Chapt. 12, Movement Across Membranes
Chapt. 12, Movement through lipid bilayer
• Two ways substances can cross
membranes
• Hydrophobic molecules
and small polar
molecules can diffuse
through a synthetic
lipid bilayer or the lipid
bilayer of a real
biological membrane.
(Fig. 12-2)
– Passing through the lipid bilayer
– Passing through the membrane as a
result of specialized proteins
1
2
Chapt. 12, Movement through lipid bilayer
Chapt. 12, Movement through lipid bilayer
• Larger polar molecules,
cannot rapidly diffuse
through the bilayer.
• Larger polar molecules,
cannot rapidly diffuse
through the bilayer.
3
4
Chapt. 12, Movement through lipid bilayer
• Ions or charged
molecules cannot
rapidly diffuse through
the bilayer. (Fig. 11-20)
Chapt. 12, Protein Based Transport
• Many charged or large polar molecules do
enter and exit cells. This requires
membrane proteins. A simple proof:
• Ions are small. Why
can’t they diffuse
through?
5
Fig 12.1
6
1
Chapt. 12, Protein Based Transport
• The two classes of membrane transport
proteins. Similarities and differences.
(Fig. 12-2)
7
Chapt. 12, Protein Based Transport
• The cellular concentrations of ions and
metabolites are very different on the
inside and the outside of the cell.
8
Table 12-1
Chapt. 12, Protein Based Transport
• Ions
– The inside has much less Na+ and much more
K+ than outside.
– Other ions more common outside include Ca++,
Mg++, and Cl-.
– Fixed anions are much more common inside
(but never diffuse out)
– Summary table 12-1
9
10
Chapt. 12, Protein Based Transport
• Metabolites or other organic molecules
– One of the major functions of the plasma
membrane is to contain metabolites or other
molecules necessary for cellular functioning.
Chapt. 12, Carrier Proteins
• Carrier Proteins are largely responsible
for the differences in concentration of
substances inside and outside of cells.
– Some organic substances are rapidly imported
into certain cells.
11
12
2
Chapt. 12, Carrier Proteins
• Some examples of carrier proteins in cells.
(Fig. 12-5)
13
Chapt. 12, Carrier Proteins
• Nomenclature -- types of transport
mediated by carrier proteins. (Fig. 12-14)
14
Chapt. 12, Carrier Proteins
• Mechanism of action. (Fig. 12-7)
– Molecular recognition/binding
– Allosteric conformational change
– Solute release
– Return to original conformation
15
16
Chapt. 12, Carrier Proteins
• Sound familiar? (I hope)
Chapt. 12, Carrier Proteins
• Similarities between enzymes and carrier
proteins:
– Specificity in binding
– Release of products
– Can only carry out events with a negative ∆G
– Can be coupled to an energy source to carry
out half reactions that otherwise would have a
positive ∆G.
17
18
3
Chapt. 12, Carrier Proteins
• Further similarities between enzymes and
carrier proteins:
Chapt. 12, Carrier Proteins
• Compare typical reaction: A ----> B with
carrier based transport: Xin ----> Xout
– Speeds up a “permissible” (=spontaneous)
reaction.
– It does so by lowering the energy of the
transition state.
19
20
Chapt. 12, Carrier Proteins
• Similarities in kinetics:
– Vmax
Chapt. 12, Carrier Proteins
• Active and “Passive” Transport
(=facilitated diffusion) Fig. 12-4
– Km
• Design an experiment to determine Vmax
and Km. Be specific.
21
22
Chapt. 12, Carrier Proteins
• There is something wrong with this figure.
What is it?
23
Chapt. 12, Carrier Proteins
• For uncharged molecules the free energy
gradient is really the same as the
concentration gradient and the diagram is
O.K.
24
4
Chapt. 12, Carrier Proteins
• However, for any charged particle, the
free energy differences is a composite of
the concentration gradient and the charge
gradient. This combined gradient is called
the electrochemical gradient, and the
energy difference for the particle is
called the electrochemical potential. (Fig
12-7)
25
Chapt. 12, Carrier Proteins
• (Fig 12-8; alternative version)
---
+ + +
Extra panel
26
Chapt. 12, Carrier Proteins
• Passive transport thus can be defined as
transport in which the transported molecule
drops down the electrochemical gradient
(and thus the free energy gradient)
Chapt. 12, Carrier Proteins
• Active transport can be powered by:
– Co-transport of another substance down its
energy gradient
– ATP hydrolysis
– Light energy
• Active transport can be defined as
transport in which the transported molecule
is moved up the electrochemical gradient.
27
– Fig 12-9
28
Chapt. 12, The Na+/K+ Pump
• A reminder: K+ is much more common
inside cells than outside; Na+ is much more
common outside cells than inside. How did
it get that way?
Chapt. 12, The Na+/K+ Pump
• Lets us consider what this fact alone can
tell us.
– We have seen that an ion can diffuse up its
concentration gradient in response to an
electrical gradient. Could this explain these
results?
• Lets us consider what this fact alone can
tell us.
29
– No! Both ions are positive. You cannot attract
both ions in different directions with an
electrical gradient.
30
5
Chapt. 12, The Na+/K+ Pump
Chapt. 12, The Na+/K+ Pump
• Lets us consider what this fact alone can
tell us.
• Lets us consider what this fact alone can
tell us.
– If these ion distributions cannot be brought
about by facilitated diffusion, what is the
other alternative?
– If you had to guess, how do you suppose that
this pump would be powered?
– A: at least one (and probably both) ions must
be pumped against their electrochemical
gradients.
31
– ATP is a logical choice.
32
Chapt. 12, The Na+/K+ Pump
Chapt. 12, A Model for the Na+/K+ Pump
• Lets us consider what this fact alone can
tell us.
– Where should the K+ binding site be located?
(On the portion of the pump facing the
cytosolic or non-cytosolic side?)
– Where should the Na+ binding site be located?
33
– Where should the ATP binding site be located?
34
Fig. 12.12
Chapt. 12, Functions of the Na+/K+ Pump
• This pump is very expensive -- it can use
30% to 70% of the ATP used by an animal
cell. What are these gradients used for?
35
36
6
Chapt. 12, Functions of the Na+/K+ Pump
• This pump is very expensive -- it can use
30% to 70% of the ATP used by an animal
cell. What are these gradients used for?
– Powering co-transport. (Fig. 12-14, 12-15)
Fig.
12.15
38
37
Chapt. 12, Functions of the Na+/K+ Pump
• What are these gradients used for?
– The ion gradients are responsible for
electrically active cells (considered in
more detail later).
39
40
Chapt. 12, Functions of the Na+/K+ Pump
• What are these gradients used for?
– In many animals, the pump is necessary to
prevent osmotic lysis.
• Typically more non-water molecules inside than
outside; water flows down its own concentration
gradient into the cell and the cell bursts.
Chapt. 12, Other Important Pumps
• The H+ pump.
– Importance in some organelles.
– Importance in plants, fungi and bacteria. (Fig.
12-17)
• Made worse by Na+ and Cl- diffusing in.
• Na+/K+ Pump pumps out Na+, also results in negative
membrane charge which repels Cl-.
41
42
7
Chapt. 12, Other Important Pumps
• Ion channels are like doors
• The Ca++
pump.
– They are often gated.
– Well
understood
– They can
be gated
in
different
ways.
– Importance
43
Chapt. 12, Ion Channels
44
Fig 12-6
Chapt. 12, Ion Channels
Fig 12-24
Chapt. 12, Ion Channels
• Ion channels can be in either open or
closed states. The evidence (Fig. 12-22)
• Ion channels
are like doors
– They show ion
selectivity.
• Sometimes
pass only 1
particular ion.
45
• Sometimes
pass multiple
similar ions.
Fig 12-19
46
Chapt. 12, Ion Channels
• Channels are either all they way open or all
the way closed. (Fig. 12-23)
Fig. 12.22
47
48
8
Chapt. 12, Ion Channels and Membrane
Potential
Cell inside
Cell outside
10,000 Na+
140,000 K+
• What is membrane potential?
– Membrane potential can easily be measured (as
we just saw).
– Where does the membrane potential come
from?
10,000 Cl-
110,000 Cl-
139,999 other
neg charges
42,000 other
neg charges
total net charge = 0
50
Suppose a Na+ channel opened and 1000 Na+ diffused
down their electrochemical gradient....
Cell inside
144,000 Na+
10,000 Na+
145,000 Na+
140,000 K+
5,000 K+
110,000 Cl-
139,999 other
neg charges
42,000 other
neg charges
10,000 Na+
Difference in charges = 1000 minus - 1000 = 2,000
• What have we learned?
– The membrane potential is due to differing net
charges on each side of the membrane.
– Changes in membrane potential are due to ions
moving across the membrane.
– Because ions do not penetrate the hydrophobic
interior of the lipid bilayer, they must pass
through carrier proteins or channel proteins.
6,000 K+
140,000 K+
5,000 K+
1,000 Ca++
10,000 Cl-
110,000 Cl-
139,999 other
neg charges
42,000 other
neg charges
total net charge = -1000
52
Cell outside
145,000 Na+
139,000 K+
1 Ca++
total net charge =- 1000
Chapt. 12, Ion Channels and Membrane
Potential
53
Cell inside
1,000 Ca++
10,000 Cl-
total net charge = +1000
Suppose a K+ channel opened and 1000 K+ diffused down
their electrochemical gradient....
Cell outside
11,000 Na+
1 Ca++
51
total net charge =0
Difference in charges
= 0- 0 or none
• Cannot find free negative or positive charges on the
shelf of chemicals.
49
5,000 K+
1,000 Ca++
1 Ca++
– The difference in total charges on the
opposite sides of a membrane.
145,000 Na+
total net charge =+1000
Difference in charges = - 1000 minus + 1000 = - 2,000
Chapt. 12, Ion Channels and Membrane
Potential
• The equilibrium potential:
– Let us consider again this
figure. The inside of the
cell is to the left. There is
a large difference in Na+
concentrations. What
happens if we open up if we
open up Na+ channels only?
Cell inside
Cell outside
10,000 Na+
140,000 K+
145,000 Na+
5,000 K+
1,000 Ca++
1 Ca++
10,000 Cl-
110,000 Cl-
139,999 other
neg charges
42,000 other
neg charges
total net charge = 0
total net charge =0
Difference in charges
= 0- 0 or none
54
9
Chapt. 12, Ion Channels and Membrane
Potential
• What happens if we open
up Na+ channels only?
– Na+ flows in.
– Changes membrane
potential.
– Changes
Na+
Cell inside
144,000 Na+
10,000 Na+
145,000 Na+
1 Ca++
=
5,000 K+
1,000 Ca++
10,000 Cl-
110,000 Cl-
139,999 other
neg charges
42,000 other
neg charges
total net charge = +1000
in
Cell outside
11,000 Na+
140,000 K+
concentration.
• Will Na+ continue to
diffuse in until [Na+]
[Na+] out ?
Suppose a Na+ channel opened and 1000 Na+ diffused
down their electrochemical gradient....
total net charge =- 1000
Difference in charges = 1000 minus - 1000 = 2,000
Chapt. 12, Ion Channels and Membrane
Potential
• Will Na+ continue to
diffuse in until [Na+]
[Na+] out ?
in
=
• No, before long the
positive interior of the
cell will balance out the
greater concentration of
Na+ on the outside.
Suppose a Na+ channel opened and 1000 Na+ diffused
down their electrochemical gradient....
Cell inside
Cell outside
11,000 Na+
144,000 Na+
10,000 Na+
145,000 Na+
140,000 K+
1 Ca++
5,000 K+
1,000 Ca++
10,000 Cl-
110,000 Cl-
139,999 other
neg charges
42,000 other
neg charges
total net charge = +1000
total net charge =- 1000
Difference in charges = 1000 minus - 1000 = 2,000
55
56
Chapt. 12, Ion Channels and Membrane
Potential
Chapt. 12, Ion Channels and Membrane
Potential
• There are so many ions on the inside and
outsides of cells that this usually does not
changes the ion’s concentration very much.
Fig 12-27
• So, now we can define the equilibrium
potential:
– The membrane charge where the component of
the electric portion of the electrochemical
gradient exactly balances the concentration
portion of the electrochemical gradient.
– Different for every ion. Depends on:
• The relative concentrations of the ion on the inside
v.s. the outside of the cell.
• The charge on that ion.
57
58
Chapt. 12, Ion Channels and Membrane
Potential
Chapt. 12, Ion Channels and Membrane
Potential
• The “resting potential” of most cells is
negative.
– The Na+/K+ pump (a minor contributor)
• The voltage gated Na+ channel is responsible
for the action potential of electrically active
cells including nerve and muscle.
– K+ leak channels
• What is an action potential? Fig. 12-32
59
60
10
Fig. 12-32
Chapt. 12, Ion Channels and Membrane
Potential
• The three states of the voltage gated Na+
channel.
61
62
• Movement
of the Na+
ion and the
action
potential.
63
Fig 12-34
• The explanation for unidirectional propagation.
Fig 12-23
• The action potential propagates (=regenerates)
along the membrane in one direction.
64
Fig. 12-38
Chapt. 12, Ion Channels and Membrane
Potential
• Other channels participate in nerve
transmission.
– The voltage gated K+ channel.
– The voltage gated Ca++ channel at the axon
terminus. (Fig. 12-40)
65
66
11
Fig 12-40
Chapt. 12, Ion Channels and Membrane
Potential
• Other channels participate in nerve
transmission (cont.)
– The acetlycholine gated cation channel.
Fig. 12-42
67
68
Chapt. 12, Ion Channels and Membrane
Potential
– How does the acetylcholine gated cation
channel initiate a response?
69
Fig. 12-41
Chapt. 12, Ion Channels
• There are synapses that make an action
potential more likely (excitatory) or less
likely
(inhibitory)
70
Fig. 12-43
12