and can easily diffuse through the cell membrane. Once inside the cell, the ligand binds with its receptor, causing the receptor to change shape; in the case of steroid signals, the ligand receptor complex moves into the nucleus if it is not there already, to activate specific gene transcription.

Self-Assessment Question 8

List several ways in which a signal may be amplified in an intracellular signaling pathway.

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Model 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 Figure 9.10.

Self-Assessment Question 9

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

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Model 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 de-phosphorylation) or any of the downstream enzymes (like cAMP or Protein kinase A) could become inactivated.