Download Transport Across The Cell Membrane

Anatomy & Physiology
101-805
Unit 4
Transport Across
The Cell Membrane
Paul Anderson
2017
METHODS OF MEMBRANE TRANSPORT
• Simple Diffusion
• Facilitated Diffusion
• Osmosis
Passive Transport
No ATP needed
• Dialysis
• Active Transport
• Vesicle Transport
(Exo & Endocytosis)
ATP required
Simple Diffusion
• A sample of dye dissolves in water then spreads by simple
diffusion throughout the water.
• Simple Diffusion is the tendency for molecules or ions to
randomly distribute because of their kinetic (heat) energy.
Martini & Bartholomew Figure 3.4
Stages of Simple Diffusion-1 • SIMPLE DIFFUSION of molecules/ions occurs in all directions.
• NET DIFFUSION is the overall movement down a
CONCENTRATION GRADIENT.
STAGE 1
Maximum Concentration Gradient & Net Diffusion
A
A
B
A
B = 4 - 0 = 4 mols/t
Net Diffusion
Maximum
rate of net
diffusion
Maximum
Concentration
gradient
B
Stages of Simple Diffusion-2 Reduced CONCENTRATION GRADIENT
causes a reduced rate of NET DIFFUSION
STAGE 2
Reduced Concentration Gradient & Net Diffusion
A
A
A
B
B = 3 - 1 = 2 mols/t
Net Diffusion
Reduced rate of
net diffusion
Reduced
Concentration
gradient
B
Stages of Simple Diffusion-3 LAW OF DIFFUSION
Substances show a NET DIFFUSION down their CONCENTRATION
GRADIENT until they reach a DIFFUSION EQUILIBRIUM
STAGE 3
Diffusion Equilibrium
No Net Diffusion
A
A
A
B
B
No Concentration
Gradient
B = 2 - 2 = 0 mols/t
• Net Diffusion ceases but
• Simple Diffusion continues to maintain a dynamic equilibrium.
Summary of Simple Diffusion
•  Simple Diffusion is the tendency for molecules or ions to randomly
distribute because of their kinetic (heat) energy.
•  Simple Diffusion occurs in all directions.
•  in Simple Diffusion substances obey the Law of Diffusion:
•  Simple Diffusion does not require a membrane or a carrier.
•  Simple Diffusion does not use ATP so is Passive Transport.
Molecules
Simple Diffusion through a membrane
Membrane
Concentration
Gradient
Net Diffusion
Reduced
Concentration
Gradient
Reduced Net Diffusion
Diffusion Equilibrium
Simple Diffusion across the Cell Membrane
Cell Membrane
SIMPLE DIFFUSION occurs across the cell
membrane in two ways:
• HYDROPHOBIC MOLECULES diffuse through
the membrane lipids, e.g. O2, CO2, lipids.
• SMALL HYDROPHILIC POLAR MOLECULES
or IONS diffuse through channels in
membrane proteins, e.g. H2O, Na+, K+, Cl-.
Concentration
gradient
Law of Diffusion & Gas Exchange
In body fluids each molecule or ion shows a NET
DIFFUSION down its own concentration gradient
until it reaches a DIFFUSION EQUILIBRIUM.
In the lungs
• O2 shows a net
diffusion into
blood from alveoli.
• CO2 shows a net
diffusion out of
blood into alveoli.
O2
CO2
Conc
gradient
air
Air
sac& Reece fig 7.11
Campbell
(alveolus)
blood
Pulmonary
capillary
Glucose Enters Cells by Facilitated Diffusion
• FACILITATED DIFFUSION is the diffusion of molecules cross the cell
membrane using a specific membrane CARRIER PROTEIN.
• Glucose enters most cells from the IF by this method.
• The rate of Facilitated Diffusion depends on the number of carriers.
• Insulin increases the number of glucose carriers in target cells which
increases the rate of glucose uptake.
Glucose molecule
attaches to
receptor site
Receptor site
INTERSTITIAL
FLUID
Carrier
protein
Martini & Bartholomew Figure 3.8
Change in shape
of carrier
protein
CYTOPLASM
Glucose released
into cytoplasm
Facilitated Diffusion: Carrier Mediated Transport
Cell Membrane
Carrier
protein
Carrier
protein
Carrier
protein
Carrier
protein
Net diffusion down
Concentration gradient
• Unlike simple diffusion Facilitated
Diffusion only occurs across cell
membranes and requires a
membrane carrier protein.
• Molecules crossing by Facilitated
Diffusion are transported using
kinetic (heat) energy so they obey
the LAW of DIFFUSION.
• Facilitated Diffusion does NOT
use ATP so is PASSIVE
TRANSPORT.
Cell Membrane
ATP
Carrier protein
ATP
Carrier protein
Active Transport
• Unlike Diffusion ACTIVE TRANSPORT
moves molecules or ions in one direction up
their concentration gradient opposing net
diffusion.
• Like Facilitated Diffusion Active
Transport requires a specific carrier
protein.
• The carrier is like an enzyme but
catalyses the physical process of
membrane transport.
• Unlike Diffusion Active Transport is a
form of biological work and so requires
energy in the form of ATP.
Concentration
gradient
• Energy from ATP is used to change the
shape of the carrier so that the
molecules/ions can only be transported
up their concentration gradient .
Comparison of 3 Methods of Membrane Transport
Channel
protein
Carrier
protein
ATP
Cell Membrane
Simple Diffusion
Facilitated Diffusion
Active Transport
Three Examples of Active Transport in the Body
1.  The Na+/K+ pump is a carrier protein in all body cells
that moves Na+ out of cells and moves K + into cells.
2.  Amino acids enter all cells by active transport.
3.  Glucose enters intestinal cells from the GI tract by
secondary active transport (co-transported with Na)
indirectly dependent on the ATP- driven Na pump.
Intestinal lumen
Na+
Common carrier for
Na & glucose
Intestinal
epithelial
cell
Interstitial fluid
ATP
Na+
glucose
• Na moves into intestinal
cells by facilitated
diffusion down its
concentration gradient
created by the Na pump.
• Glucose moves up its
concentration gradient
by active transport
using the same carrier.
Na/K pump
The Na+/K+ Pump Moves Na Out & K Into Cells
The Na+/K+PUMP
is a protein
carrier which
moves Na+ out of
cells and
moves K+ into
cells
by ACTIVE
TRANSPORT
opposing net
diffusion for
each ion.
Na+
INTERSTITIAL
+
Na
FLUID
Na+3 Na+
Sodium–
Potassium
Exchange
Pump
2
K+
K+
K+
ATP
ADP
CYTOPLASM
Martini & Bartholomew, fig 3-9
Functions of The Na+/K+PUMP
The Na+/K+pump has two important functions.
1.  It concentrates Na+ in the ECF and
concentrates K+ in the ICF.
2.  It helps create a polarized membrane in
which the external cell surface is positively
charged with respect to the inside of the
membrane.
The Na+/K+ Pump Helps Create a Polarized Cell Membrane
• The Na/K pump moves 3 Na+ out of cells for every 2 K+ moved in
so adding more positive charge to the ECF than to the ICF.
• ACTIVE TRANSPORT of Na+, K+ helps create a POLARIZED
CELL.
•  In a polarized cell the exterior has a slight excess of positive
charge and the interior an excess of negative charge.
• This is called the cell’s Resting Membrane Potential, important for
nerve & muscle cell function.
K+
K+
ICF
--
Carrier
ATP Protein
Na K pump
Cell Membrane
+
+
+
+
+
Na+
Na+
Na+
ECF
Figure 3.5
Summary
of Methods
for Crossing Cell
Membrane
Hydrophobic molecules can
diffuse through membrane lipids.
INTERSTITIAL FLUID
cell membrane
INTRACELLULAR
FLUID
Steroids,
fats O2
CO2
• Glucose
• amino acids
• Na+ & K+ (via pump)
Hydrophilic molecules/ions
may cross membrane using a
membrane protein carrier.
Martini &
Bartholomew,
fig 3-5
Channel
protein
• Na+
• K+
• Cl• H2O
Small hydrophilic
Molecules/ions
can diffuse through
membrane channels.
Vesicle Transport
Large molecules or particles move in or out of cells via
Vesicle Transport.
- Endocytosis requires ATP: three types exist
1. Receptor Mediated Endocytosis: specific
uptake of large solutes & requires membrane
receptors.
2. Pinocytosis: non - specific uptake of large
solutes.
3. Phagocytosis: uptake of particles, e.g. bacteria,
by phagocytic cells of the immune system.
- Exocytosis: secretion of large molecules from the
cell via secretory vesicles.
Receptor-Mediated Endocytosis
• Although Cholesterol is hydrophobic it enters cells
from the IF by Receptor-Mediated Endocytosis.
• This ensures that cells can take in cholesterol even
if the ECF concentration is lower than the ICF.
Lipoprotein
with
cholesterol
Receptor protein
Martini & Bartholomew, fig 3-10
Definition of OSMOSIS
OSMOSIS is the simple diffusion of solvent (water)
molecules through a SEMIPERMEABLE MEMBRANE.
Martini & Bartholomew, fig 3-6
2 glucose
solutions
separated by a
semipermeable
(= selectively
permeable)
membrane
solvent (H2O)
can cross
membrane
A SEMIPERMEABLE
MEMBRANE is
selectively permeable to
the solvent (e.g. H2O) but
does not allow passage of
the solute, (e.g. sugar).
solute
(glucose)
cannot
cross
membrane
Concentrations of Solute & Solvent in a Solution
• A more concentrated solution (e.g. 5% glucose) has
• a greater solute concentration but
• a lower solvent concentration than a less concentrated
(i.e. more dilute) solution (e.g. 1% glucose).
Solution A
• 1% glucose
• Lower concentration
of solute (glucose)
• Greater concentration
of solvent (water)
A
Water
molecules
B
Glucose
molecules
Martini & Bartholomew, fig 3-6
Solution B
5% glucose
• Greater concentration
of solute (glucose)
• Lower concentration
of solvent (water)
Semipermeable
membrane
Solute Particles Lower the Concentration of Water
• Solute particles attract water molecules
forming hydration shells.
• This effectively reduces the concentration
of freely diffusible water.
H2O molecules form
hydration shell
around solute particle
Freely diffusible
solvent (H2O) can
cross membrane
Solute particle
Water molecule
Hydration shell
lowers freely
diffusible [H2O ]
H2O in hydration
shells cannot
cross membrane
NET OSMOSIS
NET OSMOSIS is the NET DIFFUSION of WATER (or
solvent) through a semi-permeable membrane
• from a region of HIGH H2O concentration to LOW H2O
concentration or
• from LOW SOLUTE concentration to a HIGH SOLUTE
concentration, until an OSMOTIC EQUILIBRIUM occurs.
Solution A (1%)
• Low [solute]
• High [solvent]
Solution B (5%)
• High [solute]
• Low [solvent]
A
B
Volume
increased
Volume
decreased
net
osmosis
3%
glucose
Martini & Bartholomew, fig 3-6
• At equilibrium, the
solvent & solute
concentrations on each
side of the membrane
are equal.
• Net Osmosis has caused
the volume of solution B
to increase and volume of
solution A to decrease.
3% glucose
OSMOTIC PRESSURE
• OSMOTIC PRESSURE (=osmotic potential) is a measure of the
ability of a given solution to attract H2O molecules by net osmosis.
• Therefore NET OSMOSIS will occur from a solution of lower
osmotic pressure into a solution of higher osmotic pressure until an
OSMOTIC EQUILIBRIUM is reached.
1%
glucose
3%
glucose
5%
glucose
Net
osmosis
1% glucose solution
• HIGH H2O concentration
• LOW SOLUTE concentration
• LOW OSMOTIC PRESSURE
time
net
osmosis
3%
glucose
Osmotic
equilibrium
5% glucose solution
• LOW H2O concentration
• HIGH SOLUTE concentration
• HIGH OSMOTIC PRESSURE
OSMOLAR CONCENTRATION
The most important variable influencing the osmotic
pressure of a solution is the total concentration of
solute particles, called the osmolar concentration.
Osmolar concentration
= molar concentration x particles per molecule.
For electrolytes Osmolar concentration
= molar concentration x # ions per molecule.
1 molar glucose → 1 osmolar (1 osmol/L)
1 molar NaCl → 2 osmolar (2 osmol/L) i.e. Na+ + Cl1 molar CaCl2 → 3 osmolar (3 osmol/L) i.e. Ca++ + 2Cl-
OSMOSIS
Summary
Osmosis
simple diffusion of solvent (water) molecules
through a SEMIPERMEABLE MEMBRANE.
Osmotic
Equilibrium
Solution 1
↑[H2O ]
↓[solute]
Solution 2
↓ [H2O ]
↑[solute]
hydration shell
lowers freely
diffusible [H2O ]
↓ Volume in
solution 1
Net osmosis
into solution 2
↑ Volume in
solution 2
------ Semipermeable Membrane
Allows diffusion of solvent H2O but NOT solute
Solute Particles
attract H2O molecules
forming hydration shells
OSMOSIS Summary -2
1
1%
glucose
solutions
2
1
Net osmosis
Osmotic
pressure
[solute]
[H2O]
≪
5%
glucose
solutions
3%
glucose
1
2
3%
glucose
Osmotic equilibrium
Osmotic
pressure
Osmotic
pressure
[solute]
[solute] =
[solute]
[H2O]
[H2O]
≫ [H2O]
In net osmosis H2O
molecules obey the
law of diffusion.
=
=
Osmotic
pressure
↓volume
↑volume
1
2
• OSMOTIC PRESSURE (osmotic potential) is a measure of
the ability of a given solution to attract H2O molecules.
• Osmotic pressure is determined by the total osmolar [solute].
Diffusion & Osmosis in the Body
• Simple Diffusion and Osmosis occur in
the body to transport molecules over
short distances within cells and across
membranes.
• Simple Diffusion and Osmosis therefore
occur across the Cell membrane and the
Capillary wall.
OSMOSIS across the Cell Membrane
•  The cell membrane is a semipermeable (=selectively permeable)
membrane since
•  although Na+ and K+ can freely diffuse through the cell
membrane, active transport pumps them in the opposite
direction i.e. up their concentration gradients.
•  The cell membrane thus acts as if it were impermeable to these
ions, to Cl- which follows Na+ and to large organic molecules (e.g.
proteins) which cannot diffuse across the membrane due to their
size and charge.
Cl+
Na
Net Diffusion
+
Cl
K
K+
K+
ICF
K+
Protein anions
H2 O
Na+
H2 O
K+
Na+
Na+
Cell
membrane
IF
Cl-
ClClNa+
Cl-
Active Transport
Osmotic Equilibrium
Osmosis Across The Cell Membrane
•  Na+, K+, and proteins are “non-penetrating”
solutes for the cell membrane.
•  Any change in the concentrations of any of
these non-penetrating solutes results in net
osmosis across the cell membrane and a change
in cell volume.
Effect of Hypertonic Solutions on the Cell & ECF
•  Raising the extracellular [Na+] (sodium retention by the body) raises
the extracellular osmotic pressure and causes net osmosis out of cells
(cellular dehydration) & cellular shrinking (CRENATION).
•  The ECF in this case is HYPERTONIC to cells and since it has a
higher osmotic pressure than the ICF is HYPEROSMOTIC to the ICF.
Hypernatremia
↓ ICF vol
↑ ECF vol
crenation
(= 0.2 M)
Cell shrinks
↓ ICF vol
Osmolarity of ICF
= 0.3 osmoles/L
net osmosis out of cells
Solution is
hypertonic
edema
↑ ABP
Hypertonic
ECF
↓ [H2O ]
↑ [solute]
↑ OP
Effect OSMOSIS
of Hypotonic
across
Solutions
the Cell
onMembrane
the Cell & -ECF
2
•  Lowering the extracellular [Na+] lowers the extracellular osmotic
pressure which causes net osmosis into the cells (cellular
overhydration) and the cell membrane may burst (HEMOLYSIS).
•  The ECF in this case is HYPOTONIC and HYPOOSMOTIC to the ICF.
Hyponatremia
↑ ICF vol
↓ ECF vol
hemolysis
↓ ABP
Cell expands
(= 0.1 M)
Hypotonic
ECF
↑ [H2O ]
net osmosis
into cells
Solution is
hypotonic
↓[solute]
↓ OP
Effect
OSMOSIS
of Isotonic
across
Solutions
the Cell
on Membrane
the Cell & -ECF
3
•  A solution in which the extracellular osmolar [Na+] is equal to the
intracellular osmolar [non -diffusible solutes] will be in OSMOTIC
EQUILIBRIUM, cause no change in cell volume and is said to be
ISOTONIC.
•  The ECF in this case has the same osmotic pressure as the ICF so
is ISOSMOTIC to the ICF.
0.15M NaCl
8.7g/L
0.87 g%
Isotonic saline
(= 0.15 M)
Normal RBC
osmotic equilibrium
iso
Summary of OSMOSIS & RED BLOOD CELLS
CELLS are normally in OSMOTIC EQUILIBRIUM
with the ECF, i.e. the ICF & ECF are ISOTONIC.
If blood cells are exposed to non - isotonic fluids net
osmosis will occur and they will expand and burst or
else shrink.
RED
BLOOD
CELL
Hypotonic
solution
Hypertonic
solution
Isotonic
solution
H2 O
Hemolysed cell
H2 O
H2 O
Normal cell
H2 O
Crenated cell
OSMOSIS across the Capillary Wall
•  The body normally maintains an ISOTONIC ECF and removes
excess water and electrolytes by excretion (via kidneys).
•  Plasma proteins are non – penetrating solutes for the capillary
membrane & determine the osmotic pressure of the blood.
•  Plasma proteins attract water into blood at capillaries so
helping to prevent edema.
•  Any drop in plasma protein level (e.g. from hepatic disease) will
lower the osmotic pressure of the blood causing net osmosis
out of the blood and edema.
Hepatic
Disease
(cirrhosis)
Decreased
plasma
proteins
Decreased Blood
Osmotic Pressure
Net osmosis
Edema
DIALYSIS
• Dialysis is the process of separating SMALL
MOLECULES (crystalloids, e.g. NaCl, glucose,
amino acids) from LARGE MOLECULES (colloids,
mainly proteins) across a SEMIPERMEABLE
(DIALYSING) MEMBRANE.
• HEMODIALYSIS is the process of purifying the
blood by allowing dialysis to occur between the
blood and a dialysing fluid.
•  HEMODIALYSIS RESTORES HOMEOSTASIS
in cases of kidney failure.
Arterial blood
enters
HEMODIALYSIS
Net diffusion
Dialysing
Fluid leaves
Net
osmosis
Net diffusion
Blood cells
& colloids
do not
cross
membrane
Purified blood
returned to vein
Dialysing
membrane Allows
small molecules
across
Dialysing
Fluid enters
HEMODIALYSIS
Semipermeable
Dialysing membrane
separates blood
from dialysate fluid.
Source MedBroadcast.com
Comparison of Methods for Crossing the Cell membrane
NEEDS
NEEDS
PROCESS MEMBRANE
ATP
SIMPLE
DIFFUSION
FACILITATED
DIFFUSION
OSMOSIS
ACTIVE
TRANSPORT
DIALYSIS
UP or
ONLY in
ACTIVE
DOWN
LIVING
or
PASSIVE CONC GRAD CELLS
NEEDS a
CARRIER EXAMPLES