Chapter 5
Learning Objectives:
1,Principles of membrane transport;
2,Passive transport and active transport;
3,Two main classes of membrane transport proteins:
Carriers and Channels;
4,The ion transport systems;
5,Endocytosis and Phagocytosis,cellular uptake of
macromolecules and particles.
A,The Movement of Substances
Across Cell Membranes
A motor neuron cell body in the spinal cord,(A) Many thousands of nerve
terminals synapse on the cell body and dendrites,These deliver signals from other
parts of the organism to control the firing of action potentials along the single axon
of this large cell,(B) Micrograph showing a nerve cell body and its dendrites
stained with a fluorescent antibody that recognizes a cytoskeletal protein (green).
Thousands of axon terminals (red) from other nerve cells (not visible) make
synapses on the cell body and dendrites; they are stained with a fluorescent
antibody that recognizes a protein in synaptic vesicles,
1,Principles of membrane transport
A,The plasma membrane is a selectively permeable
barrier,It allows for separation and exchange of
materials across the plasma membrane.
Figure 11-1 The relative
permeability of a synthetic
lipid bilayer to different
classes of molecules,The
smaller the molecule and,
more important,the fewer
hydrogen bonds it makes with
water,the more rapidly the
molecule diffuses across the
bilayer,
B,The protein-free lipid bilayers are highly
impermeable to ions.
If uncharged solutes are small enough,
they can move down their concentration
gradients directly across the lipid
bilayer by simple diffusion,
Most solutes can cross the membrane
only if there is a membrane transport
protein to transfer them,
Passive transport,in the same
direction as a concentration gradient.
Active transport,is mediated by
carrier proteins,against a concentration
gradient,require an input of energy.
Diffusion of small
molecules across
phospholipid
bilayers
Figure 11-2 Permeability coefficients
(cm/sec) for the passage of various
molecules through synthetic lipid
bilayers,The rate of flow of a solute
across the bilayer is directly proportional
to the difference in its concentration on the
two sides of the membrane,Multiplying
this concentration difference (in mol/cm3)
by the permeability coefficient (cm/sec)
gives the flow of solute in moles per
second per square centimeter of
membrane,A concentration difference of
tryptophan of 10-4 mol/cm3 (10-4/10-3 L =
0.1 M),for example,would cause a flow of
10-4 mol/cm3 x 10-7 cm/sec = 10-11 mol/sec
through 1 cm2 of membrane,or 6 x 104
molecules/sec through 1 microns2 of
membrane,
C,The energetics of solute movement:
Diffusion is the spontaneous movement of material from a region of high concentration to a region of low
concentration.
The free-energy change during diffusion of nonelectrolytes depends on the concentration grdient.
The free-energy change during diffusion of electrolytes depends on the electrochemical grdient.
D,Transport processes within an eukaryotic cell
2,Passive transport and active transport
A,Comparison of two classes of transport.
Figure 11-7 Kinetics of simple diffusion compared to carrier-mediated
diffusion,Whereas the rate of the former is always proportional to the solute
concentration,the rate of the latter reaches a maximum (Vmax) when the carrier
protein is saturated,The solute concentration when transport is at half its
maximal value approximates the binding constant (KM) of the carrier for the
solute and is analogous to the KM of an enzyme for its substrate,The graph
applies to a carrier transporting a single solute; the kinetics of coupled transport
of two or more solutes (see text) are more complex but show basically similar
phenomena,
B,Two classes of membrane transport proteins
Carrier proteins are responsible for both the
passive and the active transport.
Channel proteins are only responsible for passive
transport.
Figure 11-8 Three types of carrier-mediated transport,The
schematic diagram shows carrier proteins functioning as uniports,
symports,and antiports,
Carrier proteins bind one or more solute molecules
on one side of the membrane and then undergo a
conformational change that transfer the solute to
the other side of the membrane.
The carrier protein,the Glucose transporter (GluT1 ) in the erythrocyte
PM,alter conformation to facilitate the transport of glucose.
Facilitate diffusion,Protein-mediated movement,
movement down the gradient
Most of the channel proteins are ion channels,
including three types,with ion channels that they
can be opened and closed
Figure 11-36,A model for the structure of the acetylcholine receptor,Five
homologous subunits (a,a,b,g,d) combine to form a transmembrane aqueous pore,
The pore is lined by a ring of five transmembrane a helices,one contributed by
each subunit,In its closed conformation,the pore is thought to be occluded by the
hydrophobic side chains of five leucines,one from each a helix,which form a gate
near the middle of the lipid bilayer,The negatively charged side chains at either end
of the pore ensure that only positively charged ions pass through the channel,Both
of the a subunits contain an acetylcholine-binding site; when acetylcholine binds to
both sites,the channel undergoes a conformational change that opens the gate,
possibly by causing the leucines to move outward,
电压门控离子通道：铰链细胞失水应力激活的离子通道,2X1013N,0.04nm
3,Active transport,Carrier protein-
mediated movement up the gradient
A,This process differs from facilitated
diffusion in two crucial aspects:
Active transport maintains the gradients for potassium,
sodium,calcium,and other ions across the cell membrane,
Always moves solutes up a concentration or
electrochemical gradient;
Active transport couples the movement of substances
against gradients to ATP hydrolysis,i.e Always requires
the input of energy.
B,Cells carry out active transport in
three main ways
Couple the uphill transport of one solute across membrane
to the downhill transport of another.
Couple uphill transport to the hydrolysis of ATP.
Mainly in bacteria,couple uphill transport to an input of
energy from light.
C,Direct active transport depends on four types
of transport ATPases
The four classes of ATP-powered transport proteins:
―P‖type stands for phosphorylation;
ABC (ATP-binding Cassette) superfamily,bacteria—humans,
Two transmembrane (T) domains and two cytosolic ATP-binding (A) domains
The Na+-K+ ATPase
---A coupling active transport to ATP hydrolysis.
The Na+-K+ ATPase requires K+
outside,Na+ and ATP inside,and
is inhibited by ouabain.
The ratio of Na+:K+ pumped is
3:2 for each ATP hydrolyzed.
The Na+-K+ ATPase is a P-type
pump.This ATPase seruentially
phosphorylates and dephosphory-
lates itself during the pumping
cycle,
The Na+-K+ ATPase is found
only in aniimals,
The active transport of Na+/K+ ATPase is used to
maintains electrochemical ion gradients,and
thereby maintains cell’s excitability.
The Na+/K+ pumo is required to maintain osmotic
balance and stabilize cell volume
The biological functions of Na+/K+ pump
forming a phosphorylated protein intermediate
A Model Mechanism for
the Na+/K+ ATPase
Other P-type punps,including H+ and Ca+ ATPases,
and H+/K+ ATPases
Plant cells have a H+-transporting plasma membrane pump,
This proton pump plays a key role in the secondary transport of
solutes,in the control of cytosolic pH,and possibly in control of cell
growth by means of acidification of the plant cell wall.
Ca2+ pump,Ca2+-ATPase present in both the plasma membrane and
the membranes of the ER,It contain 10 transmembrane? helices.
This Ca2+ pump functions to actively transport Ca2+ out of the
cytosol into either the extracellular space or the lumen of the ER,
H+/K+ ATPases (epithelial lining of the stomach),which secretes a solution of
concentrated acid (up to 0.16N HCl) into the stomach chamber.
The V-type pump,utilize the energy of ATP without
forming a phosphorylated protein intermediate.
Vacuolar(V-type) pump actively transport H+
across the membranes of cytoplasmic organelles
and vacuoles.
They precent in lysosomes,secretory granules,
and plant cell vacuoles,have also been found in
the plasma membranes of a variety of cells
(kidney tubules).
4,Indirect active transport is driven by Ion
gradients ----- Cotransport
A,Sugars,amino acids,and other organic
molecules into cells,?The inward transport of such
molecules up their
concentration gradients is often
coupled to,and driven by,the
concomitant inward movement
of these ions down their
electrochemical gradients:
Animal cells-----Sodium ions (Na+/K+
ATPase)
Plant,fungi,bacterium-----Protons(H+
ATPase)
Gradients created by active ion pumping store energy that can
be coupled to other transport processes,
The difference between animal and plant cells
to absorb nutrients
B,Cotransport,Symport and antiport
Na+-linked symporters
import amino acids and
glucose into many animal
cells
Na+-linked antiporter
exports Ca+ from
cardiac muscle cells
Medicine
Ouabain and digoxin
increase the force of
heart muscle contraction
by inhibiting the Na+/K+
ATPase,Fewer Ca+ ions
are exported
5,Endocytosis,
Large molecules enter into cells
A,Endocytosis imports
extracellular molecules
dissolved or suspended in
fluid by forming vesicles
from the plasma membrane
Bulk-phase endocytosis
does not require surface
membrane recognition.It is the
nonspecific uptake of
extracellular fluids,
Receptor-mediated
endocytosis (RME) follows
the binding of substances to
membrane receptors.
B,Phagocytosis,The uptake of large particles
Including,macromolecules,
cell debris,even microorganisms
and other cells.
Phagocytosis is usually
restricted to specialized cells
called Phagocytes.
Phagocytosis is initiated by
cellular contact with an
appropriate target.
Phagocytosis may be
stimulated by the opsonins
Phagocytosis is driven by
contractile activities of MF.
C,Receptor-mediated endocytosis
Structure of a clathrin –coated vesicle
Model for the
formation of a clathrin-
coated pit and the
selective incorporation
of integral membrane
proteins into clathrin-
coated vesicles
The endocytic pathway is divided into the
early endosomes and late endosomes pathway
Materials in the early endosomes are sorted,
Integral membrane proteins are shipped back to
the membrane;
Other dissolved materials and bound ligands?
Multivesicular body (MT mediated transport)?
the late endosomes.
Molecules that reach the late endosomes are moved
to lysosomes.
6,Exocytosis
A,Constitutive exocytosis pathway
B,Regulated exocytosis pathway
7,Membrane Potentials and Nerve Impulses
A,K+ gradients maintained by the Na+-K+ ATPase are responsible
for the resting membrane potential.
Resting state,All Na+ and K+ channels
closed.
Depolarizing phase,Na+ channels
open,triggering an action potential.
Repolarizing phase,Na+ channels
inactivated,K+ channels open.
Hyperpolarizing phase,K+ channels
remain open,Na+ channels inactivated.
B,The action potential,The changes in
ion channels and membrane potential,
The sequence of events during synaptic transmission,
Excitable membranes exhibit ―all-or-none‖ behavior.
Propagation of action potentials as an impulse.
Chapter 5 B,Cell Signaling
Learning Objectives:
1,Some of the basic characteristics of cell signaling
2,The types of signal molecules,receptors,molecular
switches and effectors;
3,The different signal transduction pathways;
4,The convergence,divergence,and cross talking between
different signaling pathways.
1,Overview of cell signaling
A,Some of the basic
characteristics of cell
signaling
Cell must respond
appropriately to external
stimuli to survive.
Cells respond to stimuli
via cell signaling
1,Recognition of the stimulus
by a specific plasma
membrane receptor.
2,Transfer of a signal across
the plasma membrane.
3,Transmission of the signal to
effector molecules within the
cell,which causes a change in
cellular activities.
4,Cessation of the cellular
response due to inactivation
of the signal molecule,
Signal transduction pathways consist
of a series of steps
signal magnification
Each cell is programmed to respond to specific
combinations of exreaceluular signal molecules
Different cells can respond differently
to the same extracellular signal molecule
Figure 15-9 The same signaling molecule
can induce different responses in different
target cells,In some cases this is because
the signaling molecule binds to different
receptor proteins,as illustrated in (A) and (B),
In other cases the signaling molecule binds to
identical receptor proteins that activate different
response pathways in different cells,as
illustrated in (B) and (C),In all of the cases
shown the signaling molecule is acetylcholine
(D),
A cell can remember the effect of some signals,
after the signal has disappeared,(Ca2+)
Protein kinase activited by Ca2+ to phosphorylate itself and
other proteins,the autophosphorylation keeps the kinase active
long after Ca2+ levels return to normal,providing a memory
trace of the initial signal.
Transient extracellular signals often induce much longer-term
changes in cells during the development of a multicellular
organism.They usually depend on self-activating memory
mechanisms that operate further downstream in a signaling
pathway,at the level of gene transcription.
B,The forms of cell communication----- Different types
of chemical signals can be received by cells
Gap junction
C,Signal Molecules and Receptors
signal molecules:
Lipid-soluble hormones
Water-soluble hormones
nitric oxide (NO) and carbon
monoxide(CO) as cellular messengers
Receptors include three classes,glycoproteins
D,Two types of intracellular signaling
proteins that act as Molecular Switches
Phosphorylation and dephosphorylation
via protein kinases and phosphatases,
Thereby stimulating or inhibiting the
activities
GAPs inactivate G-protein; GEFs
activates G-protein; GDIs(guanine
nucleotide-dissociation inhibitors)
maintain the G-protein inactive.
2,Signal transdution mediated by
the receptors within cells
A,Some small hydrophobic hormones (steroid
hormones) whose receprors are intracellular
gene regulatory proteins.
Figure 15-13 Early primary response (A) and delayed secondary response
(B) that result from the activation of an intracellular receptor protein,The
response to a steroid hormone is illustrated,but the same principles apply for all ligands
that activate this family of receptor proteins,Some of the primary-response proteins turn
on secondary-response genes,whereas others turn off the primary-response genes,The
actual number of primary- and secondary-response genes is greater than shown,As
expected,drugs that inhibit protein synthesis suppress the transcription of secondary-
response genes but not primary-response genes,
B,Nitric oxide couples G protein-linked receptor
stimulation in endothelial cells to relaxation of
smooth muscle cells in blood vessels
It has been known for many years that acetylcholine
dilate blood vessels by causing their smooth muscles
to relax,In 1980,Furchgott concluded that blood
vessels are dilated because the endothelial cells
produce a signal molecule that makes smooth muscle
cells relax,In 1986 work by Furchgott and parallel
work by Louis Ignarro identified NO as the signal
that cause relaxation of the vascular smooth muscle.
1998,Received Nobel Prize
The action of Nitric oxide on blood vessels
The mechanism by which acetylcholine stimulation of the
endothelial cells leads to smooth muscle relaxation also
explains the mechanism of action of the chemical
nitroglycerin.
The drug sildenafil,sold under the trade name Viagra,is an
inhibitor of a cyclic GMP-specific phosphodiesterase that
normally catalyzes the breakdown of cyclic GMP.
The carbon monoxide(CO) acts as a cellular
messenger to stimulate the production of cGMP by
stimulating G-cyclase.
3,Signal transduction mediated by
the receptors on the cell surface
A,Mediated by the Ion-Linked Receptors which
convert chemical signals into electrical ones
4 or 6-helix transmembrane
receptor
B,Signal transduction mediated by G
protein-linked receptors
The structure
of G protein-linked
receptors,
Seven-helix transmembrane;
C-terminal,Ser- and Thr-
rich
---the sites of phosphorylation
make for the desensitization
of GPLR.
Figure 15-18 Two major pathways by which G-protein-linked cell-surface
receptors generate small intracellular mediators,In both cases the binding of
an extracellular ligand alters the conformation of the cytoplasmic domain of the receptor,
causing it to bind to a G protein that activates (or inactivates) a plasma membrane
enzyme,In the cyclic AMP (cAMP) pathway the enzyme directly produces cyclic AMP,In
the Ca2+ pathway the enzyme produces a soluble mediator (inositol trisphosphate) that
releases Ca2+ from the endoplasmic reticulum,
The structure and activation of G proteins
Figure 15-23 A current model of how Gs
couples receptor activation to adenylyl
cyclase activation,As long as the
extracellular signaling ligand remains bound,
the receptor protein can continue to activate
molecules of Gs protein,thereby amplifying
the response,More important,an alphas can
remain active and continue to stimulate a
cyclase molecule for many seconds after the
signaling ligand dissociates from the receptor,
providing even greater amplification.
C,Cyclic AMP signaling pathway
G-protein activation and inactivation cycle
The activation of protein kinase A by cyclic AMPs
Second messengers (cAMP),an effector,amplify the response to a
single extracellular ligand by cAMP to trigger a reaction cascade.
The cascade starts with the binding of cAMP to cAMP-dependent
protein kinase A.
PKA inhibits glycogen synthase and activates phosphorylase kinase.
Double Messenger system
Figure 15-32 Two intracellular pathways by which activated C-kinase can
activate the transcription of specific genes,In one (red arrows) C-kinase
activates a phosphorylation cascade that leads to the phosphorylation of a pivotal
protein kinase called MAP-kinase,which in turn phosphorylates and activates the gene
regulatory protein Elk-1,Elk-1 is bound to a short DNA sequence in association with
another DNA-binding protein,In the other pathway (green arrows) C-kinase activation
leads to the phosphorylation of Ik-B,which releases the gene regulatory protein NF-kB
so that it can migrate into the nucleus and activate the transcription of specific genes,
The mechanisms that shut off a signal are as
important as the mechanisms that turn it on.
Cholera is caused by a bacterium that multiplies
in the intestine,where it produces a protein called
cholera toxin,This enters the cells lining the
intestine and modifies the α subunit of G protein
so that it can no longer hydrolyze its bound GTP,
The altered α subunit thus remains in the active
state indefinitely,continuing to transmit a signal
to its target proteins,Outflow of Na+ and water
into the gut.
D,The pathway through phospholipase C
results in a rise in intracellular Ca+
Cytolasmic calcium
levels are determined
by events within a
membrane.
IP3-Ca2+ pathway and DG-PKC pathway
Elevation of cytosolic Ca2+
via the IP signaling pathway
Signals?GPLR?GP?PLC
IP3 and DAG (twin signals).
IP3?IP3 receptor(Ca2+ channel,
located at the surface of sER)?
Elevation of cytosolic Ca2+;
DAG?activates PKC?to
phosphoralate Ser and Thr on
target proteins.
Calcium binds to calcium-
binding proteins(CaM) which
affects other proteins.
Structure of calmodulin,a cytosolic protein of 148 amino
acids that bind Ca2+ ions
Regulating calcium concentrations in plant cells
Cytosolic calcium changes in response to several stimuli,including
light,pressure,gravity,and hormones.
Calcium signaling aids in decreasing turgor pressure in guard cells.
Ca2+/CaM dep,protein kinase
(CaM-kinase) mediate many of the
actions of Ca2+ in animal cells.
The functions of increase the levels
of cytosolic calcium-CaM,?start-
up embryo development after the
fecundation,?excitating contract
of muscle cells;?excitating
secretion of endocrine and nerve
cells.
G-protein-linked receptor desensitization depends on
receptor phosphorylation by PKA,PKC,CaMK2 or G-
protein-linked receptor kinases(GRKs)
The target cells can become desensitized to a signal molecule by five
ways.
Three general ways of the desensitization:
1,Receptor inactivation by alteration;
2,Receptor sequestration by internalization;
3,Receptor down-regulation by destroying in Ls.
Sequestration; down-regulation; inactivation; inactivation; inhibitory protein
E,Receptor tyrosine kinase (RTK) and
RTK-Ras signaling pathway
RTK are the second major type of cell-
surface receptors
Signaling ligands of RTKs:
1.Nerve growth factor (NGF)
2.Platelet-derived growth factor
(PDGF)
3.Fibroblast growth factor (FGF)
4.Epidermal growth factor (EGF)
5.insulin and insulin-like GF(IGF-1)
6.ephrins(Eph)
7.vascular endothelial factor(VEGF)
Six classes of enzyme-linked receptors
have thus far been identified:
Receptor tyrosine kinase(RTK);
Tyrosine-kinase-associated receptor;
Receptorlike tyrosine phosphatases;
Receptor serine/threonine kinase;
Receptor guanylyl cyclases;
Histidine-kinase-associated receptor
Figure 15-47 Six subfamilies of receptor tyrosine kinases,Only one or two
members of each subfamily are indicated,Note that the tyrosine kinase domain
is interrupted by a "kinase insert region" in some of the subfamilies,The
functional significance of the cysteine-rich and immunoglobulinlike domains is
unknown,
Ligand binding leads to autophosphorylation
of RTK
RTK activity stimulated by cross-phosphorylation.
Phsphorylated Tyrosine
Serve as docking sites for
protein with SH2 domains
(Src homology region).
Other protein modules
such as SH3 binds to
proline-rich motifs in
intracellular proteins,dimerization
Steps in activation of Ras by RTKs
Phosphotyrosines of RTK act as
binding sites for a specific SH2
protein called GRB2 (Growth factor
receptor binding protein in mammalian).
GRB2 is not a protein with
catalytic activity,but one that
functions solely as an adapter
molecule that links other proteins
into a complex.
Sos(son of sevenless) is a guanine
nucleotide exchange factor for Ras
(Ras-GEF)
When a ligand binds to the RTK
and recruits the Grb2-Sos to the inner
surface of the membrane,the Sos
protein binds to Ras causing GDP/GTP
exchange,thus activating Ras.
In the cell,Ras activity is regulated by GAPs( GTPase –
Activating proteins)—?100 000-fold
MAP-kinase serine/threonine phosphorylation
Pathway activated by Ras
Ras-activated
phosphorylation
cascade
MAP kinase=mitogen-activated protein kinase; MAP-KKK=Raf (Ser/Thr-PK)
RTK-Ras signaling pathway
F,Jak-STAT signaling pathway
Janus kinases (Jaks) and STATs,
Jaks,A class of cytoplasmic tyrosine kinases,including Jak1,
Jak2,Jak3,and Tyk2,and each is associated with particular
cytokine receptors;
STATs,Jaks then phosphorylate and activate a set of latent gene
regulatory proteins called STATs(signal transducers and activators
of transcription,which have an SH2 domain),which move into the
nucleus and stimulate the transcription of specific genes.
Signaling ligands and Cytokine receptors:
Ligands,more than 30 cytokines and hormones activate the Jak-
STAT pathway by binding to cytokine receptors.
Cytokine receptors,they are composed of two or more poly
peptide chains,All receptor are associated with one or more Jaks.
The Jak-STAT signaling pathway avtivated by?-interferon,
Providing a fast track to the nucleus.
MBC 885,15-63
4,Signals that originate from contacts
between the cell surface and the substratum
A,Integrins are receptors at sites of cell-substrate
and cell-cell contact.
Interaction between the extracellular domain of integrin and an
extracellular ligand generate a variety of signals.
The interaction leads to clustering of integrins and the rapid
tyrosine phosphorylation of proteins at the cytoplasmic face of focal
adhesions by the tyrosine kinase,Src.
Focal adhesion kinase (FAK) is an effector in integrin-mediated
responses.
Another signal pathway activated by integrin engagement leads to
the protein-synthesizing machinery of the cytoplasm.
1.The mechanical-
structural function of
focal adhesion,which
is carried out by actin
filaments and
associated proteins.
2,Signaling function
from extracellular
surface?nucleus
(where they stimulate
the transcription of
genes involved in cell
growth and
proliferation),which
is carried out by
tyrosine kinase (Src
and FAK)
Controlling the assembly of focal adhesions
Signal pathway
that lead to the
assembly of the
fibers of a focal
adhesion.
Rho is involved
in regulating the
organization of
the cell’s actin
cytoskeleton.
5,Convergence,divergence,and crosstalk
among different signaling pathway
A,Convergence:
Signals from a
variety of unrelated
receptors can
converge to activate
a common effector.
B,Divergence,Signals from the same ligand can
diverge to activate a variety of different effectors.
C,Crosstalk,Signals can be passed back and
forth between different pathways
6,In actual fact,signaling pathways in the
cell are much more complex.
Learning Objectives:
1,Principles of membrane transport;
2,Passive transport and active transport;
3,Two main classes of membrane transport proteins:
Carriers and Channels;
4,The ion transport systems;
5,Endocytosis and Phagocytosis,cellular uptake of
macromolecules and particles.
A,The Movement of Substances
Across Cell Membranes
A motor neuron cell body in the spinal cord,(A) Many thousands of nerve
terminals synapse on the cell body and dendrites,These deliver signals from other
parts of the organism to control the firing of action potentials along the single axon
of this large cell,(B) Micrograph showing a nerve cell body and its dendrites
stained with a fluorescent antibody that recognizes a cytoskeletal protein (green).
Thousands of axon terminals (red) from other nerve cells (not visible) make
synapses on the cell body and dendrites; they are stained with a fluorescent
antibody that recognizes a protein in synaptic vesicles,
1,Principles of membrane transport
A,The plasma membrane is a selectively permeable
barrier,It allows for separation and exchange of
materials across the plasma membrane.
Figure 11-1 The relative
permeability of a synthetic
lipid bilayer to different
classes of molecules,The
smaller the molecule and,
more important,the fewer
hydrogen bonds it makes with
water,the more rapidly the
molecule diffuses across the
bilayer,
B,The protein-free lipid bilayers are highly
impermeable to ions.
If uncharged solutes are small enough,
they can move down their concentration
gradients directly across the lipid
bilayer by simple diffusion,
Most solutes can cross the membrane
only if there is a membrane transport
protein to transfer them,
Passive transport,in the same
direction as a concentration gradient.
Active transport,is mediated by
carrier proteins,against a concentration
gradient,require an input of energy.
Diffusion of small
molecules across
phospholipid
bilayers
Figure 11-2 Permeability coefficients
(cm/sec) for the passage of various
molecules through synthetic lipid
bilayers,The rate of flow of a solute
across the bilayer is directly proportional
to the difference in its concentration on the
two sides of the membrane,Multiplying
this concentration difference (in mol/cm3)
by the permeability coefficient (cm/sec)
gives the flow of solute in moles per
second per square centimeter of
membrane,A concentration difference of
tryptophan of 10-4 mol/cm3 (10-4/10-3 L =
0.1 M),for example,would cause a flow of
10-4 mol/cm3 x 10-7 cm/sec = 10-11 mol/sec
through 1 cm2 of membrane,or 6 x 104
molecules/sec through 1 microns2 of
membrane,
C,The energetics of solute movement:
Diffusion is the spontaneous movement of material from a region of high concentration to a region of low
concentration.
The free-energy change during diffusion of nonelectrolytes depends on the concentration grdient.
The free-energy change during diffusion of electrolytes depends on the electrochemical grdient.
D,Transport processes within an eukaryotic cell
2,Passive transport and active transport
A,Comparison of two classes of transport.
Figure 11-7 Kinetics of simple diffusion compared to carrier-mediated
diffusion,Whereas the rate of the former is always proportional to the solute
concentration,the rate of the latter reaches a maximum (Vmax) when the carrier
protein is saturated,The solute concentration when transport is at half its
maximal value approximates the binding constant (KM) of the carrier for the
solute and is analogous to the KM of an enzyme for its substrate,The graph
applies to a carrier transporting a single solute; the kinetics of coupled transport
of two or more solutes (see text) are more complex but show basically similar
phenomena,
B,Two classes of membrane transport proteins
Carrier proteins are responsible for both the
passive and the active transport.
Channel proteins are only responsible for passive
transport.
Figure 11-8 Three types of carrier-mediated transport,The
schematic diagram shows carrier proteins functioning as uniports,
symports,and antiports,
Carrier proteins bind one or more solute molecules
on one side of the membrane and then undergo a
conformational change that transfer the solute to
the other side of the membrane.
The carrier protein,the Glucose transporter (GluT1 ) in the erythrocyte
PM,alter conformation to facilitate the transport of glucose.
Facilitate diffusion,Protein-mediated movement,
movement down the gradient
Most of the channel proteins are ion channels,
including three types,with ion channels that they
can be opened and closed
Figure 11-36,A model for the structure of the acetylcholine receptor,Five
homologous subunits (a,a,b,g,d) combine to form a transmembrane aqueous pore,
The pore is lined by a ring of five transmembrane a helices,one contributed by
each subunit,In its closed conformation,the pore is thought to be occluded by the
hydrophobic side chains of five leucines,one from each a helix,which form a gate
near the middle of the lipid bilayer,The negatively charged side chains at either end
of the pore ensure that only positively charged ions pass through the channel,Both
of the a subunits contain an acetylcholine-binding site; when acetylcholine binds to
both sites,the channel undergoes a conformational change that opens the gate,
possibly by causing the leucines to move outward,
电压门控离子通道：铰链细胞失水应力激活的离子通道,2X1013N,0.04nm
3,Active transport,Carrier protein-
mediated movement up the gradient
A,This process differs from facilitated
diffusion in two crucial aspects:
Active transport maintains the gradients for potassium,
sodium,calcium,and other ions across the cell membrane,
Always moves solutes up a concentration or
electrochemical gradient;
Active transport couples the movement of substances
against gradients to ATP hydrolysis,i.e Always requires
the input of energy.
B,Cells carry out active transport in
three main ways
Couple the uphill transport of one solute across membrane
to the downhill transport of another.
Couple uphill transport to the hydrolysis of ATP.
Mainly in bacteria,couple uphill transport to an input of
energy from light.
C,Direct active transport depends on four types
of transport ATPases
The four classes of ATP-powered transport proteins:
―P‖type stands for phosphorylation;
ABC (ATP-binding Cassette) superfamily,bacteria—humans,
Two transmembrane (T) domains and two cytosolic ATP-binding (A) domains
The Na+-K+ ATPase
---A coupling active transport to ATP hydrolysis.
The Na+-K+ ATPase requires K+
outside,Na+ and ATP inside,and
is inhibited by ouabain.
The ratio of Na+:K+ pumped is
3:2 for each ATP hydrolyzed.
The Na+-K+ ATPase is a P-type
pump.This ATPase seruentially
phosphorylates and dephosphory-
lates itself during the pumping
cycle,
The Na+-K+ ATPase is found
only in aniimals,
The active transport of Na+/K+ ATPase is used to
maintains electrochemical ion gradients,and
thereby maintains cell’s excitability.
The Na+/K+ pumo is required to maintain osmotic
balance and stabilize cell volume
The biological functions of Na+/K+ pump
forming a phosphorylated protein intermediate
A Model Mechanism for
the Na+/K+ ATPase
Other P-type punps,including H+ and Ca+ ATPases,
and H+/K+ ATPases
Plant cells have a H+-transporting plasma membrane pump,
This proton pump plays a key role in the secondary transport of
solutes,in the control of cytosolic pH,and possibly in control of cell
growth by means of acidification of the plant cell wall.
Ca2+ pump,Ca2+-ATPase present in both the plasma membrane and
the membranes of the ER,It contain 10 transmembrane? helices.
This Ca2+ pump functions to actively transport Ca2+ out of the
cytosol into either the extracellular space or the lumen of the ER,
H+/K+ ATPases (epithelial lining of the stomach),which secretes a solution of
concentrated acid (up to 0.16N HCl) into the stomach chamber.
The V-type pump,utilize the energy of ATP without
forming a phosphorylated protein intermediate.
Vacuolar(V-type) pump actively transport H+
across the membranes of cytoplasmic organelles
and vacuoles.
They precent in lysosomes,secretory granules,
and plant cell vacuoles,have also been found in
the plasma membranes of a variety of cells
(kidney tubules).
4,Indirect active transport is driven by Ion
gradients ----- Cotransport
A,Sugars,amino acids,and other organic
molecules into cells,?The inward transport of such
molecules up their
concentration gradients is often
coupled to,and driven by,the
concomitant inward movement
of these ions down their
electrochemical gradients:
Animal cells-----Sodium ions (Na+/K+
ATPase)
Plant,fungi,bacterium-----Protons(H+
ATPase)
Gradients created by active ion pumping store energy that can
be coupled to other transport processes,
The difference between animal and plant cells
to absorb nutrients
B,Cotransport,Symport and antiport
Na+-linked symporters
import amino acids and
glucose into many animal
cells
Na+-linked antiporter
exports Ca+ from
cardiac muscle cells
Medicine
Ouabain and digoxin
increase the force of
heart muscle contraction
by inhibiting the Na+/K+
ATPase,Fewer Ca+ ions
are exported
5,Endocytosis,
Large molecules enter into cells
A,Endocytosis imports
extracellular molecules
dissolved or suspended in
fluid by forming vesicles
from the plasma membrane
Bulk-phase endocytosis
does not require surface
membrane recognition.It is the
nonspecific uptake of
extracellular fluids,
Receptor-mediated
endocytosis (RME) follows
the binding of substances to
membrane receptors.
B,Phagocytosis,The uptake of large particles
Including,macromolecules,
cell debris,even microorganisms
and other cells.
Phagocytosis is usually
restricted to specialized cells
called Phagocytes.
Phagocytosis is initiated by
cellular contact with an
appropriate target.
Phagocytosis may be
stimulated by the opsonins
Phagocytosis is driven by
contractile activities of MF.
C,Receptor-mediated endocytosis
Structure of a clathrin –coated vesicle
Model for the
formation of a clathrin-
coated pit and the
selective incorporation
of integral membrane
proteins into clathrin-
coated vesicles
The endocytic pathway is divided into the
early endosomes and late endosomes pathway
Materials in the early endosomes are sorted,
Integral membrane proteins are shipped back to
the membrane;
Other dissolved materials and bound ligands?
Multivesicular body (MT mediated transport)?
the late endosomes.
Molecules that reach the late endosomes are moved
to lysosomes.
6,Exocytosis
A,Constitutive exocytosis pathway
B,Regulated exocytosis pathway
7,Membrane Potentials and Nerve Impulses
A,K+ gradients maintained by the Na+-K+ ATPase are responsible
for the resting membrane potential.
Resting state,All Na+ and K+ channels
closed.
Depolarizing phase,Na+ channels
open,triggering an action potential.
Repolarizing phase,Na+ channels
inactivated,K+ channels open.
Hyperpolarizing phase,K+ channels
remain open,Na+ channels inactivated.
B,The action potential,The changes in
ion channels and membrane potential,
The sequence of events during synaptic transmission,
Excitable membranes exhibit ―all-or-none‖ behavior.
Propagation of action potentials as an impulse.
Chapter 5 B,Cell Signaling
Learning Objectives:
1,Some of the basic characteristics of cell signaling
2,The types of signal molecules,receptors,molecular
switches and effectors;
3,The different signal transduction pathways;
4,The convergence,divergence,and cross talking between
different signaling pathways.
1,Overview of cell signaling
A,Some of the basic
characteristics of cell
signaling
Cell must respond
appropriately to external
stimuli to survive.
Cells respond to stimuli
via cell signaling
1,Recognition of the stimulus
by a specific plasma
membrane receptor.
2,Transfer of a signal across
the plasma membrane.
3,Transmission of the signal to
effector molecules within the
cell,which causes a change in
cellular activities.
4,Cessation of the cellular
response due to inactivation
of the signal molecule,
Signal transduction pathways consist
of a series of steps
signal magnification
Each cell is programmed to respond to specific
combinations of exreaceluular signal molecules
Different cells can respond differently
to the same extracellular signal molecule
Figure 15-9 The same signaling molecule
can induce different responses in different
target cells,In some cases this is because
the signaling molecule binds to different
receptor proteins,as illustrated in (A) and (B),
In other cases the signaling molecule binds to
identical receptor proteins that activate different
response pathways in different cells,as
illustrated in (B) and (C),In all of the cases
shown the signaling molecule is acetylcholine
(D),
A cell can remember the effect of some signals,
after the signal has disappeared,(Ca2+)
Protein kinase activited by Ca2+ to phosphorylate itself and
other proteins,the autophosphorylation keeps the kinase active
long after Ca2+ levels return to normal,providing a memory
trace of the initial signal.
Transient extracellular signals often induce much longer-term
changes in cells during the development of a multicellular
organism.They usually depend on self-activating memory
mechanisms that operate further downstream in a signaling
pathway,at the level of gene transcription.
B,The forms of cell communication----- Different types
of chemical signals can be received by cells
Gap junction
C,Signal Molecules and Receptors
signal molecules:
Lipid-soluble hormones
Water-soluble hormones
nitric oxide (NO) and carbon
monoxide(CO) as cellular messengers
Receptors include three classes,glycoproteins
D,Two types of intracellular signaling
proteins that act as Molecular Switches
Phosphorylation and dephosphorylation
via protein kinases and phosphatases,
Thereby stimulating or inhibiting the
activities
GAPs inactivate G-protein; GEFs
activates G-protein; GDIs(guanine
nucleotide-dissociation inhibitors)
maintain the G-protein inactive.
2,Signal transdution mediated by
the receptors within cells
A,Some small hydrophobic hormones (steroid
hormones) whose receprors are intracellular
gene regulatory proteins.
Figure 15-13 Early primary response (A) and delayed secondary response
(B) that result from the activation of an intracellular receptor protein,The
response to a steroid hormone is illustrated,but the same principles apply for all ligands
that activate this family of receptor proteins,Some of the primary-response proteins turn
on secondary-response genes,whereas others turn off the primary-response genes,The
actual number of primary- and secondary-response genes is greater than shown,As
expected,drugs that inhibit protein synthesis suppress the transcription of secondary-
response genes but not primary-response genes,
B,Nitric oxide couples G protein-linked receptor
stimulation in endothelial cells to relaxation of
smooth muscle cells in blood vessels
It has been known for many years that acetylcholine
dilate blood vessels by causing their smooth muscles
to relax,In 1980,Furchgott concluded that blood
vessels are dilated because the endothelial cells
produce a signal molecule that makes smooth muscle
cells relax,In 1986 work by Furchgott and parallel
work by Louis Ignarro identified NO as the signal
that cause relaxation of the vascular smooth muscle.
1998,Received Nobel Prize
The action of Nitric oxide on blood vessels
The mechanism by which acetylcholine stimulation of the
endothelial cells leads to smooth muscle relaxation also
explains the mechanism of action of the chemical
nitroglycerin.
The drug sildenafil,sold under the trade name Viagra,is an
inhibitor of a cyclic GMP-specific phosphodiesterase that
normally catalyzes the breakdown of cyclic GMP.
The carbon monoxide(CO) acts as a cellular
messenger to stimulate the production of cGMP by
stimulating G-cyclase.
3,Signal transduction mediated by
the receptors on the cell surface
A,Mediated by the Ion-Linked Receptors which
convert chemical signals into electrical ones
4 or 6-helix transmembrane
receptor
B,Signal transduction mediated by G
protein-linked receptors
The structure
of G protein-linked
receptors,
Seven-helix transmembrane;
C-terminal,Ser- and Thr-
rich
---the sites of phosphorylation
make for the desensitization
of GPLR.
Figure 15-18 Two major pathways by which G-protein-linked cell-surface
receptors generate small intracellular mediators,In both cases the binding of
an extracellular ligand alters the conformation of the cytoplasmic domain of the receptor,
causing it to bind to a G protein that activates (or inactivates) a plasma membrane
enzyme,In the cyclic AMP (cAMP) pathway the enzyme directly produces cyclic AMP,In
the Ca2+ pathway the enzyme produces a soluble mediator (inositol trisphosphate) that
releases Ca2+ from the endoplasmic reticulum,
The structure and activation of G proteins
Figure 15-23 A current model of how Gs
couples receptor activation to adenylyl
cyclase activation,As long as the
extracellular signaling ligand remains bound,
the receptor protein can continue to activate
molecules of Gs protein,thereby amplifying
the response,More important,an alphas can
remain active and continue to stimulate a
cyclase molecule for many seconds after the
signaling ligand dissociates from the receptor,
providing even greater amplification.
C,Cyclic AMP signaling pathway
G-protein activation and inactivation cycle
The activation of protein kinase A by cyclic AMPs
Second messengers (cAMP),an effector,amplify the response to a
single extracellular ligand by cAMP to trigger a reaction cascade.
The cascade starts with the binding of cAMP to cAMP-dependent
protein kinase A.
PKA inhibits glycogen synthase and activates phosphorylase kinase.
Double Messenger system
Figure 15-32 Two intracellular pathways by which activated C-kinase can
activate the transcription of specific genes,In one (red arrows) C-kinase
activates a phosphorylation cascade that leads to the phosphorylation of a pivotal
protein kinase called MAP-kinase,which in turn phosphorylates and activates the gene
regulatory protein Elk-1,Elk-1 is bound to a short DNA sequence in association with
another DNA-binding protein,In the other pathway (green arrows) C-kinase activation
leads to the phosphorylation of Ik-B,which releases the gene regulatory protein NF-kB
so that it can migrate into the nucleus and activate the transcription of specific genes,
The mechanisms that shut off a signal are as
important as the mechanisms that turn it on.
Cholera is caused by a bacterium that multiplies
in the intestine,where it produces a protein called
cholera toxin,This enters the cells lining the
intestine and modifies the α subunit of G protein
so that it can no longer hydrolyze its bound GTP,
The altered α subunit thus remains in the active
state indefinitely,continuing to transmit a signal
to its target proteins,Outflow of Na+ and water
into the gut.
D,The pathway through phospholipase C
results in a rise in intracellular Ca+
Cytolasmic calcium
levels are determined
by events within a
membrane.
IP3-Ca2+ pathway and DG-PKC pathway
Elevation of cytosolic Ca2+
via the IP signaling pathway
Signals?GPLR?GP?PLC
IP3 and DAG (twin signals).
IP3?IP3 receptor(Ca2+ channel,
located at the surface of sER)?
Elevation of cytosolic Ca2+;
DAG?activates PKC?to
phosphoralate Ser and Thr on
target proteins.
Calcium binds to calcium-
binding proteins(CaM) which
affects other proteins.
Structure of calmodulin,a cytosolic protein of 148 amino
acids that bind Ca2+ ions
Regulating calcium concentrations in plant cells
Cytosolic calcium changes in response to several stimuli,including
light,pressure,gravity,and hormones.
Calcium signaling aids in decreasing turgor pressure in guard cells.
Ca2+/CaM dep,protein kinase
(CaM-kinase) mediate many of the
actions of Ca2+ in animal cells.
The functions of increase the levels
of cytosolic calcium-CaM,?start-
up embryo development after the
fecundation,?excitating contract
of muscle cells;?excitating
secretion of endocrine and nerve
cells.
G-protein-linked receptor desensitization depends on
receptor phosphorylation by PKA,PKC,CaMK2 or G-
protein-linked receptor kinases(GRKs)
The target cells can become desensitized to a signal molecule by five
ways.
Three general ways of the desensitization:
1,Receptor inactivation by alteration;
2,Receptor sequestration by internalization;
3,Receptor down-regulation by destroying in Ls.
Sequestration; down-regulation; inactivation; inactivation; inhibitory protein
E,Receptor tyrosine kinase (RTK) and
RTK-Ras signaling pathway
RTK are the second major type of cell-
surface receptors
Signaling ligands of RTKs:
1.Nerve growth factor (NGF)
2.Platelet-derived growth factor
(PDGF)
3.Fibroblast growth factor (FGF)
4.Epidermal growth factor (EGF)
5.insulin and insulin-like GF(IGF-1)
6.ephrins(Eph)
7.vascular endothelial factor(VEGF)
Six classes of enzyme-linked receptors
have thus far been identified:
Receptor tyrosine kinase(RTK);
Tyrosine-kinase-associated receptor;
Receptorlike tyrosine phosphatases;
Receptor serine/threonine kinase;
Receptor guanylyl cyclases;
Histidine-kinase-associated receptor
Figure 15-47 Six subfamilies of receptor tyrosine kinases,Only one or two
members of each subfamily are indicated,Note that the tyrosine kinase domain
is interrupted by a "kinase insert region" in some of the subfamilies,The
functional significance of the cysteine-rich and immunoglobulinlike domains is
unknown,
Ligand binding leads to autophosphorylation
of RTK
RTK activity stimulated by cross-phosphorylation.
Phsphorylated Tyrosine
Serve as docking sites for
protein with SH2 domains
(Src homology region).
Other protein modules
such as SH3 binds to
proline-rich motifs in
intracellular proteins,dimerization
Steps in activation of Ras by RTKs
Phosphotyrosines of RTK act as
binding sites for a specific SH2
protein called GRB2 (Growth factor
receptor binding protein in mammalian).
GRB2 is not a protein with
catalytic activity,but one that
functions solely as an adapter
molecule that links other proteins
into a complex.
Sos(son of sevenless) is a guanine
nucleotide exchange factor for Ras
(Ras-GEF)
When a ligand binds to the RTK
and recruits the Grb2-Sos to the inner
surface of the membrane,the Sos
protein binds to Ras causing GDP/GTP
exchange,thus activating Ras.
In the cell,Ras activity is regulated by GAPs( GTPase –
Activating proteins)—?100 000-fold
MAP-kinase serine/threonine phosphorylation
Pathway activated by Ras
Ras-activated
phosphorylation
cascade
MAP kinase=mitogen-activated protein kinase; MAP-KKK=Raf (Ser/Thr-PK)
RTK-Ras signaling pathway
F,Jak-STAT signaling pathway
Janus kinases (Jaks) and STATs,
Jaks,A class of cytoplasmic tyrosine kinases,including Jak1,
Jak2,Jak3,and Tyk2,and each is associated with particular
cytokine receptors;
STATs,Jaks then phosphorylate and activate a set of latent gene
regulatory proteins called STATs(signal transducers and activators
of transcription,which have an SH2 domain),which move into the
nucleus and stimulate the transcription of specific genes.
Signaling ligands and Cytokine receptors:
Ligands,more than 30 cytokines and hormones activate the Jak-
STAT pathway by binding to cytokine receptors.
Cytokine receptors,they are composed of two or more poly
peptide chains,All receptor are associated with one or more Jaks.
The Jak-STAT signaling pathway avtivated by?-interferon,
Providing a fast track to the nucleus.
MBC 885,15-63
4,Signals that originate from contacts
between the cell surface and the substratum
A,Integrins are receptors at sites of cell-substrate
and cell-cell contact.
Interaction between the extracellular domain of integrin and an
extracellular ligand generate a variety of signals.
The interaction leads to clustering of integrins and the rapid
tyrosine phosphorylation of proteins at the cytoplasmic face of focal
adhesions by the tyrosine kinase,Src.
Focal adhesion kinase (FAK) is an effector in integrin-mediated
responses.
Another signal pathway activated by integrin engagement leads to
the protein-synthesizing machinery of the cytoplasm.
1.The mechanical-
structural function of
focal adhesion,which
is carried out by actin
filaments and
associated proteins.
2,Signaling function
from extracellular
surface?nucleus
(where they stimulate
the transcription of
genes involved in cell
growth and
proliferation),which
is carried out by
tyrosine kinase (Src
and FAK)
Controlling the assembly of focal adhesions
Signal pathway
that lead to the
assembly of the
fibers of a focal
adhesion.
Rho is involved
in regulating the
organization of
the cell’s actin
cytoskeleton.
5,Convergence,divergence,and crosstalk
among different signaling pathway
A,Convergence:
Signals from a
variety of unrelated
receptors can
converge to activate
a common effector.
B,Divergence,Signals from the same ligand can
diverge to activate a variety of different effectors.
C,Crosstalk,Signals can be passed back and
forth between different pathways
6,In actual fact,signaling pathways in the
cell are much more complex.
