How molecules move through body and across plasma membranes

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How molecules move through body and across plasma
membranes

• Molecular Movement
• Bulk Flow
• Simple Diffusion (including OSMOSIS)
• Mediated Transport
• Facilitated Diffusion
• Active Transport

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Gradients
A GRADIENT is a difference in any parameter over
distance
Molecules move down gradients from Hi to
Lo, spontaneously
e.g. Pressure, concentration, temperature,
energy
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Bulk Flow
Many molecules moving simultaneously in one
direction, from an area of high P to low P
PATMOS mm Hg
Lo P Expiration Hi P
Hi P Inspiration Lo P
PLUNGS mm Hg
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Poiseulles Law of Bulk Flow Its all about
PRESSURE
P1 P2
FB kB(P1 - P2) L/min FB Bulk Flow L/min kB
bulk flow constant tube diameter
If P1 gt P2, flow goes from 1 to 2 If P1 lt P2,
flow goes from 2 to 1 If P1 P2, no flow occurs
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How molecules move through body and across plasma
membranes

• Molecular Movement
• Bulk Flow
• Simple Diffusion (including OSMOSIS)
• Mediated Transport
• Facilitated Diffusion
• Active Transport

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Simple Diffusion
Net movement from an area of high concentration
to low concentration
e.g. smell of coffee in the morning sugar in
coffee cup molecules across membranes
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Bulk Flow vs. Simple diffusion

• Bulk flow
• requires a pressure gradient for mass molecular
  movement
• is efficient over long distances

• Diffusion (simple)
• requires a concentration gradient
• never requires a metabolic energy source
• is efficient over short distances only

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Simple Diffusion Across a Membrane
Co gt Ci
Outside
Inside
Cell Membrane
Net flux (Jnet ) occurs from high to low
concentration and will continue until equilibrium
is reached
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Diffusion
A
B
10
A
Glucose mmol/L
5
B
0
time
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Diffusion Equilibrium
Outside
Inside
Co Ci




Cell Membrane
Influx (ji ) efflux (je ) Net flux (Jnet )
equals zero
drmunro
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Diffusion
A
B
10
A
Glucose mmol/L
Diffusion Equilibrium CA CB
Jnet 0 j A to B j B to A
5
B
0
time
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Ficks First Law of Diffusion
J µ
Co - Ci
Jnet kp(Co Ci)
Jnet net flux rate kp permeability
constant Co - Ci concentration gradient
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Factors Altering the rate of Jnet
1. Membrane permeability kp
2. Concentration gradient DC
Ficks Law Jnet kp (?C)
3. Membrane Surface Area More Faster
Diffusion
drmunro
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• Factors affecting kp
• Membrane Solubility
• to Non-Polar Molecules

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Polar vs. Non-Polar Molecules
Polar Molecules have UNEVEN arrangement of
charges Examples Water, All IONS!, Glucose
Non-Polar Molecules have EVEN, Balanced
arrangement Examples Lipids, Cholesterol
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Membrane Structure
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Lipid solubility and kp
Hi Lo
kp
Lipid soluble, non-polar molecules
Lo Hi Lipid Solubility
drmunro
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• Factors affecting kp
• Membrane Solubility
• to Non-Polar Molecules
• 2) Molecule Size
• POLAR Molecules

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Channel Proteins
Examples Aquaporin Water Ion Channels Ca2
Figure 5-10
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Polar molecules molecular size vs. kp
Hi Lo
Small Polar Molecules can move freely via Channels
kp
Large Polar Molecules Move VERY SLOWLY Mediated
Transport by Specialized Proteins!
0 0.2 0.4 0.8 1.0 1.2 1.4 molecular diameter
(nm)
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How molecules move through body and across plasma
membranes

• Molecular Movement
• Bulk Flow
• Simple Diffusion (including OSMOSIS)
• Mediated Transport
• Facilitated Diffusion
• Active Transport

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Most molecules (polar non-polar) can fit
between Polar Heads!
Non-Polar Molecules (Lipids, Hormones) Solubility
Polar Molecules cannot diffuse through Non-Polar
layer
High Kp
Any Polar Molecule
Low Kp
Polar Heads
Non-Polar Tails
Fast
Slow
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Polar Molecules cannot diffuse DIRECTLY Need
Channels/Pumps/Carriers
Any Polar Molecule
Polar Heads
Non-Polar Tails
Special Proteins allow POLAR molecules to move
through
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Simple diffusion through CHANNEL PROTIENS Moves
SMALL POLAR MOLECULES (H20, Ions)
Mediated Transport Moves Large Polar
Molecules (Glucose)
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Mediated Transport Systems
Properties
All use membrane transporters (proteins)
Demonstrate 1. Specificity
2. Competition
3. Saturation
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1. Specificity
Transport protein is specific for a
SINGLE molecule or a family of RELATED molecules
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Specificity
Outside
Inside
Glucose
Leucine
Membrane
Transporter (carrier) molecule
drmunro
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Specificity
Hi
glucose
Co
Ci mmol/L
Diffusion equilibrium Ci Co ji je Jnet
0 passive
Lo
leucine
time
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2. Competition
Two different, but chemically related,
compounds bind to the same carrier
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Competition
Glucose only present
Out
In
Glucose
Glucose
Glucose only present 2 glucose molecules cross
per msecond
Membrane
Transporter (carrier) molecule
drmunro
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Competition
Glucose and Galactose present
Out
In
Glucose
Galactose
Glucose and Galactose present 1 glucose
molecule crosses per msecond
Glucose
Galactose
Membrane
Transporter (carrier) molecule
drmunro
drmunro
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Competition
Diffusion equilibrium Ci Co ji je Jnet
0 passive
Hi
Co
Ci glucose mmol/L
Glucose only
Glucose in presence of galactose
Lo
time
drmunro
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3. Saturation
The number of available transporter
proteins determines the maximum rate of transport
into the cell
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Maximum rate of entry into cell
All carriers in use at this concentration
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Types of mediated transport
1. Facilitated diffusion passive (no ATP )
2. Active Transport
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How can you differentiate Simple from Facilitated
Diffusion??
F.D. Shows Competition, S.D. doesnt!
Hi
Co
Ci glucose mmol/L
Glucose only
Glucose in presence of galactose
Lo
time
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Simple Diffusion No competition!
Hi
Co
Ci H2O mmol/L
H20 only
H2O in presence of galactose
Lo
time
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Active Transport
The net movement of molecules against a chemical
or electrical gradient
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Active Transport
Outside
Inside
Co less than Ci
Cell Membrane
Net flux (Jnet ) occurred from low to high
concentration
drmunro
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Active transport
(requires the use of ATP)
Steady State Ci Co ji je jnet 0
ATP use maintains the conc. difference
Ci mmol/L leucine
Co
time
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Plasma Membrane Structure
Non-Polar Molecules (Lipids, Hormones) Solubility
Polar Molecule SIZE
small
large
High Kp
Low Kp
Polar Heads
Non-Polar Tails
Fast
Slow
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Osmosis Diffusion of Water
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Osmosis
Diffusion of H2O down its concentration
gradient across a membrane that is IMPERMEABLE to
the solute involved. OUT IN Hi
H2O Hi solute Lo solute Lo H2O
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Penetrating vs. Non-Penetrating

• Penetrating Particles Move freely throughout
  plasma membrane!
• Non-penetrating Particles Cannot move through
  plasma membrane!

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Volume changes during osmotic flow
HiS LoH2O
HiH2O LoS
Figure 5-29
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Volume changes during osmotic flow
Figure 5-29
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Osmotic concentration and cell size
The Effects of Osmosis on Cell Volume
100
200
300
400
500
mosmol/L NaCl
Of external solution (ECF)
Hypotonic
Hypertonic
Isotonic Physiologically NORMAL!
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1. 300 mosmol/L NaCl
Isotonic
2. 200 mosmol/L Leucine
Hypotonic
3. 300 mosmol/L Leucine
Isotonic
4. 400 mosmol/L Glucose
Hypertonic
150
2
1
100
3
Cell volume
4
0
time
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Trans-epithelial transport
3
1
H2O
1
Diffusion
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H2O
Na
H2O
3
Osmosis
H2O Na
4
4
Active transport
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Bulk flow
5
H2O Na
capillary
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Excitable Membranes
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What is an excitable membrane?

• Any plasma membrane that can hold a charge and
  propagate electrical signals.

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Two types of Excitable Membranes
Both work in similar ways.

• Muscle Cells excite and then contract.
• Neurons transmit electrical impulses throughout
  the body (sensory and motor)

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Excitable Membrane Function Outline

• Resting Membrane Potential
• Graded Potentials
• Action Potentials

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Resting Membrane Potential

• All excitable membranes maintain a non-0 resting
  membrane potential

Neurons -70 mV Muscle Cells -85 mV
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How do we measure membrane potentials?

• ALWAYS REFER TO INSIDE relative to
  OUTSIDE!!!!!!!!!!

How is this resting potential achieved?
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Resting Membrane Potential Ionic Concentration
Gradients
K
Na Cl -
Proteins
This is an example of Physiological Steady State!
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Resting Membrane Potential Membrane Channels

• LOTS OF K Leaks out by Diffusion
• Na cannot leak in
• Cl Leaks out electrical repulsion due to Protiens

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1
2
K
Na Cl -