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13 The Plasma Membrane
Chapt. 13 Student learning outcomes:
• Diagram structure, explain function of lipid bilayer
plasma membrane: types of phospholipids, proteins
• Explain mechanisms for transport of small molecules:
• Passive transport
• Active transport
• Describe uptake of
larger molecules:
• Phagocytosis
• Receptor-mediated endocytosis
Fig 13.2 Lipid components of plasma membrane
Bilayers are viscous fluids, not solid
Asymmetric plasma membrane phospholipids: (Table 1)
• Outer leaflet — phosphatidylcholine (37%), sphingomyelin.
•
Inner leaflet — phosphatidylethanolamine,
phosphatidylserine, phosphatidylinositol.
Glycolipids — only in outer leaflet, carbohydrate on surface.
Cholesterol — lot
Bilayer structure of the plasma membrane
Cells are surrounded by plasma membrane:
• separates cell from environment
• selective barrier, mediates interactions with environment.
Fundamental structure is phospholipid bilayer:
• Proteins embedded in bilayer carry out functions:
Mammalian red blood cells (erythrocytes) good model
• no nuclei or internal membranes,
• easy to isolate pure plasma membranes.
Fig. 13.1
.
Polar head groups: dark lines because
bind electron-dense metal stains.
Hydrophobic fatty acid chains in
center are lightly stained
Figure 13.4 Visualization of lipid rafts in plasma membrane
Cholesterol and sphingolipids (sphingomyelin and
glycolipids) cluster in small patches (lipid rafts)
• Rafts highly-ordered versus phospholipid bilayer.
• Sphingolipids - different melting temperatures than
phospholipids derived from glycerol.
• Visualize lipid rafts with fluorescent probe Laurdan, sensitive
to rigidity of phospholipid bilayer (false color: red = ordered).
Figs. 13.3,4:
• 23% epithelial cell
• Affects fluidity
Fig. 13.2: note negative
charge on PI, PS
1
Fluid mosaic model of plasma membrane
Solubilization of integral membrane proteins by detergents
Plasma membrane ~ 50% lipid, 50% protein by weight.
Integral membrane proteins insert into lipid bilayer;
• Proteins larger than lipids, → ~1 protein per 50–100 lipids
• Dissociate with reagents that disrupt hydrophobic interactions
– detergents
Detergents - amphipathic (both hydrophobic and hydrophilic):
solubilize integral, transmembrane proteins
ex. Octyl glucoside, sodium dodecyl sulfate (SDS)
Fluid mosaic model of membrane structure
Integral proteins (often transmembrane)
Peripheral proteins
Distinguish by ease of
disruption of protein
from membraneIntegral need detergents
Some integral anchored
behave as peripheral
Fig. 13.6:
detergents
solubilize integral
membrane
proteins
*Fig. 13.5: membrane proteins
Integral membrane proteins of red blood cells
Transmembrane proteins:
•
•
•
•
Membrane-spanning s α helices (~ 20 to 25 hydrophobic aa)
Inserted into ER membrane during synthesis (chapt. 10)
Carbohydrate groups added in ER and Golgi
Most are glycoproteins: oligosaccharides on cell surface
Ex. Red blood cell:
Glycophorin unknown function (131 aa; 16 chains carbohydrates)
Band 3 anion transporter for HCO3 – and Cl– ions
Bacterial outer membranes
Porins: Transmembrane proteins in outer membrane
of bacteria such as E. coli. (Gram-negative)
• Porins cross membrane as β barrels.
• Very permeable to ions, small polar molecules
• Porins also in outer membrane of mitochondria and
chloroplasts
(929 aa; dimer).
Fig. 13.8: red blood
cell transmembrane
Fig. 13.10: bacterial
porins
2
Proteins anchored in plasma membrane by lipids and glycolipids
Outer leaflet has proteins anchored by glycosylphosphatidylinositol (GPI) anchors on C terminus.
• Proteins glycosylated in ER, Golgi, exposed on cell surface
• In lipid rafts
Inner leaflet has proteins anchored
by covalently attached lipids.
• From free ribosomes, modified by
myristic acid, prenyl, palmitic acid.
• Often roles in signal transmission
(ex. Src and Ras)
• May behave as peripheral proteins
Fig. 13.11: membrane-anchored proteins
See also Figs. 10.17, 8.33, 8.36
Mobility of membrane proteins
Lateral movement of proteins and lipids
in membrane:
• First demonstrated in 1970.
• Fused human and mouse cells in culture
• Analyzed membrane proteins
using fluorescent antibodies
Some membrane proteins have
restricted movement:
•
•
•
•
bound to cytoskeleton,
other membrane proteins
proteins on adjacent cells,
extracellular matrix.
Fig. 13.12:
A polarized intestinal epithelial cell
EX. Polarized epithelial cell plasma membranes divided
into apical and basolateral domains.
Small intestine:
• apical surface covered by microvilli –
increase surface area for absorption.
• basolateral surface mediates
transfer of nutrients to blood
Tight junctions restrict movement
of proteins between domains
(tighter than adherens junctions)
(Fig. 14.26, 27
Fig. 12.16)
Structure of the Plasma Membrane
Lipid composition can affect protein movement.
• GPI-anchored proteins cluster in lipid rafts.
• Proteins may move in and out of rafts, facilitating processes
such as cell movement, cell signalling.
Figs. 13.3, 13.11
Fig. 13.13
3
The glycocalyx
Binding of selectins to oligosaccharides
Glycocalyx: coat of carbohydrates of glycolipids,
glycoproteins on outer face of plasma membrane:
Glycocalyx oligosaccharides participate in cell-cell
interactions.
• Protects cell from ionic and mechanical stress
• Barrier to invading microorganisms
• Cell-cell recognition
Ex. White blood cells (leukocytes) adhere to endothelial cells
lining blood vessels
• leave circulatory system
• mediate inflammatory responses.
• adhesion involves transmembrane
proteins - selectins
• Selectins bind sugars
(details Ch. 14)
Figs. 13.14 Intestinal
epithelium
Membrane Transport: Permeability of phospholipid bilayers
2. Selective permeability of plasma membrane:
Maintains internal composition of cell
Passive diffusion:
• Molecules dissolve in phospholipid bilayer, diffuse across
• Direction of transport from high concentration to low
• Small, relatively hydrophobic molecules passively diffuse
Figs. 13.15 selectins bind
glycosylated transmembrane proteins
Transport of Small Molecules
Facilitated diffusion:
• Concentration gradients control direction
of movement (from high to low).
• Transmembrane proteins take polar and
charged molecules across membrane
• Carrier proteins bind molecules on one
side; undergo conformational change to
allow molecule to pass through
membrane to other side.
Figs. 13.15 permeability
See also Fig. 2.27
• Channel proteins form open pores
through membrane; allow free diffusion of
molecule of appropriate size and charge.
Fig. 2.28
4
Structure of glucose transporter
Model for facilitated diffusion of glucose
*Carrier proteins
Glucose transporter alternates between two
conformational states:
Facilitated diffusion: sugars, amino acids, nucleosides.
• Glucose transporter has 12 α-helical
transmembrane segments (typical carrier protein).
• Once glucose is taken up, it is rapidly metabolized
• Intracellular glucose concentration remains low;
• Glucose continues to be transported into cell.
Figs. 13.17: purple = polar
residues in lipid bilayer; bind
glucose
See Fig. 2.28 also
[Glucose can be
transported in
opposite direction]
Fig. 13.18 carriers
See Fig. 2.28 also
Channel Protein: Structure of an aquaporin
Transport Small Molecules
Channel proteins
• Open pores in membrane, molecules pass freely
• Porins, Gap junctions (also Ch. 14)
Aquaporins:
• H2O molecules cross membrane
more rapidly than diffusion
through phospholipid bilayer;
• Impermeable to charged ions
Fig. 13.19 aquaporin
Red = water molecules
See Fig. 2.28 also
Ion channels:
Well-studied in nerve
and muscle cells;
Fig. 13.20
• Open, close for transmission of electric signals.
• Transport extremely rapid: >million ions per second.
• Ion channels very selective; different channel
proteins allow passage of Na+, K+, Ca2+, and Cl–
• Most have “gates” (open if specific stimuli).
• Ligand-gated channels open in response to binding
of neurotransmitters or other signaling molecules.
• Voltage-gated channels open in response to
changes in electric potential across membrane.
5
study ion channels: the patch clamp technique
Role of ion channels in transmitting electric impulses elucidated
using giant squid axons (Hodgkin and Huxley,1952).
Electrodes inserted in axon measured changes in membrane
potential, from opening, closing of Na+ and K+ channels
Patch clamp technique:
• Study of activity of individual ion channels
• Micropipette isolates small patch of membrane, allows
analysis of ion flow through single channel
Fig. 13.20 Patch Clamp;
(Neher and
Sakmann,1976)
Ion gradients, resting membrane potential of giant squid axon
Ions are electrically charged → transport results in
electric gradient across membrane.
Resting squid axons have electric potential ~ -60 mV:
inside of cell is negative with respect to outside
• Potential arises from ion pumps (Na+/K+ pump)
& open membrane channels
• Resting membrane
has open K+ channels,
so flow of K+ (out) makes
largest contribution
to resting potential.
Fig. 13.22* squid axon
resting potential
Transport of Small Molecules
Ion pumps
• Use energy from ATP
hydrolysis to actively
transport ions across
plasma membrane to
maintain concentration
gradients (Na+/K+ pump)
• Ionic composition of
cytoplasm very different
from extracellular fluids.
Membrane potential and ion channels during action potential
As nerve impulses (action potentials) travel along
axons, membrane depolarizes:
• Membrane potential changes from
–60 mV to +30 mV in < 1 msec
• Rapid sequential opening, closing
of voltage-gated Na+ and K+ channels
propagates impulse along axon.
Fig. 13.23
action
potential
6
Fig 13.24 Signaling by neurotransmitter release at synapse
Model of the nicotinic acetylcholine receptor
• Depolarization of adjacent regions of plasma membrane
(voltage-gated Na+/ K+ channels) allows action potentials to
travel length of nerve cell.
Ligand-gated ion channel
• At nerve end, chemical neurotransmitters release
into synapse, bind receptors on another cell to open
ligand-gated ion channels
• 5 subunits in cylinder
• Closed, pore is blocked by side
chains of hydrophobic amino acids.
• Acetylcholine binding induces
conformational change, hydrophobic
side chains shift out of channel,
opens a pore for positive ions
(negative aa line channel)
ex. Acetylcholine release binds channel protein on muscle cell:
(Na+ enters; opens
voltage-gated Ca2+ channel,
activates myosin binding
to actin, contraction
(Fig. 12.28)
Fig. 13.24*
neurotransmitter
Ion selectivity of Na+ channels
Voltage-gated Na+ and K+ channels very selective.
• Na+ (0.95 Å) is smaller than K+ (1.33 Å),
• Na+ channel pore is too narrow for K+ or larger ions
Fig. 13.26 Selectivity of Na+ channel
Ex. Nicotinic acetylcholine receptor:
Fig. 13.25 nicotinic acetylcholine receptor:
Nicotine keeps channel open;
Neurotoxins block receptor (curare)
Selectivity of K+ channels
Voltage-gated K+ channels
• Part of channel pore lined with carbonyl oxygen
(C=O) atoms from polypeptide backbone; displace
water to which K+ is bound, and K+ goes through.
• Na+ too small to interact, remains bound to water.
Fig. 13.27 Selectivity of K+ channel
(3-D structure of by X-ray crystallography).
7
Transport of Small Molecules
Voltage-gated
Na+,
K +,
Transport of Small Molecules
and
Ca2+
channels:
Family of related proteins
1 protein each
K+ channel 4 subunits
α-helix 4 has positive aa,
senses voltage
** Active transport
• Molecules transported against
concentration gradients.
• Energy provided by
coupled reaction (ATP hydrolysis)
Ion pumps examples:
• Na+-K+ pump (Na+-K+ ATPase)
uses energy from ATP hydrolysis
to transport 3 Na+ and 2 K+
against electrochemical gradient
Fig. 13.28
Fig. 13.29 α subunit blue;
β green; γ (red) regulatory;
Fig 13.30 Model for operation of the Na+-K+ pump
Na+-K+
pump uses ATP-driven conformational change:
• 3 Na+ transported out of cell and 2 K+ transported into
cell for every ATP used
Na+-K+ pump uses 25% of ATP in many animal cells.
Ion gradients across plasma membrane of typical mammalian cell
Differences in ion concentrations:
• balance high concentrations of organic molecules
inside cells,
• equalize osmotic pressure, prevent net influx of H2O
gradients necessary:
• for propagation of
electric signals in nerve
and muscle cells
• active transport of
molecules
Fig. 13.31 mammalian cell
ionic composition
• maintain osmotic
balance and cell volume.
Fig. 13.30
8
Structure of the Ca 2+ pump
Ca2+ pump structurally related
•
to Na+-K+ pump, also powered
by ATP hydrolysis
H+ Ion pumps in bacteria, yeasts, and plants
actively transport H+ out of cell
Ca2+
• H+ pumped out of cells lining stomach, → acid gastric fluids.
• Structurally distinct pumps actively transport H+ into lysosomes
and endosomes.
transported out of cell or
into ER lumen, so intracellular
Ca2+ concentrations are
extremely low (0.1 µM)
• Transient, localized increases
in intracellular Ca2+ important in
cell signaling (muscle contraction)
Fig. 13.32 3 cytosolic subunits;
3 transmembrane subunits
Structure of an ABC transporter
ABC transporters: highly conserved ATP-binding
domains or ATP-binding cassettes.
•
•
•
•
Transport of Small Molecules
More than 100 members of family; prokaryotic, eukaryotic
ATP hydrolysis transports molecules in one direction.
Bacteria transport nutrients in: ions, sugars, amino acids.
Eukaryotes transport toxic substances out of cell:
Ex. products of mdr (multidrug resistance) genes.
– Often expressed high
levels in cancer cells
– Can remove variety of
chemotherapy drugs.
Fig. 2.29 H+ pump
in bacteria
ATP synthases of mitochondria and chloroplasts also H+ pumps:
• pumps operate in reverse, movement of ions down
electrochemical gradient drives ATP synthesis.
cystic fibrosis transmembrane conductance regulator (CFTR)
Cystic fibrosis (CFTR protein):
• Defective Cl– transport in epithelial cells → very
thick, sticky mucus, obstructs respiratory passages
• Protein CFTR
(cystic fibrosis transmembrane
conductance regulator)
in ABC transporter family
• Autosomal recessive disease:
• Mutant protein not fold properly
• Gene therapy targets
Fig. 13.33 ABC
transporters
9
Fig 13.34 Active transport of glucose
Active transport of glucose can be driven by Na+
gradient (or H+ gradient in prokaryotes).
• Glucose transporters in apical domain of intestine epithelials
transport 2 Na+ and 1 glucose into cell.
• Flow of Na+ down electrochemical gradient provides energy
for transport , accumulation of high intracellular glucose
concentrations.
[Cell will need to pay ATP
to pump out those Na+
(Na+/K+ pump)]
Fig. 13.34 use of ion
gradient to drive
accumulation of nutrient
Examples of antiport
Antiport : 2 molecules transported opposite directions.
Ca2+
•
is exported from cells by
Ca2+ pump, Na+-Ca2+ antiporter
that transports Na+ into cell
and Ca2+ out.
• Na+-H+ exchange protein
helps regulate intracellular pH.
Fig. 13.36 antiport
Two kinds of
Glucose transport by intestinal epithelial cells
Apical domain uses active
uptake of glucose by
cotransport with Na+
(symport)
Basolateral domain: glucose is
transferred to underlying tissue
by facilitated diffusion
(uniport)
System is driven by Na+-K+
pump, also found in
basolateral domain.
Fig. 13.35 uptake glucose by
intestinal epithelial, transfer to blood
Phagocytosis
3. Endocytosis: cells take up
macromolecules, fluids, large
particles such as bacteria.
• Material is surrounded by plasma
membrane, which buds off inside cell to
form vesicle with ingested material
• Pinocytosis (cell drinking):
property of all eukaryotic cells.
• Phagocytosis (cell eating)
occurs in specialized cell types:
– Phagosome vesicle formed from
pseudopodia after bind particle
– Phagolysosome – after fuse with
lysosome
Fig. 13.37 endocytosis
10
Examples of phagocytic cells
Amoebas use phagocytosis to capture food particles
such as bacteria (role of Actin)
Multicellular animals use phagocytosis as defense
against invading microorganisms, to eliminate aged
or damaged cells.
Macrophages and neutrophils (white blood cells)
are “professional phagocytes.”
Endocytosis
* Receptor-mediated endocytosis: mechanism
for selective uptake of specific macromolecules.
• Cell surface receptors in regions (clathrin-coated pits).
• Pits bud as clathrin-coated vesicles, fuse with early endosome
• Dynamin, GTP help
Fig. 13.38 Phagocytosis:
A, amoeba eating protist;B,
macrophage and red blood cells
Fig. 13.39 Receptor-mediated
endocytosis; see also Fig. 10.37
for clathrin and lysosomal proteins
The LDL receptor
Endocytosis
Receptor-mediated endocytosis first elucidated in
patients with familial hypercholesterolemia
• they have defective LDL receptor (LDLR gene):
• not bind LDL, or not internalize LDL after bound
• Cholesterol transported through bloodstream mostly
in form of low-density lipoprotein, or LDL particle
• 1500 cholesteryl esters
• 500 cholesterols
• 800 phospholipids
• 1 protein (apoprotein B100)
Mutant LDL receptors that do not bind in coated pits have
altered internalization signal in cytoplasmic tail of receptor.
Internalization signal is 6 aa, including tyr (-> cys in mutant).
[Similar internalization signals are found in other receptors taken
up via clathrin-coated pits]
Fig. 13.41
Fig. 13.40 LDL particle
11
Formation of clathrin-coated pits
• Internalization signals bind adaptor
proteins, which bind clathrin.
• Clathrin assembles into basketlike
structure, forms invaginated pits.
• Dynamin-GTP releases
coated vesicles into cell
• Clathrin-coated pits occupy to 2%
of cell surface; lifetime 1 to 2 minutes
Endocytosis
Early endosomes acidic pH
Figs.
13.42,44
• After internalization, clathrin-coated
vesicles shed coats, fuse with early
endosomes
• Molecules sorted, recycled to
plasma membrane, or remain in
early endosomes, mature to late
endosomes and lysosomes
(6.0 to 6.2), (membrane H+ pump)
• Dissociates many ligands
from receptors.
• Receptors return to plasma
membrane via transport vesicles.
• Ligands (LDL) remain,
degraded to release cholesterol
• Late endosomes more acidic
(pH 5.5 to 6.0);
• mature into lysosomes (pH 5) :
endocytosed materials are
degraded by acid hydrolases.
Fig. 13.44
Review questions
Endocytosis
In nerve cells, after synaptic vesicles release
neurotransmitters, are recovered in clathrin-coated
vesicles which fuse with early endosomes.
• Synaptic vesicles
regenerated by budding
from endosomes.
Fig. 13.45
7. Curare binds nicotinic acetylcholine receptors and prevents
them from opening; how would it affect contraction of muscles?
9. How can glucose be transported against its concentration
gradient without the direct expenditure of ATP in intestinal
epithelial cells?
10. How does the mdr gene confer drug resistance upon cancer
cells?
12. What have studies on cells from children with familial
hypercholesterolemia told us about the mechanisms of
receptor-mediated endocytosis?
12