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Cell Signaling | Principles of Biology from Nature Education
Principles of Biology
20
contents
Cell Signaling
Cells use signaling molecules and receptors to communicate.
An ion channel viewed from above.
Ion channels permit communication of information across the cell membrane. They are special pore­like structures
that only allow certain ions to pass through them. Shown is a view down the center of an ion channel, a short
tunnel that an ion must flow through to pass across a cell membrane.
© 2009 Nature Publishing Group Hilf, R.J.C. & Dutzler, R. Structure of a potentially open state of a proton­activated
pentameric ligand­gated ion channel. Nature 457, 115–118 (2009) doi: 10.1038/nature07461. Used with
permission.
Topics Covered in this Module Cell­Cell Communication
Receptors
Major Objectives of this Module Describe different ways cells can communicate with each other.
Outline the steps of signal transduction.
Compare and contrast receptor types found in cells.
Explain how enzyme­linked receptors lead to signal transduction.
Explain how activation of a G protein­coupled receptor leads to signal transduction.
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Principles of Biology
20 Cell Signaling
contents
Cell­Cell Communication
Trillions of cells in the human body, with hundreds of different cell types, must recognize each other, coordinate activity
and respond to stimuli. The consequences can be dire if there is a mistake in this intricate system. For example,
cancerous tumors grow when macrophages fail to recognize cancer cells as "non­self." So how do cells communicate
and respond to each other? Can cells in an organism also interact with cells from other organisms? Does cellular
structure influence communication? Using environmental cues, or "signals," and signal receptors, cells interact with
each other. These interactions occur primarily between molecules.
How do cells communicate with each other?
A dolphin whistles to identify itself to a group. Birds convey alarm with calls to the flock when danger is near. Cells
likewise communicate with each other in response to environmental cues. For example, single­celled organisms such
as bacteria may coordinate a group response using bioluminescence or aggregation. Many single­celled eukaryotes,
such as yeasts, are surprisingly social organisms that interact with each other to find mating partners. Cues that
stimulate responses and communication include light or heat, touch or pressure, and — most frequently — chemicals.
The mechanisms that cells use to share information are common to many diverse species and underscore the
interconnectedness among all life.
The underlying mechanism used by cells to share information is signal transduction, which produces cellular
responses to extracellular signals. Chemical cell signaling in multicellular eukaryotes involves information exchange
between neighboring cells as well as with those farther afield. Together, the various signaling mechanisms allow the
cells of a multicellular organism to coordinate activities that maintain the organism's overall functioning.
Numerous chemicals are involved in the language of cells. Cells that respond to a stimulus are called sensory cells.
These cells secrete ligands, signaling molecules that bind directly to receptors on target cells to produce biological
responses. The assortment of signaling molecules includes hormones, chemical messengers secreted by glands, and
neurotransmitters, chemicals produced by the nervous system. These chemicals, among others, serve as signals that
cue target cells to respond in a particular way to alter the physiology of an organism. These signals are received by
receptors on the target cell that act as signal detectors.
Different types of signals are grouped according to the distance they travel to reach their targets. Autocrine signaling
targets receptors in the same cell that originated the signal. Paracrine signaling targets cells near the signaling cell.
Paracrine signals diffuse through extracellular fluid to reach their destination. Endocrine signaling involves hormones
that target distant cells. The circulatory system ferries hormones to facilitate long­distance communication.
Adjacent cells can use direct contact to communicate locally. In humans, gap junctions provide transmission channels
between adjacent cells. Gap junctions are pores in the cell membrane that allow cells to share molecules and ions. A
protein called connexin aggregates in groups of six in the plasma membrane. The resulting structure, called a
connexon, pairs with a connexon from the adjoining cell. The paired connexons create a pore through which small
molecules and ions can travel. Plasmodesmata are the plant counterpart to gap junctions. A third mechanism by which
direct contact can be used to transmit information is through cell­cell recognition. For example, in human
embryogenesis, large cell surface proteins called cadherins allow similar cells to recognize each other and aggregate
to form specialized tissues.
The nervous system plays a key role in multicellular animals. Communication between the synapses of neurons is
accomplished with the release of neurotransmitters, which diffuse across the synaptic cleft. Although this synaptic
contact does not involve a physical connection between two neurons, it provides the functional connection between the
neurons. For example, an action potential can transmit a signal along a presynaptic axon. At the synapse between
neurons, the information is transmitted from the presynaptic site to the postsynaptic site using neurotransmitters. Such
long­distance signals, originating in the brain and extending to an animal's farthest extremities, allow for centralized
communication in the nervous system.
Do cells transmit signals internally? Downstream activation is a cascading sequence in which an initial signaling event
triggers further events that transmit the signal to other proteins in the cell. Ligands initiate intracellular messaging, and
other chemicals called relay molecules pass around information within the cell.
Test Yourself
Explain three ways by which cells communicate with each other.
Submit
Like pieces of a puzzle, signaling ligands must be specific to their receptors (Figure 1). Once the receptor receives its
specific chemical signal, it changes its three­dimensional shape, which in turn alters its activity. Receptor­ligand binding
is reversible, which is essential because it allows the biological activity of the receptor to be switched off (or on) after the
ligand is released. A ligand that activates the biological activity of a receptor is known as an agonist. In some cases,
agonist binding can also be prevented by binding of an inhibitory molecule, called an antagonist, to the receptor. In this
situation, the antagonist blocks binding of the agonist to the receptor, blocking the normal function of the receptor.
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Figure 1: Receptor structure enables binding of appropriate ligand.
The chemical structure of a receptor matches a specific ligand. When the ligand binds to the correct receptor,
subsequent molecular reactions inside the cell can occur. Ligands that activate and deactivate a receptor's
biological function are known as agonists and antagonists, respectively.
© 2011 Nature Education All rights reserved.
How is a cell able to respond to the signals it receives?
Internal and external cellular structures determine how a cell responds to environmental signals. Receptors are located
on the cell surface as well as within the cell, in the cytoplasm or the nucleus. Transmembrane receptors bind to large,
hydrophilic molecules that cannot enter the cell by crossing through the lipid bilayer of the plasma membrane. There are
three major types of transmembrane receptors: ion channel­linked receptors, enzyme­linked receptors and G protein­
coupled receptors. The binding sites of transmembrane receptors are typically oriented toward the exterior of the cell,
although the receptor directs the signal inward.
Intracellular nuclear and cytoplasmic receptors directly bind smaller, hydrophobic molecules that readily diffuse
through the cell membrane. Such molecules include the steroid hormones estrogen and testosterone, which are
derivatives of cholesterol, a lipid­soluble molecule. Steroid hormones bind to intracellular receptors, causing those
receptors to change shape. The hormone­receptor complex then moves into the nucleus, where it binds to a specific
DNA region called the hormone response element (HRE). By binding to the HRE, the hormone­receptor complex
induces or represses the expression of specific genes (Figure 2). Those genes then either begin or cease the synthesis
of their corresponding proteins, resulting in a metabolic response that ultimately alters the physiology of the body.
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Figure 2: Steroid hormone signaling pathway.
The chemical structure of a receptor matches a specific ligand. When the ligand binds to the correct receptor,
subsequent molecular reactions inside the cell can occur.
© 2014 Nature Education All rights reserved.
Test Yourself
What are the differences between transmembrane and cytoplasmic receptors? How do these
differences influence function?
Submit
Signal transduction occurs in three major stages (Figure 3). Similar to how a key fits into a lock, signal reception occurs
when a signaling molecule binds to its receptor. This action initiates the signal transduction pathway, usually by
inducing a conformational change in the receptor. Transduction occurs when the receptor is activated as a result of its
conformational change and transmits, or transduces, the signal to other molecules within the cell. Transduction often
involves phosphorylation, dephosphorylation, or another form of chemical transformation. Phosphorylated and
dephosphorylated molecules can become relay molecules, inducing subsequent cell reactions. The final phase in the
pathway is the response, when the transduced signal initiates a reaction from the cell, such as a change in gene
expression or metabolic activity.
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Figure 3: Signal transduction occurs in three steps.
Reception, transduction and response are the three steps common to all signal transduction pathways. The cellular
responses vary depending on the particular pathway but usually result in changes in gene expression.
© 2014 Nature Education All rights reserved.
Figure Detail
Test Yourself
What three steps make up signal transduction?
Submit
Various receptors bind signaling molecules with different degrees of affinity. Affinity refers to the strength with which a
ligand binds to an enzyme, receptor, or transport protein. Most enzymes and receptors have specific conformations that
confer high affinity. For example, most enzymes have high affinity for their substrates, so that the substrate remains in
the active site long enough for the enzyme to catalyze a reaction. In contrast, many of the common transport proteins
have low affinity, which allows the release of the transported molecule from the protein to proceed relatively easily. The
concentrations of ligands also influence their binding to the receptors on target cells. As the ligand concentration
decreases, the likelihood of ligands binding to the receptors on target cells and thus triggering a response also
decreases.
What kinds of responses can result? Most cellular responses to extracellular stimulation require signal transduction.
Such responses include short­term effects such as enzyme activation and cell movement. Long­term responses include
gene activation and gene expression. Cell survival, reproduction and death are also mediated by signal transduction.
Stimuli from the environment, such as the volatile chemicals in perfume, can initiate signal transduction and prompt a
response. When receptors on olfactory sensory neurons bind these chemicals and send that information to the brain,
the event is perceived by the brain as a fragrance.
The sophistication of an organism's signal transduction pathways generally corresponds to the organism's structural
complexity. In single­celled organisms, such as bacteria, signal transduction allows for an effective mode of
communication. For example, the bacterium Salmonella enteritidis uses the chemical acyl homoserine lactone to
signal growth and increase virulence (Figure 4). Individual bacteria secrete acyl homoserine lactone to communicate
their presence to other bacteria and induce them to form aggregates with one another. When a critical density, or
quorum, of bacteria is reached, acyl homoserine lactone concentration reaches a threshold, changing its function. At
this higher concentration, acyl homoserine lactone instead signals the aggregated bacteria to begin invading their host
organism, causing disease. Yeasts, such as Candida albicans, use a similar mechanism of quorum sensing. When
nutrient­deprived, Saccharomyces cerevisiae, brewer's yeast, transmits mating factors to initiate reproduction.
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Figure 4: Salmonella enteritidis.
This bacteria's ability to communicate to other individuals via signal transduction has led to increased incidence of
food poisoning in the developed world.
Scimat/Science Source.
Basic research: Sensory transduction.
Sensory biologists study the molecular processes that drive sensory transduction, focusing on the role of those
processes in cellular architecture. One example is the transient receptor potential (TRP) ion channel family in the fruit fly
Drosophila melanogaster. In photoreceptor cells, TRP channels transduce light stimuli into electric signals. TRP
channels are widely expressed in the sensory systems of vertebrates, including humans. They function in both auditory
transduction and olfaction as well as touch and temperature detection. As progress continues toward understanding the
mechanisms underlying TRP channel function, disorders such as Usher syndrome, which can cause deafness and
blindness, might be targeted for therapy. Many more TRP channel mechanisms await discovery.
Controversy.
With so much still unknown about sensory transduction, uncertainty surrounds proposed TRP channel function theories.
In particular, scientists dispute mechanisms underlying cold temperature perception. A Spanish team of sensory
biologists published a paper in 2008 arguing that temperature signal transduction differs between visceral sensory
neurons, found in many internal organs, and somatic sensory neurons found elsewhere. These neurons use subsets of
the TRP channel family — TRPA1 and TRPM8, respectively — and scientists hypothesize that those channels play a
key role in the perception of pain caused by cold temperatures. More recently, in 2010, members of the team suggested
that TRPM8 also plays a role in generating tears in the cold to keep the eyes moist. Although other researchers dispute
the theories, scientists believe research on TRPM8 and TRPA1 could help develop drugs that target abdominal pain
receptors.
IN THIS MODULE
Cell­Cell Communication
Receptors
Summary
Test Your Knowledge
WHY DOES THIS TOPIC MATTER?
Synthetic Biology: Making Life from
Bits and Pieces
Scientists are combining biology and
engineering to change the world.
Stem Cells
Stem cells are powerful tools in
biology and medicine. What can
scientists do with these cells and their
incredible potential?
Cancer: What's Old Is New Again
Is cancer ancient, or is it largely a
product of modern times? Can
cutting­edge research lead to prevention
and treatment strategies that could make
cancer obsolete?
PRIMARY LITERATURE
http://www.nature.com/principles/ebooks/principles­of­biology­104015/29144952/1
5/6
2015/3/8
Cell Signaling | Principles of Biology from Nature Education
Innovation in Cannabis medicine
Cannabinoid potentiation of glycine
receptors contributes to cannabis­induced
analgesia.
View | Download
Inhibitors may block entry of
hepatitis C into cells
EGFR and EphA2 are host factors for
hepatitis C virus entry and possible targets
for antiviral therapy.
View | Download
How can nematodes help reduce
obesity in humans?
A whole­organism screen identifies new
regulators of fat storage.
View | Download
Classic paper: Breakthrough
enables tiny measurements of ion
channel activity (1976)
Single­channel currents recorded from
membrane of denervated frog muscle
fibers.
View | Download
Adaptor proteins regulate cell
signaling
Structural basis for regulation of the Crk
signaling protein by a proline switch.
View | Download
Mitochondria change shape to help
the cell survive
During autophagy mitochondria elongate,
are spared from degradation and sustain
cell viability.
View | Download
SCIENCE ON THE WEB
What Do Your Teeth Have to Do with
Bacterial Communication?
Scientist Bonnie Bassler explains quorum
sensing, a bacterial communication
phenomenon
An Interactive on Cell Responses.
Send a signal to a plant of animal cells.
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20 Cell Signaling
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Receptors
Three major types of transmembrane receptors — the ion channel­linked receptors, enzyme­linked receptors and G
protein­coupled receptors — play a role in signal transduction. Why does the cell need three kinds of receptors, and
what specific role does each play in facilitating cell signaling?
What are ion channel­linked receptors?
Ubiquitous among cells, ion channels are transmembrane proteins that act as pores, allowing passage of ions such as
sodium, calcium and potassium into and out of cells. However, not all ion channels are open at all times. Many are
gated — they are initially closed and require an external stimulus before they open and permit ions to pass through. An
important class of ion channel is the ion channel­linked receptors (Figure 5). These ion channels, initially closed,
contain a receptor site that accepts a specific ligand. Upon binding of the ligand, the ion channel undergoes a
conformational change that results in the opening of the channel. The ligands that activate ion channel­linked receptors
are frequently neurotransmitters, and ion channel­linked receptors are often associated with the nervous system.
Research suggests that ion channel­linked receptors mediate the effects of alcohol and anesthesia, among other
functions.
Figure 5: Ion channel­linked receptors permit ion flow depending on the presence of specific ligands.
Before an ion channel­linked receptor binds with its specific ligand, the channel is closed, preventing ion flow (1).
When the ligand binds to receptors in the channel protein, a conformational change causes the channel to open (2),
and ions begin flowing through the channel protein (3). When the ligand is no longer present, the conformational
change reverses, and the channel closes, ending ion flow (4).
© 2014 Nature Education All rights reserved.
What do enzyme­linked receptors do?
Like all transmembrane receptors, enzyme­linked receptors contain two domains. The extracellular domain contains a
receptor for a signaling ligand, such as a hormone or a growth factor. Upon binding, the ligand induces enzymatic
activity in the intracellular domain. Most frequently, these receptors are linked to kinases, which are enzymes that
phosphorylate other molecules by transferring a phosphate group from another molecule, usually adenosine
triphosphate (ATP), to them. Receptor tyrosine kinases (Figure 6) are an important class of enzyme­linked receptors.
When two receptor tyrosine kinase proteins are activated by ligand binding, they dimerize, forming a complex and
activating their kinase domains. The close proximity of the two receptor tyrosine kinases allows them to phosphorylate
each other at specific tyrosine residues. Inactive relay or adapter proteins then bind the phosphorylated tyrosine
residues and become activated, relaying the signal to other parts of the cell and initiating cellular responses.
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Figure 6: Receptor tyrosine kinases.
Receptor tyrosine kinases are transmembrane receptors. When they bind to their ligands, they phosphorylate each
other, triggering cellular events.
© 2014 Nature Education All rights reserved.
Test Yourself
How does a receptor tyrosine kinase initiate signal transduction?
Submit
What is the purpose of G protein­coupled receptors?
G protein­coupled receptors (GPCRs) perform a wide variety of functions, but their structures always feature a receptor
containing seven membrane­spanning domains. Inactive G proteins are bound to guanosine diphosphate (GDP), a
nucleotide closely related to ATP. An example of a GPCR in action is the binding of epinephrine to its receptor to
increase the availability of blood glucose during times of stress (Figure 7). When epinephrine binds to the GPCR, the
GPCR undergoes a conformational change that allows it to bind to the inactive G protein. The G protein is induced to
exchange its GDP with guanosine triphosphate (GTP) from the cytoplasm, which results in activation of the G protein.
The activated G protein then diffuses along the membrane to activate an enzyme, which in turn initiates the next steps in
the signaling cascade. At the end of the process, the G protein hydrolyzes its GTP into GDP, thereby returning the G
protein to the inactive state.
Figure 7: The epinephrine signal transduction pathway in liver cells.
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The epinephrine receptor is a membrane­spanning G protein­coupled receptor. Binding of epinephrine to the
receptor triggers the activation of a G protein. The activated G protein activates the enzyme adenylyl cyclase, which
catalyzes the formation of the second messenger cyclic AMP (cAMP) from ATP. cAMP activates other enzymes
including protein kinase A, which phosphorylates and activates other signal transduction molecules. The signaling
cascade initiated by epinephrine binding induces many physiological and gene expression changes that result in
increased availability of glucose in the blood, including breakdown of glycogen and inhibition of glycogen
synthesis.
© 2014 Nature Education All rights reserved.
Sometimes the cell must deactivate a signal receptor through any one of a variety of mechanisms. Deactivation can
serve to control a signal. Through such control, the cell can coordinate multiple signals and pathways.
Test Yourself
How does activation of a G protein­coupled receptor initiate signal transduction?
Submit
Cell signaling also exhibits two other hallmarks. The initial signaling event triggered by ligand­receptor binding is
frequently amplified during subsequent transduction steps. For example, the binding of one hormone molecule to a
receptor tyrosine kinase may result in the phosphorylation of 10 proteins, and each of the 10 proteins may activate
another 10 proteins, so that one hormone molecule indirectly triggers the activation of 100 proteins. This amplification
enables a small signal to elicit a large cellular response and is an important mechanism in positive feedback responses.
In addition, it is critical that cells be able to easily deactivate cell signaling, and they are able to do so through a variety
of mechanisms, such as the degradation of a ligand or the dephosphorylation of a receptor. Deactivation prevents a
cellular response from inappropriately persisting after the signaling event has ended. For example, if a cell could not
inactivate a growth factor signal directing it to divide, the cell would divide uncontrollably, resulting in tumor formation
and cancer. Through amplification and deactivation of cell signaling events, a cell can coordinate multiple signals and
pathways and formulate an appropriate physiological response.
Future perspectives.
Discoveries in genomics often lead to novel drug therapies. A particularly promising area of research is G protein­
coupled receptors. Nearly half of all drugs currently in use target this receptor superfamily, and the biological
importance of GPCRs is highlighted by the 2012 Nobel Prize in Chemistry, awarded to Brian Kobilka and Robert
Lefkowitz for their work on GPCRs. Thousands of GPCRs are encoded by the human genome, many of which are
"orphan" receptors that lack any known ligand. The functions of these receptors are not clearly understood. Scientists
are trying to assess the potential functions the receptors perform. Without detailed genetic information, however,
researchers must use a reverse approach to determine which orphan receptors may be effective drug targets. By
developing chemicals that serve as agonists and/or antagonists to a receptor, scientists can identify the receptor's active
ligands, leading them to better understand the receptor's function and its implications in health and disease.
IN THIS MODULE
Cell­Cell Communication
Receptors
Summary
Test Your Knowledge
WHY DOES THIS TOPIC MATTER?
Synthetic Biology: Making Life from
Bits and Pieces
Scientists are combining biology and
engineering to change the world.
Stem Cells
Stem cells are powerful tools in
biology and medicine. What can
scientists do with these cells and their
incredible potential?
Cancer: What's Old Is New Again
Is cancer ancient, or is it largely a
product of modern times? Can
cutting­edge research lead to prevention
and treatment strategies that could make
cancer obsolete?
PRIMARY LITERATURE
Innovation in Cannabis medicine
Cannabinoid potentiation of glycine
receptors contributes to cannabis­induced
analgesia.
View | Download
Inhibitors may block entry of
hepatitis C into cells
EGFR and EphA2 are host factors for
hepatitis C virus entry and possible targets
http://www.nature.com/principles/ebooks/principles­of­biology­104015/29144952/2
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Cell Signaling | Principles of Biology from Nature Education
for antiviral therapy.
View | Download
How can nematodes help reduce
obesity in humans?
A whole­organism screen identifies new
regulators of fat storage.
View | Download
Classic paper: Breakthrough
enables tiny measurements of ion
channel activity (1976)
Single­channel currents recorded from
membrane of denervated frog muscle
fibers.
View | Download
Adaptor proteins regulate cell
signaling
Structural basis for regulation of the Crk
signaling protein by a proline switch.
View | Download
Mitochondria change shape to help
the cell survive
During autophagy mitochondria elongate,
are spared from degradation and sustain
cell viability.
View | Download
SCIENCE ON THE WEB
What Do Your Teeth Have to Do with
Bacterial Communication?
Scientist Bonnie Bassler explains quorum
sensing, a bacterial communication
phenomenon
An Interactive on Cell Responses.
Send a signal to a plant of animal cells.
page 104 of 989
2 pages left in this module
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Principles of Biology
20 Cell Signaling
contents
Summary
OBJECTIVE
Describe different ways cells can communicate with each other.
Autocrine and paracrine signals are forms of local cell communication. Autocrine signaling targets receptors within the
same cell that originated the signal. Paracrine signaling targets receptors near the cell that originated the signal.
Endocrine signals are a form of long­distance cell communication.
OBJECTIVE
Outline the steps of signal transduction.
Signal transduction takes place in three basic steps. Reception occurs when a signaling molecule binds to a receptor.
Transduction occurs when the receptor is activated as a result of its conformational change and transmits the signal to
another molecule. The response is the final phase in the pathway, occurring when the transduced signal initiates a
physiological reaction from the cell.
OBJECTIVE
Compare and contrast receptor types found in cells.
Transmembrane receptors, including ion channel­linked receptors, enzyme­linked receptors and G protein­coupled
receptors, bind to large, hydrophilic molecules that cannot enter the cell by crossing through the lipid bilayer of the
plasma membrane. The binding sites of transmembrane receptors are usually situated on the cell surface, although the
receptor directs the signal inward. Intracellular nuclear and cytoplasmic receptors directly bind smaller, lipid­soluble
molecules, such as steroid hormones, that readily diffuse through the cell membrane.
OBJECTIVE
Explain how enzyme­linked receptors lead to signal transduction.
Receptor tyrosine kinases are examples of enzyme­linked receptors that must dimerize to transmit a signal. This
receptor contains an extracellular domain that binds ligands and an intracellular domain that acts as the kinase. When
ligands bind to two receptor tyrosine kinase molecules, the receptors dimerize and form a complex, activating the kinase
domain of the receptors and allowing the receptors to phosphorylate each other. The resulting phosphorylation of the
complex leads to signal transduction and subsequent cellular responses.
OBJECTIVE
Explain how activation of a G protein­coupled receptor leads to signal transduction.
G protein­coupled receptors feature seven membrane­spanning domains. Upon binding signaling molecules, the
receptor facilitates an inactive G protein's exchange of GDP for GTP. The G protein, now activated, diffuses along the
membrane and binds to and activates enzymes to trigger the next signal transduction event.
Key Terms
affinity
The strength with which a ligand binds to its receptor, enzyme, transport protein or other binding partner.
autocrine signaling
A form of cell signaling in which the target cell is the same cell that originated the signal.
cytoplasmic receptor
An intracellular receptor that binds directly to smaller hydrophobic ligands, such as steroid hormones, that can
diffuse through the lipid bilayer of the membrane.
direct contact
A form of cell signaling in which adjacent cells communicate with each other via cell surface proteins that bind
each other.
downstream activation
Cascading sequence of intracellular signaling events triggered after the initial binding of a signaling molecule to its
receptor.
endocrine signaling
A form of cell signaling that involves long­range signals (hormones) that are transmitted to distant cells through the
circulatory system.
enzyme­linked receptor
One of three major types of transmembrane receptors; ligand­receptor binding activates an enzymatic activity in
the receptor that results in further signal transduction events.
G protein­coupled receptor (GPCR)
One of three major types of transmembrane receptors; uses G proteins as an intracellular signaling molecule.
hormone
Substance produced in a small amount in one site and transported via the circulatory system to a target at another
site in the organism.
ion channel­linked receptor
Transmembrane protein that opens and closes in response to the binding of specific ligands; allows passage of
specific ion types across a cell membrane.
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kinase
A type of enzyme that catalyzes the phosphorylation of proteins and other molecules.
ligand
A molecule that binds directly to a receptor or other binding partner.
paracrine signaling
A form of cell signaling in which a cell signals to other cells in its immediate vicinity via diffusible signaling
molecules.
reception
The first phase of signal transduction in which a signaling molecule binds to a receptor.
response
The third and final phase of signal transduction in which a cell experiences a physiological change, such as
changes in metabolism or gene expression, as a result of an extracellular signal.
signal transduction
The overall process by which cells respond to extracellular signaling molecules or other stimuli by executing a
physiological response.
synaptic contact
A form of cell signaling in which two adjacent cells are not physically linked but separated by a small gap; diffusible
molecules such as neurotransmitters are used to communicate between the two cells.
transmembrane receptor
A receptor that spans the plasma membrane of a cell; binds to hydrophilic molecules that cannot pass through the
lipid bilayer of the membrane; triggers intracellular responses.
IN THIS MODULE
Cell­Cell Communication
Receptors
Summary
Test Your Knowledge
WHY DOES THIS TOPIC MATTER?
Synthetic Biology: Making Life from
Bits and Pieces
Scientists are combining biology and
engineering to change the world.
Stem Cells
Stem cells are powerful tools in
biology and medicine. What can
scientists do with these cells and their
incredible potential?
Cancer: What's Old Is New Again
Is cancer ancient, or is it largely a
product of modern times? Can
cutting­edge research lead to prevention
and treatment strategies that could make
cancer obsolete?
PRIMARY LITERATURE
Innovation in Cannabis medicine
Cannabinoid potentiation of glycine
receptors contributes to cannabis­induced
analgesia.
View | Download
Inhibitors may block entry of
hepatitis C into cells
EGFR and EphA2 are host factors for
hepatitis C virus entry and possible targets
for antiviral therapy.
View | Download
How can nematodes help reduce
obesity in humans?
A whole­organism screen identifies new
regulators of fat storage.
View | Download
Classic paper: Breakthrough
enables tiny measurements of ion
channel activity (1976)
Single­channel currents recorded from
membrane of denervated frog muscle
fibers.
View | Download
Adaptor proteins regulate cell
signaling
Structural basis for regulation of the Crk
signaling protein by a proline switch.
http://www.nature.com/principles/ebooks/principles­of­biology­104015/29144952/3
2/3
2015/3/8
Summary of Cell Signaling | Principles of Biology from Nature Education
View | Download
Mitochondria change shape to help
the cell survive
During autophagy mitochondria elongate,
are spared from degradation and sustain
cell viability.
View | Download
SCIENCE ON THE WEB
What Do Your Teeth Have to Do with
Bacterial Communication?
Scientist Bonnie Bassler explains quorum
sensing, a bacterial communication
phenomenon
An Interactive on Cell Responses.
Send a signal to a plant of animal cells.
page 105 of 989
1 pages left in this module
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3/3
2015/3/8
Cell Signaling | Principles of Biology from Nature Education
Principles of Biology
20 Cell Signaling
contents
Test Your Knowledge
1. What is the significance of distance in cell­to­cell communication?
Types of signals can be defined according to the distance they travel to reach their
targets.
Neighboring cells can communicate with each other, while distant cells are unable to
communicate with each other.
Gap junctions allow transduction of the most distant signal.
Synaptic transmission is only effective as a mechanism for distant communication.
None of the answers are correct.
2. Which of the following is true of a receptor tyrosine kinase?
Part of it is exposed to the extracellular space, and part of it is located on the inside
of the cell.
It is a cytoplasmic receptor.
It has seven transmembrane­spanning domains.
Dimerization is an optional step in receptor tyrosine kinase activation.
None of the answers are correct.
3. How are G protein­coupled receptors continually made available for reuse?
The G protein­mediated activation of the enzyme is only temporary.
The G protein opens multiple binding sites simultaneously.
When a G protein activates, it diffuses away from the receptor, leaving it available for
reuse.
Any signal can bind a G protein­coupled receptor.
None of the answers are correct.
4. Why is the kinase so important in a receptor tyrosine kinase?
Tyrosine kinases activate synapses, triggering cellular responses.
Tyrosine kinase is the signal that the receptor binds.
Kinases transfer phosphates, phosphorylating the receptor.
The tyrosine kinase domains are active in their monomeric states.
The tyrosine kinase uses guanosine triphosphate to become activated.
Submit
IN THIS MODULE
Cell­Cell Communication
Receptors
Summary
Test Your Knowledge
WHY DOES THIS TOPIC MATTER?
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