chapter 11 cell communication - hcc learning web
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BIOLOGY I. Chapter 11 – Cell Communication
Why is cell-to-cell communication important?
• Communication between cells is important for multicellular organisms as well as for unicellular organisms.
• Cells communicate with each other to coordinate their activities in a way that enables an organism to develop, survive and reproduce.
– Share nutrients, ions, etc.
– Influence other cells through signals
– Interact to work together in tissues and organs (e.g. the heart, the muscles)
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BIOLOGY I. Chapter 11 – Cell Communication
Evolution of Cell Signaling
• Signal transduction pathway:
the process by which a signal on
a cell’s surface is converted into a
specific cellular response.
• Cell signaling (signal transduction)
in microbes (such as yeast, a type
of fungus) has much in common
with processes in multicellular
organisms, including animals,
suggesting an early origin of
signaling mechanisms.
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BIOLOGY I. Chapter 11 – Cell Communication
Local and Long Distance Signaling
1. Cell communication by direct contact between cells:
– Cell junctions: allow molecules to pass freely between adjacent cells.
• Gap junctions,
plasmodesmata, etc.
– Cell-cell recognition: cells communicate by interaction between cell surface molecules.
• Very important in such
processes as embryonic
development and immune
responses.
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BIOLOGY I. Chapter 11 – Cell Communication
Local and Long Distance Signaling
2. Cell communication by indirect contact through messenger molecules:
a) Paracrine signaling = local regulators released by a secreting cell travel only short distances to influence neighboring cells.
• Example: growth factors.
b) Synaptic signaling = neurotransmitters released by a nerve cell stimulate a target cell.
• The neurotransmitter is
released into the synapse,
the space between the
nerve cell and its target cell.
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A nerve signal can travel along a
series of nerve cells, some of which
can be quite long; therefore, it can also
be considered long-distance signaling.
BIOLOGY I. Chapter 11 – Cell Communication
Local and Long Distance Signaling
c) Endocrine (hormonal) signaling = chemicals called hormones are released by specialized endocrine cells into blood vessels.
– The hormones may travel long
distances to other parts of the body.
– Insulin, a hormone that regulates
sugar levels in blood, is an example
of a mammalian hormone.
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BIOLOGY I. Chapter 11 – Cell Communication
The Three Stages of Cell Signaling (Communication): Overview
• From the perspective of the cell receiving the message, the process of cell signaling can be divided into three stages:
1) Reception: The target cell’s detection of a signal molecule (ligand) from outside the cell, when it binds to a receptor protein.
2) Transduction: The conversion of a signal from outside the cell to a form that can bring about a specific cellular response.
3) Response: The change in a specific cellular activity brought about by a transduced signal from outside the cell.
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BIOLOGY I. Chapter 11 – Cell Communication
Cell Signaling (Communication): (1) Reception
• In reception, a signal molecule, called a ligand, binds to a receptor protein (located on the cell’s surface or inside the cell) causing it to change its shape (conformation).
– * This binding is highly specific.
– * Most signal receptors are plasma membrane proteins. However, some signal receptors are intracellular (located inside the cell, in the cytoplasm or the nucleus).
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BIOLOGY I. Chapter 11 – Cell Communication
Intracellular Receptors
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• Intracellular receptors are
cytoplasmic or nuclear
proteins.
• Signal molecules that are
small enough or hydrophobic
enough and can readily cross
the plasma membrane use
these receptors.
– Examples include: steroid
hormones (such as
testosterone) and thyroid
hormones.
BIOLOGY I. Chapter 11 – Cell Communication
Receptors in the Plasma Membrane
• Most water-soluble signaling molecules, generally too
large to pass freely through the plasma membrane, bind
to specific sites on receptor proteins embedded in the
plasma membrane of the cell.
• The specific ligand binds to the receptor on the plasma
membrane and the receptor either changes shape or
aggregates.
• Three major types of plasma membrane receptors are:
– G-protein-coupled receptors
– Receptor tyrosine kinases
– Ion channel receptors
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BIOLOGY I. Chapter 11 – Cell Communication
Receptors in the Plasma Membrane: G-Protein-Coupled Receptors
• A G-protein-coupled
receptor is a membrane
receptor that works with
the help of a cytoplasmic
G protein, which binds
the energy-rich molecule
GTP (guanosine
triphosphate).
• Ligand binding activates
the receptor, which then
activates a specific G
protein, which activates
yet another protein, thus
propagating the signal
along a signal
transduction pathway.
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G-protein-linked receptors are widespread in organisms and diverse in their functions, including roles in sensory reception. They are also involved in many human diseases, including bacterial infections.
BIOLOGY I. Chapter 11 – Cell Communication
Receptors in the Plasma Membrane: Receptor Tyrosine Kinases
• Receptor tyrosine kinases have enzymatic activity and react to the binding of signal molecules by forming dimers and then adding phosphate groups to tyrosines on the cytoplasmic side of the other subunit of the dimer.
• Relay proteins in the cell can then be activated by binding to different phosphorylated tyrosines, allowing this receptor to trigger several pathways at once. These receptors are involved in cell growth and reproduction.
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A tyrosine kinase is an enzyme that catalyzes the transfer of a phosphate group from ATP to the amino acid tyrosine on a substrate protein.
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BIOLOGY I. Chapter 11 – Cell Communication
Receptors in the Plasma Membrane:
Ion Channel Receptors
• A ligand-gated ion channel receptor in a
membrane opens or closes when a specific
signal molecule binds to the receptor
protein, regulating the flow of specific ions.
• These receptors have a region that can act
as a “gate” when the receptor changes
shape upon binding of the ligand. The gate
opens or closes, allowing or blocking the
flow of specific ions, such as Na+ or Ca2+
through a channel in the receptor.
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BIOLOGY I. Chapter 11 – Cell Communication
Cell Signaling (Communication): (2) Transduction
• Transduction is the conversion of a signal from outside the cell to a form that can bring about a specific cellular response.
– It is initiated when the binding of the signaling molecule changes the receptor protein in some way.
• Transduction often occurs through a sequence of changes requiring different relay molecules—a signal transduction pathway.
– Cascades of molecular interactions relay signals from receptors to target molecules in the cell.
– At each step in a pathway, the signal is transduced into a different form, commonly a conformational change in a protein.
• Advantage of multistep pathways:
– Greatly amplifying a signal (and thus producing a large cellular response), more opportunities for coordination and regulation.
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BIOLOGY I. Chapter 11 – Cell Communication
Cell Signaling (Communication): (2) Transduction
• Phosphorylation and dephosphorylation of proteins is a widespread cellular mechanism for regulating protein activity.
– Phosphorylation: The addition of a phosphate group to a protein or another molecule.
• Many signal transduction pathways include “phosphorylation
cascades”; a series of enzymes called protein kinases each
transfer a phosphate group (from ATP) to the next protein in line,
activating it.
– Dephosphorylation: The removal of a phosphate group.
• Enzymes called protein phosphatases rapidly remove the
phosphates from the proteins kinases, making them inactive and
available for reuse (turning off the signal transduction pathway
when the initial signal is no longer present).
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BIOLOGY I. Chapter 11 – Cell Communication
Cell Signaling (Communication): (2) Transduction
• Small Molecules and Ions as Second Messengers
– In addition to proteins (kinases, phosphatases), signal transduction pathways may involve small nonprotein water-soluble molecules or ions called second messengers (the “first messenger” is the extracellular signal molecule that binds to the membrane receptor).
– Second messengers, diffuse readily through the cell and thus help broadcast signals quickly.
– Examples: cyclic AMP (cAMP) and calcium ions (Ca2+).
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BIOLOGY I. Chapter 11 – Cell Communication
Small Molecules and Ions as Second Messengers
• Cyclic AMP (cyclic adenosine
monophosphate)
– The first messenger
activates a G-protein-linked
receptor, which activates a
specific G protein.
– In turn, the G protein
activates adenylyl cyclase,
an enzyme embedded in the
plasma membrane, which
converts ATP to cAMP.
– The cAMP then activates
another protein.
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BIOLOGY I. Chapter 11 – Cell Communication
Small Molecules and Ions as Second Messengers
• Calcium Ions (Ca2+)
– Many signal molecules in
animals (e.g. neurotransmitters,
growth factors, some hormones),
induce responses in their target
cells via signal transduction
pathways that increase the
cytosolic concentration of Ca2+.
This causes many responses in
animal cells, such as muscle cell
contraction, secretion of certain
substances, and cell division.
– Cells use Ca2+ as a second
messenger in both G-protein
and tyrosine kinase pathways.
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Figure 11.11. The maintenance of Ca2+
concentration in an animal cell. The Ca2+ concentration in the cytosol is usually much lower than in the extracellular fluid and ER.
BIOLOGY I. Chapter 11 – Cell Communication
Small Molecules and Ions as Second Messengers
• Calcium Ions (Ca2+) and Inositol Trisphosphate (IP3)
– In response to a signal
relayed by a signal
transduction pathway, the
cytosolic calcium level may
rise, usually by a mechanism
that releases Ca2+ from the
cell’s endoplasmic reticulum.
– Calcium release in the cell
involves two other second
messengers, IP3 and DAG
(diacylglycerol). DAG
functions as a second
messenger in other pathways.
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BIOLOGY I. Chapter 11 – Cell Communication
Cell Signaling (Communication): (3) Response
• Signal transduction ultimately triggers a cellular response,
leading to the regulation of one or more cellular activities.
• Cytoplasmic and Nuclear Responses
– In the cytoplasm: Enzyme activity, rearrangement of the
cytoskeleton, protein synthesis, protein activity, and many
other activities are regulated.
– In the nucleus: Activation or inactivation of specific genes,
by transcription factors (special proteins that control which
genes are turned on in a particular cell at a particular time).
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BIOLOGY I. Chapter 11 – Cell Communication
Cell Signaling (Communication): (3) Response in the Cytoplasm or the Nucleus
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BIOLOGY I. Chapter 11 – Cell Communication
Cell Signaling (Communication):
Fine-Tuning of the Response
• Signal Amplification
– One of the benefits of multistep
signaling pathways is signal
amplification: each catalytic protein
in a signaling pathway amplifies the
signal by activating multiple copies of
the next component of the pathway.
• The Specificity of Cell Signaling and Coordination of the Response
– The particular collection of proteins in
a cell gives the cell great specificity in
both the signals it detects and the
responses it carries out.
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BIOLOGY I. Chapter 11 – Cell Communication
Cell Signaling (Communication): Fine-Tuning of the Response
• Signaling Efficiency: Scaffolding Proteins and Signaling Complexes
– Scaffolding proteins are large relay proteins to which several other
relay proteins are simultaneously attached to increase the efficiency of
signal transduction.
– In this figure, the scaffolding protein simultaneously binds to a specific
activated membrane receptor and three different protein kinases. This
physical arrangement facilitates signal transduction by these molecules.
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BIOLOGY I. Chapter 11 – Cell Communication
Cell Signaling (Communication):
Fine-Tuning of the Response
• Termination of the Signal
– If a signaling pathway component becomes locked into one
state, whether active or inactive, dire consequences for the
organism can result, therefore, the changes that signals
produce are reversible.
– Signal response is terminated quickly by the reversal of
ligand binding. When signal molecules leave the receptor,
the receptor reverts to its inactive form and relay molecules
are inactivated.
– In this way, the cell is soon ready to respond to a new signal.
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BIOLOGY I. Chapter 11 – Cell Communication
The Three Stages of Cell Signaling: An Analogy
Cells Analogy
Reception The target cell’s detection of a
signal molecule from outside
the cell, when it binds to a
receptor protein.
A TV camera (receptor)
is shooting a scene.
Transduction The conversion of a signal
from outside the cell to a form
that can bring about a specific
cellular response.
The picture is converted
to electrical signals
(transduction pathway)
that are understood by
the TV in your house.
Response The change in a specific
cellular activity brought about
by a transduced signal from
outside the cell.
The electrical signals are
converted to a picture on
your TV screen (the
response).
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BIOLOGY I. Chapter 11 – Cell Communication
References
• Audesirk, Teresa; Audesirk, Gerald & Byers, Bruce E. (2005). Biology: Life on Earth. Seventh Edition. Pearson Education, Inc.-Prentice Hall. NJ, USA.
• Brooker, Robert J.; Widmaier, Eric P.; Graham, Linda E.; Stiling, Peter D. (2008). Biology. The McGraw-Hill Companies, Inc. NY, USA.
• Campbell, Neil A.; Reece, Jane B., et al. (2011). Biology. Ninth Edition. Pearson Education, Inc.-Pearson Benjamin Cummings. CA, USA.
• Ireland, K.A. (2011). Visualizing Human Biology. Second Edition. John Wiley & Sons, Inc. NJ, USA.
• Mader, Sylvia S. (2010). Biology. Tenth Edition. The McGraw-Hill Companies, Inc. NY, USA.
• Martini, Frederic H.; Nath, Judi L. (2009). Fundamentals of Anatomy & Physiology. Eighth Edition. Pearson Education, Inc. – Pearson Benjamin Cummings. CA, USA.
• Solomon, Eldra; Berg, Linda; Martin, Diana W. (2008). Biology. Eighth Edition. Cengage Learning. OH, USA.
• Starr, Cecie. (2008). Biology: Concepts and Applications , Volume I. Thompson Brooks/Cole. OH, USA.
• Tortora, Gerard J.; Derrickson, Bryan. (2006). Principles of Anatomy and Physiology. Eleventh Edition. John Wiley & Sons, Inc. NJ, USA. www.wiley.com/college/apcentral.
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