Signal Transduction

Signal Transduction

1. When a platelet contacts a damaged blood vessel, it is stimulated to release thromboxane A2. Thromboxane A2 in turn stimulates vascular spasm and attracts additional platelets to the injured site. In this example thromboxane A2 is acting as a(n) _____.

A. neurotransmitter

B. transcription factor

C. protein kinase

D. local regulator

E. G protein

2. Cells use different signaling strategies to achieve different goals. In hormonal signaling _____.

A. numerous cells can receive and respond to a signal produced in their vicinity

B. the signal can be directed to a very specific target because a narrow space separates

the target cell from the transmitting cell

C. a concentration gradient between the signaling cell and its target cells is established,

causing cells along the gradient to respond in different ways

D. specialized cells release hormone molecules into the circulatory system, permitting

distant cells to be affected

E. special molecules are passed through cell junctions

3. 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?

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

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

C. Nontarget cells lack the intracellular receptors that, when activated by the signal

molecule, can interact with genes in the cell's nucleus.

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

achieved.

E. In nontarget cells these signal molecules cross the membranes of the endoplasmic

reticulum and are captured by vesicles.

4. Steroid hormones can enter a cell by simple diffusion. Therefore steroids _____.

A. are not an example of signaling molecules

B. do not bind to receptors

C. directly bind to DNA

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

E. act by phosphorylating DNA

5. G-protein-linked receptors _____; whereas tyrosine-kinase receptors _____.

A. act by phosphorylating a protein ... open an ion channel when bound to a signal

molecule

B. are transmembrane proteins ... are found only on the cytoplasmic side of the plasma

membrane

C. are not enzymes ... have enzymatic function

D. form a dimer when activated ... catalyze the transfer of a phosphate group from ATP

to an amino acid

E. phosphorylate the amino acid guanine ... phosphorylate the amino acid threonine

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

A. ion-channel receptors

B. protein phosphatase receptors

C. G-protein-linked receptors

D. adenylyl cyclase receptors

E. tyrosine-kinase receptors

7. The binding of a signal molecule to a ligand-gated ion channel _____.

A. phosphorylates an amino acid

B. results in the formation of a dimer

C. promotes the binding of a steroid hormone to its receptor in the cytoplasm

D. alters the expression of genes, especially in neurons

E. affects the membrane potential

8. A G protein is active when _____.

A. GDP replaces GTP

B. it is bound by its ligand and transported to the nucleus

C. GTP is bound to it

D. it is phosphorylated by protein kinase

E. Ca2+ binds to a G-protein-linked receptor

9. 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 _____.

A. the inactivation of ligand-gated ion channels

B. the inactivation of G-protein-linked signaling pathways

C. that the activated G proteins would remain locked in the "on" position, transmitting

signal even in the absence of signaling molecule

D. the inhibition of pathways stimulated by tyrosine-kinase receptors

E. receptor tyrosine kinases would be stimulated by the additional phosphate groups

present in the modified GTP

10. What event would activate a G protein?

A. hydrolysis of GTP to GDP

B. hydrolysis of GDP to GTP

C. phosphorylation of GDP to GTP

D. replacement of GDP with GTP

E. phosphorylation of GTP to GDP

11. The cellular response of a signal pathway that terminates at a transcription factor would be _____.

A. the synthesis of mRNA

B. the activation of an inactive enzyme

C. alteration of the cytoskeleton

D. a change in the chemical composition of the cytosolic environment

E. the activation of a metabolic pathway

12. The general name for an enzyme that transfers phosphate groups from ATP to a protein is _____.

A. protein dehydrogenase

B. protein phosphatase

C. protein kinase

D. peptidase

E. protein cyclase

13. Phosphorylation _____.

A. always inactivates a protein

B. activates G-protein-linked receptors

C. can either activate or inactivate a protein

D. is accomplished by protein phosphatases

E. always activates a protein

14. The source of phosphate for a phosphorylation cascade is _____.

A. cAMP

B. ATP

C. protein kinase

D. GTP

E. protein phosphatase

15. Second messengers tend to be both water-soluble and small. This accounts for their ability to _____.

A. rapidly cross the plasma membrane

B. rapidly move throughout the cell by diffusion

C. pass quickly from cell to cell

D. move from substrate to substrate during a phosphorylation cascade

E. cross the nuclear membrane and interact with DNA

16. cAMP usually directly activates _____.

A. phosphodiesterase

B. receptor tyrosine kinases

C. G proteins

D. adenylyl cyclase

E. protein kinase A

17. In a typical cell, calcium ions _____.

A. are far more abundant in the cytoplasm compared to blood and other extracellular

fluid

B. are rapidly transported into the endoplasmic reticulum in response to G-protein-

mediated signals

C. are often concentrated within the endoplasmic reticulum

D. are not very widely used as second messengers

E. all of the above

18. During the transduction of a signal, one molecule or ion may be closely associated with the activity of another. Select the pair that is correctly combined.

A. calcium—IP3

B. cAMP—adenylyl cyclase

C. cAMP—protein kinase A

D. DAG—IP3

E. all of the above

19. A difference between the mechanisms of cAMP and Ca2+ in signal transduction is that cAMP_____ and Ca2+ _____.

A. is always present at high levels in the cytosol ... is present at low levels in the

absence of a signal

B. is synthesized by an enzyme in response to a signal ... released from intracellular

stores

C. is stored in the endoplasmic reticulum ... is never stored in the cell

D. is tyrosine-kinase-receptor linked ... is G-protein-receptor linked

E. enters the cell via a transmembrane protein channel ... enters the cell by diffusing

across the plasma membrane

20. IP3 (inositol trisphosphate) is produced as a result of _____.

A. protein kinase A activation

B. Ca2+

C. the cleavage of PIP2

D. DAG

E. phospholipase C

21. IP3 (inositol trisphosphate) acts by _____.

A. activating cAMP

B. phosphorylating signal receptors

C. opening Ca2+ channels

D. activating PIP2

E. activating DAG

22. In eukaryotic cells, which one of the following is a second messenger that is produced as a response to an external signal such as a hormone? (Concept 11.3) [Hint]

A. glycogen

B. cyclic AMP

C. tRNA

D. epinephrine

E. glucose

23. Which of the following sequences is correct?

A. binding of a growth factor to its receptor ® activation of transcription factor ®

phosphorylation cascade ® transcription

B. binding of a growth factor to its receptor ® phosphorylation cascade ® activation of

transcription factor ® transcription

C. G-protein activation ® phosphorylation cascade ® binding of a signaling molecule to

its receptor ® activation of transcription factor ® transcription

D. testosterone binds to its receptor ® G-protein activation ® adenylyl cyclase

activation ® levels of cAMP in the cytoplasm rise

E. binding of a signaling molecule to a receptor tyrosine kinase ® phosphorylation of G

protein ® adenylyl cyclase activation ® levels of cAMP in the cytoplasm

rise

24. Cells of the gastrointestinal (GI) tract and cells of the heart respond differently to epinephrine because _____.

A. there are differences in the proteins found in the two types of cells

B. the GI tract does not have epinephrine receptors

C. in cells of the GI tract epinephrine operates via a cytosolic receptor, whereas in cells

of the heart epinephrine acts via a plasma membrane receptor

D. the concentration of Ca2+ is lower in the cytosol of GI-tract cells than in the cytosol

of heart cells

E. cells of the GI tract lack cAMP