Chapter 11 - Cell Communication

Question 1

Question 2

How do plant hormones travel between secreting cells and target cells?

Answer 2

Plant hormones (often called growth regulators) may reach their target cells by traveling through vessels, through cells via plasmodesmata, or even through the air as a gas.

Question 3

Explain why G-protein-regulated pathways shut down rapidly in the absence of a signal molecule.

Answer 3

The G protein is also a GTPase enzyme that hydrolyzes its bound GTP to GDP and inactivates itself. It then dissociates from the enzyme it had activated, and that enzyme returns to its original state.

Question 4

Label the parts in this diagram of an activated receptor tyrosine kinase dimer.

Answer 4

a. signaling molecules
b. α helix in the membrane
c. phosphates
d. ATP → ADP
e. tyrosines
f. activated relay proteins
g. cellular responses

Question 5

a. What does a protein kinase do?
b. What does a protein phosphate do?
c. What is a phosphorylation cascade?

Answer 5

a. A protein kinase transfers a phosphate group from ATP to a protein; adding a charged phosphate group causes a shape change that usually activates the protein.
b. A protein phosphatase removes a phosphate group from a protein, usually inactivating the protein.
c. A phosphorylation cascade is a series of protein kinase relay molecules that are sequentially phosphorylated.

Question 6

Label the components in this diagram of the steps in a signal transduction pahtway that uses cAMP as a second messenger.

Answer 6

a. signaling molecule (first messenger)
b. G-protein-linked receptor
c. activated G protein (GTP bound)
d. adenylyl cyclase
e. ATP
f. cAMP (second messenger)
g. protein kinase A
h. phosphorylation cascade to cellular response

Question 7

Fill in the blanks to review a G-protein-linked pathway that uses Ca²⁺ as a second messenger.

A a. ________ binds to a G-protein-linked receptor. An activated b. ________ activates the enzyme phospholipase C, which cleaves a c. ________ into DAG and d. ________, which binds to and opens a ligand-gated channel, releasing e. ________ from the f. ________.

Answer 7

a. signaling molecule
b. G protein
c. membrane phospholipid
d. IP₃
e. Ca²⁺
f. endoplasmic reticulum

Question 8

How do the following mechanisms or molecules discontinue a cell’s response to a signal and maintain a cell’s ability to respond to fresh signals?

a. reversible binding of signaling molecules
b. GTPase activity of G protein
c. phosphodiesterase
d. protein phosphatases

Answer 8

a. Signaling molecules reversibly bind to receptors, and when they leave a receptor, the receptor reverts to its inactive form. The concentration of signaling molecules influences how many are bound at any time.
b. Activated G proteins are inactivated when the GTPase portion of the protein converts GTP to GDP.
c. Phosphodiesterase converts cAMP to AMP, thus damping out this second messenger.
d. Protein phosphatases remove phosphate groups from activated proteins. The balance of active protein kinases and active phosphatases regulates the activity of many proteins.

Question 9

a. What can one conclude from the fact that the mitochondrial apoptosis proteins in mammals are homologous to the Ced-3, Ced-4, and Ced-9 proteins of nematodes?
b. Give some examples of programmed cell death in humans.

Answer 9

a. The basic mechanism for programmed cell death evolved early in animal evolution.
b. Programmed cell death in humans is important in the normal development of the nervous system, finger, and toes; for normal functioning of the immune system; and to prevent the development of cancerous cells (by the internal triggering) of apoptosis in cells with DNA damage).

Question 10

Why is cell signaling such an important component of a cell’s life?

Answer 10

Cell signaling is essential for communication between cells.

Unicellular organisms use signals to relay environmental or reproductive information.

Communication in multicellular organisms allows for the development and coordination of specialized cells.

Extracellular signals control the crucial activities of cells, such as cell division, differentiation, metabolism, and gene expression.

Question 11

Briefly describe the three stages of cell signaling.

Answer 11

Cell signaling occurs through signal transduction pathways that include reception, transduction, and response.

First a signaling molecule binds to a specific receptor.

The message is transduced as the activated receptor activates a protein that may relay the message through a sequence of activations, finally leading to the activaction of proteins that produce the specific cellular response.

Question 12

Some signaling pathways alter a protein’s activity; others may result in the production of new proteins.

Explain the mechanisms for these two different responses.

Answer 12

The sequence described in the SYK #2 results in the activation of cellular proteins. When the signal is transduced to activate a transcription factor, however, the cellular response is a change in gene expression and the production of new proteins.

Question 13

How does an enzyme cascade produce an amplified response to a signal molecule?

Answer 13

In an enzyme cascade, each step in the pathway activates multiple substrates of the next step, thus amplifying the original message to produce potentially millions of activated proteins and thus a large cellular response to a few signals.

Question 14

When epinephrine binds to cardiac (heart) muscle cells, it speeds their contraction. When it binds to muscle cells of the small intestine, it inhibits their contraction. How can the same hormone have diffent effects on muscle cells?

a. Cardiac cells have more receptors for ephinephrine than do intestinal cells.
b. Epinephrine circulates to the heart first and is in higher concentration around cardiac cells.
c. The two types of muscles cells have different signal transduction pathways for epinephrine and thus have different cellular responses.
d. Cardiac muscle is stronger than intestinal muscle and thus has a stronger response to epinephrine.
e. Epinephrine binds to G-protein-linked receptors in cardiac cells, and these receptors always increase a response to the signal. Epinephrine binds to receptor tyrosine kinases in intestinal cells, and these receptors always inhibit a response to the signal.

Answer 14

c. The two types of muscles cells have different signal transduction pathways for epinephrine and thus have different cellular responses.

Question 15

Which of the following would be used in the type of local signaling called paracrine signaling in animals?

a. the neurotransmitter acetylcholine
b. the hormone epinephrine
c. the neurotransmitter norepinephrine
d. a local regulator such as a growth factor
e. both a and c

Answer 15

d. a local regulator such as a growth factor

Question 16

A signaling molecule that binds to a plasma-membrane protein functions as a

a. ligand.
b. second messenger.
c. protein phosphatase.
d. protein kinase.
e. receptor protein.

Answer 16

a. ligand.

Question 17

What is a G protein?

a. a specific type of membrane-receptor protein
b. a protein on the cytoplasmic side of a membrane that becomes activated by a receptor protein
c. a membrane-bound enzyme that converts ATP to cAMP
d. a tyrosine kinase relay protein
e. a guanine nucleotide that converts between GDP and GTP to activate and inactivate relay proteins

Answer 17

b. a protein on the cytoplasmic side of a membrane that becomes activated by a receptor protein

Question 18

How do receptor tyrosine kinases transduce a signal?

a. They transport the signaling molecule into the cell, where it binds to and activates a transcription factor. The transcription factor then alters gene expression.
b. Signaling molecule binding causes a shape change that activates membrane-bound tyrosine kinase relay proteins that phosphorylate serine and threonine amino acids.
c. Their activated tyrosine kinases convert ATP to cAMP; cAMP acts as a second messenger to activate other protein kinases.
d. When activated, they cleave a membrane phospholipid into two second-messenger molecules. One of the molecules opens Ca²⁺ ion channels on the endoplasmic reticulum.
e. They form a dimer; they phosphorylate each other’s tyrosines; specific proteins bind to and are activated by the phosphorylated tyrosines.

Answer 18

e. They form a dimer; they phosphorylate each other’s tyrosines; specific proteins bind to and are activated by the phosphorylated tyrosines.

Question 19

Which of the following can activate a protein by transferring a phosphate group to it?

a. G protein
b. phosphodiesterase
c. protein phosphatase
d. protein kinase
e. both a and c

Answer 19

d. protein kinase

Question 20

Many signal transduction pathways use second messengers to

a. transport a signaling molecule through the lipid bilayer portion of the plasma membrane.
b. relay a signal from the outside to the inside of the cell.
c. relay the message from the inside of the membrane throughout the cytoplasm.
d. amplify the message by phosphorylating cascades of proteins.
e. damplen the message once the signaling molecule has left the receptor.

Answer 20

c. relay the message from the inside of the membrane throughout the cytoplasm.

Question 21

What is a function of the second messenger IP₃?

a. bind to and activate protein kinase A
b. activate transcription factors
c. activate other membrane-bound relay molecules
d. convert ATP to cAMP
e. bind to and open ligand-gated calcium channels on the ER

Answer 21

e. bind to and open ligand-gated calcium channels on the ER

Question 22

Many human diseases, including bacterial infections, and also many medicines used to treat these diseases produce their effects by influencing which of the following?

a. cAMP concentrations in the cell
b. Ca²⁺ concentrations in the cell
c. G-protein-linked receptor pathways
d. gene expression
e. receptor tyrosine kinases

Answer 22

c. G-protein-linked receptor pathways

Question 23

Signal amplification is most often achieved by

a. an enzyme cascade involving multiple protein kinases.
b. the binding of multiple signaling molecules.
c. branching pathways that produce mulitiple cellular responses.
d. activating transcription factors that affect gene expression.
e. the action of adenylyl cyclase in converting ATP to ADP.

Answer 23

a. an enzyme cascade involving multiple protein kinases.

Question 24

From studying the effects of epinephrine on liver cells, Sutherland concluded that

a. there is a one-to-ten correlation between the number of epinephrine molecules bound to receptors and the number of glucose molecules released from glycogen.
b. epinephrine enters liver cells and binds to receptors that function as transcription factors to turn on the gene for glycogen phosphorylase.
c. there is a “second messenger” that transmits the signal of epinephrine binding on the plasma membrane to the enzymes involved in glycogen breakdown inside the cell.
d. the signal transduction pathway through which epinephrine signals glycogen breakdown involves receptor tyrosine kinases and Ca²⁺ that activate glycogen phosphorylase.
e. epinephrine functions as a ligand to open ion channels in the plasma membrane that allow Ca²⁺ to enter and initiate a response.

Answer 24

c. there is a “second messenger” that transmits the signal of epinephrine binding on the plasma membrane to the enzymes involved in glycogen breakdown inside the cell.

Question 25

Which of the following is a similarity between G-protein-linked receptors and receptor tyrosine kinases?

a. signaling molecule-binding sites specific for steroid hormones
b. formation of a dimer following binding of a signaling molecule
c. activation that results from binding of GTP
d. phosphorylation of specific amino acids in direct response to ligand binding
e. α-helix regions of the receptor that span the plasma membrane

Answer 25

e. α-helix regions of the receptor that span the plasma membrane

Question 26

Which of the following is incorrectly matched with its description?

a. scaffolding protein — large relay protein that may bind with several other relay proteins to increase the efficiency of a signaling pathway
b. protein phosphatase — enzyme that transfers a phosphate group from ATP to a protein, causing a shape change that usually activates that protein
c. adenylyl cyclase — enzyme attached to plasma membrane that converts ATP to cAMP in response to an extracellular signal
d. phospholipase C — enzyme that may be activated by a G protein or receptor tyrosine kinase and cleaves a plasma-membrane phospholipid into the second messengers IP₃ and DAG
e. G protein — relay protein attached to the inside of plasma membrane that, when activated by an activated G-protein-linked receptor, binds GTP and then usually activates another membrane-attached protein

Answer 26

b. protein phosphatase — enzyme that transfers a phosphate group from ATP to a protein, causing a shape change that usually activates that protein

Question 27

The gene of ced-9 codes for a protein that inactivates the proteins of suicide genes found in C. elegans. For development to proceed normally, ced-9

a. should be activated in all cells, but its protein product must remain inactive.
b. should be activated in all cells, but its product will be inactivated when cells programmed to die receive the proper signal.
c. should be inactivated in all cells except when a cell receives a signal to die.
d. should be inactivated only in those cells that must die for proper development to occur.
e. should code for a transcription factor that attaches to the enhancers of other suicide genes.

Answer 27

b. should be activated in all cells, but its product will be inactivated when cells programmed to die receive the proper signal.