Transcript 3 - Dr. Jerry Cronin

PowerPoint® Lecture Slides
prepared by
Barbara Heard,
Atlantic Cape Community
College
CHAPTER
3
Cells: The
Living Units:
Part B
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
Membrane Transport: Active Processes
• Two types of active processes
– Active transport
– Vesicular transport
• Both require ATP to move solutes across a
living plasma membrane because
– Solute too large for channels
– Solute not lipid soluble
– Solute not able to move down concentration
gradient
© 2013 Pearson Education, Inc.
Active Transport
• Requires carrier proteins (solute pumps)
– Bind specifically and reversibly with
substance
• Moves solutes against concentration
gradient
– Requires energy
© 2013 Pearson Education, Inc.
Active Transport: Two Types
• Primary active transport
– Required energy directly from ATP hydrolysis
• Secondary active transport
– Required energy indirectly from ionic
gradients created by primary active transport
© 2013 Pearson Education, Inc.
Primary Active Transport
• Energy from hydrolysis of ATP causes
shape change in transport protein that
"pumps" solutes (ions) across membrane
• E.g., calcium, hydrogen, Na+-K+ pumps
© 2013 Pearson Education, Inc.
Primary Active Transport
• Sodium-potassium pump
– Most well-studied
– Carrier (pump) called Na+-K+ ATPase
– Located in all plasma membranes
– Involved in primary and secondary active
transport of nutrients and ions
© 2013 Pearson Education, Inc.
Sodium-Potassium Pump
• Na+ and K+ channels allow slow leakage
down concentration gradients
• Na+-K+ pump works as antiporter
– Pumps against Na+ and K+ gradients to
maintain high intracellular K+ concentration
and high extracellular Na+ concentration
• Maintains electrochemical gradients essential for
functions of muscle and nerve tissues
• Allows all cells to maintain fluid volume
© 2013 Pearson Education, Inc.
Figure 3.10 Primary active transport is the process in which solutes are moved across cell
membranes against electrochemical gradients using energy supplied directly by ATP.
Extracellular fluid
Na+
Na+–K+ pump
K+
Na+ bound
ATP-binding site
Cytoplasm
1 Three cytoplasmic Na+ bind to pump
protein.
P
K+ released
2 Na+ binding promotes hydrolysis of ATP.
The energy released during this reaction
phosphorylates the pump.
6 Pump protein binds ATP; releases K+ to
the inside, and Na+ sites are ready to bind
Na+ again. The cycle repeats.
Na+ released
K+ bound
P
Pi
K+
5 K+ binding triggers release of the
phosphate. The dephosphorylated pump
resumes its original conformation.
3 Phosphorylation causes the pump to
change shape, expelling Na+ to the outside.
P
4 Two extracellular K+ bind to pump.
© 2013 Pearson Education, Inc.
Slide 1
Figure 3.10 Primary active transport is the process in which solutes are moved across cell
membranes against electrochemical gradients using energy supplied directly by ATP.
Extracellular fluid
Na+
Na+–K+ pump
K+
ATP-binding site
Cytoplasm
1 Three cytoplasmic Na+ bind to pump
protein.
© 2013 Pearson Education, Inc.
Slide 2
Figure 3.10 Primary active transport is the process in which solutes are moved across cell
membranes against electrochemical gradients using energy supplied directly by ATP.
Extracellular fluid
Na+
Na+–K+ pump
K+
Na+ bound
ATP-binding site
Cytoplasm
1 Three cytoplasmic Na+ bind to pump
protein.
P
2 Na+ binding promotes hydrolysis of ATP.
The energy released during this reaction
phosphorylates the pump.
© 2013 Pearson Education, Inc.
Slide 3
Figure 3.10 Primary active transport is the process in which solutes are moved across cell
membranes against electrochemical gradients using energy supplied directly by ATP.
Extracellular fluid
Na+
Na+–K+ pump
K+
Na+ bound
ATP-binding site
Cytoplasm
1 Three cytoplasmic Na+ bind to pump
protein.
P
2 Na+ binding promotes hydrolysis of ATP.
The energy released during this reaction
phosphorylates the pump.
Na+ released
P
3 Phosphorylation causes the pump to
change shape, expelling Na+ to the outside.
© 2013 Pearson Education, Inc.
Slide 4
Figure 3.10 Primary active transport is the process in which solutes are moved across cell
membranes against electrochemical gradients using energy supplied directly by ATP.
Extracellular fluid
Na+
Na+–K+ pump
K+
Na+ bound
ATP-binding site
Cytoplasm
1 Three cytoplasmic Na+ bind to pump
protein.
P
2 Na+ binding promotes hydrolysis of ATP.
The energy released during this reaction
phosphorylates the pump.
Na+ released
P
K+
3 Phosphorylation causes the pump to
change shape, expelling Na+ to the outside.
P
4 Two extracellular K+ bind to pump.
© 2013 Pearson Education, Inc.
Slide 5
Figure 3.10 Primary active transport is the process in which solutes are moved across cell
membranes against electrochemical gradients using energy supplied directly by ATP.
Extracellular fluid
Na+
Na+–K+ pump
K+
Na+ bound
ATP-binding site
Cytoplasm
1 Three cytoplasmic Na+ bind to pump
protein.
P
2 Na+ binding promotes hydrolysis of ATP.
The energy released during this reaction
phosphorylates the pump.
Na+ released
K+ bound
P
Pi
K+
3 Phosphorylation causes the pump to
change shape, expelling Na+ to the outside.
5 K+ binding triggers release of the
phosphate. The dephosphorylated pump
resumes its original conformation.
P
4 Two extracellular K+ bind to pump.
© 2013 Pearson Education, Inc.
Slide 6
Figure 3.10 Primary active transport is the process in which solutes are moved across cell
membranes against electrochemical gradients using energy supplied directly by ATP.
Extracellular fluid
Na+
Na+–K+ pump
K+
Na+ bound
ATP-binding site
Cytoplasm
1 Three cytoplasmic Na+ bind to pump
protein.
P
K+ released
6 Pump protein binds ATP; releases K+ to
the inside, and Na+ sites are ready to bind
Na+ again. The cycle repeats.
2 Na+ binding promotes hydrolysis of ATP.
The energy released during this reaction
phosphorylates the pump.
Na+ released
K+ bound
P
Pi
K+
5 K+ binding triggers release of the
phosphate. The dephosphorylated pump
resumes its original conformation.
3 Phosphorylation causes the pump to
change shape, expelling Na+ to the outside.
P
4 Two extracellular K+ bind to pump.
© 2013 Pearson Education, Inc.
Slide 7
Figure 3.10 Primary active transport is the process in which solutes are moved across cell
membranes against electrochemical gradients using energy supplied directly by ATP.
Extracellular fluid
Na+
Na+–K+ pump
K+
Na+ bound
ATP-binding site
Cytoplasm
1 Three cytoplasmic Na+ bind to pump
protein.
P
K+ released
6 Pump protein binds ATP; releases K+ to
the inside, and Na+ sites are ready to bind
Na+ again. The cycle repeats.
2 Na+ binding promotes hydrolysis of ATP.
The energy released during this reaction
phosphorylates the pump.
Na+ released
K+ bound
P
Pi
K+
5 K+ binding triggers release of the
phosphate. The dephosphorylated pump
resumes its original conformation.
3 Phosphorylation causes the pump to
change shape, expelling Na+ to the outside.
P
4 Two extracellular K+ bind to pump.
© 2013 Pearson Education, Inc.
PLAY
A&P Flix™: Resting Membrane Potential
Secondary Active Transport
• Depends on ion gradient created by
primary active transport
• Energy stored in ionic gradients used
indirectly to drive transport of other solutes
© 2013 Pearson Education, Inc.
Secondary Active Transport
• Cotransport—always transports more
than one substance at a time
– Symport system: Substances transported in
same direction
– Antiport system: Substances transported in
opposite directions
© 2013 Pearson Education, Inc.
Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary active
transport.
Extracellular fluid
Slide 1
Glucose
Na+-K+
pump
Na+-glucose
symport
transporter
loads glucose
from extracellular
fluid
Na+-glucose
symport transporter
releases glucose
into the cytoplasm
Cytoplasm
1 Primary active transport
The ATP-driven Na+-K+ pump
stores energy by creating a
steep concentration gradient for
Na+ entry into the cell.
© 2013 Pearson Education, Inc.
2 Secondary active transport
As Na+ diffuses back across the membrane
through a membrane cotransporter protein, it
drives glucose against its concentration gradient
into the cell.
Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary active
transport.
Extracellular fluid
Na+-K+
pump
Cytoplasm
1 Primary active transport
The ATP-driven Na+-K+ pump
stores energy by creating a
steep concentration gradient for
Na+ entry into the cell.
© 2013 Pearson Education, Inc.
Slide 2
Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary active
transport.
Extracellular fluid
Slide 3
Glucose
Na+-K+
pump
Na+-glucose
symport
transporter
loads glucose
from extracellular
fluid
Na+-glucose
symport transporter
releases glucose
into the cytoplasm
Cytoplasm
1 Primary active transport
The ATP-driven Na+-K+ pump
stores energy by creating a
steep concentration gradient for
Na+ entry into the cell.
© 2013 Pearson Education, Inc.
2 Secondary active transport
As Na+ diffuses back across the membrane
through a membrane cotransporter protein, it
drives glucose against its concentration gradient
into the cell.
Vesicular Transport
• Transport of large particles,
macromolecules, and fluids across
membrane in membranous sacs called
vesicles
• Requires cellular energy (e.g., ATP)
© 2013 Pearson Education, Inc.
Vesicular Transport
• Functions:
– Exocytosis—transport out of cell
– Endocytosis—transport into cell
• Phagocytosis, pinocytosis, receptor-mediated
endocytosis
– Transcytosis—transport into, across, and
then out of cell
– Vesicular trafficking—transport from one
area or organelle in cell to another
© 2013 Pearson Education, Inc.
Endocytosis and Transcytosis
• Involve formation of protein-coated
vesicles
• Often receptor mediated, therefore very
selective
• Some pathogens also hijack for transport
into cell
• Once vesicle is inside cell it may
– Fuse with lysosome
– Undergo transcytosis
© 2013 Pearson Education, Inc.
Figure 3.12 Events of endocytosis mediated by protein-coated pits.
1 Coated pit ingests
substance.
Protein coat
(typically
clathrin)
2 Protein-coated
vesicle detaches.
Extracellular fluid
Plasma
membrane
Cytoplasm
3 Coat proteins are
recycled to plasma
membrane.
Transport
vesicle
Uncoated endocytic
vesicle
Endosome
4 Uncoated vesicle
fuses with a sorting
vesicle called an
endosome.
Lysosome
5 Transport
vesicle containing
membrane compone
-nts moves to the plasma
membrane for recycling.
6 Fused vesicle may (a)
fuse with lysosome for
digestion of its contents,
or (b) deliver its contents
to the plasma membrane
on the opposite side of the
cell (transcytosis).
© 2013 Pearson Education, Inc.
Slide 1
Figure 3.12 Events of endocytosis mediated by protein-coated pits.
1 Coated pit ingests
substance.
Protein coat
(typically
clathrin)
© 2013 Pearson Education, Inc.
Extracellular fluid
Plasma
membrane
Cytoplasm
Slide 2
Figure 3.12 Events of endocytosis mediated by protein-coated pits.
1 Coated pit ingests
substance.
Protein coat
(typically
clathrin)
2 Protein-coated
vesicle detaches.
© 2013 Pearson Education, Inc.
Extracellular fluid
Plasma
membrane
Cytoplasm
Slide 3
Figure 3.12 Events of endocytosis mediated by protein-coated pits.
1 Coated pit ingests
substance.
Protein coat
(typically
clathrin)
2 Protein-coated
vesicle detaches.
© 2013 Pearson Education, Inc.
Extracellular fluid
Plasma
membrane
Cytoplasm
3 Coat proteins are
recycled to plasma
membrane.
Slide 4
Figure 3.12 Events of endocytosis mediated by protein-coated pits.
1 Coated pit ingests
substance.
Protein coat
(typically
clathrin)
2 Protein-coated
vesicle detaches.
Uncoated endocytic
vesicle
4 Uncoated vesicle
fuses with a sorting
vesicle called an
endosome.
© 2013 Pearson Education, Inc.
Extracellular fluid
Plasma
membrane
Cytoplasm
3 Coat proteins are
recycled to plasma
membrane.
Endosome
Slide 5
Figure 3.12 Events of endocytosis mediated by protein-coated pits.
1 Coated pit ingests
substance.
Protein coat
(typically
clathrin)
2 Protein-coated
vesicle detaches.
Extracellular fluid
Plasma
membrane
Cytoplasm
3 Coat proteins are
recycled to plasma
membrane.
Transport
vesicle
Uncoated endocytic
vesicle
4 Uncoated vesicle
fuses with a sorting
vesicle called an
endosome.
© 2013 Pearson Education, Inc.
Endosome
5 Transport
vesicle containing
membrane compone
-nts moves to the plasma
membrane for recycling.
Slide 6
Figure 3.12 Events of endocytosis mediated by protein-coated pits.
1 Coated pit ingests
substance.
Protein coat
(typically
clathrin)
2 Protein-coated
vesicle detaches.
Extracellular fluid
Plasma
membrane
Cytoplasm
3 Coat proteins are
recycled to plasma
membrane.
Transport
vesicle
Uncoated endocytic
vesicle
Endosome
4 Uncoated vesicle
fuses with a sorting
vesicle called an
endosome.
Lysosome
5 Transport
vesicle containing
membrane compone
-nts moves to the plasma
membrane for recycling.
6 Fused vesicle may (a)
fuse with lysosome for
digestion of its contents,
or (b) deliver its contents
to the plasma membrane
on the opposite side of the
cell (transcytosis).
© 2013 Pearson Education, Inc.
Slide 7
Endocytosis
• Phagocytosis
– Pseudopods engulf solids and bring them into
cell's interior
– Form vesicle called phagosome
• Used by macrophages and some white
blood cells
– Move by amoeboid motion
• Cytoplasm flows into temporary extensions
• Allows creeping
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Figure 3.13a Comparison of three types of endocytosis.
Receptors
Phagosome
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Phagocytosis
The cell engulfs a large particle
by forming projecting pseudopods
("false feet") around it and enclosing
it within a membrane sac called a
phagosome. The phagosome is
combined with a lysosome.
Undigested contents remain in
the vesicle (now called a residual
body) or are ejected by exocytosis.
Vesicle may or may not be protein
coated but has receptors capable of
binding to microorganisms or solid
particles.
Endocytosis
• Pinocytosis (fluid-phase endocytosis)
– Plasma membrane infolds, bringing
extracellular fluid and dissolved solutes inside
cell
• Fuses with endosome
– Most cells utilize to "sample" environment
– Nutrient absorption in the small intestine
– Membrane components recycled back to
membrane
© 2013 Pearson Education, Inc.
Figure 3.13b Comparison of three types of endocytosis.
Pinocytosis
The cell "gulps" a drop of
extracellular fluid containing solutes
into tiny vesicles. No receptors are
used, so the process is nonspecific.
Most vesicles are protein-coated.
Vesicle
© 2013 Pearson Education, Inc.
Endocytosis
• Receptor-mediated endocytosis
– Allows specific endocytosis and transcytosis
• Cells use to concentrate materials in limited supply
– Clathrin-coated pits provide main route for
endocytosis and transcytosis
• Uptake of enzymes, low-density lipoproteins, iron,
insulin, and, unfortunately, viruses, diphtheria, and
cholera toxins
© 2013 Pearson Education, Inc.
Receptor-Mediated Endocytosis
• Different coat proteins
– Caveolae
• Capture specific molecules (folic acid, tetanus
toxin) and use transcytosis
• Involved in cell signaling but exact function
unknown
– Coatomer
• Function in vesicular trafficking
© 2013 Pearson Education, Inc.
Figure 3.13c Comparison of three types of endocytosis.
Vesicle
© 2013 Pearson Education, Inc.
Receptor-mediated endocytosis
Extracellular substances bind to
specific receptor proteins, enabling
the cell to ingest and concentrate
specific substances (ligands) in
protein-coated vesicles. Ligands
may simply be released inside the
cell, or combined with a lysosome to
digest contents. Receptors are
recycled to the plasma membrane in
vesicles.
Exocytosis
• Usually activated by cell-surface signal or
change in membrane voltage
• Substance enclosed in secretory vesicle
• v-SNAREs ("v" = vesicle) on vesicle find
t-SNAREs ("t" = target) on membrane and
bind
• Functions
– Hormone secretion, neurotransmitter release,
mucus secretion, ejection of wastes
© 2013 Pearson Education, Inc.
Figure 3.14 Exocytosis.
Slide 1
The process of exocytosis
Plasma membrane
Extracellular
SNARE (t-SNARE)
fluid
Secretory
vesicle
Vesicle
SNARE
(v-SNARE)
Molecule to
be secreted
Cytoplasm
Fused
v- and
t-SNAREs
© 2013 Pearson Education, Inc.
Fusion pore formed
1 The membranebound vesicle
migrates to the
plasma membrane.
2 There, proteins at
the vesicle surface
(v-SNAREs) bind
with t-SNAREs
(plasma membrane
proteins).
3 The vesicle
and plasma
membrane
fuse and a pore
opens up.
4 Vesicle
contents are
released to the
cell exterior.
Figure 3.14 Exocytosis.
Slide 2
The process of exocytosis
Plasma membrane
Extracellular
SNARE (t-SNARE)
fluid
Secretory
vesicle
Vesicle
SNARE
(v-SNARE)
Molecule to
be secreted
Cytoplasm
© 2013 Pearson Education, Inc.
1 The membranebound vesicle
migrates to the
plasma membrane.
Figure 3.14 Exocytosis.
Slide 3
The process of exocytosis
Plasma membrane
Extracellular
SNARE (t-SNARE)
fluid
Secretory
vesicle
Vesicle
SNARE
(v-SNARE)
Molecule to
be secreted
Cytoplasm
Fused
v- and
t-SNAREs
© 2013 Pearson Education, Inc.
1 The membranebound vesicle
migrates to the
plasma membrane.
2 There, proteins at
the vesicle surface
(v-SNAREs) bind
with t-SNAREs
(plasma membrane
proteins).
Figure 3.14 Exocytosis.
Slide 4
The process of exocytosis
Plasma membrane
Extracellular
SNARE (t-SNARE)
fluid
Secretory
vesicle
Vesicle
SNARE
(v-SNARE)
Molecule to
be secreted
Cytoplasm
Fused
v- and
t-SNAREs
© 2013 Pearson Education, Inc.
Fusion pore formed
1 The membranebound vesicle
migrates to the
plasma membrane.
2 There, proteins at
the vesicle surface
(v-SNAREs) bind
with t-SNAREs
(plasma membrane
proteins).
3 The vesicle
and plasma
membrane
fuse and a pore
opens up.
Figure 3.14 Exocytosis.
Slide 5
The process of exocytosis
Plasma membrane
Extracellular
SNARE (t-SNARE)
fluid
Secretory
vesicle
Vesicle
SNARE
(v-SNARE)
Molecule to
be secreted
Cytoplasm
Fused
v- and
t-SNAREs
© 2013 Pearson Education, Inc.
Fusion pore formed
1 The membranebound vesicle
migrates to the
plasma membrane.
2 There, proteins at
the vesicle surface
(v-SNAREs) bind
with t-SNAREs
(plasma membrane
proteins).
3 The vesicle
and plasma
membrane
fuse and a pore
opens up.
4 Vesicle
contents are
released to the
cell exterior.
Figure 3.14b Exocytosis.
Photomicrograph
of a secretory
vesicle releasing
its contents
by exocytosis
(100,000x)
© 2013 Pearson Education, Inc.
Table 3.2 Active Membrane Transport Processes (1 of 2)
© 2013 Pearson Education, Inc.
Table 3.2 Active Membrane Transport Processes (2 of 2)
© 2013 Pearson Education, Inc.
Generation of a Resting Membrane Potential
• Resting membrane potential (RMP)
– Produced by separation of oppositely charged
particles (voltage) across membrane in all
cells
• Cells described as polarized
– Voltage (electrical potential energy) only at
membrane
• Ranges from –50 to –100 mV in different cells
– "–" indicates inside negative relative to outside
© 2013 Pearson Education, Inc.
Selective Diffusion Establishes RMP
• Electrochemical gradient established
– Electro (charge); chemical (ion concentration)
• K+ diffuses out of cell through K+ leakage
channels, proteins cannot  inside cell
membrane more negative 
• K+ attracted back as inner face more
negative
• K+ equalizes across membrane at –90 mV
when K+ concentration gradient balanced
by electrical gradient = RMP
© 2013 Pearson Education, Inc.
Figure 3.15 The key role of K+ in generating the resting membrane potential.
1 K+ diffuse down their steep
concentration gradient (out of the cell)
via leakage channels. Loss of K+
results in a negative charge on the
inner plasma membrane face.
Extracellular fluid
+
+
–
+
+
+
+
+
–
Cytoplasm
© 2013 Pearson Education, Inc.
–
Slide 1
–
–
Potassium
leakage
channels
–
–
+
–
Protein anion (unable to
follow K+ through the
membrane)
2 K+ also move into the cell
because they are attracted to the
negative charge established on the
inner plasma membrane face.
3 A negative membrane potential
(–90 mV) is established when the
movement of K+ out of the cell equals
K+ movement into the cell. At this
point, the concentration gradient
promoting K+ exit exactly opposes the
electrical gradient for K+ entry.
Figure 3.15 The key role of K+ in generating the resting membrane potential.
1 K+ diffuse down their steep
concentration gradient (out of the cell)
via leakage channels. Loss of K+
results in a negative charge on the
inner plasma membrane face.
Extracellular fluid
+
+
–
+
+
+
+
+
–
Cytoplasm
© 2013 Pearson Education, Inc.
–
Slide 2
–
–
Potassium
leakage
channels
–
–
+
–
Protein anion (unable to
follow K+ through the
membrane)
Figure 3.15 The key role of K+ in generating the resting membrane potential.
1 K+ diffuse down their steep
concentration gradient (out of the cell)
via leakage channels. Loss of K+
results in a negative charge on the
inner plasma membrane face.
Extracellular fluid
+
+
–
+
+
+
+
+
–
Cytoplasm
© 2013 Pearson Education, Inc.
–
Slide 3
–
–
Potassium
leakage
channels
–
–
+
–
Protein anion (unable to
follow K+ through the
membrane)
2 K+ also move into the cell
because they are attracted to the
negative charge established on the
inner plasma membrane face.
Figure 3.15 The key role of K+ in generating the resting membrane potential.
1 K+ diffuse down their steep
concentration gradient (out of the cell)
via leakage channels. Loss of K+
results in a negative charge on the
inner plasma membrane face.
Extracellular fluid
+
+
–
+
+
+
+
+
–
Cytoplasm
© 2013 Pearson Education, Inc.
–
Slide 4
–
–
Potassium
leakage
channels
–
–
+
–
Protein anion (unable to
follow K+ through the
membrane)
2 K+ also move into the cell
because they are attracted to the
negative charge established on the
inner plasma membrane face.
3 A negative membrane potential
(–90 mV) is established when the
movement of K+ out of the cell equals
K+ movement into the cell. At this
point, the concentration gradient
promoting K+ exit exactly opposes the
electrical gradient for K+ entry.
Selective Diffusion Establishes RMP
• In many cells Na+ affects RMP
– Attracted into cell due to negative charge 
RMP to –70 mV
– Membrane more permeable to K+ than Na+,
so K+ primary influence on RMP
• Cl– does not influence RMP—
concentration and electrical gradients
exactly balanced
© 2013 Pearson Education, Inc.
Active Transport Maintains Electrochemical
Gradients
• Na+-K+ pump continuously ejects 3Na+
from cell and carries 2K+ in
• Steady state maintained because rate of
active transport equal to and depends on
rate of Na+ diffusion into cell
• Neuron and muscle cells "upset" RMP by
opening gated Na+ and K+ channels
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Cell-Environment Interactions
• Cells interact directly or indirectly by
responding to extracellular chemicals
• Always involves glycocalyx
– Cell adhesion molecules (CAMs)
– Plasma membrane receptors
– Voltage-gated channel proteins
© 2013 Pearson Education, Inc.
Roles of Cell Adhesion Molecules
•
•
•
•
•
Thousands on approximately every cell in body
Anchor to extracellular matrix or each other
Assist in movement of cells past one another
Attract WBCs to injured or infected areas
Stimulate synthesis or degradation of adhesive
membrane junctions
• Transmit intracellular signals to direct cell
migration, proliferation, and specialization
© 2013 Pearson Education, Inc.
Roles of Plasma Membrane Receptors
• Contact signaling—touching and recognition of
cells; e.g., in normal development and immunity
• Chemical signaling—interaction between
receptors and ligands (neurotransmitters,
hormones, and paracrines) to alter activity of cell
proteins (e.g., enzymes or chemically gated ion
channels)
– Same ligand can cause different cell responses
– Response determined by what receptor linked to
inside cell
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Chemical Signaling
• Ligand binding  receptor structural change 
protein alteration
– Catalytic receptor proteins become activated
enzymes
– Chemically gated channel-linked receptors open and
close ion gates  changes in excitability
– G protein–linked receptors activate G protein,
affecting an ion channel or enzyme, or causing
release of internal second messenger, such as cyclic
AMP
© 2013 Pearson Education, Inc.
Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second
messengers that cause responses within the cell.
Slide 1
The sequence described here is like a molecular
relay race. Instead of a baton passed from runner
to runner, the message (a shape change) is
passed from molecule to molecule as it makes its
way across the cell membrane from outside to
inside the cell.
1 Ligand* (1st messenger) binds to the receptor.
The receptor changes shape
and activates.
Ligand (1st Receptor G protein Enzyme
messenger)
2 The activated receptor
binds to a G protein and activates it. The G protein changes
shape (turns “on”), causing it to
release GDP and bind GTP (an
energy source).
2nd
messenger
3 Activated G protein
activates (or inactivates)
an effector protein by
causing its shape to
change.
Extracellular fluid
Effector protein
(e.g., an enzyme)
Ligand
Receptor
G protein
GDP
Inactive 2nd
messenger
Active 2nd
messenger
Activated
kinase
enzymes
* Ligands include
hormones and
neurotransmitters.
© 2013 Pearson Education, Inc.
4 Activated effector enzymes
catalyze reactions that produce
2nd messengers in the cell.
(Common 2nd messengers include
cyclic AMP and Ca2+.)
5 Second messengers
activate other enzymes or ion
channels. Cyclic AMP typically
activates protein kinase enzymes.
6 Kinase enzymes activate
other enzymes. Kinase enzymes
transfer phosphate groups from ATP
to specific proteins and activate a
Cascade of cellular responses series of other enzymes that trigger
(The amplification effect is
various metabolic and structural
tremendous. Each enzyme
catalyzes hundreds of reactions.) changes in the cell.
Intracellular fluid
Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second
messengers that cause responses within the cell.
Slide 2
Ligand (1st Receptor G protein Enzyme
messenger)
2nd
messenger
1 Ligand* (1st messenger) binds to the receptor.
The receptor changes shape
and activates.
Extracellular fluid
Ligand
Receptor
* Ligands include
hormones and
neurotransmitters.
© 2013 Pearson Education, Inc.
Intracellular fluid
Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second
messengers that cause responses within the cell.
Slide 3
Ligand (1st Receptor G protein Enzyme
messenger)
1 Ligand* (1st messenger) binds to the receptor.
The receptor changes shape
and activates.
Ligand
2 The activated receptor
binds to a G protein and activates it. The G protein changes
shape (turns “on”), causing it to
release GDP and bind GTP (an
energy source).
2nd
messenger
Extracellular fluid
Receptor
G protein
GDP
* Ligands include
hormones and
neurotransmitters.
© 2013 Pearson Education, Inc.
Intracellular fluid
Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second
messengers that cause responses within the cell.
Slide 4
Ligand (1st Receptor G protein Enzyme
messenger)
1 Ligand* (1st messenger) binds to the receptor.
The receptor changes shape
and activates.
2 The activated receptor
binds to a G protein and activates it. The G protein changes
shape (turns “on”), causing it to
release GDP and bind GTP (an
energy source).
2nd
messenger
3 Activated G protein
activates (or inactivates)
an effector protein by
causing its shape to
change.
Extracellular fluid
Effector protein
(e.g., an enzyme)
Ligand
Receptor
G protein
GDP
* Ligands include
hormones and
neurotransmitters.
© 2013 Pearson Education, Inc.
Intracellular fluid
Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second
messengers that cause responses within the cell.
Slide 5
Ligand (1st Receptor G protein Enzyme
messenger)
1 Ligand* (1st messenger) binds to the receptor.
The receptor changes shape
and activates.
2 The activated receptor
binds to a G protein and activates it. The G protein changes
shape (turns “on”), causing it to
release GDP and bind GTP (an
energy source).
2nd
messenger
3 Activated G protein
activates (or inactivates)
an effector protein by
causing its shape to
change.
Extracellular fluid
Effector protein
(e.g., an enzyme)
Ligand
Receptor
G protein
GDP
* Ligands include
hormones and
neurotransmitters.
© 2013 Pearson Education, Inc.
Inactive 2nd
messenger
Active 2nd
messenger
4 Activated effector enzymes
catalyze reactions that produce
2nd messengers in the cell.
(Common 2nd messengers include
cyclic AMP and Ca2+.)
Intracellular fluid
Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second
messengers that cause responses within the cell.
Slide 6
Ligand (1st Receptor G protein Enzyme
messenger)
1 Ligand* (1st messenger) binds to the receptor.
The receptor changes shape
and activates.
2 The activated receptor
binds to a G protein and activates it. The G protein changes
shape (turns “on”), causing it to
release GDP and bind GTP (an
energy source).
2nd
messenger
3 Activated G protein
activates (or inactivates)
an effector protein by
causing its shape to
change.
Extracellular fluid
Effector protein
(e.g., an enzyme)
Ligand
Receptor
G protein
GDP
Inactive 2nd
messenger
Active 2nd
messenger
Activated
kinase
enzymes
* Ligands include
hormones and
neurotransmitters.
© 2013 Pearson Education, Inc.
4 Activated effector enzymes
catalyze reactions that produce
2nd messengers in the cell.
(Common 2nd messengers include
cyclic AMP and Ca2+.)
5 Second messengers
activate other enzymes or ion
channels. Cyclic AMP typically
activates protein kinase enzymes.
Intracellular fluid
Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second
messengers that cause responses within the cell.
Slide 7
Ligand (1st Receptor G protein Enzyme
messenger)
1 Ligand* (1st messenger) binds to the receptor.
The receptor changes shape
and activates.
2 The activated receptor
binds to a G protein and activates it. The G protein changes
shape (turns “on”), causing it to
release GDP and bind GTP (an
energy source).
2nd
messenger
3 Activated G protein
activates (or inactivates)
an effector protein by
causing its shape to
change.
Extracellular fluid
Effector protein
(e.g., an enzyme)
Ligand
Receptor
G protein
GDP
Inactive 2nd
messenger
Active 2nd
messenger
Activated
kinase
enzymes
* Ligands include
hormones and
neurotransmitters.
© 2013 Pearson Education, Inc.
4 Activated effector enzymes
catalyze reactions that produce
2nd messengers in the cell.
(Common 2nd messengers include
cyclic AMP and Ca2+.)
5 Second messengers
activate other enzymes or ion
channels. Cyclic AMP typically
activates protein kinase enzymes.
6 Kinase enzymes activate
other enzymes. Kinase enzymes
transfer phosphate groups from ATP
to specific proteins and activate a
Cascade of cellular responses series of other enzymes that trigger
(The amplification effect is
various metabolic and structural
tremendous. Each enzyme
catalyzes hundreds of reactions.) changes in the cell.
Intracellular fluid