3. Membrane transport: The set of transport proteins in the plasma
membrane, or in the membrane of an intracellular organelle, determines
exactly what solutes can pass into and out of that cell or organelle. Each
type of membrane therefore has its own characteristic set of transport
Each type of
particular type of
selective set of
transported in or
9. Mediated Transport
• Passive mediated transport
▫ Facilitated diffusion
▫ DOWN a conc. gradient
• Active transport
▫ Primary active transport—uses ATP
▫ Secondary active transport—uses a different
▫ Pumps things UP a conc. gradient
10. How to tell mediated transport vs.
• Saturation kinetics
• Competition kinetics
18. Consequences of Na/K ATPase
• Two ion gradients
▫ Used as energy source
▫ Electrical signaling
• Charge difference across membrane
▫ Membrane potential difference
▫ Negative on inside
-60 to –90 mV in animal cells
~ -150 mV in bacteria
-200 to –300 mV in plants
▫ Not just due to these ions
Phosphatidylserine on inside of PM
22. Thermodynamic treatment
Mass transport across the membrane must be discussed fundamentally on
the basis of the thermodynamics because the thermodynamics describes
the rule of energy changes inevitably generating in the mass transport.
The classical thermodynamics have introduced a lot of information on
the basis of the first and the second law. However, we have to notice that
the classical thermodynamics discusses only reversible phenomena and it
does not treats the transport rate, in other words, it does not includes the
parameter of “time”. Many phenomena (including membrane
phenomena) generating in the natural world are far away from reversible
states, so the reversible thermodynamics was ineffective to discuss their
mechanism. On other hand, the irreversible thermodynamics (Non-equilibrium
thermo-dynamics) came to succeed in discussing the mass
transport by introducing the concept of “time” in its system.
The irreversible thermodynamics is not general theory applicable to
analyze every reaction in the natural world. Further it should be noticed
that the applicable limit of the irreversible thermodynamics must be
determined through experiments.
23. Biochemistry: 09/25/08
• What goes through and what doesn’t?
• Nonpolar gases (CO2, O2) diffuse
• Hydrophobic molecules and small
uncharged molecules mostly pass
• Charged molecules blocked
24. Biochemistry: 09/25/08
Types of Transport (Table 9.3)
Type ProteinSaturable Movement Energy
Carrierw/substr. Rel.to conc. Input?
Diffusion No No Down
Channels Yes No Down
Passive Yes Yes Down No
Active Yes Yes Up Yes
26. Biochemistry: 09/25/08
passive and active transport
• If you think of the transport as a
chemical reaction Ain Aout or Aout Ain
• It makes sense that the free energy
equation would look like this:
• Gtransport = RTln([Ain]/[Aout])
• More complex with charges;
see eqns. 9.4 through 9.6.
27. Biochemistry: 09/25/08
• Suppose [Aout] = 145 mM, [Ain] = 10 mM,
T = body temp = 310K
• Gtransport = RT ln[Ain]/[Aout]
= 8.325 J mol-1K-1 * 310 K * ln(10/145)
= -6.9 kJ mol-1
• So the energies involved are moderate
compared to ATP hydrolysis
28. Biochemistry: 09/25/08
• Charged species give rise to a factor that
looks at charge difference as well as
chemical potential (~concentration)
• Most cells export cations so the inside of
the cell is usually negatively charged
relative to the outside
29. Biochemistry: 09/25/08
Quantitative treatment of charge
• Membrane potential (in volts J/coul):
Y = Yin - Yout
• Gibbs free energy associated with change in
electrical potential is
Ge = zFY
where z is the charge being transported and F is
Faraday’s constant, 96485 JV-1mol-1
• Faraday’s constant is a fancy name for 1.
30. Biochemistry: 09/25/08
• Relating energy per mole
to energy per coulomb:
• Energy per mole of charges,
e.g. 1 J mol-1, is
1 J / (6.022*1023 charges)
• Energy per coulomb, e.g, 1 V = 1 J coul-1, is
1 J / (6.241*1018 charges)
• 1 V / (J mol-1) =
(1/(6.241*1018)) / (1/(6.022*1023) = 96485
• So F = 96485 J V-1mol-1
31. Biochemistry: 09/25/08
Total free energy change
• Typically we have both a chemical potential
difference and an electrical potential difference
• Gtransport = RTln([Ain]/[Aout]) + zFY
• Sometimes these two effects are opposite in sign,
but not always
32. Biochemistry: 09/25/08
Pores and channels
• Transmembrane proteins with central
passage for small molecules,
possibly charged, to pass through
▫ Bacterial: pore. Usually only weakly selective
▫ Eukaryote: channel. Highly selective.
• Usually the Gtransport is negative so they don’t
require external energy sources
• Gated channels:
▫ Passage can be switched on
▫ Highly selective, e.g. v(K+) >> v(Na+)
33. Biochemistry: 09/25/08
• All involve negative Gtransport
▫ Uniport: 1 solute across
▫ Symport: 2 solutes, same direction
▫ Antiport: 2 solutes, opposite directions
• Proteins that facilitate this are like
enzymes in that they speed up
reactions that would take place slowly
• These proteins can be inhibited,
reversibly or irreversibly
35. Thermodynamics of Transport,
GA = RT ln ([A]in/[A]out) + ZA
Z= charge on A
F = Faraday's constant, the charge in a mole of electrons
Y = membrane potential, difference in charge
between in and out, generally negative
36. Two major classes of membrane transmembrane proteins
Carrier proteins bind a solute on
one side and deliver it to the other
side through a change in shape.
Cells can also transport
macromolecules across the
Channel proteins form tiny
hydrophilic pores in the
membrane and the specific
molecules pass through by
diffusion from high to low
concentration. Most are ion
37. The ion concentrations inside a cell are very different from those
Because ions are
their movements can
create powerful electric
forces across the
membrane. Important in
nerve cells, muscle cells,
and in the mitochondria,
for ATP synthesis using
the electron transport
38. Ion transport across cell
membranes is of central
importance in biology. Cells
maintain an internal ion
composition very different
from that in the fluid around
them and these differences are
crucial for the cell’s survival
Animal cells pump Na+ out.
If the pumping fails, water
flows in by osmosis and
causes the cell to swell and
The positive and negative
charges must be balanced by
an almost exactly equal
quantity both inside and
outside of the cell
Differential ion concentrations crucial.
nucleic acids, proteins, etc.
39. Carrier proteins are required for the transport of almost all small organic
molecules across the cell membranes. Each carrier is highly selective,
often transporting just one type of molecule.
Each cell, each
40. The membrane transport proteins studied have polypeptide chains that
traverse the lipid bilayer multiple times, forming a continuous protein-lined
pathway allowing selected small hydrophilic molecules to cross
without coming into contact with the hydrophobic lipid bilayer.
A basic difference
proteins and channel
proteins is the way they
proteins mainly go on
size and electric charge.
actually bind their
molecules it transfers,
and then changes
41. Passive transport – no energy needed, with the concentration gradient
Active transport - energy needed, against the concentration gradient
Passive transport with a carrier
protein = facilitated transport
42. Passive transport of glucose: carrier exists in at least to states
(shapes). Example: liver cell after a large meal – lots of glucose
outside in the extracellular fluid.
Opposite in liver when blood glucose becomes low. Liver cells
breakdown glycogen. Glucose levels are higher inside the cell now.
Passive transport moves glucose outside the cell.
43. Electrically charged molecules diffuse according to their concentration
gradient and according to their charge – electrochemical gradient.
Na+ is at a higher
outside of the cell so
it tends to enter the
cell if given the
K+ is present at a
inside the cell –
therefore there is little
45. Animal cells use the Energy of ATP hydrolysis to pump Na+ out of the
cell to maintain the electrochemical gradient. This gradient is then used
to pump other molecules into or out of the cell against their
or Na+-K+ pump
other ATP driven
gradients of H+
ions. Next chapter
51. • Cytosolic Ca+2 concentrations are kept low by
• Influx of Ca+2 is tightly regulated, since Ca+2
binds molecules (enzymes) and alters their
activities (activation or inhibition).
• Influx of Ca+2 through Ca+2 channels is often
used as a signal to trigger other intracellular
events (muscle contraction).
• The cell maintains a low concentration, so
that signaling via increases is kept sensitive.
52. Ion channels are ion selective and gated. Tthey show ion selectivity
depending on the diameter and shape of the ion channel and on the
distribution of charged amino acids in its lining.. Most ion channels are
gated: they can switch between an open and a closed state by a change
in conformation, which is regulated by conditions inside and outside the
54. If the plasma membrane of animal cells was made permeable
to Na+ and K+, the Na+-K+ pump would:
A. be completely inhibited.
B. begin to pump Na+ in both directions.
C. begin synthesizing ATP instead of hydrolyzing it.
D. continue to pump ions and to hydrolyze ATP but the energy
would be wasted, as it would generate heat rather than ion
E. continue to pump ions but would not hydrolyze ATP.
55. Ca2+ pumps in the plasma membrane and endoplasmic
reticulum are important for:
A. maintaining osmotic balance.
B. preventing Ca2+ from altering the behavior of molecules in
C. providing enzymes in the endoplasmic reticulum with Ca2+
ions that are necessary
for their catalytic activity.
D. maintaining a negative membrane potential.
E. helping cells import K+.