carmel m. mcnicholas, ph.d. department of physiology & biophysics contact information: mclm 868...

Carmel M. McNicholas, Ph.D.Department of Physiology &

Biophysics

Contact Information:MCLM 868934 1785

cbevense@uab.edu

BIOLOGICAL MEMBRANES AND PRINCIPLES OF SOLUTE AND WATER

MOVEMENT

Sept. ‘11

OUTLINE• Biological Membranes and Principles of Solute and Water Movement

• Diffusion and Osmosis

• Principles of Ion Movement

• Membrane Transport

• Nerve Action Potential

• HANDOUT AND PROBLEM SET

The Cell: The basic unit of life

(i) obtaining food and oxygen, which are used to generate energy(ii) eliminating waste substances(iii) protein synthesis(iv) responding to environmental changes (v) controlling exchange of substances (vi) trafficking materials (vii) reproduction.

[Na+] = 15 mM[K+] = 120 mM[Cl-] = 20 mM

[protein] = 4 mMOsmolality = 290

mOsm

EXTRACELLULAR (~40%) INTRACELLULAR (~60%)

[Na+] = 142 mM[K+] = 4.4 mM[Cl-] = 102 mM

[protein] = 1 mMOsmolality = 290 mOsm

Capillary endothelium

BLOODPLASMA

~3 LINTRACELLULAR

FLUID~25 L

TRANSCELLULAR FLUID~1 L

Composition: variable

[Na+] = 145 mM[K+] = 4.5 mM[Cl-] = 116 mM

[protein] = 0 mMOsmolality = 290 mOsm

INTERSTITIAL FLUID~13 L

Plasma membraneEpithelial cells

The fluid compartments of a 70kg adult human

TOTAL BODY WATER (~42 L)Modified from: Boron & Boulpaep, Medical Physiology, Saunders, 2003.

Solute composition of key fluid compartments

• Osmolality constant

• Cell proteins – 10-20% of the cell mass• Structural and functional

Membranes are selectively permeable

Gas molecules are freely permeable

Large / charged molecules need ‘assistance’ to traverse the plasma membrane

Small uncharged molecules are freely permeable

Gas molecules are freely permeable

Large / charged molecules need ‘assistance’ to traverse the plasma membrane

Gas molecules are freely permeable

Small uncharged molecules are freely permeable

Gas molecules are freely permeable

Large / charged molecules need ‘assistance’ to traverse the plasma membrane

Small uncharged molecules are freely permeable

Gas molecules are freely permeable

Structure of the Plasma Membrane

The Extracellular Matrix

The extracellular matrix (ECM) of animal cells functions in support, adhesion, movement and regulation

Epithelial cell

Basement

membraneCapillary

endotheliumConnective tissue and ECM

Fibroblast

The Extracellular MatrixThe ECM is an organized meshwork of

polysaccharides and proteins secreted by fibroblasts. Commonly referred to as connective tissue.

COMPOSITION:Proteins: Collagen (major protein comprising the

ECM), fibronectin, laminin, elastinTwo functions: structural or adhesive

Polysaccharides: Glycosaminoglycans, which are mostly found covalently bound to protein backbone (proteoglycans).

Cells attach to the ECM by means of transmembrane glycoproteins called integrins

• Extracellular portion of integrins binds to collagen, laminin and fibronectin.

• Intracellular portion binds to actin filaments of the cytoskeleton

The CytoskeletonIntracellular network of protein filaments

RoleSupports and stiffens the cellProvides anchorage for proteinsContributes to dynamic whole cell activities (e.g., dividing and crawling of cells and moving vesicles and chromosomes)

Three Types Of Cytoskeletal Fibers

Microtubules (tubulin - green)

Microfilaments (actin-red)

Intermediate filaments

Structural JunctionsTight

JunctionsAdhering Junctions

Desmosome

Zonula Adherens (belt)

Gap Junctions

ROLE: Passage of solutes (MW<1000) from cell to cell.• Cell-cell communication• Propagation of electrical signal

Carbohydrates are:• Covalently attached to membrane proteins and lipids• Sugar chains added in the ER and modified in the golgi Oligo and polysaccharide chains absorb water and form

a slimy surface coating, which protects cell from mechanical and chemical damage.

Membrane Carbohydrates and Cell-Cell Recognition – crucial in the functioning of an organism. It is the

basis for: > Sorting embryonic cells into tissues and

organs. > Rejecting foreign cells by the immune

system.

The Membrane Glycocalyx - cell coat

Alberts et al., Molecular Biology of the Cell, 4th Ed. Garland Science, 2002)

Transport of large molecules

EXOCYTOSIS: Transport molecules migrate to the plasma membrane, fuse with it, and release their contents.

ENDOCYTOSIS: The incorporation of materials from outside the cell by the formation of vesicles in the plasma membrane. The vesicles surround the material so the cell can engulf it.

Exocytosis

Endocytosis

Principles of Solute and Water Movement

Diffusion and Osmosis

Membranes are selectively permeable

Gas molecules are freely permeable

Large / charged molecules need ‘assistance’ to traverse the plasma membrane

Small uncharged molecules are freely permeable

Gas molecules are freely permeable

Large / charged molecules need ‘assistance’ to traverse the plasma membrane

Gas molecules are freely permeable

Small uncharged molecules are freely permeable

Gas molecules are freely permeable

Large / charged molecules need ‘assistance’ to traverse the plasma membrane

Small uncharged molecules are freely permeable

Gas molecules are freely permeable

Diffusion

Diffusion is the net movement of a substance (liquid or gas) from an area of higher conc. to one

of lower conc. due to random thermal motion.

Kinetic characteristic of diffusion of an uncharged

soluteModel: compartments separated by permeable

glass

A = cross sectional area of the glass discCs = concentration of uncharged solutex = thickness

compartment 1 compartment 2

x

Cs1 Cs

2

According to kinetics, the rate of movement can be described as follows: rate of diffusion from 1 2 = kCs

1

-{rate of diffusion from 2 1 = kCs2}

----------------------------------------------------------------------------net rate of diffusion across barrier = k(Cs

1-Cs2) = kCs

where k is a proportionality constant.

compartment 1 compartment 2

x

Cs1 Cs

2

Diffusion is proportional to the surface area of the barrier (A) and inversely proportional to its thickness (x).

k can thus be expressed as ADs/x, where Ds is the diffusion coefficient of the solute.

The concentration gradient across the membrane is the driving force for net

diffusion.

FLUX (Js) describes how fast a solute moves, i.e. the number of moles crossing a unit area of membrane

per unit time (moles/cm2.s) Therefore, net diffusion rate = ADsCs/x.

Dividing both sides by A (to obtain flux), we obtain:Fick’s first law of diffusion:

Flux = Js = DsCs/x“The rate of flow of an uncharged solute due to diffusion is directly proportional to the rate of

change of concentration with distance in direction of flow”

When the concentration gradient of a substance is zero the system must be in equilibrium and the net

flux must also be zero.

Diffusion of an uncharged soluteModel: compartments separated by a

lipid bilayer

Biological membranes are composed of a lipid bilayer of phospholipids interspersed with integral and peripheral proteins (“Fluid Mosaic Model”).

compartment 1 compartment 2

x

Cs1 Cs

2

The partition coefficient, Ks will increase or decrease the driving force of the solute S across the

membrane:Js = KsDsCs/x

Because it is difficult to measure Ks, Ds and x, these terms are often combined into a permeability

coefficient, Ps = KsDs/x. It follows that:Js = PsCs

Cs1

HydrophilicKs < 1 Cs

2

LipophilicKs > 1

Partitioning of an uncharged solute across a lipid bilayer

Ks lies between 0 and 1

Solute movement across a lipid bilayer through entry into the lipid phase occurs by simple

diffusion.

This movement occurs downhill and is passive.

Osmosis: The flow of volumeOsmosis refers to the net movement of water across a semi-permeable membrane (or displacement of volume) due to the solute concentration difference.

1 2 1 2

The solute concentration difference causes water to move from compartment 2 1. The pressure

required to prevent this movement is the osmotic pressure.

Time

Osmosis. The flow of volume

Here the membrane is only permeable to water which will flow down its concentration gradient from 2 1.

The volume flow can be prevented by applying pressure to the piston. The pressure required to stop

the flow of water is the osmotic pressure of solution 1.

(The piston applies pressure to stop water flow)

H2O

Cs2Cs

1

Compartment 1 Compartment 2

Osmosis. The flow of volumeAN IDEAL MEMBRANE

Piston

(Compartment 2 is open to the atmosphere)

(Meniscus)

The osmotic pressure () required is determined from the van’t Hoff equation:

= RTCS = (25.4)CS atm at 37°C.

Where, R = the gas constant (0.082 L.atm.K-1.mol-1), T = absolute temperature (310 K @ 37 ºC) and CS

(mol.L-1) is the concentration difference of the uncharged solute

φic = osmotically effective concentration

φ is the osmotic coefficient‘i’ is the number of ions formed by dissociation of a single solute molecule ‘c’ is the molar concentration of solute (moles of solute per liter of solution)

e.g. what is the osmolarity of a 154 mM NaCl solution, where φ = 0.93

→ 154 x 2 x 0.93 = 286.4 mOsm/l

Osmosis. Importance of osmolarity

Osmosis. The flow of volumeA NONIDEAL MEMBRANE

Piston H2O

Cs2Cs

1

S

The osmotic pressure depends on the ability of the membrane to distinguish between solute and solvent.

If the membrane is entirely permeable to both, then intercompartmental mixing occurs and = 0.

The ability of the membrane to “reflect” solute S is defined by a reflection coefficient S that has values from 0 (no reflection) to 1 (complete reflection).

Thus, the effective osmotic pressure for nonideal membranes is:

eff = SRTCS

Osmotic and hydrostatic pressure differences in volume flow

Volume flow across a membrane is described by:JV = KfP

where Kf is the membrane’s hydraulic conductivity and P is the sum of pressure differences.

These pressure differences can be hydrostatic (PH), osmotic (eff) or a combination of both. There is

equivalence of osmotic and hydrostatic pressure as driving forces for volume flow, hence Kf applies to

both forces.

Thus, JV = Kf(eff – PH) (Starling equation)

and (eff – PH) is the driving force for volume flow.

Arteriole VenuleInterstitial

space

Starling Forces

Hydrostatic pressure

Osmotic (oncotic) pressure

= fluid movement

Filtration dominates Absorption dominates

Importance of plasma proteins!

Interstitial fluid pressure under normal conditions ~0

mmHg

Tonicity

Principles of Ion Movement

K+

Cs1=100mM

Ac-

Cs2=10mM

Diffusion of Electrolytes

V+–

For charged species, both electrical and chemical forces govern diffusion.

All solutions must obey the principle of bulk electroneutrality: the number of positive charges in a solution must be the same as the number of

negative charges.

The Principle of Bulk Electroneutrality

Cs1=100m

MCs

2=10mM

K+

Ac-

V +–

Diffusion of Electrolytes

Law of electroneutrality (for a bulk solution) must be maintained. In the above model in which the membrane becomes permeable to sodium (K+) and acetate (Ac–),

both ions will move from side 1 2.The concentration gradient between compartment 1 and

2 is the driving force.K+ (with the smaller radius) will move slightly ahead of

Ac–, thereby creating a diffusing dipole. A series of dipoles will generate a diffusion potential.

Eventually, equilibrium is reached and Cs1 = Cs

2 = 55mM

Ac- K+

Cs1=100mM Cs

2=10mM

K+

Ac-

V +–

Diffusion of Electrolytes

When the membrane is permeable to only one of the ions (e.g., K+) an equilibrium potential is reached. Here, the

chemical and electrical driving forces are equal and opposite.

Equilibrium potentials (in mV) are calculated using the Nernst equation:

2

1

log3.2

S

SionCC

zFRTE

R = gas constant; T = absolute temp.; F = Faraday’s constant; z = charge on the ion (valence); 2.3RT/F = 60 mV at 37ºC

2

1

log60

S

SionCC

zE

The Nernst Equation is satisfied for ions at equilibrium and is used to compute the electrical force that is equal and opposite to the concentration force.

At the Nernst equilibrium potential for an ion, there is no net movement because the electrical and chemical driving forces are equal and opposite.

2

1

log60

S

SionCC

zE

• Even when there is a potential difference across a membrane, charge balance of the bulk solution is maintained.

• This is because potential differences are created by the separation of a few charges adjacent to the membrane.

Cs1 =

100mMCs

2 = 10mM

Na+

Ac-

V +–

Calculating a Nernst Equilibrium Potential

For the model above, the Nernst potential for Na+,

ENa = 60 log(100/10) = +60 mV

2

1

log60

S

SionCC

zE

Taking valence of the ion into account in calculating a Nernst

potential

[Cl-]i = 10 mM[Cl-]o = 100 mMi

oCl

ClClE log60

mVECl 6010100

log60

Here, z = -1

[K+]i = 100 mM

I ON Extracellular Conc. (mM)

I ntracellular Conc. (mM)

Equilibrium Potential (mV)

Na+ 145 12 +67 Cl- 116 4.2 -89 K+ 4.5 155 -95

Ca2+ 1 1x10-4 +123

[K+]o = 10 mMi

oKKKE

][][

log60

Equilibrium potentials of various ions for a mammalian cell

mVEK 6010010

log60

Remember:

Log 10/100 = log 0.1 = –1Log 100/10 = log 10 = +1

A 10-fold concentration gradient of a monovalent ion

is equivalent, as a driving force, to an electrical potential

of 60 mV.

Membrane potential vs. equilibrium potential

When a cell is permeable to more than one ion then all permeable ions contribute to the

membrane potential (Vm).

Membrane Transport Mechanisms I

1. Most biologic membranes are virtually impermeable to:

Hydrophilic molecues having molecular radii > 4Å e.g. glucose, amino acids)

Charged molecules2. The intracellular concentration of many water soluble solutes differ from the medium in which they are bathed.Thus, mechanisms other than simple diffusion

across the lipid bilayer are required for the passage of solutes across the membrane.

Transport across cell membranes

from: Boron, W.F. & Boulpaep, E.L., eds., Medical Physiology, 2003.

Transport through poresA general characteristic of pores is that they are always open.Examples:

1) Porins are found in the outer membrane of gram-negative bacteria and mitochondria..2) Monomers of Perforin are released by cytotoxic T lymphocytes to kill target cells

Transport Through Channels

General Characteristics of ion channels:

1) Gating determines the extent to which the channel is open or closed.

2) Sensors respond to changes in Vm, second messengers, or ligands.

3) Selectivity filter determines which ions can access the pore.

4) The channel pore determines selectivity.

Source: Boron, W.F. & Boulpaep, E.L., eds., Medical Physiology, 2003.

Why do we need to know how ion channels influence cells……..?

Na+ channel blocker

Macular degeneration

Solute movement through pores and channels occurs via simple diffusion, is passive and downhill. Metabolic

energy is not required.

Transport through carriersCarriers never display a continuous transmembrane

path. Transport is relatively slow (compared to pores and channels) because solute movement across the

membrane requires a cycling of conformation changes of the carrier to allow the binding and unbinding of a

limited number of solutes.

Carrier mediated transport

Cotransporter ExchangerFacilitated diffusion: the carrier transports solute

from a region of higher to lower concentration. No additional energy sources are required.

Such proteins are important for:1) the transport of cell nutrients and multivalent ions2) ion and solute asymmetry across membranes While diffusion processes display a linear relationship between flux and solute concentration, carrier transport exhibit saturation kinetics. Hyperbolic plots of transport activity Jx vs. [X] are

indicative of Michaelis-Menten enzyme kinetics. Carrier-mediated transporters display competitive inhibition

Carrier-mediated transport: Facilitated diffusion

Fick’s 1st law][][max

XKXJJ

m

x

Carrier mediated transport:Active Transport

• Movement of an uncharged solute from a region of lower concentration to higher concentration (uphill)

• Movement of a charged solute against combined chemical and electrical driving forces

• Requires metabolic energy• Two classes: primary and secondary

Primary Active Transport – Na-K ATPase

• ATP-dependent• Electrogenic• Important for maintaining ionic gradients

(conduction, nutrient uptake)• Important for maintaining osmotic balance

An example of a secondary active transporter is the electroneutral Na/Cl cotransporter.

The energy released from Na+ moving down its electrochemical gradient is used to fuel the transport of Cl– against its electrochemical

gradient. Note that the Na+ pump plays an important role in maintaining a continual Na+ gradient.

Secondary Active Transport-Symport

Na+ Cl- Na+

Comparison of Pores, Channels, and Carriers

PORE CHANNEL CARRIER

Conduit through

membraneAlways open Intermittently

open Never open

Unitary eventNone

(Continuously open)

Open/closeCycle of

conformational changes

Particles translocated per ‘event’ --- 60,000 * 1-5

Particles translocated per second

Up to 2 billion 1-100 million 200-50,000

* Assuming a 100 pS channel, a driving force of 100 mV and an open time of 1 ms

The “pump-leak” model(generating the membrane

potential)

The Na-pump that pumps 2 K+ into the cell in exchange for 3 Na+ out. Under steady-state conditions, the diffusion of each ion in the opposite direction through its channel-mediated “leak” must be equal to the amount transported.For most cells, however, PK > Pna. In the absence of a membrane potential, K+ would diffuse out of the cell faster than Na+ would diffuse in, thereby violating the law of electroneutrality. Thus, a Vm is generated that reduces the diffusion of K+ out of the cell and simultaneously increases the diffusion of Na+ in.Vm is generated by the ionic asymmetries across the membrane, which are established by the Na-pump.

Na+Na+

K+K+Cl–~

Pr–

Gibbs-Donnan Membrane Equilibrium

• Proteins are not only large, osmotically active particles but they are also negatively charged anions

• Proteins can influence the distribution of other ions so that electrochemical equilibrium is maintained

Gibbs-Donnan Equilibrium

In the simple model system above, Cl– will diffuse from 1 2, and Na+ will follow to maintain electroneutrality. In compartment 2 then, Cl– will be present and [Na+]equil. > [Na+]initial at Donnan equilibrium.Because of the asymmetrical distribution of the permeant ions, there must be a Vm that simultaneously satisfies their equilibrium distributions.

1 2

Na+Na+

Cl–P–

Initially

Na+Na+

Cl–P–

Equilibrium

Cl–

1 2

At equilibrium, the increase in osmotically active particles leads to the flow of water into compartment

2.

In animal cells, the presence of large impermeant intracellular anions tends to lead to cell swelling due to Donnan forces. However, the Na+ pump actively extrudes osmotic solutes and counteracts the cell

swelling.

Gibbs-Donnan equilibrium(the tendency for cells to swell)

Na+Na+

Cl–

P–

Equil.:Cl–

H2O

1 2

P-

[Na+][K+][Cl-]

~Na+

2K+

3Na+

Cl-H2O

K+

Equal number of +ve and –ve charges move: Equilibrium

P-

↑[Na+]↓[K+]↑[Cl-]

Na+

Cl-

H2O

K+

Inhibition of the Na-pump (ouabain) → cell swelling

The Na-pump (Na-K pump) is essential for maintaining cell

volume

~

Membrane Transport Mechanisms II

and the Nerve Action Potential

Epithelia

Basal Lamina

MicrovilliTight junction

• Lie on a sheet of connective tissue (basal lamina)

• Tight Junctional Complexes: Structural Allow paracellular transport

• Apical membrane; brush border (microvilli) – increases surface area

• Apical (mucosal, brush border, lumenal) and basolateral (serosal, peritubular) membranes have different transport functions

• Capable of vectorial transport

Apical

Basolateral

Models of Ion Transport in Mammalian Cellse.g. Cl- secretory cell

Na+

K+Cl-K+Na+

K+

Cl-

Na+

H2O

TranscellularParacellular

Transepithelial potential difference

APICAL/MUCOSAL

SIDE

BASOLATERAL/SEROSAL/

BLOOD SIDE

NEGATIVE POSITIVE

Absorptive Epithelia - e.g. Villus cell of the small intestine

(Modified from: Alberts et al., Molecular Biology of the Cell, 4th Ed. Garland Science, 2002)

Na+-driven glucose symport

Lateral domain

Basal domain

Carrier protein mediating passive transport of glucose

Common Gating Modes of Ion Channels

(Source: Alberts et al., Molecular Biology of the Cell, 4th Ed. Garland Science, 2002)

Diffusion of electrolytes through membrane channels

The following are three important features of ion channels that influence flux :1) Open probability (Po). Opening and closing of channels are random processes. The Po is the probability that the channel is in an open state.2) Conductance. 1/R to the movement of ions. Where V=IR (Ohms law)

3) Selectivity. The channel pore allows only certain ions to pass through.

I

V

Electrophysiological Technique: Patch Clamp

Terminology and Electrophysiological Conventions

-100 mV

+100 mV

0 mV

Membrane potential (Vm)

Depolarize

Hyperpolarize

V

IOUTWARD CURRENT

INWARD CURRENT

(Positive)

(Negative)

+100 mV-100 mV

Reversal Potential (I=0)

How the behavior of an ion channels can be modified to permit an increased ion flux:

Closed stateOpen state

Control/ Wild-type:

An increase in conductance (more current flows/opening) but the open probability stays the same:

An increase in open probability (the channel spends more time in the open state, or less time in the closed state) but the conductance stays the same:

Closed stateOpen state

Closed stateOpen state

Ionic currents through a single channel sum to make macroscopic currents

Na+ Channel K+ ChannelVOLTAGE-GATED CHANNELS

VOLTAGE-dependent closure

TIME-depende

nt closure

The resting membrane potential (Vm) describes a steady state condition with no flow of electrical current across the membrane.

Vm depends at any time depends upon the distribution of permeant ions and the

permeability of the membrane to these ions relative to the Nernst equilibrium potential for

each.

Depo

lariz

ing

phas

e

Restingpotential

Threshold

After-hyperpolarization

Overshoot

RepolarizingPhase

The Nerve Action Potential

-5 0 5 10 15 20-80

-60

-40

-20

0

20M

embr

ane

Pote

ntia

l (m

V)

Time (ms)

Changes in the underlying conductance of Na+ and K+ underlie the nerve action

potential

Na+ K+

+

Chemical and electrical gradients prior to initiation of an action

potential

• At rest, the cell membrane potential (Vm-rest) is generated by ion gradients established by the Na- pump.

• The K+ conductance (permeability) is high, Na+ conductance is extremely low, hence Vm-rest is strongly negative.

Na+

+

A stimulus raises the intracellular potential to a threshold level and voltage-gated Na+ channels open

instantaneouslyStimulus

1. The membrane becomes permeable to Na+ and there is a rapid Na+ influx due to due to both electrical and chemical gradients. The cell membrane potential becomes progressively, but rapidly, more positive - i.e. it depolarizes

+ ++

++

+

++

Na+

Na+

Na+

Na+

0 5 10 15 20-80

-60-40

-20

0

20M

embr

ane

Pote

ntia

l (m

V)

Time (ms)

-100 +1000 +150-50 +50Eion

K+ Na+Cl-

The rapid upstroke, or depolarizing phase, is due to an increase in Na+ conductance of the cell membrane due to activation of voltage-gated Na+ channels. An all-or-none response. The cell potential moves toward ENa due to chemical and electrical driving forces. Vm does not reach ENa.

+ +++

+

+

++

Na+Na+

+ ++

++

+

++ K+

K+

- - ---

-

--K+

K+

K+

5. Cell repolarizes

3. Outward K+ gradient

4. Outward K+ flux as voltage-dependent K+ channels open hyperpolarization

2. Na+ channels begin to close:

-100 +1000 +150-50 +50Eion

K+ Na+ Ca2+Cl-

As the cell depolarizes, the Na+ channels inactivate and the permeability to Na+ is reduced. Voltage-gated K+ channels open and the cell membrane potential becomes permeable to K+ thereby driving Vm toward EK. The continued opening of K+ channel causes a brief after-hyperpolarization before the cell returns to its resting membrane potential.

0 5 10 15 20-80-60-40-200

20

Mem

bran

e Po

tent

ial (

mV)

Time (ms)

Activation gateInactivation gate

REST

ACTIVATED(UPSTROKE) INACTIVATED

REPOLARIZATION→HYPERPOLARIZATION

DEPOLARIZING Vm

Na+

out

in

Gates Regulating Ion Flow Through Voltage-gated Na+

Channels

REFRACTORY PERIODSDuring RP the cell is incapable of eliciting a normal action potential

• Absolute RP: no matter how great the stimulus an AP cannot be elicited. Na+ channel inactivation gate is closed.

• Relative RP: Begins at the end of the absolute PR and overlaps with the after-hyperpolarization. An action potential can be elicited but a larger than normal stimulus is required to bring the cell to threshold.

REVIEW AND PROBLEM SET

Solute Intracellular conc. (mM)

Extracellular conc. (mM)

A+ 7 104 B+ 110 8 C++ 1 0.01 D- 5 10 E- 10 100 F- 2 2

G (uncharged) 4 4 H (uncharged) 3 1

Review Question 1

A. If the membrane potential of a hypothetical cell is –60 mV (cell interior negative):a) Given the extracellular concentration listed on the table

above, what would the predicted intracellular concentration of each of the solutes A-H have to be for passive diffusion across the membrane.

b) Given the intracellular concentrations calculated in part a), what can we conclude about the transport mode of each of the solutes that are not passively distributed.

B. Calculate the Nernst equilibrium potential for each solute.

Consider a closed system bound by rigid walls and a rigid membrane separated the two compartments. Assume the membrane is freely permeable to water and impermeable to sucrose.

A) If both compartments contain pure water and a pressure is applied to the piston establishing a hydrostatic pressure difference across the membrane, which direction will water flow in? What will the initial rate of water flow depend on?B) If no force is applied to the piston and 100 mM sucrose is placed in compartment A, which direction will the meniscus in compartment B move? What concentration of NaCl (also impermeant) would have to be added to compartment B to prevent volume displacement? What hydrostatic pressure must be applied to the solution in compartment A to prevent this volume flow?

Piston

BA

Review Question 2

Consider two compartments of equal volume separated by a membrane that is impermeant to anions and water

A) If in addition the membrane is not permeant to Na+, what is the orientation and the magnitude of the potential difference across the membrane at 37C? What is the composition of compartment B when the system reaches equilibrium?

B) If the properties of the membrane change and now the membrane is only permeant to Na+, what is the orientation and magnitude of the potential difference?

C) If both Na+ and K+ are permeable, but PNa>PK what will be the orientation of the potential difference initially? What will be the orientation of the potential difference and the composition of compartments A and B when electrochemical equilibrium is reached?

A B100 mM NaCl

10 mM KCl

100 mM KCl

10 mM NaCl

Review Question 3