10/5/2011
MEMBRANE TRANSPORT
MEMBRANE TRANSPORT


Expected Learning Outcomes



Explain what is meant by a selectively permeable
membrane.
Describe the various mechanisms for transporting material
through the plasma membrane.
membrane
Define osmolarity and tonicity and explain their importance.
Plasma membrane—a barrier and a gateway between the
cytoplasm and ECF



Passive transport mechanisms require no ATP

Random molecular motion of particles provides the necessary energy

Filtration, diffusion, osmosis
Active transport mechanisms consumes ATP


Selectively permeable—allows some things through, and prevents other
things from entering and leaving the cell
Active transport and vesicular transport
Carrier-mediated mechanisms use a membrane protein to
transport substances from one side of the membrane to the
other
3-1
3-2
FILTRATION


SIMPLE DIFFUSION
Filtration—process in which
particles are driven through
a selectively permeable
membrane by hydrostatic
pressure (force exerted
on a membrane by water)
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Solute
Water
Capillary wall
Filtration of nutrients through
gaps in blood capillary walls
into tissue fluids

Filtration of wastes from the
blood in the kidneys while
holding back blood cells and
proteins
Simple diffusion—the net
movement of particles from
area of high concentration to
area of low concentration

Red blood
cell
Examples


Blood pressure in capillary
forces water and small
solutes such as salts through
narrow clefts between
capillary cells.
Clefts hold back
larger particles
such as red blood
cells.

Figure 3.13
Down
gradient
Due to their constant,
spontaneous
t
motion
ti
Also known as movement
down the concentration
gradient—concentration of a
substance differs from one
point to another
Up
gradient
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Figure 3.14
3-3
3-4
SIMPLE DIFFUSION

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Factors affecting diffusion rate through a membrane

Temperature:  temp.,  motion of particles

Molecular weight: larger molecules move slower

Steepness of concentrated gradient: difference,
difference  rate

Membrane surface area:  area,  rate

Membrane permeability:  permeability,  rate
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10/5/2011
OSMOSIS
SIMPLE DIFFUSION
Osmosis—flow of water from
one side of a selectively
permeable membrane to the
other
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
Diffusion through lipid bilayer
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

Diffusion through channel proteins


Nonpolar, hydrophobic, lipid-soluble substances diffuse
through lipid layer
Water and charged, hydrophilic solutes diffuse through
channel proteins in membrane

Cells control permeability by regulating number of
channel proteins or by opening and closing gates

From side with higher water
concentration to side with lower
water concentration
Reversible attraction of water to
solute particles forms hydration
spheres
Makes those water molecules
less available to diffuse back to
the side from which they came
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Side A
Side B
Solute
Water
(a) Start
3-7
3-8
OSMOSIS
Aquaporins—channel proteins in
plasma membrane specialized for
passage of water

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

Cells can increase the rate of
osmosis by installing more
aquaporins
Decrease rate by removing them
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Side B
Solute
Water
(a) Start
Significant amounts of water
diffuse even through the
hydrophobic, phospholipid
regions of the plasma membrane

Side A
Figure 3.15a
3-10
OSMOLARITY AND TONICITY
OSMOSIS
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

Osmotic pressure—amount
of hydrostatic pressure
required to stop osmosis
Reverse osmosis—pressure
applied to one side,
overrides pressure, drives
against concentration
gradient

Heart drives water out of
capillaries by reverse
osmosis—capillary
filtration
Tonicity—ability of a solution to affect fluid volume and pressure in a
cell

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
Osmotic
pressure
Hydrostatic
pressure
Depends on concentration and permeability of solute
Hypotonic solution

Has a lower concentration of nonpermeating solutes than
intracellular fluid (ICF)

Cells absorb water, swell, and may burst (lyse)


(b) 30 minutes later
Hypertonic solution

Has a higher concentration of nonpermeating solutes

Cells lose water + shrivel (crenate)

Figure 3.15b

Low water concentration
Isotonic solution


3-11
High water concentration

Concentrations in cell and ICF are the same
Cause no changes in cell volume or cell shape
Normal saline
3-12
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10/5/2011
EFFECTS OF TONICITY ON RBCS
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(a) Hypotonic
(b) Isotonic
(c) Hypertonic
© Dr. David M. Phillips/Visuals Unlimited
Figure 3.16a
Figure 3.16b
Figure 3.16c
Hypotonic, isotonic, and hypertonic solutions affect the fluid volume of
a red blood cell. Notice the crenated and swollen cells.
3-13
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
Transport proteins in the plasma membrane that carry
solutes from one side of the membrane to the other
Specificity



Transport proteins specific for a certain ligand
Solute binds to a specific receptor site on carrier protein
Differs from membrane enzymes because carriers do not
chemicallyy change
g their ligand
g


Simply picks them up on one side of the membrane, and releases
them, unchanged, on the other
Saturation


CARRIER-MEDIATED TRANSPORT
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Rate of so
olute transport
(molecule
es/sec passing
through plassma membrane)
CARRIER-MEDIATED TRANSPORT
As the solute concentration rises, the rate of transport rises,
but only to a point—transport maximum (Tm)
Figure 3.17
Two types of carrier-mediated transport

Transport maximum (Tm)
Concentration of solute
Facilitated diffusion and active transport

3-15
Transport maximum—transport rate when all carriers
are occupied
3-16
CARRIER-MEDIATED TRANSPORT

Uniport

Symport



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Carries two or more solutes simultaneously in same direction
(cotransport)
Antiport



Carries only one solute at a time
Carries two or more solutes in opposite directions
(countertransport)
Sodium-potassium pump brings in K+ and removes Na+ from cell
Carriers employ two methods of transport


Facilitated diffusion
Active transport
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10/5/2011
CARRIER-MEDIATED TRANSPORT

Facilitated diffusion—carrier-mediated transport of
solute through a membrane down its concentration
gradient

Does not consume ATP

Solute attaches to binding site on carrier, carrier
changes confirmation,
confirmation then releases solute on other
side of membrane
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ECF
Figure 3.18
ICF
1 A solute particle enters
the channel of a membrane
protein (carrier).
2 The solute binds to a receptor
site on the carrier and the
carrier changes conformation.
3 The carrier releases the
solute on the other side of
the membrane.
3-19
CARRIER-MEDIATED TRANSPORT
CARRIER-MEDIATED TRANSPORT
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


Active transport—carrier-mediated transport of solute
through a membrane up (against) its concentration
gradient
ATP energy consumed to change carrier
Examples of uses:




Each pump cycle consumes one ATP
and exchanges three Na+ for two K+
Glucose
Na+
Apical surface

Keeps the K+ concentration higher
and the Na+ concentration lower
within the cell than in ECF

Necessary because Na+ and K+
constantly leak through membrane
SGLT
Cytoplasm
Sodium–potassium pump keeps K+ concentration higher
inside the cell
Bring amino acids into cell
Pump Ca2+ out of cell

Half of daily calories utilized for
Na+−K+ pump
Na+– K+
pump
ATP
ADP + Pi
Basal surface
Na+
3-22
3-21
CARRIER-MEDIATED TRANSPORT

Secondary active transport
Steep concentration gradient maintained between one
side of the membrane and the other (water behind a
dam)
 Sodium–glucose transport protein (SGLT) simultaneously
binds Na+ and glucose and carries both into the cell
 Does not consume ATP

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
Regulation of cell volume


“Fixed anions” attract cations causing osmosis
Cell swelling stimulates the Na+−K+ pump to
 ion concentration,  osmolarity and cell swelling
3-24
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10/5/2011
VESICULAR TRANSPORT
CARRIER-MEDIATED TRANSPORT

Vesicular transport—processes that move large particles, fluid
droplets, or numerous molecules at once through the membrane
in vesicles—bubblelike enclosures of membrane

Endocytosis—vesicular processes that bring material into the cell
Maintenance of a membrane potential in all cells
Pump keeps inside more negative, outside more
positive
 Necessary for nerve and muscle function




Phagocytosis—“cell eating,” engulfing large particles

Pinocytosis—“cell
drinking,”
of ECF containingg
y
g, takingg in droplets
p
molecules useful in the cell

Receptor-mediated endocytosis—particles bind to specific
receptors on plasma membrane

Heat production

Na+−K+
Thyroid hormone increases number of
pumps
 Consume ATP and produce heat as a by-product




Pseudopods; phagosomes; macrophages
Pinocytic vesicle
Clathrin-coated vesicle
Exocytosis—discharging material from the cell
Utilizes motor proteins energized by ATP
3-25
3-26
VESICULAR TRANSPORT
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Particle
7 The indigestible
residue is voided by
exocytosis.
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1 A phagocytic cell encounters a
particle of foreign matter.
Pseudopod
Residue
2 The cell surrounds
the particle with its
pseudopods.
Nucleus
Phagosome
g
6 The phagolysosome
fuses with the
plasma membrane.
3 The particle is phagocytized
and contained in a
phagosome.
Lysosome
Vesicle fusing
with membrane
Phagolysosome
5 Enzymes from the
lysosome digest the
foreign matter.
4 The phagosome fuses
with a lysosome and
becomes a phagolysosome.
3-28
Phagocytosis keeps tissues free of debris and infectious microorganisms
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10/5/2011
VESICULAR TRANSPORT
VESICULAR TRANSPORT





Taking in droplets of ECF
Occurs in all human cells



Receptor-mediated endocytosis
Pinocytosis or “cell-drinking”
More selective endocytosis
Enables cells to take in specific molecules that bind to
extracellular receptors
Clathrin-coated vesicle in cytoplasm
Membrane caves in, then pinches off into the
cytoplasm as pinocytotic vesicle

Uptake of LDL from bloodstream

Familial Hypercholesterolemia
Receptor-mediated endocytosis
3-31
VESICULAR TRANSPORT
3-32
VESICULAR TRANSPORT
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Capillary endothelial cell

Intercellular cleft
Exocytosis
Capillary lumen


Pinocytotic vesicles
Secreting material
Replacement of plasma membrane removed by endocytosis
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Muscle cell
Fusion pore
Dimple
Tissue fluid
Secretion
Plasma
membrane
Linking
protein
© Don Fawcett/Photo Researchers, Inc.


0.25 m
Transport of material across the cell by capturing it on one side
and releasing it on the other
Receptor-mediated endocytosis moves it into the cell and
exocytosis moves it out the other side

Insulin
3-33
Secretory
vesicle
(a)
1
2
(b)
3-34
6