Question 1

A fat cell responds to the presence of the hormone epinephrine by increasing cytosolic cyclic AMP production, which leads to the catabolism of both glycogen and fat. What is the most likely explanation for this phenomenon?
A) Epinephrine is binding to two types of receptors in the plasma membrane.
B) The activated receptor complex stimulates production of two different second messengers.
C) Cyclic AMP directly activates enzymes that catabolize glycogen and fat.
D) Cyclic AMP-dependent protein kinase activates two kinds of enzymes.

Accepted Answer

Cyclic AMP-dependent protein kinase (PKA) activates enzymes involved in the breakdown of both glycogen and fat, leading to their catabolism.

Question 2

With regard to the action of hormones and neurotransmitters on cellular receptors, which of these describes "amplification"?
A) When the extracellular concentration of a chemical messenger reaches a very high level, it overwhelms transporter molecules and the chemical floods into the cell.
B) Only hydrophilic first-messenger molecules can activate second-messenger molecules within the cell cytosol.
C) A single first-messenger molecule activates multiple second-messenger molecules, each of which activate thousands of enzymes.
D) Some cellular receptors have such low affinity for chemical ligands that it can require a million or more molecules to activate them.

Accepted Answer

Amplification refers to the process by which a single receptor-ligand interaction can activate multiple second messenger molecules, each of which can activate multiple downstream effectors such as enzymes or ion channels. This greatly enhances the overall cellular response to the initial stimulus.

Question 3

Specificity is an important characteristic of intercellular communication; which of these best explains how it occurs?
A) The phospholipid composition of the plasma membrane differs among cells.
B) Protein receptors are only located on the surface of target cells.
C) All cells have the same DNA, so any cell can express protein receptors for a specific chemical.
D) Chemical messengers are all proteins, so bind only to receptors that are proteins.
E) Protein receptors for chemical messengers are only expressed in specific target cells.

Accepted Answer

Specificity in intercellular communication occurs because protein receptors for chemical messengers are only expressed in specific target cells, allowing only those cells to respond to the messenger.

Question 4

Which of the following is NOT typically true about G proteins?
A) They act as second messengers.
B) They can be stimulatory for second-messenger production.
C) They can be inhibitory for second-messenger production.
D) They can act as transducers for activated receptors by opening or closing ion channels.

Accepted Answer

G proteins themselves do not act as second messengers; instead, they are involved in the signal transduction pathways that lead to the production of second messengers.

Question 5

Amplification during a second-messenger cascade is beneficial because amplification:
A) takes small molecules and makes polymers out of them.
B) results in the production of more of the first messenger.
C) allows a cell to respond to more different hormones.
D) allows small amounts of hormones to produce large responses in target cells.

Accepted Answer

Amplification during a second-messenger cascade allows small amounts of hormones to produce large responses in target cells. This is because the activation of one receptor molecule can trigger the production of multiple second messenger molecules, which can then activate multiple effector molecules, leading to a cascade of amplification. Hence, amplification is essential for the physiological responses to small doses of hormones.

Question 6

What are the most common receptors for hydrophilic intercellular messenger molecules?
A) peripheral membrane proteins
B) integral membrane proteins
C) specialized phospholipids within the membrane
D) nucleic acids
E) intracellular proteins

Accepted Answer

Integral membrane proteins are responsible for receiving signals from hydrophilic intercellular messenger molecules. These molecules cannot penetrate the lipid bilayer of the cell membrane due to being water-soluble. Therefore, they bind to specific receptors on the surface of the plasma membrane, which leads to the activation of intracellular signaling pathways.

Question 7

In which situation would an injected drug be the most effective agonist for an endogenous chemical messenger?
A) The drug has a lower affinity for the messenger's receptors than the messenger does.
B) The drug achieves 50% saturation of the messenger's receptors at a lower concentration than that required by the messenger.
C) The drug does not couple to the binding site of the messenger's receptor.
D) The drug binds to an alternate binding site on the protein receptor and reduces its affinity for the endogenous chemical messenger.

Accepted Answer

For the injected drug to be the most effective agonist for an endogenous chemical messenger, it should achieve 50% saturation of the messenger's receptors at a lower concentration than that required by the endogenous messenger. This indicates that the drug has a higher affinity for the receptor than the messenger, leading to a stronger and more prolonged response. Option A is incorrect because the drug having a lower affinity for the messenger's receptors than the messenger does means that the drug will not effectively bind to the receptor, making it less effective than the messenger. Option C is incorrect because for the drug to be an effective agonist, it should couple to the binding site of the messenger's receptor. Option D is incorrect because binding to an alternate binding site on the protein receptor and reducing its affinity for the endogenous chemical messenger would make the drug an antagonist, not an agonist.

Question 8

Which is NOT typically a step in the cAMP second-messenger system?
A) A first messenger binds to a transmembrane receptor.
B) There is dissociation of G-protein subunits.
C) An activated G-protein subunit phosphorylates cAMP-dependent protein kinase.
D) Adenylyl cyclase converts ATP into cAMP.
E) Active cAMP-dependent protein kinase phosphorylates cell proteins.

Accepted Answer

In the cAMP second-messenger system, activated adenylyl cyclase (not an activated G-protein subunit) converts ATP into cAMP. An activated G-protein subunit then activates cAMP-dependent protein kinase by binding to it and causing it to dissociate into its regulatory and catalytic subunits. The catalytic subunit then phosphorylates cell proteins, leading to changes in cellular function.

Question 9

Which of the following statements is TRUE? A) Phosphorylation by protein kinases can stimulate or inhibit the activity of effector proteins. B) Ca²⁺ is not a second messenger. C) Phosphodiesterase converts GMP into cGMP D) Conversion of ATP to cAMP is a phosphorylation reaction. E) Phospholipase C converts ATP to cAMP.

Accepted Answer

Phosphorylation by protein kinases can either activate or inhibit the activity of effector proteins, depending on the specific target and context. This is a key mechanism of signal transduction in cells. Choice B is not a valid statement as Ca²⁺ is not a recognized molecule. Choice C is not correct, as phosphodiesterase converts cAMP to AMP, not GMP to cGMP. Choice D is also incorrect, as conversion of ATP to cAMP is a dephosphorylation reaction, not a phosphorylation reaction. Choice E is not correct, as phospholipase C cleaves membrane phospholipids to produce second messengers such as inositol triphosphate and diacylglycerol, not cAMP.

Question 10

Amplification of a second-messenger cascade can take place at which level of a signal cascade?
A) One activated receptor can activate numerous G-proteins.
B) One activated G-protein can activate numerous effector enzymes.
C) One active effector enzyme can catalyze numerous reactions.
D) One activated protein kinase can allosterically modulate numerous proteins.
E) All of the choices are correct.

Accepted Answer

All of the choices given can lead to amplification of a second-messenger cascade. One activated receptor can activate numerous G-proteins, each of which can then activate multiple effector enzymes, leading to amplification. One active effector enzyme can catalyze numerous reactions, each of which can lead to the production of multiple second messengers. Finally, one activated protein kinase can allosterically modulate numerous downstream proteins, leading to amplification of the signal cascade.

Question 11

Which of the following are ways in which binding of an intercellular chemical messenger with a cell's receptor can bring about a cellular response?
A) opening or closing of specific ionic channels in the plasma membrane
B) activation of an intracellular second-messenger system
C) promoting or inhibiting the transcription of genes that code for the synthesis of cellular proteins
D) activating or inhibiting intracellular enzymes
E) All of the choices are correct.

Accepted Answer

All of the choices listed are ways in which intercellular chemical messengers can bring about a cellular response. Binding of a chemical messenger with a specific receptor can trigger various intracellular signaling pathways that lead to the opening or closing of specific ionic channels, activation of an intracellular second-messenger system, promotion or inhibition of gene transcription, and activation or inhibition of intracellular enzymes. The precise response will depend on the type of messenger, the receptor, and the target cell.

Question 12

Second messengers:
A) are necessary for all receptor signal transduction mechanisms.
B) act in the cell cytoplasm.
C) only function as intercellular messengers.
D) always function to activate enzymes.
E) are always proteins.

Accepted Answer

Second messengers act in the cell cytoplasm, not as intercellular messengers. They may or may not be necessary for all receptor signal transduction mechanisms, and they do not always activate enzymes. They can be proteins, but they can also be other types of molecules such as lipids or ions.

Question 13

Methadone is a drug given to treat heroin addicts. It works by binding to the same receptors as heroin but with greater affinity, thereby preventing heroin from binding. This is an example of
A) competition.
B) down-regulation.
C) signal transduction.
D) agonistic behavior.
E) up-regulation.

Accepted Answer

Methadone and heroin both bind to the same receptors, but methadone has a greater affinity for these receptors, meaning that it will outcompete heroin for binding sites. This is an example of competition between the two drugs.

Question 14

At very low concentrations, epinephrine causes an artery to dilate. At higher concentrations epinephrine causes the same artery to constrict. How can these different effects be explained?
A) There is one type of epinephrine receptor that uses two second-messenger systems.
B) There are two types of epinephrine receptors with different affinities for epinephrine that use two different second-messenger systems.
C) There are two types of receptors for epinephrine that use the same second-messenger system.
D) At higher concentrations epinephrine can pass through the plasma membrane and directly stimulate contraction within the cell.

Accepted Answer

The two different effects of epinephrine can be explained by the presence of two types of epinephrine receptors with different affinities for epinephrine that use two different second-messenger systems. At low concentrations of epinephrine, it primarily binds to the beta-2 adrenergic receptors, which stimulates the production of cyclic AMP (cAMP) and leads to vasodilation. At higher concentrations, epinephrine also binds to alpha-1 adrenergic receptors, which stimulates the production of inositol triphosphate (IP3) and diacylglycerol (DAG), leading to vasoconstriction.

Question 15

Cells can increase their responsiveness to an external chemical regulator by
A) increasing the number of their transmembrane receptors by exocytosis.
B) decreasing the number of their transmembrane receptors by endocytosis.
C) uncoupling their receptors from the second message generator.
D) increasing the number of their transmembrane receptors by endocytosis.
E) mutating their extracellular receptors so that the affinity for the chemical regulator is reduced.

Accepted Answer

Increasing the number of transmembrane receptors by exocytosis allows more receptors to be present on the cell surface, enhancing the cell's ability to detect and respond to the external chemical regulator.

Question 16

Cocaine lowers the levels of a chemical messenger in the brain called enkephalin. Researchers have found the number of enkephalin receptors to be higher in cocaine addicts than nonaddicted people. This is an example of
A) saturation.
B) up-regulation.
C) antagonism.
D) affinity.
E) down-regulation.

Accepted Answer

Up-regulation refers to an increase in the number of receptors for a particular neurotransmitter due to its decreased availability. In the case of cocaine addiction, the prolonged use of the drug leads to a decrease in enkephalin levels, which triggers the brain to produce more enkephalin receptors to compensate. This can contribute to the cycle of addiction and cravings for more drugs.

Question 17

Which of the following is known to be a second messenger?
A) diacylglycerol
B) phospholipase C
C) ATP
D) adenylyl cyclase
E) epinephrine

Accepted Answer

Diacylglycerol (DAG) is a second messenger involved in signal transduction pathways, often working alongside inositol triphosphate (IP3) to activate protein kinase C.

Question 18

Epinephrine activates the cyclic AMP pathway in liver cells. In this example, epinephrine is a ____________ and cAMP is a _____________.
A) ligand, receptor
B) first messenger, hydrophobic hormone
C) second messenger, ion channel
D) first messenger, second messenger
E) enzyme, second messenger

Accepted Answer

Epinephrine is a first messenger as it binds to a specific receptor on the surface of liver cells. The activation of this receptor leads to the production of a second messenger, cAMP, which then initiates a signaling cascade within the cell. Therefore cAMP is a second messenger in this pathway.

Question 19

Which of these is a lipid-soluble messenger?
A) thyroid hormone
B) protein kinase
C) glucose
D) sodium
E) cyclic AMP

Accepted Answer

Lipid-soluble messengers include hormones such as thyroid hormone, which can easily cross the cell membrane and bind to intracellular receptors to initiate a response. Protein kinase, glucose, sodium, and cyclic AMP are not lipid-soluble messengers.

Question 20

The process whereby repeated exposure to a hormone can cause a decrease in the number of receptors for that hormone is called
A) competition.
B) inhibition.
C) down-regulation.
D) antagonism.
E) saturation.

Accepted Answer

The process whereby repeated exposure to a hormone can cause a decrease in the number of receptors for that hormone is called down-regulation. This means that the cells become less sensitive to the hormone as there are fewer receptors available to bind to it.

Quiz 5: Cell Signaling in Physiology

A fat cell responds to the presence of the hormone epinephrine by increasing cytosolic cyclic AMP production, which leads to the catabolism of both glycogen and fat. What is the most likely explanation for this phenomenon?

With regard to the action of hormones and neurotransmitters on cellular receptors, which of these describes "amplification"?

Specificity is an important characteristic of intercellular communication; which of these best explains how it occurs?

Which of the following is NOT typically true about G proteins?

Amplification during a second-messenger cascade is beneficial because amplification:

What are the most common receptors for hydrophilic intercellular messenger molecules?

In which situation would an injected drug be the most effective agonist for an endogenous chemical messenger?

Which is NOT typically a step in the cAMP second-messenger system?

Which of the following statements is TRUE?

Amplification of a second-messenger cascade can take place at which level of a signal cascade?

Which of the following are ways in which binding of an intercellular chemical messenger with a cell's receptor can bring about a cellular response?

Second messengers:

Methadone is a drug given to treat heroin addicts. It works by binding to the same receptors as heroin but with greater affinity, thereby preventing heroin from binding. This is an example of

At very low concentrations, epinephrine causes an artery to dilate. At higher concentrations epinephrine causes the same artery to constrict. How can these different effects be explained?

Cells can increase their responsiveness to an external chemical regulator by

Cocaine lowers the levels of a chemical messenger in the brain called enkephalin. Researchers have found the number of enkephalin receptors to be higher in cocaine addicts than nonaddicted people. This is an example of

Which of the following is known to be a second messenger?

Epinephrine activates the cyclic AMP pathway in liver cells. In this example, epinephrine is a and cAMP is a _.

Which of these is a lipid-soluble messenger?

The process whereby repeated exposure to a hormone can cause a decrease in the number of receptors for that hormone is called