10. Cell Signaling

Classes of Signaling Receptors

G Protein Coupled Receptors(GPCRs) - Structure, Function, Mechanism of Action. Everything!

 the inactivation of ligand-gated ion channels 

 No. GTP is not involved in the regulation of ligand-gated ion channels. 

 the inactivation of G-protein-coupled signaling pathways 

 No. You should review the mechanism of G-protein activation. 

 the inhibition of pathways stimulated by tyrosine-kinase receptors 

 No. GTP is not involved in the signaling mediated by receptor tyrosine kinases. 

 receptor tyrosine kinases would be stimulated by the additional phosphate groups present in the modified GTP 

 that the activated G proteins would remain locked in the "on" position, transmitting signal even in the absence of a signaling molecule 

Which of the three major types of cell-surface transmembrane receptors requires GTP?

If a modified form of GTP that cannot be enzymatically converted to GDP were added to a culture of cells, the likely result would be __________.

Common cell signaling pathway

 No. Ion-channel receptors open and close in response to a signal. They do not form dimers. 

 No. Protein phosphatases reverse the effects of protein kinases. 

 No. These receptors do not form dimers. G proteins are activated in response to the signal molecule binding to the receptor. 

 No. Adenylyl cyclase is an enzyme that converts ATP to cAMP. 

Review the basic mechanisms of the three types of cell-surface transmembrane receptors.

Which of the following is activated when the binding of single molecules causes it to form a dimer?

Animation: Nuclear Response: Activating a Gene

 inhibits the enzyme that normally breaks down cAMP 

 No. However, abnormally high levels of cAMP are characteristic of cholera. 

 inhibits adenylyl cyclase, preventing the cell from producing cAMP 

 No. Cholera results from too much cAMP, not too little. 

 No. Cholera results from a process that produces too much cAMP. If a cAMP receptor site were blocked, the cell would behave as if there were too little cAMP. 

 No. cAMP is a second messenger. It is not phosphorylated. 

 prevents G-protein inactivation, which leads to the continuous production of cAMP 

The action of the cholera toxin affects the functions of G proteins.

Cholera develops when the bacterial toxin __________.

 are not an example of signaling molecules 

 No. Steroid hormones can trigger a specific cellular response. 

 No. Steroid hormone receptors are involved. 

 No. Steroid hormones do not directly bind to DNA. 

 No. Steroid hormones do not phosphorylate DNA. 

 do not initiate cell signaling by interacting with a receptor in the plasma membrane 

Consider how such a molecule can trigger a specific cellular response.

Steroid hormones can enter a cell by simple diffusion. Therefore, steroids __________.

Which type of receptor leads directly to a change in the distribution of ions on opposite sides of the membrane?a) Receptor tyrosine kinase.b) G protein-coupled receptor.c) Ligand-gated ion channel.d) Steroid receptor.e) Intracellular receptor.

Lipid-soluble signaling molecules, such as aldosterone, cross the membranes of all cells but affect only target cells because
a. only target cells retain the appropriate DNA segments.
b. intracellular receptors are present only in target cells.
c. only target cells have enzymes that break down aldosterone.
d. only in target cells is aldosterone able to initiate the phosphorylation cascade that turns genes on.

The molecules that convert extracellular signals into intracellular signals are:a) Neurotransmitters.b) Hormones.c) Cell surface receptors.d) Intracellular receptors.

Receptor Tyrosine Kinases (Newer Version)

Moss sperm are motile and capable of swimming short distances to fertilize an egg. However, it was unknown until recently how sperm make their way from male to female gametophytes that may be separated by a distance of several centimeters or more.

What structure do moss sperm use for motility, and how does that reflect the evolution of land plants from green algae?

Receptors: Types & Functions

 No. When the GTP that is bound to the active G protein is hydrolyzed to form GDP, the G protein is inactivated. 

 it is bound by its ligand and transported to the nucleus 

 No. This describes the activation of a steroid hormone receptor. 

 it is phosphorylated by a protein kinase 

 No. G proteins are not activated by phosphorylation. 

Ca2+ binds to a G-protein-coupled receptor

 No. Calcium ions are second messengers. They do not interact with G proteins. 

Review the mechanism of G-protein activation.

A G protein is active when __________.

Moss sperm are motile and capable of swimming short distances to fertilize an egg. However, it was unknown until recently how sperm make their way from male to female gametophytes that may be separated by a distance of several centimeters or more.

Researchers hypothesized that small animals called springtails (a type of arthropod) facilitate sperm transfer in mosses. To test this hypothesis, the researchers grew patches of male and female mosses at varying distances from each other and in the presence or absence of springtails. They later checked to see how many sporophytes were found in each female patch (as an indicator that fertilization had occurred). Their results are shown in the graph here (** means P<0.01, * means P>0.05. What conclusions can be drawn about the role of the springtails during the reproductive cycle of the mosses? <IMAGE>

Only target cells have the cell surface receptor able to bind with testosterone.

Only target cells contain the genes regulated by testosterone. 

Only target cells possess the phosphorylation cascade uniquely activated by testosterone. 

Only target cells possess the intracellular receptor able to bind with testosterone.

Why does testosterone, a lipid-soluble / hydrophobic signaling molecule, not affect all cells in the body but only specific cells?a) Only target cells have the cell surface receptor able to bind with testosterone.b) Only target cells contain the genes regulated by testosterone.c) Only target cells possess the phosphorylation cascade uniquely activated by testosterone.d) Only target cells possess the intracellular receptor able to bind with testosterone.

 act by phosphorylating a protein; open an ion channel when bound to a signal molecule 

 No. G proteins do not have kinase activity. Ligand-gated ion channels are the only type of receptor that allows ions to cross a membrane directly in response to a signal. 

 are transmembrane proteins; are found only on the cytoplasmic side of the plasma membrane 

 No. Tyrosine kinase receptors are transmembrane proteins. 

 form a dimer when activated; catalyze the transfer of a phosphate group from ATP to an amino acid 

 No. G-protein-coupled receptors do not form dimers. 

 phosphorylate the amino acid guanine; phosphorylate the amino acid threonine 

 No. G proteins do not phosphorylate. Tyrosine kinases only add phosphate groups to tyrosines. 

 are not enzymes; have enzymatic function 

G-protein-coupled receptors __________, whereas receptor tyrosine kinases __________.

Active, Passive, and Bulk Cell Transport

Intracellular Receptors

Receptors and Second Messenger system; G-protein, Enzyme linked and Ligand gated ion channels

 Nontarget cells possess enzymes that immediately degrade the molecules as they enter the cell. 

 Nontarget cells lack the inactive enzymes that the signal molecules activate. 

 The signal molecules diffuse from the cell before an effective concentration can be achieved. 

 In nontarget cells, these signal molecules cross the membranes of the endoplasmic reticulum and are captured by vesicles. 

 Nontarget cells lack the intracellular receptors that, when activated by the signal molecule, can interact with genes in the cell's nucleus. 

Consider how cells that respond to these hormones differ from those that do not.

Testosterone and estrogen are lipid-soluble signal molecules that cross the plasma membrane by simple diffusion. If these molecules can enter all cells, why do only specific cells respond to their presence?

Animation: Cytoplasmic Response: Glycogen Breakdown

Animation: Reception

 No. For example, G-protein-coupled receptors do not act via protein kinases. 

 are never found in the nucleus of a cell 

 No. After steroids bind to their receptors in the cytoplasm, the activated hormone-receptor complex is transported into the nucleus, where it regulates gene expression. 

 No. Ion-channel receptors are not the only kind of receptor that permits or restricts the passage of ions across membranes. 

 are only found associated with the plasma membrane 

 No. This is not the only location of signal receptors. 

 may be found embedded in the plasma membrane or found within the cytoplasm or nucleus 

Receptors for signal molecules can be found in more than one location in a cell.

Receptors for signal molecules __________.

 testosterone cannot cross the plasma membrane 

 No. Testosterone is hydrophobic and can cross the plasma membrane by simple diffusion. 

 not all cells in the body have membrane receptors for testosterone 

 No. You are on the right track, but go back and review steroid hormone receptors. 

 No. Testosterone is produced in the gonads and adrenal glands but exerts its effects at various other parts of the body. 

 it affects only cells that have ion-channel receptors 

 No. Testosterone does not work via ion-channel receptors. 

 not all cells have cytoplasmic receptors for testosterone 

Testosterone is a steroid.

Testosterone does not affect all cells of the body because __________.

Cell-Surface Receptors

 Calcium is actively pumped into the cell by the kainate receptor. 

 No. Recall the relative differences in calcium concentration inside and outside the cell and whether that would require the pumping of calcium into the cell. 

 The kainate receptor is a GPCR that stimulates adenylate cyclase to produce cAMP that serves to amplify the signal and to speed the signal's transduction through the cytoplasm. 

 No. The receptor facilitates the movement of calcium across a membrane, so it cannot be a GPCR. 

 The first two listed responses are correct. 

 No. Although one of those responses is correct, one is incorrect. 

 The second and third responses are correct. 

 No. Neither of those responses is correct. 

 The kainate receptor is a ligand-gated ion channel, and domoic acid is a ligand. 

Consider the function of adenylyl cyclase.

Domoic acid is a neurotoxin produced by some species of alga. Domoic acid binds to the kainate receptor on neurons in parts of the brain. The kainate receptor facilitates the movement of calcium into the cell. Excess domoic acid–induced stimulation of the neural kainate receptors causes neural damage and short-term memory loss. Based on this, which of the following is likely true?

 A mutation that leads to Ras being sent to the endomembrane system 

 No. There's no reason that targeting Ras to the endomembrane system would lead to cancer.  

 A mutation that means Ras cannot bind to GTP 

 No. If Ras could not bind GTP, then it could not be activated, and the cell would not be signaled to divide. 

 A mutation that means GDP is constantly bound to Ras 

 No. Such a mutation means that Ras could not bind to GTP and would therefore be turned off all the time. 

 A mutation in which Ras cannot bind to its GPCR 

 No. In this case, Ras could not be activated and would be in the "off" state all the time. This means there would be no signal to activate cell division. 

 A mutation that means Ras cannot hydrolyze GTP to GDP 

How do you think the G protein got its name?

Ras, a small G protein located at the plasma membrane, is often mutated in different types of cancer. Ras normally signals to a cell that it should divide. Cancer cells divide uncontrollably. Which of the following changes to Ras could explain uncontrolled division in a cancer cell that has mutated Ras present?

 No, just the opposite is true. Activated G proteins hydrolyze bound GTP to GDP; this hydrolysis terminates the activation. 

 No. GTP cannot be produced by the hydrolysis of GDP. 

 No. However, the conversion of GDP to GTP would require phosphorylation. 

 No. The phosphorylation of GTP would not produce GDP. 

What event would activate a G protein?

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