12 - Membrane Transport

- Lipid bilayer is ideally suited to prevent loss of charged &
polar solutes (ions, sugars, AA)
- Must allow movement of nutrients, respiratory gases,
hormones, wastes, etc

•membranes are semipermeable

•Approximately equal quantities of +/- charges
inside and outside of cell
•Tiny excesses of + or - charges do occur near
PM and have imp. electrical effects

carrier proteins

- Transporters and channels move inorganic
ions and small, polar molecules cross
membrane
A — channels either open or close; faster transport rates than
transporters
B — transporters undergo a series of conformational changes
(alternate access)

- two means for movement both of which lead to net
flux of ions/compounds (influx - into cell; efflux -
out of cell)
1. Passively by diffusion
2. Actively by energy-coupled transport process

•substance moves from region of high conc. to region of
low conc.; eventually eliminating conc. difference
•driven by random thermal motion leading to increase in
entropy
•electrochemical gradient
--chemical - compartment concentration difference
-- electrical - compartment charge difference

1. Simple diffusion through lipid bilayer
2. Facilitate transport through aqueous,
protein-lined channel; “channels”
3. Facilitated transporters; “transporters”
4. Active transport; “pumps

•must be concentration difference
•must be membrane permeable
•directly through lipid
-- partition coefficient - measure of polarity;
ratio of solubility in nonpolar solvent/water
-- molecule size (smaller is faster)

•Ions (e.g. Na+, K+, Ca2+, & Cl-) cannot pass freely
through the lipid bilayer, but must still enter and exit
cells.
• ion channels

•transport is passive and proceeds down the
concentration gradient of the ion being transported
•Usually 107 - 108 ions/s
•gated -- open/close with the proper stimulus
• bidirectional -- depending on the gradient, channels
may transport ions in either// both of the two possible
directions.

- Potassium channel has a selectivity filter for ion specificity
Unsolvated = water removed

1. Voltage-gated channels -- opens due to change in
voltage difference
2. Ligand-gated channels -- opens due to ligand
binding; usually not the transported species
3. Stress (mechanically) gated -- opens due to
mechanical stimulation

(used to monitor ion channel
activity)

-- a ligand gated ion
channel

- us to hear
- Stress-gated ion channels allow us
to hear

•binds to membrane-spanning protein
that changes its shape; facilitative
transporter
•passive -- doesn’t require energy
•bidirectional -- depends on conc.
•102 - 104 molecules/s
•molecule-specific; (AA, sugars)
• activity regulatable

•Solute binding on one side of membrane
may or may not be required to change
protein shape, exposing solute to other
surface
ex. gluclose transporter

•Changes shape randomly even without glucose; glucose binding
induces shape change
•Side of membrane with highest glc conc. will tend to bind
most glc
•If glc higher outside, will move glc inward and vice versa

•down concentration
gradient (High --> Low)
•14 related
transporters
(isoforms; GLUT 1-14)
in humans

• (INC) glucose
levels triggers
insulin secretion
by pancreas
• (DEC) glucose
uptake into
various cells (e.g.,
skeletal muscle
and fat cells)

•uses energy to move molecules “up” conc.
gradient (Low --> High)
•energy source may be ATP hydrolysis or light
absorbance
•uses molecule-specific transmembrane
proteins; usually called “pumps

- changes conformation as it
exchanges Na+ for K+

•Pump is unidirectional; pumps Na+ ions out & K+ ions in
•phosphorylation-induced shape change alters ion
affinity
• If 3 Na+ ions out for every 2 K+ ions in, what does this
do to net charge on each side of the membrane? Na+/K+-ATPase
• As the pump operates, it makes the cell interior
more negative
• electrogenic -- contributes to charge differences
across PM — pump changes shape when autophosphorylated
• establish steep gradients needed for nerve-muscle impulses
• uses 1/3 of most animal cells' energy (2/3 in nerve cells) -100 cycles/sec

— Think of the pump keeping internal Na+ low, (not keeping external high)
• functions like bilge pump on a leaky ship; constantly pumping out Na+ that came in thru channels or carrier proteins

- gradient can be used to transport other molecules
• Humans have hundreds of different transporters that use this Na+ gradient to move specific molecules against
their concentration gradients

can function as uniports, symports, or antiports

drives coupled transporters

uses the electrochemical Na+ gradient to drive the import of glucose • Unlikely to close unless entirely empty or entirely full
• Glucose is “dragged” into cell along with N ——— actually 2 Na+ per 1 glucose

Coupling active transport to existing ion gradients • Na+ moves glucose up its gradient as Na+ flows down its gradient across apical membrane (2° active transport system)
•Transport protein (Na+lucose cotransporter) binds 2 Na+ ions & 1 glucose molecule on outer apical surface • Movement of 2 moles of Na+ ions into a cell can generate a glucose conc. that is 30,000 times higher inside than out

• Nerve cells (neurons) specialized for the transmission of information, which is coded in the form of fast-moving electrical impulses
• electricity in metals is carried by electrons, electricity in aqueous solutions is carried by ions (positive cations and negative anions)

• Thin (<1nm) layer of ions (~ 10 ions-thick)
• When ions of one type cross membrane, charge difference creates membrane potential - If balancing of charges are same on both side of the membrane the potential is = 0

potassium leak channels. •Membrane potential is determined by K+ gradient •Electrochemical gradient for K+ is zero
•In animals resting potential is -20 to -200mV (inside negative) •If a Na+ channel were to open, a new potential would be reached •Membrane potential is determined by state of ion channels and ion concentrations

(–) inside cell because:
1. Na+/K+-ATPase pumps more Na+ out than K+ in (10% contribution)
2. K+ ions leak out; excess (+) outside favors K+ remaining inside; balances; (90% contribution)
3. (–) proteins trapped inside

•inside of PM (–) 60-70 mV relative to outside
•magnitude and direction determined by differences in [ion] and permeability

The axon is big, not the squid— so it is not the giant squid axon and you needn’t be afraid

•Learned that of all of the ions in seawater that Na+ and K+ levels needed for an action potential
•resting potential determined by [K+] and height of action potential voltage change determined by extracellular [Na+

•Resting potential of axon -60-70 mV (neg. inside)
•Voltage-gated sodium channel — opens at -40 mV (so closed in resting axon)
•Voltage-gated potassium channel — opens at +40 mV (so closed in resting axon)

1. stimulus causes opening of Na+ channels causing depolarization of membrane (gets more (+))
(if small amt, then rapidly returns to resting potential)
2. if depolarization reaches -40 mV threshold, voltage-gated Na+ channels open & Na+ rushes in; depolarization to +40 mV; all-or- none law
3. At +40 mV Na+ stops entering because electrical gradient and conc. gradient balance
4. 1 msec later, Na+ channels inactivated and voltage-gated K+ channels open to let K+ out and -60 mV returns

•total % of cell’s ions doesn’t change much
•Internal Na+ is mM, one action potential lets in pM amounts of Na+
•many consecutive action potentials without Na+ /K+ pump requirement
•in a 10 um3 cell 1/100,000 of internal K+ must flow out to change voltage by 100 mV

•Change in voltage in one part of axon triggers opening of channels in next part of axon
•Does not weaken (decrease in size) as it travels
•Note that it moves one direction. Why?

•At +40 mV even though voltage would keep channel open, it becomes inactivated
•Na+ stops entering cell and channel cannot be reopened initially

Voltage-gated Na+ channel change conformation during an action potential

rom onset of action potential to end of refractory period ~2 msec; thus a neuron can produce 100’s per sec.

•the greater the axon diameter, the faster the impulse
•wrapping axon with myelin sheath allows small axons to transmit faster (~20X or more)

•action potential at one node triggers next node
•Ion channels are concentrated at nodes
•saltatory conduction

•neurons are connected to their target cells at junctions called synapses
•presynaptic cells (neuron or sensory cell) and postsynaptic cells (neuron, muscle or gland cell) are separated by a synaptic cleft

•Voltage-gated Ca++ channels at terminal open to increase cytosolic Ca++
•Increased Ca++ causes synaptic vesicles to fuse with PM •synaptic vesicles contain neurotransmitte

electrical -> chemical -> electrical transmission