Download 1 TRANSPORT ACROSS MEMBRANES Cell or organelle is not

Note Set 9
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TRANSPORT ACROSS MEMBRANES
Cell or organelle is not wholly closed or wholly open to surroundings
Interior must be protected from certain toxic cmpds, metabolites must be taken in,
products are sometimes removed, waste products need to be removed, thousands
of substances involved
Three categories:
Passive
Facilitated
Active
The Thermodynamics of Transport
G = RT ln
C2
C1
Free energy change, ∆ G, for transporting 1 mol of substance from region where
concentration is C1 to region where concentration is C2
If C2 < C1 then ∆ G is negative
Favorable
As more is transferred (between two finite compartments) C1 decreases and C2
increases until C2 =C1
Then ∆ G = 0 and system is at equilibrium
Unless other factors involved, this is ultimate state, will eventually be same
concentration on each side
"Kinetic" way to see this result:
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If molecules wandering into membrane at random, number entering from each side
is proportional to concentration on that side
When concentrations are equal, rates of transport/movement in both directions is the
same, and there is no net transport in either direction
The rate of transport, J (moles/cm2/sec, is proportional to the difference between C2
and C1
J = -KD1 (C2 - C1)/l
C1 and C2 in mol/cm3
l = thickness of membrane (cm)
D1 is the diffusion coeeficient (cm2/sec)
K is the partition coefficient, a ratio of the solubilities in lipid and water
Symplified by use of a term P (cm/sec), permeability constant so that ( P = KD1/l)
J = -P(C1 - C2) see Table 10.6
Three "other factors" can circumvent this equalization:
1. Substance is bound by macromolecules confined to one side of membrane
Not "free", so doesn't count in equation
O2 in red blood cells bound to Hb; total O2 in cell much higher than in plasma
"Free" O2 in erythrocyte and plasma is the same
2. Membrane electrical potential affects distribution of ions
If inside negative w/ respect to outside, cations will be drawn inside, and anions
want to leave
Affects ∆ G
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G = RT ln
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C2
+ZF
C1
F = Faraday constant (96.5 kJ mol-1 V-1)
∆ψ = membrane potential in volts
Z = charge on ion being considered
If ∆ψ is negative going from ouside to in, and Z is positive, then ZF
makes a
negative contribution to ∆ G and transport in of cations is favored
In this case, equilibrium state will not correspond to equal concentration of ions on
both sides of membrane
But energy must be used to maintain membrane potential
This is a special case of active transport
Can also be interpreted to mean if different ionic concentrations are maintained,
potential. will develop accross membrane that can be used by a system not
directly connected to the pump (usually ATPase-coupled) maintaining the
potential
If some other process is coupled to transport, its ∆ G must be included
General case of active transport
G = RT ln
C2
+ G'
C1
For example, ATP hydrolysis could be the ∆ G'
The two cases of active transport mean is that under the right circumstances,
substances can be transported into or out of a cell or compartment against an
unfavorable concentration difference
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For example, cells bring K+ in and move Na+ out against their concentration gradients
Passive Transport: Diffusion
This is molecular diffusion; random wandering of molecules through membrane due
to Brownian motion of molecules in any fluid
First equation applies (10.1)
Problem with diffusion is that the lipid bilayer does not like ions and other hydrophilic
substances so diffusion of these substances is very slow
Exception is water: membranes are very permeable to water and no one knows
exactly why
Fortunate for life, however, as cells can exchange water very readily with
surroundings
Although leaves of desert plants have waxy substance to prevent water loss
Membranes are also very permeasble to oxygen
Facilitated Transport: Accelerated Diffusion
Increases rate of diffusion: it's just a special form of diffusion
Energy not required
First equation still applies
Pore-Facilitated Transport (Fig 10.20)
A channel or ion pore is used
For example, the uptake and release of chloride and bicarbonate in red blood cells: in
capillaries, HCO3- taken up from erythrocyte release, and Cl- taken in to maintain
anion balance in cell; process reversed in lungs; very selective, occurs on 1:1 basis
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A glucose pore called glucose transport protein is used by erthyrocyte to increase the
diffusion of glucose 50,000-fold
Some pores are gated: they open and close in response to control mechanisms
Gap junctions between animal cells: they're open most of the time, but will close
under some circumstances, like if the cell is damaged and Ca2+ increases
Gated pores play a major role in the propagation of nerve impulses
Ionophores
Protein toxins that act by making hole in membrane and allowing indiscriminate
movement of specific ions...kills cells. Made by bacteria to kill other bacteria, specific
for bacterial membranes so they are used as antibiotics
Can be another kind of pore
Gramicidin A (Fig 10.22) produced by Bacillus brevis is a cation specific pore:
screws up K+ gradient--2 molecules of gA needed to form pore, one from each
membrane leaflet, together form helical pore
15-residue polypeptide that contains alternating L- and D-amino acids
Carrier-Facilitated Transport
Valinomycin (Fig 10.23) produced by Streptomyces : also screws up K+ gradient--out
side is hydrophobic (lots of methyl groups), inside nicely accomodates K+ (Ns and
Os) and carrier picks up an ion on one side of membrane, diffuses to other side and
releases ion (does NOT "flip")
Also has D- and L- amino acids
Looks a little like a clathrate water cage
Distinguishing Different Types of Diffusion (Fig 10.24)
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How can you tell the difference between diffusion and facilitated transport??
If it is facilitated it is saturable, by analogy to enzyme catalysis
How can you tell the difference between a pore and a carrier?
Pore does not depend on membrane fluidity for efficient function, carrier does.
Fluidity influenced by various factors including lipid composition and temperature
Active Transport: Transport Against a Concentration Gradient
Ion pumps, cotransport, and transport by modification are all ways that a cell can
transport a substance against a concentration gradient
Ion pumps: Direct Coupling of ATP Hydrolysis to Transport
Most cells spend 25% of ATP on active transport!
Direct coupling of ATP hydrolysis to transport
Sodium-potassium pump maintains sodium and potassium gradient in cell:
called Na+ - K+ ATPase (Fig 10.25)
Has binding sites for ouabain and digitalis (cardiotonic steroids)
Fig 10.26 Na outside cell = 140mM, inside = 10mM and Na is transported out
K out = 5mM, inside = 100mM and K is trans in
Inside of memb. is more negative than ouside
for Na+ out, membrane potential opposes flow:
140mM
∆ G = RT ln 10mM +1 x F x 0.07volts = 13.55 kJ/mol
for 3 mol , need 40.65 kJ
for K+ in:
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100mM
∆ G = RT ln 5mM +1 x F x (-0.07volts) = 0.97 kJ/mol
for 2 mol , need 1.94 kJ
For total process need 42.59 kJ
-why is 1 ATP enuf?? isn't ∆ G = --30 kJ/mol??
-in most cells, [ATP] is much > than [ADP] and [Pi], so ∆ G of hydrolysis is more like
45-50 kJ/mol
Pumps can run backwards and generate ATP
Probably the major way that ATP is made in living organisms
Cotransport Systems
A substance to be transported against a concentration gradient "piggybacks" by
moving with an ion that is going in the direction of a favorable gradient
Sodium-glucose transport in the small intestine--transport of each molecule of
glucose into cells is accompanied by the simultanious movement of one sodium
ion in the same direction
[Na] is lower in cell than outside...so energy is provided
Many amino acids and sugars transported this way. Same direction = symport,
opposite direction = antiport
Transport by modification
Transported substance is modified as soon as it gets into the cell, and thus is no
longer free to leave, and no longer contributes to the equilibrium...therefore the
substance can be accumulated against a concentration gradient
Occurs in transport of many sugars (see Fig 10.27)
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Phosphorylation is common modification
Glucose is phosphorylated to G-6-P, which is also the 1st step in the metabolic
utilization of glucose
Overview
Transport very pervasive, continue to encounter during met. reg.,cells utilize all the
kinds we discussed
See Fig 10.28
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