Bio 122 Chapter 11 Study Guide: Cell Communication

What is a target cell?

A cell that can receive the signal. In order to receive the signal (hear the message), the cell needs to have a receptor for that signal.

Amplification of a signal

- Cascades spread an intracellular signal throughout the cell and amplify it or magnify it. - Signal transduction usually involves multiple steps. - Multistep pathways can amplify a signal: a few molecules can produce a large cellular response. - Definition of a cascade: a process that uses multiple steps, AND there is an amplification of the signal or response at each step.

The Specificity of Cell Signaling and Coordination of the Response

- Different kinds of cells have different signal transduction pathways. - These different pathways allow cells to detect and respond to different signals. - Even the same signal can have different effects in different cells, due to different signal transduction pathways. - Pathway branching and "cross-talk" further help the cell coordinate incoming signals

Signal transduction for G-protein coupled receptors

- Many G protein-coupled receptors trigger formation of cAMP. - cAMP activates protein kinase A, which phosphorylates various target proteins. - Phosphorylation turns the target proteins ON.

What are the functions of signal transduction pathways?

- Signal transduction pathways allow different types of cells to respond differently to the same signal molecule. - Signal transduction pathways amplify the effect of a signal molecule. - Signal transduction pathways convert a signal on a cell's surface to a specific cellular response

Hydrophilic signals turn ON a series of proteins, analogous to a chain of ON switches.

- The signal binds to the receptor and turns it ON or activates it. - The activated receptor then turns ON other proteins. - These other proteins then turn ON yet more proteins, creating a response. - Every protein that turns ON must also turn OFF, otherwise the cell will get sick or die.

What are the four stages of cell signaling for hydrophilic signals?

1) Sending the signal (extracellular signal) 2) Receiving the signal 3) Intracellular signals (signal transduction) 4) Response

Nuclear responses to a signal: the activation of a specific gene by a growth factor.

1) Signal reception: The initial signaling molecule triggers a phosphorylation cascade 2) Signal transduction: Once activated, the last kinase in the cascade enters the nucleus and activates a transcription factor 3) Cellular response: The transcription factor stimulates transcription of a specific gene 4) The resulting mRNAs direct the synthesis of a particular protein in the cytoplasm

Signals fall into four classes based upon how far they travel in the body.

1) direct contact 2) synaptic signaling: only neurons 3) paracrine signaling: only within a tissue 4) endocrine signaling: only hormones, the hormone must travel in the blood

Signals fall into two classes based upon their general chemical properties:

1) hydrophilic 2) hydrophobic

The process by which a signal is converted to a specific cellular response involves three stages:

1) reception of a signaling molecule 2) transduction of the signal 3) response to the signal

What sequence below best describes the sequential steps (numbered 1-5) in a signal transduction pathway that utilizes a G protein-coupled receptor? 1. The signal-receptor complex activates a G protein. 2. Protein kinases are activated. 3. A signal molecule binds to a receptor. 4. Target proteins are phosphorylated.5. cAMP is produced.

3, 1, 5, 2, 4

Which of the following signal transduction proteins are activated by phosphorylation after binding to a signal molecule? A) receptor tyrosine kinases B) ligand-gated ion channels C) steroid receptors D) G protein-coupled receptors

A

A drug could inhibit the release of glucose from liver cells by having which of the following effects on the liver cells? A) blocking G protein activity B) increasing glycogen phosphorylase activity C) increasing cAMP production D) activating epinephrine receptors

A

A plasma membrane protein does all of the following: binds a signal, acts as a kinase, and activates (turns ON) cytoplasmic proteins. Choose the correct name for this protein. A) tyrosine kinase receptors B) ligand-gated ion channels C) G protein-coupled receptors D) steroid receptors

A

A signal molecule is also known as a(n) _____. A) ligand B) key C) protein D) initiator E) receptor

A

Apoptosis can be turned on in response to which of the following? A) hormonal signals B) bacterial infections C) excess ATP D) cancer

A

Binding of a signaling molecule to which type of receptor leads directly to a change in the distribution of ions on opposite sides of the membrane? A) ligand-gated ion channel B) receptor tyrosine kinase C) G protein-coupled receptor D) steroid receptor

A

Choose the correct sequence for G-protein coupled receptors. A) receptor ON → G-proteins ON → membrane-associated enzyme ON B) G-proteins ON → receptor ON → membrane-associated enzyme ON C) membrane-associated enzyme ON → receptor ON → G-proteins ON D) receptor ON → membrane-associated enzyme ON → G-proteins ON

A

Lipid-soluble signaling molecules, such as aldosterone, cross the membranes of all cells but affect only target cells because: A) intracellular receptors are present only in target cells. B) only target cells retain the appropriate DNADNA segments. 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.

A

Not all intercellular signals require transduction. Which one of the following signals may be processed without transduction? A) a lipid-soluble signal B) a signal that binds to the extracellular matrix C) a signal that binds to a receptor in the cell membrane D) a signal that is weakly bound to a nucleotide

A

The function of phosphatases in signal transduction is best described as A) turning OFF (inactivating) protein kinases and other proteins, ending a cell's response. B) stimulating the production of second messengers, such as cAMP. amplifying an intracellular signal. C) transferring a phosphate group from one protein to the next. D) activating (turning ON) a protein kinase in response to an extracellular signal.

A

Thyroid hormones bind to _____ receptors. A) intracellular B) plasma membrane ion-channel C) tyrosine-kinase D) G-protein-linked E) steroid

A

Which of the following is a substance that acts at a long distance from the site at which it is secreted? A) hormone B) synaptic signal C) local regulator D) neurotransmitter E) paracrine signal

A

Definition of a second messenger

A molecule or ion whose ONLY function in the cell is to act as an intracellular signal. The extracellular signal (ligand) that binds to the receptor is a pathway's "first messenger". Second messengers spread throughout a cell by diffusion. In this way, they relay the intracellular signal throughout the cell. Cyclic AMP and calcium ions are common second messengers.

What is a protein kinase, and what is its role in a signal transduction pathway?

A protein kinase is an enzyme that transfers a phosphate group from ATP to a protein, usually activating that protein (often a second type of protein kinase). Many signal transduction pathways include a series of such interactions, in which each phosphorylated protein kinase in turn phosphorylates the next protein kinase in the series. Such phosphorylation cascades carry a signal from outside the cell to the cellular protein(s) that will carry out the response.

Cyclic AMP (cAMP; cyclic adenosine monophosphate)

A ring-shaped molecule made from ATP that is a common intracellular signaling molecule (second messenger) in eukaryotic cells. It is also a regulator of some bacterial operons.

Phosphorylation cascade

A series of chemical reactions during cell signaling mediated by enzymes (kinases), in which each kinase, in turn, phosphorylates and activates another, ultimately leading to phosphorylation of many proteins. The signal is transmitted by a cascade of protein phosphorylations, each causing a shape change in the phosphorylated protein. The shape change results from the interaction of the newly added phosphate groups with charged or polar amino acids on the protein being phosphorylated. The shape change in turn alters the function of the protein, most often activating it.

G-protein coupled receptors

A special class of membrane receptors with an associated GTP binding protein; activation of a G protein-coupled receptor involves dissociation and GTP hydrolysis

Apoptosis

A type of programmed cell death, which is brought about by activation of enzymes that break down many chemical components in the cell.

An enzyme em​bedded in the plasma membrane, adenylyl cyclase (also known as adenylate cyclase), converts

ATP to cAMP in response to an extracellular signal.

Protein kinase

An enzyme that transfers phosphate groups from ATP to a protein, thus phosphorylating the protein.

How can a target cell's response to a single hormone molecule result in a response that affects a million other molecules?

At each step in a cascade of sequential activations, one molecule or ion may activate numerous molecules functioning in the next step. This causes the response to be amplified at each such step and overall results in a large amplification of the original signal.

A target cell for a signal is defined as a cell that has A) a cell surface receptor for the signal; intracellular receptors are excluded from the definition. B) a receptor that binds to the signal, and the ability to respond. C) an intracellular receptor for the signal; cell surface receptors are excluded from the definition. D) a kinase that binds to the signal.

B

A toxin that inhibits the production of GTP would interfere with the function of a signal transduction pathway that is initiated by the binding of a signal molecule to _____ receptors. A) intracellular B) G-protein-linked C) steroid D) ion-channel E) receptor tyrosine kinase

B

Binding of epinephrine to its receptor results in which of the following events? A) Inhibition of glycogen phosphorylase. B) Elevation of cytosolic concentrations of cyclic AMP. C) Stimulation of glycogen synthesis. D) A decrease in blood glucose levels.

B

If an animal cell suddenly loses the ability to produce GTP, what will most likely happen to its signaling system? A) It will use ATP instead of GTP to activate G proteins on the cytoplasmic side of the plasma membrane. B) It will not be able to activate G proteins on the cytoplasmic side of the plasma membrane. C) It will not be able to activate receptor tyrosine kinases. D) It will be able to carry out reception and transduction but would not be able to respond to a signal

B

When a neuron responds to a particular neurotransmitter by opening gated ion channels, the neurotransmitter is serving as which part of the signal pathway? A) transducer B) signal molecule C) response molecule D) relay molecule

B

Which of the following is the best explanation for the fact that most signal transduction pathways have multiple steps? A) Each individual step can remove excess phosphate groups from the cytoplasm. B) Multiple steps provide amplification of a signal. C) Multiple steps in a pathway require the least amount of ATP. D) Each step brings the signal closer to the nucleus.

B

Which of the following statements best describes the mechanism by which testosterone functions inside a cell? A) It acts as a signal receptor that activates tyrosine kinases. B) It binds with a receptor protein that enters the nucleus and activates expression of specific genes. C) It coordinates a phosphorylation cascade that reduces spermatogenesis. D) It acts as a steroid signal receptor that activates ion channel proteins in the plasma membrane.

B

Which of the following statements best describes why phosphorylation cascades are useful in cellular signal transduction? A) they counter the harmful effects of phosphatases B) they amplify the original signal many times C) they always lead to the same cellular response D) they are species specific

B

Which of the following statements describes why a signaling molecule can cause different responses in different cells? A) Different cells have membrane receptors that bind to different sides of the signaling molecule. B) The transduction process is unique to each cell type; to respond to a signal, different cells require only a similar membrane receptor. C) The transduction pathway in cells has a variable length. D) Different cells possess different enzymes, which modify the signaling molecule into different molecules after it has arrived. E) All of the listed choices are correct.

B

The cleavage of glycogen by glycogen phosphorylase releases _____. cellulose A) glucose-1-phosphate B) fructose-1-phosphate C) galactose-1-phosphate D) nothing: glycogen phosphorylase cannot cleave glycogen

B. Glycogen is a polysaccharide composed of glucose monomers.

Which property is necessary for protein A to bind to protein B? A) Protein A and Protein B must be identical so they can form a pair B) Protein A must have regions of shape and charge that are complementary to those on Protein B C) Protein A must have region of shape and charge that are the same as those on Protein B D) Any two molecules can bind to each other; there is no special property required

B. Molecular shape determines how biological molecules recognize and respond to one another with specificity. Biological molecules often bind temporarily to each other by forming weak interactions, but only of their shapes are complementary and of their functional groups and surface charges are compatible. The role of molecular shape illustrates how biological organization leads to a match between structure and function, one of biology's unifying themes.

How does hydrolysis of ATP drive transport? A) Transport cannot be driven by ATP hydrolysis-- substances are transported through channels in channel proteins B) A phosphate group from ATP is added to a transport protein, changing its shape and facilitating transport C) ATP hydrolysis moves protons across the membrane and the proton motive force is used to transport substances D) The transport protein synthesizes ATP as it transports ions down their concentration gradient across the membrane

B. One way ATP hydrolysis can power active transport is when its terminal phosphate group is transferred directly to the transport protein. This can induce the protein to change its shape in a manner that translocates a solute bound to the protein across the membrane. The transport proteins that move solutes against their concentration gradients are all carrier proteins rather than channel proteins.

How is ligand binding similar to the process of allosteric regulation of enzymes?

Binding of a ligand to a receptor changes the shape of the receptor, altering the ability of the receptor to transmit a signal. Binding of an allosteric regulator to an enzyme changes the shape of the enzyme, either promoting or inhibiting enzyme activity.

A type of localized signaling in which a cell secretes a signal molecule that affects neighboring cells is best described as which of the following? A) hormonal signaling B) cell-cell contact-dependent signaling C) paracrine signaling D) autocrine signaling

C

A(n) _____ is an example of a signal molecule that can bind to an intracellular receptor and thereby cause a gene to be turned on or off. A) protein B) ion C) steroid D) carbohydrate E) nucleic acid

C

At puberty, several organs systems in an adolescent female change their structure and function, primarily under the influence of changing concentrations of estrogens. How can one hormone, such as estrogen, produce so many different effects? A) Estrogen is kept away from the surface of any cells not able to bind it at the surface. B) Estrogen is produced in very large concentration and therefore diffuses widely. C) Many kinds of cells have estrogen receptors, and each cell will have a different response. D) The subcomponents of estrogen, when metabolized, can influence cell response.

C

Consider this pathway: epinephrine →→ G protein-coupled receptor →→ G protein →→ adenylyl cyclase →→ cAMP. Identify the second messenger. A) G protein B) adenylyl cyclase C) cAMP D) GTP

C

Testosterone functions inside a cell by A) acting as a steroid receptor that activates ion channel proteins. B) becoming a second messenger that activates adenylyl cyclase. activating a phosphorylation cascade. C) attaching to a receptor protein, then the receptor goes to the nucleus and changes gene transcription. D) acting as a signal that activates tyrosine kinases.

C

What is apoptosis? A) a type of membrane receptor B) a metabolic step in blood clotting C) controlled cell suicide D) a type of second messenger E) a way to stimulate transcription

C

When a neuron responds to a particular neurotransmitter by opening ion channels, the neurotransmitter is serving as which part of the signal pathway? A) second messenger B) transducer C) signal molecule D) endocrine molecule E) receptor

C

Which of the following activities would be inhibited by a drug that specifically blocks the addition of phosphate groups to proteins? A) adenylyl cyclase activity B) ligand-gated ion channel signaling pathways C) receptor tyrosine kinase activity D) binding of G proteins to G protein-coupled receptors

C

Which of the following are among the most common second messengers? A) G proteins and GTP B) kinases and phosphate groups C) calcium ions and cAMP D) kinases and phosphatases E) GTP and GDP

C

Which of the following conditions is required for a target organ to respond to a particular hormone? A) The target organ must be the same as the organ that produced the hormone. B) Cells in the target organ must modify their plasma membranes to allow the hormone to enter the cytoplasm. C) The target organ must have receptors that recognize and bind the hormone molecule. D) The target organ must have the opposite mating type of the organ that produced the hormone.

C

Which of the following enzymes essentially reverses the reaction catalyzed by adenylyl cyclase? A) protein phosphatase B) phosphorylase C) phosphodiesterase D) protein kinase

C

Which statement correctly distinguishes the roles of protein kinases and protein phosphatases in signal transduction pathways? A) Protein kinases are more critical than protein phosphatases to signal transduction enzymes. B) Protein kinases are involved in signal transduction in unicellular eukaryotes such as yeast. Protein phosphatases are involved in signal transduction in multicellular eukaryotes. C) Protein kinases activate enzymes by phosphorylating or adding phosphate groups to them. Protein phosphatases dephosphorylate or remove phosphate groups from enzymes, including protein kinases.

C

Which of these extracellular signal molecules could diffuse through a plasma membrane and bind to an intracellular receptor? A) starch B) cellulose C) estrogen D) glucose E) glycerol

C. Nonpolar molecules can diffuse through the plasma membrane and bind to intracellular receptors.

What is the function of receptor tyrosine kinases? A) enzymatic degrading of GTP to GDP B) binding to nonpolar signaling molecules such as nitric oxide or steroid hormones C) enzymatic phosphorylation of tyrosine in the receptor protein D) allowing specific ions to enter the cell after ligand binding E) enzymatic hydrolysis of the signaling molecule shortly after its arrival

C. Phosphorylated receptor tyrosine kinases then interact with relay proteins within the cell.

A signal transduction pathway is initiated when a _____ binds to a receptor. A) cyclic AMP B) calmodulin C) G protein D) signal molecule E) tyrosine kinase

D

Binding of a signaling molecule to which type of receptor leads directly to a change in the distribution of substances on opposite sides of the membrane? A) intracellular receptor B) G protein-coupled receptor C) phosphorylated receptor tyrosine kinase dimer D) ligand-gated ion channel

D

Caffeine is an inhibitor of phosphodiesterase. Therefore, the cells of a person who has recently consumed coffee would have increased levels of which of the following molecules? A) activated G proteins B) adenylyl cyclase C) phosphorylated proteins D) cAMP

D

Epinephrine acts as a signal molecule that attaches to _____ proteins. A) ion-channel receptor B) nuclear receptor C) intracellular receptor D) G-protein-linked receptor E) receptor tyrosine kinase

D

Three of the following four statements is true. Pick the FALSE statement. A) Kinases come in two general categories. B) Kinases are only intracellular. C) Kinases covalently add phosphates onto proteins. D) Kinases turn proteins OFF by phosphorylating them.

D

Which of the following statements best describes a typical cAMP-mediated signal transduction event? A) A hormone activates the second messenger by directly binding to it. B) The second messenger is the last part of the system to be activated. C) The second messenger amplifies the hormonal response by attracting more hormones to the cell being affected. D) Adenylyl cyclase is activated after the hormone binds to the cell and before phosphorylation of proteins occurs.

D

Which of the following statements best describes an aspect of signal transduction? A) In response to some G protein-mediated signals, a special type of lipid molecule associated with the plasma membrane is cleaved to form IP3 and calcium. B) When signal molecules first bind to receptor tyrosine kinases, the receptors phosphorylate a number of nearby molecules. C) In most cases, signal molecules interact with the cell at the plasma membrane, enter the cell, and eventually enter the nucleus. D) Protein kinase A activation is one possible result of signal molecules binding to G protein-coupled receptors.

D

Which of the following statements describes why there are often so many steps between the original signal event and the cell's response? A) Long, highly specific pathways minimize the possibility that a relay molecule could accidentally activate a pathway leading to a secondary response. B) Each transduction is a checkpoint. C) The accumulation of genetic mutations over time has added redundant steps to the pathway. D) Each step in a cascade produces a large number of activated products, causing signal amplification as the cascade progresses. E) Lengthy pathways provide the opportunity for the initial pathway molecules to recycle back to inactive forms should additional signaling molecules be present.

D

Which of these is the second of the three stages of cell signaling? A) gene activation B) reception C) binding of a neurotransmitter to a plasma membrane receptor D) D) transduction E) cell response

D

Describe the process of apoptosis.

During this process, cellular agents chop up the DNA and fragment the organelles and other cytoplasmic components. The cell shrinks and becomes lobed (a change called "blebbing"), and the cell's parts are packaged up in vesicles that are engulfed and digested by specialized scavenger cells, leaving no trace. Apoptosis protects neighboring cells from damage that they would otherwise suffer if a dying cell merely leaked out all its contents, including its many digestive enzymes.

________ signaling occurs when a cell releases a signal molecule into the extracellular space and cells in the immediate vicinity respond. A) hormonal B) synaptic C) autocrine D) endocrine E) paracrine

E

What are the three major types of cell-surface transmembrane receptors?

G protein-coupled receptors (GPCRs), receptor tyrosine kinases, and ion channel receptors.

Second messengers participate in pathways that are initiated by both

G protein-coupled receptors and receptor tyrosine kinases. The two most common second messengers are cyclic AMP and calcium ions, Ca2+.

Which of the following are second messengers? I. ATP II. kinases III. Ca++ IV. cAMP

III and IV

If apoptosis occurred when it should not, what types of protein defects might be the cause? What types could result in apoptosis not occurring when it should?

If a receptor protein for a death-signaling molecule were defective such that it was activated even in the absence of the death signal, this would lead to apoptosis when it wouldn't normally occur. Similar defects in any of the proteins in the signaling pathway would have the same effect if the defective proteins activated relay or response proteins in the absence of interaction with the previous protein or second messenger in the pathway. Conversely, if any protein in the pathway were defective in its ability to respond to an interaction with an early protein or other molecule or ion, apoptosis would not occur when it normally should. For example, a receptor protein for a death signaling ligand might not be able to be activated, even when ligand was bound. This would stop the signal from being transduced into the cell.

Give an example of apoptosis during embryonic development, and explain its function in the developing embryo.

In formation of the hand or paw in mammals, cells in the regions between the digits are programmed to undergo apoptosis. This serves to shape the digits of the hand or paw so that they are not webbed. (A lack of apoptosis in these regions in water birds results in webbed feet.)

How is cell-cell communication used by single-celled organisms?

In single-celled organisms, signaling allows populations of cells to coordinate with one another and work like a team to accomplish tasks no single cell could carry out on its own. Most single-celled organisms can perceive changes in nutrient availability and adapt their metabolism as needed. Some single-celled organisms may utilize environmental signals to locate a suitable mate; some send signals to make their numbers known to other members of their species.

Receptor Tryosine Kinases

Membrane receptors having enzymatic activity. Receptors that attach phosphates to tyrosines.

Nerve growth factor (NGF) is a water-soluble signaling molecule. Would you expect the receptor for NGF to be intracellular or in the plasma membrane? Explain.

NGF is water-soluble (hydrophilic), so it cannot pass through the lipid membrane to reach intracellular receptors, as steroid hormones can. Therefore, you'd expect the NGF receptor to be in the plasma membrane—which is, in fact, the case.

When a signal transduction pathway involves a phosphorylation cascade, how does the cell's response get turned off?

Protein phosphatases reverse the effects of the kinases by dephosphorylation, and unless the signaling molecule is at a high enough concentration that it is continuously rebinding the receptor, the kinase molecules will all be returned to their inactive states by phosphatases.

How does the activated hormone-receptor complex turn on genes?

Recall that the genes in a cell's DNA function by being transcribed and processed into messenger RNA (mRNA), which leaves the nucleus and is translated into a specific protein by ribosomes in the​ cytoplasm. Special pro​teins called transcription factors control which genes are turned on—that is, which genes are transcribed into mRNA—in a particular cell at a particular time. When the aldosterone receptor is activated for example, it acts as a transcription factor that turns on specific genes.

Signal reception

Reception is the target cell's detection of a signaling molecule coming from outside the cell. A chemical signal is "detected" when the signaling molecule binds to a receptor protein located at the cell's surface.

What are the three steps in communication, whether human speech or cell signals?

Reception: A cell detects a signaling molecule from the outside of the cell. Transduction: When the signaling molecule binds the receptor it changes the receptor protein in some way. Response: Finally, the signal triggers a specific cellular response.

What are the three stages of Cell Signaling for hydrophobic signals?

Sending the signal - Receiving the signal - Response Hydrophobic signals pass through the membrane on their own, then attach to a receptor that is inside the cell. This is called an intracellular receptor. The signal + receptor then move into the nucleus, where they cause the transcription of genes.

What are four aspects of response regulation?

Signaling pathways generally amplify the cell's response to a single signaling event. The degree of amplification depends on the function of the specific molecules in the pathway. Second, the many steps in a multistep pathway provide control points at which the cell's response can be further regulated, contributing to the specificity of the response and allowing coordination with other signaling pathways. Third, the overall efficiency of the response is enhanced by the presence of proteins known as scaffolding proteins. Finally, a crucial point in regulating the response is the termination of the signal.

Termination of signaling

Termination of the signal - Decrease in ligand concentration - receptor turns OFF - G proteins inactivate by themselves automatically Termination of signal transduction or intracellular signals - cAMP broken down to AMP by phosphodiesterase - Calcium ions pumped out of the cytoplasm - Dephosphorylation of phosphorylated proteins by phosphatases Termination of the response - Turn OFF gene expression - Turn OFF an enzyme

Signal transduction

The binding of the signaling molecule changes the receptor protein in some way, initiating the process of transduction. The transduction stage converts the signal to a form that can bring about a specific cellular response. In Sutherland's system, the binding of epinephrine to a receptor protein in a liver cell's plasma membrane leads to activation of glycogen phosphorylase in the cytosol. Transduction sometimes occurs in a single step but more often requires a sequence of changes in a series of different molecules—a signal transduction pathway. The molecules in the pathway are often called relay molecules.

Signal transduction pathway.

The process by which a signal on a cell's surface is converted into a specific cellular response. This occurs in a series of three major steps—signal reception, signal transduction, and cellular response. Many such pathways exist in the cells of both unicellular and multicellular organisms.

Epinephrine affects heart muscle cells by causing them to mobilize glucose, contract faster, and increase heart rate. The muscle cells around lungs and airways, on the other hand, have the opposite response to epinephrine: They relax, allowing more air to be breathed in. What might explain why respiratory (breathing-related) muscle cells can respond so differently from heart muscle cells?

The proteins in the two cells are different, so the cellular response is different. In heart muscle cells, the pathway allows glucose to fuel faster muscle contractions and heart rate. In respiratory muscles, the relay proteins must be different, so that the effect is to block muscle contraction. (In fact, the steps are the same through protein kinase A (PKA), but in respiratory muscle cells, PKA phosphorylates a protein that is required for muscle contraction—and in this case, phosphorylation inactivates that protein. So muscle contraction cannot occur.)

What is the actual "signal" that is being transduced in any signal transduction pathway? In what way is this information being passed from the exterior to the interior of the cell?

The signal that is being transduced is the information that a signaling molecule is bound to the cell-surface receptor. Information is transduced byway of sequential protein-protein interactions that change protein shapes, causing them to function in a way that passes the signal (the information) along.

Cellular response

The transduced signal finally triggers a specific cellular response. The response may be almost any imaginable cellular activity—such as catalysis by an enzyme (for example, glycogen phosphorylase), rearrangement of the cytoskeleton, or activation of specific genes in the nucleus. The cell-signaling process helps ensure that crucial activities like these occur in the right cells, at the right time, and in proper coordination with the activities of other cells of the organism.

Explain how signaling is involved in ensuring that yeast cells fuse only with cells of the opposite mating type.

The two cells of opposite mating type (a and αα) each secrete a unique signaling molecule, which can only be bound by receptors carried on cells of the opposite mating type. Thus, the a mating factor cannot bind to another a cell and cause it to grow toward the first a cell. Only an αα cell can "receive" the signaling molecule and respond by directed growth.

Cell surface receptors are usually transmembrane proteins, a type of

integral membrane protein crossing from one side of the membrane to the other.

ligand-gated ion channel

Type of membrane receptor that has a region that can act as a "gate", opening or closing the channel when the receptor changes shape. When a signaling molecule binds as a ligand to the channel receptor, the channel opens or closes, allowing or blocking the flow of specific ions, such as Na + or Ca 2+. Like other receptors, these proteins bind the ligand at a specific site on their extracellular sides. These channels are very important to the nervous system.

G-protein coupled receptors are receptors that work with the help of a

a G-protein (defined as a protein that binds GTP or GDP). The G-protein can be ON or OFF. If GDP is bound to the G-protein, the G-protein is inactive or OFF. If GTP is bound, the G-protein is active or ON.

Some G-protein couple receptors use calcium ions (Ca2+) as

a second messenger. Cells can regulate the concentration of calcium ions. An increase in the calcium concentration acts as a second messenger. Like cAMP, calcium can turn ON particular proteins.

Intracellular receptor proteins are found in either the cytoplasm or nucleus of target cells. To reach such a receptor,

a signaling molecule passes through the target cell's plasma membrane. A number of important signaling molecules can do this because they are either hydrophobic enough or small enough to cross the hydrophobic interior of the membrane. Hydro​phobic signaling molecules include steroid hormones and thyroid hormones of animals. Another chemical signaling molecule that possesses an intracellular receptor is nitric oxide (NO), a gas; this very small molecule readily passes between the membrane phospholipids. Once a hormone or other signaling molecule has entered a cell, its binding to an intracellular receptor changes the receptor into a hormone-receptor complex that is able to cause a response—in many cases, the turning on or off of particular genes.

Together, protein kinases probably regulate the activity of a large proportion of the thousands of proteins in a cell. Among these are most of the proteins that, in turn, regulate cell division. Abnormal activity of such a kinase can cause

abnormal cell division and contribute to the development of cancer.

Kinases covalently

add phosphates to a protein, a process called phosphorylation. The phosphate comes from ATP.

Subsequent research has revealed that epinephrine is only one of many hormones and many other signaling molecules that lead to activation of

adenylyl cyclase by G proteins and formation of cAMP. The immediate effect of an elevation in cAMP level is usually the activation of a serine/threonine kinase called protein kinase A. The activated protein kinase A then phosphorylates various other proteins, depending on the cell type.

Adenylyl cyclase is

an enzyme in the plasma membrane that converts ATP to cAMP in response to an extracellular signal.

Apoptosis can be triggered by

an extracellular signal (death signal), DNA damage, or protein misfolding in the endoplasmic reticulum.

Signal transduction occurs when

an extracellular signal leads to an intracellular signal, and the intracellular signal spreads inside the cell. The receptor activates another protein, which activates another, until proteins producing the response are activated.

Allosteric regulation is the term used to describe

any case in which a protein's function at one site is affected by the binding of a small regulatory molecule to a separate site. In In the simplest kind of allosteric regulation, an activating or inhibiting regulatory molecule binds to a regulatory site. The binding of an activator to a regulatory site stabilizes the shape that has functional active sites, whereas the binding of an inhibitor stabilizes the inactive form the enzyme.

Cells that are infected, are damaged, or have reached the end of their functional life span often undergo "programmed cell death". The best-understood type of this controlled cell suicide is

apoptosis.

In humans and other mammals, several different pathways, involving about 15 different caspases, can carry out

apoptosis. The pathway that is used depends on the type of cell and on the particular signal that initiates apoptosis.

Cell-to-cell communication is essential for

both multicellular and unicellular organisms.

Caspases are the proteins that

carry out apoptosis. Caspases are proteases. Proteases are enzymes that cut up other proteins.

Eukary​otic cells may communicate by direct contact, which is one type of local signaling. Many animal and plant cells have

cell junctions that directly connect the cytoplasms of adjacent cells. In these cases, signaling substances dissolved in the cytosol can pass between neighboring cells. Moreover, some animal cells may communicate by direct contact between cell-surface molecules. This type of local signaling is especially important in embryonic development, the immune response, and in maintaining adult stem cell populations.

Tyrosine kinase receptors are

cell surface receptors that attach phosphates to tyrosines. !. A receptor tyrosine kinase turns ON when the signal attaches. 2. It then phosphorylates itself, or covalently adds phosphates to itself. 3. Special "relay" proteins then attach to the phosphates. The attachment activates or turns ON the relay proteins. 4. The relay proteins then start an intracellular signal.

Cells of the yeast Saccharomyces cerevisiae—which are used to make bread, wine, and beer—identify their sexual mates by

chemical signaling when they reproduce sexually. There are two sexes, or mating types called a and α. Each type secretes a specific factor that binds only to receptors on the other type of cell. When exposed to each other's mating factors, a pair of cells of opposite type change shape, grow toward each other, and fuse (mate). The new a/αa/α cell contains all the genes of both original cells, providing advantages to the cell's descendants, which arise by subsequent cell divisions.

In a multicellular organism, communication is essential for

coordinating the activities of many different cells. This coordination is important for: the functions of cells within a tissue and the functions of tissues in the body. The body is like a company or organization, where all of the employees or members need to communicate. They communicate with each other by chemical signals.

Hydrophilic signals cannot

cross the membrane, and must attach to the external part of a receptor. The receptor then has to make an intracellular signal (otherwise the cell does not know anything happened). The intracellular signaling is also called signal transduction. The intracellular signal then leads to a response.

Steroids are nonpolar and can

diffuse through the plasma membrane.

The signal that triggers all of the complex events that occur during apoptosis can come from

either outside or inside the cell. Outside the cell, signaling molecules released from other cells can initiate a signal transduction pathway that activates the genes and proteins responsible for carrying out cell death. Within a cell whose DNA has been irretrievably damaged, a series of protein-protein interactions can pass along a signal that similarly triggers cell death.

Single cell organisms also communicate with each other. They communicate about

food, communicate to help each other, and communicate to mate.

Both animals and plants use molecules called

hormones for long-distance signaling. In hormonal signaling in animals, also known as endocrine signaling, specialized cells release hormones, which travel through the circulatory system to other parts of the body, where they reach target cells that can recognize and respond to them

Intracellular receptors are found in

in the cytoplasm or nucleus of target cells. Examples of hydrophobic signals are the steroid and thyroid hormones of animals. An activated intracellular receptor turns ON the transcription of particular genes.

Further regulation of cell metabolism is provided by other G protein systems that

inhibit adenylyl cyclase. In these systems, a different signaling molecule activates a different receptor, which in turn activates an inhibitory G protein that blocks activation of adenylyl cyclase. Cell activities can be fine-tuned by the balance between these systems.

The ability of a cell to respond is determined by whether

it has a specific receptor molecule that can bind to the signaling molecule. The information conveyed by this binding, the signal, must then be changed into another form—transduced—inside the cell before the cell can respond.

The phosphorylation-dephosphorylation system acts as a

molecular switch in the cell, turning activities on or off, or up or down, as required. At any given moment, the activity of a protein regulated by phosphorylation depends on the balance in the cell between active kinase molecules and active phosphatase molecules.

The response of a cell to signal can be

nuclear or cytoplasmic. - Ultimately, a signal leads to a change in cellular function. - The response may occur in the cytoplasm or in the nucleus. - Nucleus: signaling pathways can turn transcription ON or OFF. - Cytoplasm: signaling pathways can affect the activities of enzymes (increase or decrease catalytic activity) or affect cell shape (change cytoskeletal proteins).

Dephosphorylation turns proteins

off.

Phosphorylation turns proteins

on.

Cell communication is like human communication:

one person talks, another hears the words, then the listener responds.

In many other cases of local signaling, signaling molecules are secreted by the signaling cell. Some molecules travel only short distances; such local regulators influence cells that are nearby. This type of local signaling in animals is called

paracrine signaling.

Cells in a multicellular organism communicate using dozens of chemical signals. They can be classified by their

physiological characteristics or their chemical characteristics.

The phosphorylation is involved in the activation of receptor tyrosine kinases. In fact, phosphorylation of proteins and its reverse, dephosphorylation, are commonly used in cells to regulate protein activity. An enzyme that transfers phosphate groups from ATP to a protein is generally known as a

protein kinase.

Many signal transduction pathways use relay molecules that are

protein kinases, and they often act on other protein kinases in the pathway.

Equally important in the phosphorylation cascade are the

protein phosphatases, enzymes that can rapidly remove phosphate groups from proteins, a process called dephosphorylation.

Many signaling pathways ultimately regulate

protein synthesis, usually by turning specific genes on or off in the nucleus. Like an activated steroid receptor, the final​ activated molecule in a signaling pathway may function as a transcription factor.

Bacterial cells secrete molecules that can be detected by other bacterial cells (Figure 11.2). Sensing the concentration of such signaling molecules allows bacteria to monitor their own local cell density, a phenomenon called

quorum sensing. Quorum sensing allows bacterial populations to coordinate the behavior of all cells in a population in activities that require a given density of cells acting at the same time.

The target cells for a particular signal are cells that can

receive the signal and respond to the signal. The target cells for a signal have receptors for that signal. The binding between a signal molecule, called a ligand, and its receptor is highly specific (like a key fitting only one lock). If a cell does not have the correct receptor, the signal will float by and have no effect.

Protein kinases can either be

receptors, or be part of signal transduction. Tyrosine kinases are always receptors. Serine/threonine kinases are part of signal transduction.

Phosphatases covalently

remove phosphates from proteins, a process called dephosphorylation.

In many cases, the efficiency of signal transduction is apparently increased by the presence of

scaffolding proteins, large relay proteins to which several other relay proteins are simultaneously attached. Researchers have found scaffolding proteins in brain cells that permanently hold together network​s of signaling pathway proteins at synapses. This hardwiring enhances the speed and accuracy of signal transfer between cells because the rate of protein-protein interaction is not limited by diffusion. Furthermore, in some cases, the scaffolding proteins themselves may directly activate relay proteins.

Cyclic AMP (cAMP) is a

second messenger.

Not all components of signal transduction pathways are proteins. Many signaling pathways also involve small, nonprotein, water-soluble molecules or ions called

second messengers. (The pathway's "first messenger" is considered to be the extracellular signaling molecule—the ligand—that binds to the membrane receptor.) Because they are small and water-soluble, they can readily spread through regions of the cell by diffusion.

A highly specialized type of local signaling called

synaptic signaling occurs in the animal nervous system. An ​electrical signal along a nerve cell triggers the secretion of neurotransmitter molecules. These molecules act as chemical signals, diffusing across the synapse—the narrow space between the nerve cell and its target cell—triggering a response in the target cell.

Calcium is kept at a high concentration inside

the ER and in the extracellular fluid. The cell does this by actively pumping calcium into these places. The cell keeps the calcium concentration low in the cytoplasm.

The largest family of human cell-surface receptors is that of

the G protein-coupled receptors (GPCRs).

The binding of a specific signaling molecule to a receptor in the plasma membrane triggers the first step in the signal transduction pathway. This is

the chain of molecular interactions that leads to a particular response within the cell. Like falling dominoes, the signal-activated receptor activates another molecule, which activates yet another molecule, and so on, until the protein that produces the final cellular response is activated. The molecules that relay a signal from receptor to response, called relay molecules, are often proteins.

Many of the signaling molecules that function in animals—including neurotransmitters, growth factors, and some hormones—induce responses in their target cells through signal transduction pathways that increase

the cytosolic concentration of calcium ions (Ca2+). Calcium is even more widely used than cAMP as a second messenger. Increasing the local cytosolic concentration of Ca2+ causes many responses in animal cells, including muscle cell contraction, exocytosis of molecules (secretion), and cell division.

Ligand binding generally causes a receptor protein to undergo a change in shape. For many receptors, this shape change directly activates

the receptor, enabling it to interact with other molecules in or on the cell. For other receptors, the immediate effect of ligand binding is to cause the aggregation of two or more receptor proteins, which leads to further molecular events inside the cell. Most signal receptors are plasma membrane proteins, but others are located inside the cell.

The ion channel and receptor are

the same protein. - When a signal molecule binds to the receptor, the ion channel opens. This creates a water-filled hole in the membrane. - The channel allows one particular type of ion to flow into the cell (down its concentration gradient). - This increases the ion concentration inside the cell. The higher ion concentration acts as an intracellular signal.

When receptors for signaling molecules are plasma membrane proteins

the transduction stage of cell signaling is usually a multistep pathway involving many molecules. Steps often include activation of proteins by addition or removal of phosphate groups or release of other small molecules or ions that act as signaling molecules. One benefit of using multiple steps is that a signal caused by a small number of signaling molecules can be greatly amplified. A second benefit of using multistep pathways is that they provide more opportunities for coordination and control than do simpler systems. This allows regulation of the response

Protein kinases come in two forms:

those that add phosphate to the amino acids serine and threonine, and those that add phosphate only to the amino acid tyrosine.

Signals, signal transduction, and responses need

to be terminated. If this does not happen, the cell will die.

Both G protein-coupled receptors and receptor tyrosine kinases are

transmembrane receptors that have a binding domain located on the extracellular side of the plasma membrane. The binding of a signaling molecule to these receptors is the first step in a signaling pathway. However, what happens after a signaling molecule binds is different for each receptor. An activated G protein-coupled receptor activates a G protein inside the cell, which involves the release of GDP and the binding of GTP. The activated G protein then activates an associated enzyme, leading to a cellular response. Receptor tyrosine kinases form dimers after binding signaling molecules. The tyrosines are then phosphorylated, fully activating the receptor. Each phosphorylated tyrosine can bind a relay protein, each of which can trigger a transduction pathway. In this way, a single signaling-molecule binding event can trigger multiple signal transduction pathways and thus multiple cellular responses.

By dephosphorylating and thus inactivating protein kinases, phosphatases provide the mechanism for

turning off the signal transduction pathway when the initial signal is no longer present. Phosphatases also make the protein kinases available for reuse, enabling the cell to respond again to an extracellular signal.