CLASS: 11-12
DATE: 9-7-10
PROFESSOR: Carmel McNicholas
Biological Membranes and Principles of Solute and Water Movement
XLI.
XLII.
Scribe: Angi Gullard
Proof: Coty Cantrell
Page 1 of 4
Principles of Ion Movement [S41]:
Diffusion of Electrolytes [S42]
a. For diffusion of uncharged solutes, take into account concentration gradient
b. For charged solutes (electrolytes), take into account concentration and electrical gradients
c. Will demonstrate what happens to membrane potential using the 2-compartment model: 100mM solute on
one side of membrane, and 10mM of solute on other side, with membrane permeable to the solutes
XLIII. The Principle of Bulk Electroneutrality [S43]
a. Remember the principle of bulk neutrality: the number of positive charges in a solution must be the same as
the number of negative charges
b. Nernst equilibrium potentials are generated to counteract concentration gradients to prevent violation of this
principle
XLIV. Diffusion of Electrolytes [S44]
a. Here, the membrane is permeable to K+ and Ac-, which both start out on same side of membrane
b. Driving force of diffusion is the concentration gradient, KAc will flow down concentration gradient so that both
compartments with have equal concentrations
c. Molecular radius of K+ is smaller than Ac-, so get a dipole (charge differential), and a diffusion potential
d. Remember that movement of ion with small radius predominates, and the positive charge attracts the
negative charge, creating a diffusion potential
e. Because membrane is permeable to both ions, end up with inter-mixing and equal concentrations of both
ions in both compartments
XLV. Diffusion of Electrolytes [S45]
a. Now, the membrane is permeable to only K+, so chemical and electrical gradients drive K+ movement
b. The potential that develops to prevent movement of K+ is calculated using Nernst equilibrium potential,
which accounts for electrical and chemical gradient that balances out positive movement
c. Can’t move Ac-, so K+ moves down concentration gradient, but cannot have net movement of a charge (+)
in the absence of the opposite charge (-)
d. So get development of charge along membrane to prevent K+ movement
i. A potential develops where electrical and chemical forces are balanced and there is no net movement
of K+
ii. This small electrical force is along membrane, but no change in concentration of ions in bulk solution
e. Equilibrium potential given by Nernst equation: E ion = 60/z x log(Cs1/Cs2)
i.
Understand principles involved here
ii.
Nernst equilibrium potential develops to prevent net movement of charged molecule, to balance
electrical and chemical forces
XLVI. Untitled (Nernst equation) [S46]
a. Only a few charges need to move to generate the potential difference across cell membrane, but there is no
change in bulk concentration
b. Example of trick question: Predict change in concentration across membrane if develop electrical potential
difference of 60mV.
i. Answer – no change, negligible (there is no net movement - Nernst potential counterbalances chemical
difference across membrane)
XLVII. Calculating a Nernst Equilibrium Potential [S47]
a. NaAc – 100mM vs. 10 mM; membrane permeable only to Na+ so no net change in bulk concentration
i. Want potential that prevents movement of Na+
b. Na+ goes down its concentration gradient from 100 to 10 (forget about Ac because it’s impermeable to
membrane)
c. If move charge (Na+) from left to right, want to keep positive charge on left, so develop negative charge
on left side of membrane and positive charge on right side of membrane
d. Plug in numbers for Nernst equation: 60/+1 x 100/10 = +60mV
e. Develop 60mV potential difference with right side being positive relative to left side.
XLVIII. Taking valence of the ion into account in calculating a Nernst potential [S48]
a. Here, for Cl- movement across cell membrane, have physiological concentration gradient with high Clconcentration outside of cell relative to inside
b. Nernst equilibrium equation takes into account the outside versus inside concentrations (the way this
equation is written is more representative of biological calculations)
c. Plug in numbers for Nernst equation: 60/-1 x 100/10 = -60mV
CLASS: 11-12
Scribe: Angi Gullard
DATE: 9-7-10
Proof: Coty Cantrell
PROFESSOR: Carmel McNicholas
Biological Membranes and Principles of Solute and Water Movement
Page 2 of 4
d. Want to have negative potential inside and positive outside potential to keep Cl- outside the cell.
e. Normal biological cells: refer potential on inside of cell relative to outside of cell
i. Inside cell potential is -60mV (if membrane is only permeable to Cl-)
XLIX. Untitled [S49]
a. Here, have 10-fold K+ gradient of 100mM K+ inside and 10mM K+ outside
b. Plug in numbers for Nernst equation: 60/+1 x 10/100 = -60mV
c. K+ will want to move out of cell and to prevent movement, make cell negative on inside and positive on
the outside
d. Memorize concentrations and equilibrium potentials for ions listed in chart
e. Equilibrium potentials depend on cell type
L. Remember: [S50]
a. Memorize this slide
LI. Goldman-Hodgkin-Katz (GHK) equation [S51]
a. Do not need to remember this equation!
b. GHK equation accounts for permeabilities of multiple ions to determine the membrane potential
c. Flip o to i to maintain valence
d. What is the membrane potential for cell permeably only to K+? -80 or -90mV
e. What is the membrane potential for cell permeable only to Na+? +67
f. What is the membrane potential for cell permeable to K+ and Na+? between 25-30 mV (between the two
equilibrium potentials)
LII. Membrane Transport Mechanisms I [S52]
a. Most membranes impermeable to most molecules
LIII. Untitled [S53]
a. Passive transport – driving force for solute movement is solute gradient across cell membrane
LIV.
Transport across cell membranes [S54]
a. Passive transport across cell membrane – driving force is diffusion gradient across cell membrane (Ex:
O2, urea) or facilitated diffusion (ion channel, carrier protein)
b. Facilitated diffusion – molecule gets assistance of carrier protein to cross cell membrane
c. H2O – can move through cell membrane, but when rate of flux calculated, see true movement of water by
diffusion alone via pores in membrane – aquaporins; their activity can change rate of water movement
d. Active transport:
i. Primary – ATP hydrolysis drives solute movement
ii. Requires direct input of energy; generates ion gradients)
LV. Transport through pores [S55]
a. Conduits in cell membrane
i. Gated pores- channels
ii. Simple pores- porins
b. Hole in membrane that allows ion flow
c. Structure of pore determines what moves through it
d. Are always open; equivalent to hole punched in membrane
i. There is no conformational change that alters rate of transport
LVI.
Transport through channels [S56]
a. Gated pores – part of protein structure that form change to allow ion movement ; open or closed
b. Structure determines rate of ion movement
c. Variety of sensors that respond to Vm: 2nd messengers or ligands
i. EX: acetylcholine ligand binds, causes conformational change, and gates opens pore
ii. Change in membrane potential can change structure of pore
d. The amino acids that form molecule form selectivity filter – Na(GYG sequence only allows Na+ to move
through pore)/K channels specific or nonselective channels
e. Gated pores – not always open, allows for regulation
LVII.
Untitled [S57]
a. Solute movement through pores and channels occurs via simple diffusion, is passive and downhill, from
high to low concentration
b. Metabolic energy is not required
LVIII. Transport through carriers [S58]
a. Carriers require a conformational change to occur across the cell membrane, slowing down movement of
solute
CLASS: 11-12
Scribe: Angi Gullard
DATE: 9-7-10
Proof: Coty Cantrell
PROFESSOR: Carmel McNicholas
Biological Membranes and Principles of Solute and Water Movement
Page 3 of 4
b. Need binding of solute, conformational change, and then unbinding of solute  slower transport than ion
channels
LIX.
Carrier mediated transport [S59]
a. Different types of carrier mediated transport
i. Cotransporter (symport) – solutes go in same direction
ii. Antiporter – exchange of 1 solute in one direction and other solute in other direction; type of
facilitated diffusion
1. Facilitated diffusion – area of high to low concentration; no direct energy input
LX. Carrier-mediated transport: facilitated diffusion [S60]
a. The flux of solute across membrane
b. Case of simple diffusion – linear relationship between flux and concentration of solute, so if increase
concentration, increase flux
c. Carrier –mediated: reach max value JxMax, follow Michaelis-Menten kinetics where reach max value and
cannot make flux faster because need binding, conformational change, and unbinding  maximized rate
i. Km value – concentration at which half flux occurs; lower Km = higher affinity of transport for
solute
d. Ion channel – max out rate of Flux, but would be beyond physiological concentrations
LXI.
Carrier-mediated transport: active transport [S61]
a. In contrast to passive movement, can move solute from low to high concentration - uphill movement of
solute against electrical and chemical forces that requires an input of metabolic energy
b. 2 classes
i. Primary – direct energy input by ATP hydrolysis, attach phosphate to protein, conformational
change, movement of solute against concentration gradient
ii. Secondary - use concentration gradient set up by primary active transport to move solute
LXII.
Primary Active Transport – Na-K ATPase [S62]
a. Na/K pump – ubiquitous proteins in cells
b. Pick up Na on inside of cell, conformational change, so Na is high on outside
c. Change affinity of K from outside, move it to inside of cell
d. Electrogenic process because transporter picks up 3 Na for 2 K – net positive charge movement; needed
for nutrient uptake and electrical activity, and maintain osmotic balance
LXIII. Secondary Active Transport-Symport [S63]
a. Utilize gradient set up by primary active transporter
b. Primary transporter moves solute X (Na) out – Na/Cl cotransporter
c. Most cells rest at -80 or -90mV, an electrical gradient that would keep Cl- out of cell, so use Na/K ATPase
energy to take glucose/Cl- in cell
d. Remember it is active transport but there is no direct energy input, no hydrolysis on transporter, simply
use energy derived from concentration gradient of primary transporters
LXIV. Comparison of Pores, Channels, and Carriers [S64]
a. Pore – conduit is always open
b. Carrier – never a continuous opening; slower rate of solute movement, cycling of conform changes
c. Channel – intermittently open, can move more particles in ion channel than transporter
LXV. The “pump-leak” model (generating the membrane potential) [S65]
a. How does the Na-pump generate membrane potential? Pump-leak model
b. Typical cell with Na/K ATPase in plasma membrane with ion channels
c. GHK equation – membrane potential is determined by most permeable pathway to go through
i. typical cell resting membrane potential is more permeable to K+
d. Relative permeability of each ion determines resting membrane potential
e. Na/K pump – 3 Na+ moved for every 2 K+, ATP hydrolysis causes conformational changes
f. As pump net positive charge out, leave behind more negative intracellular charge  sets up gradient but
does not dictate membrane potential of cell
g. Permeability through ion channel dictates potential of cell membrane
h. More permeable to K+ than to Na+; if cell membrane potential was +50mV, cell membrane potential
would be more permeable to Na+
i. Most cells, if not all, have some degree of negative membrane potential because membrane has better
K+ permeability
j. Small negative intracellular potential for K+ movement, so get net flux K+ out of cell due to concentration
gradient , but to maintain equilibrium (permeability to Na+), get negative potential
k. Question: Why do most cells have negative intracellular potential? Set up gradients for Na+ and K+
CLASS: 11-12
Scribe: Angi Gullard
DATE: 9-7-10
Proof: Coty Cantrell
PROFESSOR: Carmel McNicholas
Biological Membranes and Principles of Solute and Water Movement
Page 4 of 4
i. Negative intracellular potential is closer to K+ equilibrium potential (-90mV) than for Na+ (-67mV)
so relative permeability to K+ is greater than for Na+  slight outward leak of K+
LXVI. Gibbs-Donnan Membrane Equilibrium [S66]
a. Na/K ATPase needed to generate ionic gradients across cell membrane
i. Maintains cell volume (including charged proteins, nucleic acids)
b. Proteins restricted inside cell – have osmotic effect on cell milieu and their charge can influence
distribution of other ions to maintain cell equilibrium
LXVII. Gibbs-Donnan Equilibrium [S67]
a. 2 compartments – C1 Na/Cl solution, equimolar solution
i. Make membrane permeable to Na+ and Cl- but not to protein  get flow of Cl- from C1 to C2
because Cl- goes down concentration gradient; also get movement of Na+ to follow law of
electroneutrality
b. Do not need to remember equations, but know that at some point, equilibrium potential develops across
membrane to prevent net movement of charge
LXVIII. Gibbs-Donnan equilibrium (the tendency for cells to swell) [S68]
a. In closed compartment ions move with water from C1 to C2 leading to an osmotic gradient and movement
of Na+
b. Restriction in movement of water leads to swelling, so through evolution there is a pumping mechanism to
maintain gradient and prevent swelling  need Na-pump
LXIX. The Na-pump (Na-K pump) is essential for maintaining cell volume [S69]
a. Moves net osmolytes out of cell, which is an electrogenic process because the pump moves 3 Na+ for
every 2 K+
b. As water moves, osmotic gradients need to pump net gain of osmolyte out of cell
c. If inhibit process with ouabain, can’t pump substance out of cell, get swelling of cell
d. *Na/K ATPase sets up ionic gradients that do not determine membrane potentials (contribute 5-10mV to
membrane potential) but sets up gradients that dictate ion movement to set up membrane potential
e. *Na/K ATPase maintains osmotic balance because it moves net osmotic active substance out of cell (Na+
here) but presence of plasma proteins influences net movement of ions
f. If did not counterbalance this, would draw in water and cause cell to burst
[End 46:57 mins]