Membrane Transport Lecture (4)

Front 1

Describe the permeability properties of the plasma membrane.

Back 1

-The plasma membrane and its proteins function as a selectively permeable barrier

-Molecules diffuse across the membrane at different rates

-Some are able to move through the lipid bilayer more easily than others based upon their chemical properties

-For those that cannot easily cross the membrane there are transporters in the membrane to control their entry

Front 2

What type of molecules can move across the lipid bilayer by passive diffusion?

Back 2

These molecules pass through easily

1.) Small, uncharged, nonpolar molecules

2.) Gasses (O2 or CO2)

These molecules pass through, but slowly

1.) Small, uncharged polar molecules like H2O and Urea and glycerol

Front 3

What quality of molecules slows their passive diffusion through the membrane?

Back 3

-The rate is much slower as molecules get larger or more polar or charged

-VERY SLOW (sugars, amino acids)

-EFFECTIVELY IMPERMEABLE (Ca2+, K+, Na+, Cl=)

-NEVER (proteins)

Front 4

Passive Diffusion

Back 4

-When molecules diffuse directly though the lipid bilayer with NO energy consumption and NO protein involvement

Front 5

How does solubility of the molecule in the lipid bilayer affect its rate of passive diffusion?

Back 5

-The rate of movement is directly related to the solubility of molecules in the lipid bilayer (partition coefficient)

-The more soluble a molecule is in the lipid bilayer, the faster it will diffuse across

Front 6

What is the cell's solution for the limited permeability of the membrane for some substances>?

Back 6

Membrane Transport Proteins

-The solution to the limited permeability of the membrane for some substances is for cells to have transport proteins in their membranes

Front 7

Transport Proteins

Back 7

-These proteins are enzymes that catalyze the movement of specific substances across the membrane

Front 8

Passive Transport

Back 8

"Facilitated Diffusion"

-Passive transporters allow net movement down an electrochemical gradient

-They require NO energy beyond thermal motion

1.) Carrier Proteins

2.) Ion Channels

Front 9

Active Transport

Back 9

-Active transporters can move molecules AGAINST an electrochemical gradient and require EXTRA energy input for transport

1.) ATP-dependent pumps

2.) Symporter/Co-transporter Carrier Proteins

3.) Antiporter/Exchanger Carrier Proteins

Front 10

Describe the differences between the types of passive and active transport.

Back 10

Front 11

Describe the difference between channel proteins vs. carrier proteins.

Back 11

Carrier Proteins: can be active OR passive

-Undergo conformational changes and act like enzymes with binding of substrate to active site

-SLOW transport rate

-Glycoproteins

Channel Proteins: ALWAYS passive

-Ion selective FAST rates of transport

-Solute diffuse through pore NOT bind

-Lipoproteins

-ONLY pass water soluble molecules

Front 12

Describe the kinetics of the different types of transport.

Back 12

Simple Diffusion and Channel-Mediated Transport: diffusion doesn't peak as there is NO saturation; linear trend

Carrier-Mediated Transport: transport beaks when binding sites on carrier are saturated; curved trend

Front 13

TRUE OR FALSE: Unlike facilitated diffusion through ion channels, facilitated diffusion by carrier proteins can be saturated due to binding of solute to carrier

Back 13

TRUE

Front 14

How do cells use transport proteins to generate an internal ionic environment that is different than the external environment?

Back 14

-The ionic composition is not the same inside and out

-Electroneutrality: total number of cations and anions are equal

-Osmolarity: the total number of particles inside and out is equal

-Cells exploit gradients created by their transport proteins to perform critical metabolic functions

Front 15

Describe the importance of maintaining intracellular fluid osmolarity equal to that of the extracellular fluid osmolarity

Back 15

Isotonic: the goal as there is no net loss or gain

Hypotonic Solution: water rushes into the cell; cell swells and can burst

Hypertonic Solution: water rushes out of the cell; the cell shrinks

-The water always moves to the solute (the solute is trapped behind the membrane)

Front 16

If the inside of the cell has a net _______ charge, a negative ion (anion) will be ______ and a positive ion (cation) will be ________.

Back 16

If the inside of the cell has a net negative charge, a negative ion (anion) will be repelled and a positive ion (cation) will be attracted

Front 17

Electrochemical Gradient

Back 17

-The combination of the chemical concentration gradient and the electrical gradient determines the rate and direction of transport of a charged molecule

Front 18

Membrane Potential (Vm)

Back 18

-A force exerted across a membrane by a separation of charges that generates an electrical potential

-Only a few ions (0.001%) have to flow in or out to generate the electrical force SO the concentrations DO NOT have to appreciably change

Front 19

A very ______ number of ions need to move to create the membrane potential. __________ remains essentially unchanged.

Back 19

A very small number of ions need to move to create the membrane potential. Concentration remains essentially unchanged

Front 20

Nernst Equation

Back 20

-Gives the membrane potential that is sufficient to counter a given concentration gradient for one specific ion (equilibrium potential)

Z is the charge of the ion

Front 21

Nernst Potential

Back 21

-Also called reversal potential

-The membrane potential at which there is no NET flow of that one particular ion from one side of the membrane to the other

Front 22

Describe what constitutes a true equilibrium.

Back 22

True Equilibrium: No energy is required to maintain the status quo

-There is no net ion flow (although equal amounts of the ion are stochastically passing in and out) and no net currents are flowing, regardless of whether there is still a concentration gradient

Front 23

List the Nernst Potentials for the most common ions.

Back 23

Ek= -88mV

Ecl= -75mV

Ena= +60mV

Eca= +129mV

Front 24

In real cells, ________ is determine by open conductances for multiple ions.

Back 24

In real cells, Vm is determined by open conductances for multiple ions

Front 25

TRUE OR FALSE: Typically, resting cells have a K+ cunductance (Gk) that is 10 times higher than the Na+ and Cl- conductances

Back 25

TRUE: typically, resting cells have a K+ conductance (Gk) that is 10 times higher than the Na+ and Cl- conductances

-Open K+ channels make the cell 10 times more permeable to K+ than to Na+ or Cl-

Front 26

Goldman's Equation or Current Equation

Back 26

-Can be using to estimate the membrane potential under the reality that multiple ion conductances affect membrane potential

-Plug in values for conductances and Nernst/Equilibrium potentials for the various ions

Front 27

What is the resting membrane potential?

Back 27

The resting membrane potential Vm tends to be between -70mV and -75mV

-Vm will tend to be close to the Eion for the ion that has the most conductances open in the membrane

Front 28

TRUE OR FALSE: The Vm will tend to be close to the Eion for the ion that has the most conductances open in the membrane.

Back 28

TRUE

-If the Vm is -75mV then it is likely Cl- has the highest conductance/most permeability as it has an equilibrium potential of -75mV

-If conductances for only one ion are present, then Vm is equal to the equilibrium potential for that ion

Front 29

Driving Force

Back 29

-At any given membrane potential (Vm), the driving force for an ion is its total electrochemic potential (units in millivolts)

Vm-Eion

Front 30

Describe the driving forces of the most common ions under standard ionic conditions and a normal resting membrane potential of -75mV

Back 30

K+: +13mV (OUT)

Na+: -135mV (IN)

Cl-: 0mV (NONE)

Ca2+: -195mV (IN if Ca2+ channels open)

Front 31

In real cells then, Vm reflects NOT an equilibrium but a _____ ______.

Back 31

In real cells then, Vm reflects NOT an equilibrium but a STEADY STATE.

-There IS ion flow (Na+ flowing in and K+ flowing out) but the NET current is zero (Na+ and K+ current are equal but opposite)

-The strong driving force and low conductance of Na+ is balanced by the weaker driving force but high conductance of K+

Front 32

Over time the Na+ and K+ gradients will run down. To prevent this from happening what do cells use?

Back 32

Active Transport

-In this case they would use energy from ATP hydrolysis to use Na+/K+ ATPase pumps to pump Na+ out and pump K+ in

Front 33

TRUE OR FALSE: All the Na+ that contributes the Vm is leaking in down its concentration gradient.

Back 33

FALSE:

-Not all Na+ is just "leaking in" to contribute to Vm as that would be inefficient

-Instead the energy in the electrochemical gradient for Na+ is being used by secondary active transporters to do useful work.

-The Na+ conductance in part represents the activity of thousands of these transporters

Front 34

Ion Channels

Back 34

-Multi-pass transmembrane proteins forming a pore through membrane to allow passage of ions

-Bidirectional: net direction down electrochemical gradient

-Ion Specific: there are selective ion channels and nonselective cation channels

-Interactions with ion limited to determining selectivity

-FAST: 10^7 to 10^8 ions/sec

-Regulated: can be opened and closed via gating

Front 35

Could an ion move passively against its concentration gradient?

Back 35

Yes if the electrochemical gradient in the opposite direction is strong enough

Front 36

What controls ion channel activity?

Back 36

GATING

1.) Voltage-gated

2.) Ligand-Gated (EC or IC)

3.) Mechanically Gated

Front 37

Voltage-Gated K+ Channel

Back 37

-Ion Channel

-Tetramer made up of 4 subunits, each with multiple membrane spanning domains

-Both N and C termini are intracellular

-When the subunits come together they form a channel through the membrane

-channel is opened by membrane depolarization and allows flux of greater than 10^6 ions/sex

-Selective for K+

Front 38

Ion Channel Selectivity Filter

Back 38

In the K+ channel, the selectivity filter is a 12 Angstrom long segment of pore that is line with carbonyl atoms

-These atoms acts as surrogate water molecules, allowing K+ ions to shed their hydration shell and enter into the pore

-The arrangement of O2 atoms surrounding each K+ atom mimics that of a hydrated K+ ion so the energy barrier for entry and exit of K+ ions is LOW

Front 39

Opening an ion channel requires a relatively _____ change in conformation

Back 39

Opening an ion channel requires a relatively small change in conformation

Front 40

What is the structure of a eukaryotic voltage gated K+ channel

Back 40

-The new structures are the multi-colored ones on the left

-The old model is the purple one on the right

Front 41

TRUE OR FALSE: Flux through ion channels is so fast that currents flowing through a single channel can be recorded.

Back 41

TRUE

-The different between the two is that the second one is reading multiple ion channels

Front 42

Describe the selectivity of the voltage-gated K+ channel.

Back 42

-A homotetramer with each subunit having multiple transmembrane domains and contributing to the pore

-Selectivity filter at one end that perfectly fits a dehydrated K+ ion allowing efficient transfer down its gradient to water on the other side

-Na+ is too small to interact properly with the amino acids around the filter, and cannot get through the channel

Front 43

Where does the conformational change occur in a voltage-gated K+ channel to open the pore?

Back 43

-The channel is voltage-gated

-A change in membrane charge causes a minor conformation charge in the base of the channel (S4), opening the pore

Front 44

Uniport Carriers

Back 44

-Passive Transport

-Multi-pass transmembrane proteins that act more like an enzyme than a pore (substrate binding induces reversible conformational changes)

-Bidirection: but flows down gradient

-Slower than ion channels (due to substrate-carrier interactions)

Front 45

For which substances is transport ONLY subject to the chemical concentration gradient and NOT the electrical gradient?

Back 45

-For uncharged solutes like glucose, transport is only subject to the chemical concentration gradient

-So unlike ions, uncharged molecules CANNOT move passively against their chemical/concentration gradient

Front 46

GLUT Transporters

Back 46

-An example of a uniport carrier

-12 transmembrane domains

-Intracellular N and C termini

-Look a lot like a channel

-Millimolar affinity for glucose

-One isoform GLUT 4 is stored in vesicles and inserted into muscle or fat cell membrane in response to the hormone insulin

Front 47

Describe the regulated secretion in order to put GLUT 4 transporters on the membrane surface.

Back 47

1.) Intracellular pool of glucose transporters is kept packaged in specialized recycling endosomes

2.) A signal binding to an insulin receptor causes relocalization of glucose receptors to the plasma membrane to boost glucose uptake into the cell

Front 48

Active Transporters

Back 48

-Protein complexes that act as pumps to move molecules AGAINST a chemical, electrical, or electrochemical gradient

-Requires energy input

-SLOW (1-10^3/sec)

1.) Primary Active Transporters

2.) Secondary Active Transporters

Front 49

Primary Active Transporters

Back 49

1.) ATP-dependent Pumps: ATP used as the direct energy source

2.) Light Driven Pumps: Light used as the direct energy source

-They expend energy to create a gradient which will "power" a secondary active transporter

Front 50

Secondary Active Transporters

Back 50

-Uses energy of an electrochemical gradient generated by primary active transporters to couple movement of one molecule down its gradient in order to move another molecule up its gradient

1.) Symporters (Co-transporters): moves two molecules in same direction

2.) Antiporters (Exchangers): moves two molecules in opposite directions

Front 51

ATP-dependent Pumps

Back 51

-PRIMARY ACTIVE TRANSPORTERS

-Hydrolyze ATP and use the energy to move one or more molecules across the membrane

1.) P-Type

2.) V-Type

3.) ATP Binding Cassette (ABC) Transporters

Front 52

P-Type ATP-Dependent Pumps

Back 52

P-Type: become Phosphorylated by the PO4 from ATP during the transport

-Plasma Membrane

1.) Na+/K+ ATPase

2.) K+/H+ ATPase

3.) Ca2+ ATPases

Front 53

V-Type ATP-Dependent Pumps

Back 53

V-Type: Vesicular H+ ATPases

-Do NOT become phosphorylated

-Pump H+ into membrane compartments

-Acidification of endocytic vesicles, lysosomes, golgi, exc.

Front 54

Na+/K+ ATPase

Back 54

-Moves 2K+ inward and 3 Na+ outward using energy from ATP

-BOTH ions are being moved AGAINST their electrochemical gradients

-Key role in maintaining distribution of these ions in cells

-Mechanism involves a conformational change in the shape of the protein driven by ATP and binding of ions

-ATP cleavage is actually used to phosphorylate the transporter (P-TYPE)

Front 55

Ouabain is a plant compound that blocks the Na+/K+ ATPase. What would that do to the cell?

Back 55

-Nothing immediately, but over time the gradients would slowly run down and an accumulation of Na+ in the cell would cause depolarization and swelling of the cell (increase in intracellular Na+ makes cell very positive AND swells due to water rushing in as cell is hypertonic to environment)

Front 56

K+/H+ ATPases

Back 56

P-TYPE
-Stomach acidification

-Sends K+ out and H+ in to lower pH of gastric fluid in the stomach

Front 57

Ca2+ ATPases

Back 57

P-TYPE

-Pump Ca2+ out of the cell (PMCA ATPase) or into the ER (SERCA ATPase)

1.) Plasma Membrane Ca2+ ATPase

2.) Sarcoplasmic Reticulum Ca2+ ATPase

-SERCA ATPase pumps calcium ions back into the sarcoplasmic reticulum to reduce calcium level around actin and myosin filaments and allowing the muscle to relax

Front 58

ABC (ATP Binding Cassette) Transporters

Back 58

-DISTINCT CLASS (NOT P OR V TYPE)

-Uses ATP for transport but DO NOT become phosphorylated

-5% of bacterial genome is this type of gene

-Some isoforms are important in cancer; cancer cells overproduce the transporter and can pump anti-cancer drugs out of the cell making them resistant (multidrug resistance transporters)

-The cystic fibrosis gene product is an ABC transporter for Cl- ions that is found in the lung, sweat glands, and kidneys

Front 59

What is the name for coupled transporters that run off the ion gradients established by another transporter that expended ATP?

Back 59

Secondary Active Transporters

1.) Symporters

2.) Antiporters

Front 60

Symporters/Co-transporters

Back 60

Both molecules move in the SAME direction

-Secondary Active Transport (uses the energy of one molecule moving down its existing gradient to move a second molecule up its gradient)

-Carrier Mediated: relatively large conformational changes, unlike channels which undergo minor shape changes

Front 61

SGLT1 and SGLT2 (Sodium-glucose transport proteins)

Back 61

-2Na+/1 glucose symporter

-Concentrates glucose from lumen of intestine into intestinal epithelium cells

-Works against the glucose gradient by using the Na+ gradient

-Can work against a 30,000 fold gradient (i.e. it can accumulate glucose 30,000 fold)

Front 62

Antiporters/Exchangers

Back 62

-Similar to symporters except the two ions are moving in opposite directions

-Secondary Active Transport

1.) Na+/H+ Exchangers

2.) Na+/Ca2+ Exchangers

Carrier-Mediated (pronounced conformational changes

Front 63

Na+/H+ Exchangers

Back 63

-Secondary Active Transport

-Antiporter/Exchanger

-Brings Na+ in to pump H+ out

Front 64

Na+/Ca2+ Exchanger

Back 64

-Secondary Active Transport

-Antiporters/Exchangers

-Used to pump Ca2+ out of cells

-Brings in 3Na+ to pump out 1 Ca2+

Front 65

Describe the transporters involved in the absorption of glucose by the small intestine.

Back 65

1.) Na+/K+ ATPase: active primary transporter; pumps Na+ out and brings K+ in using ATP (creates Na+ gradient)

2.) Na+/Glucose Cotransporter: secondary active transporter uses Na+ gradient created by Na+/K+ ATPase to accumulate glucose in intestinal epithelial cell at high concentrations from lumen

3.) GLUT 2: passive mediated carrier transport; glucose moves down its gradient into the blood

Front 66

Describe signaling by nerve cells.

Back 66

-Nerve cells generate, receive and transmit electrical signals throughout the body

-They can be up to a meter or more in length

-They grow that long using a special type of "motility" in which they extend an axon but never retract the cell body

-The cell body with dendrites receives the signal and the axon with terminal branches delivers the signal to the next element in the chain

Front 67

How do neurons send signals over long distances since diffusion is far too slow?

Back 67

Action Potentials

Front 68

Action Potentials

Back 68

-Action potentials are electrical signals caused by rapid changes in Vm

All or none (they fire or they don't)

-Propagate actively down nerve axons

-Action potentials can travel down axons at speeds of up to 200 miles per hour

Front 69

If action potentials are all-or-none, how do neurons encode information about signals?

Back 69

Frequency: temporal summation of action potentials

-The stronger the stimulus, the higher the frequency at which action potentials are generated

-There is NO such thing as an action potential being stronger than another; the strength of the signal is in how many action potentials there are

Front 70

Explain the steps of an action potential.

Back 70

1.) A stimulus causes the target cell to depolarize towards the threshold of excitation (-55mV)

2.) If the threshold of excitation is reached, all voltage-gated Na+ channels open and the membrane depolarizes to +40mV

3.) At the peak of the action potential Na+ channels inactivate and slow K+ channels begin to open

4.) The membrane repolarizes as K+ begins to leave the cell and hyperpolarizes (refractory period)

5.) The K+ channels close and the Na+/K+ ATPases restore the resting potential

Front 71

Describe the conductances during the steps of the action potential.

Back 71

-Conductance of Na+ rises until peak of action potential when the channels inactivate

-Conductance of K+ channels slowly rise and fall

Front 72

How is an action potential generated?

Back 72

-Initially, voltage gated Na channels are closed, with open inactivation gates

-Stimulation starts to depolarize the membrane, opening Na+ channels until threshold where all the voltage-gated Na+ channels open (runaway positive feedback cycle)

Front 73

How is the action potential terminated?

Back 73

-Na+ channel inactivation gates close with a delay following depolarization; no Na+ current flows now even if activation gates stay open

-With Na+ channels closed, Vm should return to Vrest BUT voltage-gated K+ channels open, speeding the repolarization and causing a slight hyperpolarization

-Repolarization causes Na+ channel inactivation gate to reopen and K+ voltage-gated channels to close

-System is now reset and able to fire another action potential

Front 74

What would happen if there were no voltage-gated K+ channels?

Back 74

-The membrane potential would slowly drift back down to Vrest because the Na+ would flow down gradient

Front 75

How is the action potential propagated along the nerve cell membrane?

Back 75

-The local influx of Na+ during an action potential depolarizes the membrane further down the axon

-This opens voltage gated Na+ channels at those sites, causing an action potential

-The refractory period caused by inactivated Na+ voltage-gated channels assures that there is no back propagation

Front 76

TRUE OR FALSE: While voltage-gated Na+ open during the action potential, voltage-gated, ligand-gated, or mechanically gated channels can open and create graded potentials until threshold is reached.

Back 76

TRUE

Front 77

TRUE OR FALSE: There are only excitatory graded potentials stimulating an action potential

Back 77

FALSE
-There is a summation of excitatory (depolarizing) and inhibitory (hyperpolarzing) graded potentials

-If excitatory graded potentials win out and the membrane potential reaches the threshold of -55mV then an action potential will fire

Front 78

TRUE OR FALSE: A ligand-gated ion channel is always open when it is occupied by a substrate.

Back 78

FALSE

-When its unoccupied it is closed BUT it can be occupied and open or occupied and closed (INACTIVATED)

Front 79

Describe how neurons "talk" to each other via chemical synapses.

Back 79

1.) Electrical signal is transmitted down an axon and now must jump to the next neuron

2.) When it reaches the nerve terminal it causes the opening of voltage-gated Ca2+ channels allowing Ca2+ to enter the terminal

3.) Ca2+ acts as a chemical messenger that causes the fusion of synaptic vesicles with the presynaptic membrane

4.) The chemical neurotransmitter is released into the space between the neurons

5.) The neurotransmitter diffuses to the next neuron and binds to and opens a ligand-gated Na+ channel

-This begins the action potential in the next neuron

Front 80

After how many action potentials has enough Na+ entered that it must be pumped out by increased activity of Na+/K+ ATPase?

Back 80

-After many thousands of action potentials, enough Na+ has entered that it must be pumped out by increased activity of the Na+/K+ ATPase

-Remember only a small number of ions need to be transported to appreciably change membrane potential!!!!

Front 81

Could you stimulate an action potential SHORTLY after adding Ouabain?

Back 81

-Yes because it would take awhile for the chemical gradient to run down to the point that Na+/K+ ATPase need to reestablish it

-It takes many thousands of action potentials before it is needed

Front 82

Describe cardiac action potentials.

Back 82

1.) Resting Potential (maintenance of Na+ and K+ gradients by Na+/K+ ATPase)

2.) Depolarization (voltage-gated Na+ channels open)

3.) Partial Depolarization (Na+ channels close and Ca2+ channels open)

4.) Plateau (Ca2+ channel opening is balanced by K+ channel opening causing a plateau)

5.) Repolarization (other K+ channels open and Ca2+ channels close)