Chapter 12
Membrane Transport
Defintions
• Solution – mixture of dissolved
molecules in a liquid
• Solute – the substance that is
dissolved
• Solvent – the liquid
Membrane Transport Proteins
• Many molecules must move back and
forth from inside and outside of the
cell
• Most cannot pass through without the
assistance of proteins in the
membrane bilayer
• Each cell has membrane has a
specific set of proteins depending on
the cell
Movement of Small Molecules
2 Major Classes
• Carrier proteins – move the solute
across the membrane by binding it on
one side and transporting it to the
other side
– Requires a conformation change
• Channel protein – small hydrophilic
pores that allow for solutes to pass
through
– Use diffusion to move across
– Also called ion channels
Proteins
Ion Concentrations
• The maintenance of solutes on both sides
of the membrane is critical to the cell
– Helps to keep the cell from rupturing
• Concentration of ions on either side varies
widely
– Na+ and Cl- are higher outside the cell
– K+ is higher inside the cell
– Must balance the the number of positive and
negative charges as well
Carrier Proteins
• Required for almost all small
organic molecules
– Exception – fat-soluble molecules and
small uncharged molecules that can
pass by simple diffusion
• Usually only carry one type of
molecule
• Carriers can also be in other
membranes of the cell such as the
mitochondria
Carriers in the Cell
3-D of Carrier Protein
Carrier vs Channel
• Channels, if open, will let solutes pass
if they have the right size and charge
– Trapdoor-like
• Carriers require that the solute fit in
the binding site
– Turnstile-like
– Why carriers are specific like an enzyme
and its substrate
Mechanisms of Transport
• Provided that there is a pathway,
molecules move from a higher to
lower concentration
– Doesn’t require energy
– Passive transport of facilitated diffusion
• Movement against a concentration
gradient requires energy
– Active transport
– Requires the harnessing of some energy
source by the carrier protein
Passive vs Active Transport
Glucose Carrier in Liver Cells
• Glucose carrier crosses the membrane and
has at least 2 conformations
• One conformation exposes the binding site
on the outside of the cell and the other on
the inside of the cell
How it Works (Passive)
• Glucose is high outside the cell so the
conformation is open to take in glucose and
move it to the cytosol where the
concentration is low
• When glucose levels are low in the blood,
glucagon (hormone) triggers the breakdown
of glycogen, glucose levels are high in the cell
and then the conformation moves the glucose
out of the cell to the blood stream
• Glucose moves according to the
concentration gradient across the membrane
Electrochemical Gradient
• Due to concentration gradient and the
voltage across the membrane
• This gradient determines the direction
of the solute during passive transport
Active Transport
• 3 main methods to move solutes
against a gradient
– Coupled transporters
– ATP-driven pumps
– Light-driven pumps
Transporters are Linked
• The active transport proteins are linked
together so that you can establish the
electrochemical gradient
• Example
– ATP-driven pump removes Na+ to the outside of
the cell (against the gradient) and then reenters the cell through the Na+-coupled
transporter which can bring in many other
solutes
Na+-K+ ATPase (Na+-K+ Pump)
• Requires ATP hydrolysis to maintain
the Na+-K+ equilibrium in the cell
• Transporter is also a ATPase
(enzyme)
• This pump keeps the [Na+] 10 to 30
times lower than extracellular levels
and the [K+] 10 to 30 times higher
than extracellular levels
Na+-K+ Pump
Na+ and K+ Concentrations
• The [Na+] outside the cell stores a large
amount of energy, like water behind a dam
– Even if the Na+-K+ pump is halted, there is
enough stored energy to conduct other Na+
downhill reactions
• The [K+] inside the cell does not have the
same potential energy
– Electric force pulling K+ into the cell is almost
the same as that pushing it out of the cell
Na+-K+ Pump is a Cycle
Coupled Transport
• The energy in the Na+-K+ pump can be
used to move a second solute
• Couple the movement of 2 molecules in
several ways
– Symport – move both in the same direction
– Antiport – move in opposite direction
• Carrier proteins that only carry one
molecule is called uniport (not coupled)
Coupled Transporters
Na+-Driven Symport
• If one molecule of the transport pair is
missing, the transport of the second
does not occur
2 Methods of Glucose Transport
Na+-Driven Antiport
• Also very important in cells
• Na+-H+ exchanger is used to
move Na+ into the cell and
then moves the H+ out of the
cell
– Regulates the pH of the cytosol
Osmosis
• The movement of water from region
of low solute concentration (high
water concentration) to an area of
high solute concentration (low water
concentration)
• Driving force is the osmotic pressure
caused by the difference in water
pressure
Osmotic Solutions
• Isotonic – equal solute on each side of the membrane
• Hypotonic – less solute outside cell, water rushes into
cell and cell bursts
• Hypertonic – more solute outside cell, water rushes
out of cell and cell shrivels
Osmotic Swelling
• Animal cells maintain normal cell structure with
Na+-K+ pump
• Plants have cell walls – turgor pressure is the
effect of osmosis and active transport of ions into
the cell – keeps leaves and stems upright
• Protozoans have special water collecting vacuoles
to remove excess water
Calcium Pumps
• Calcium is kept at low concentration in the
cell by ATP-driven calcium pump similar to
Na+-K+ pump with the exception that it
does not transport a second solute
• Tightly regulated as it can influence many
other molecules in the cytoplasm
• Influx of calcium is usually the trigger of
cell signaling
H+ Gradients
• Drive the movement of molecule
across the membranes of plants,
fungi and bacteria
• Similar to animal Na+-K+ pump
H+ Pumps
Channel Proteins
• Channel proteins create a hydrophilic
opening in which small water-soluble
molecules can pass into or out of the cell
– Gap junctions and porins make very large
openings
• Ion channels are very specific with regards
to pore size and the charge on the molecule
to be moved
– Move mainly Na, K, Cl and Ca
Ion Channels
• Have ion selectivity – allows some ions to
pass and restricts others
– Based on pore size and the charges on the inner
‘wall’ of the channel
• Ion channels are not always open
– Have the ability to regulate the movement of
ions so that control can be maintained on the
ion concentrations within the cell
– Channels are gated – open or closed
Ion Channels
Membrane Potential
• Basis of all electrical activity in cells
• Active transport can keep ion
concentration far from equilibrium in
the cell
• Channels open and the ions rush in
because of the gradient difference
• Allows for the electrical activity to
move across the membrane
Patch Clamp
• Technique used to
determine the
electrical current
in a cell
• Can measure the
change in voltage
that occurs when
the channels are
open or closed
3 Types of Channels
• Voltage-gated channels – controlled by
membrane potential
• Ligand-gated channels – controlled by binding
of a ligand to a membrane protein
• Stress activated channel – controlled by
mechanical force on the cell
Auditory Hair Cells
• Sound waves cause the stereocilia to tilt
and this causes the channels to open and
transport signal to the brain