010-membrane transport 2

8/8/2019 010-Membrane Transport 2

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University of Balamand

Faculty of Medicine and Medical Sciences

Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

Processes by WhichParticles Move AcrossCell Membranes

Membrane Transport

Osmosis: Definition

The flow of water across a semipermeablemembrane from a compartment in which the soluteconcentration is lower to one in which the soluteconcentration is greater

A semipermeable membrane is defined as amembrane permeable to water but impermeable tosolutes


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

Osmosis

The left chamber contains asolution

The right chamber containspure water

Water will flow by osmosisfrom the pure water chamberinto the solution

Schematic Representation of the Definition

of Osmotic Pressure of a Solution

The osmotic pressure of asolution can be obtained by

determining the pressurethat must be applied to a

solution to prevent waterfrom entering it across anideal semipermeablepermeable

When the hydrostaticpressure applied to thesolution in chamber A isequal to the osmoticpressure of that solution,

there will be no net waterflow across themembrane


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

Colligative Properties of a Solution

Properties of a solution that depend on theconcentration of the solute present rather thanon its chemical nature

Colligative properties of a solution:

vapor pressure

freezing point

boiling point

osmotic pressure

Computing the Osmotic Pressure of a

Solution

1. Vant Hoffs Law for calculation of osmotic pressure

2. Use of freezing point depression for estimation ofosmotic pressure


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

Illustration of Osmotic Pressure

The concentration of water in the leftcompartment is less than in the rightcompartment

There is a concentration gradientfor water

Water moves in the tube from right toleft

Water continues to move from the regionof high water concentration to theregion of low water concentrationUNTIL the difference in height betweenthe 2 columns of water is 26 cm

At this height, a column of water exerts a

hydrostatic pressure (P) that is equal tothe difference between the osmotic

pressures ( ) of the 2 solutionsPhydrostatic = L- R

Vant Hoffs Law

= RT( i c) Osmotic pressureR Ideal gas constantT Absolute temperature

Osmotic coefficient i Number of particles formed by dissociation

of a solute molecule

c Molar concentration of a solute (moles ofsolute/liter of solution)


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

The Osmotic Coefficient ()

depends on the:nature of the solute

concentration of the solute

Values of may begreater than 1

less than 1

values can be obtained from handbooks thatlist values of for different substances asfunctions of concentration

Osmotic Coefficients of Certain Solutes of

Physiological Interest

values for solutes of physiological importance apply toconcentrations of these solutes in the extracellular fluids ofmammals


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

The Osmotic Pressure of a Solution

The osmotic pressure of a solution depends on thenumber of particles in solution (i)

Every solute particle in solution contributes to theosmotic pressure

The degree of ionization of a solute must be takeninto account when calculating osmotic pressure

The Osmotic Pressure of a Solution

= RT( i c) The osmotic pressure of a solution is often referred

to as osmolarity

(ic) can be regarded as the osmotically effectiveconcentration of the solution

(ic) is called the osmolarity of the solution (ic) is expressed in osmoles/liter Sometimes, a less precise estimate of osmotic

pressure is computed assuming that is equalto 1


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

Sample Calculations

1. What is theosmotic pressureat 0oC of a 154mMNaCl solution? Use=0.93 for NaCl

2. What is theosmolarity of thissolution?

1. = RT( i c) = 22.4 L.atm/mole x 0.93 x 2

x 0.154 mole/L = 6.42 atm

2. Osmolarity = ( i c)( i c)= 0.93 x 2 x 0.154

mole/L

( i c)= 0.286 osmole/L = 286

mOsmole/L

Sample Calculation

Sometimes, a less precise estimate of osmotic pressureis computed assuming that is equal to 1

1 mole of NaCl is dissolved in 1 liter of solution.

What is the osmotic pressure of the solution?

The salt will dissociate into 1 mole of Na+ and1mole of Cl-

The osmotic pressure of the solution will be 2osmoles/L:

1 osmole/L contributed by Na+

1 osmole/L contributed by Cl-


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

Depression of Freezing Point of Water by Solute

ic = Tf /1.86 ic osmolarity of solution Tf freezing point depression in 0C

A 1 osmolar solution has an osmotic pressure of22.4 atm at 00C and freezes at 1.860C

Isosmotic / Hypoosmotic/ Hyperosmotic

Solutions

Solutions A and C areisosmotic

Solutions A and C arehypoosmotic with respectto solution B

Solution B is hyperosmoticwith respect to solutions Aand C


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

Water Permeability of a Cell

Biological membranes are verypermeable to water

A change in soluteconcentration will cause water tomove and abolish any differencein osmotic pressure between 2compartments

As a result, athough theconcentrations and types ofsubstances in the intracellularand extracellular

compartments DIFFERsignificantly, the osmolarity ofthese compartments is thesame

Tonicity of a Solution Determined by the response of a cell when it is placed

in the solution Consider the response of a red blood cell when it is

placed in 3 different types of solutionsRBCs are convenient to use because they can be

isolated and observed If a RBC swells and stays swollen or (bursts) when

placed in a particular solution, that solution is said tobe hypotonic

If a RBC shrinks and stays shrunken when placed in a

particular solution, that solution is said to behypertonic If the volume of a RBC stays the same when added to

a particular solution, that solution is said to beisotonic


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

Osmotic Behavior of RBCs

A RBC is placed in a solution containing 0.25M sucrose, a solutionthat is isosmotic with the intracellular milieu of the RBC

Sucrose cannot penetrate the cell membrane (impermeantsolute)

Most of the solutes found within the RBC are alsomembrane impermeant

The RBC will:

Shrink?

Swell?

Maintain a constant volume?

The RBC is placed in a solution that is:

Hypertonic? Hypotonic?

Isotonic?


A RBC is placed in a solution containing 0.25M urea, a solution that

is isosmotic with the intracellular milieu of the RBC.

Urea is a permeant solute

Typically, there is very little urea inside the RBC

Most of the solutes found within the RBC are membraneimpermeant

The RBC will:

Shrink?

Swell?

Maintain a constant volume? The RBC is placed in a solution that is:

Hypertonic?

Hypotonic?

Isotonic?


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag


A RBC is placed in a solution containing 0.5M sucrose, a solutionthat is hyperosmotic with the intracellular milieu of the RBC

Sucrose cannot penetrate the cell membrane (impermeantsolute)

Most of the solutes found within the RBC are alsomembrane impermeant

The RBC will:

Shrink?

Swell?

Maintain a constant volume?

The RBC is placed in a solution that is:

Hypertonic? Hypotonic?

Isotonic?

Clinical Correlation

Isotonic NaCl solution, also known as isotonic saline, isused for intravenous rehydration or for administration ofdrugs to patients


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

Osmotic Effects of Solutes

Magnitude of OsmoticFlows

Rate of Osmotic Flow Caused by Permeating

Solutes

When a difference of hydrostatic pressure (P) causeswater to flow across a membrane, the rate of water flow(Vw) is:

Vw = L P L is a constant of proportionality, called the hydraulic

conductivity


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

Reflection Coefficients

Vw = L is the reflection coefficient

is a dimensionless number that ranges from 1 forcompletely impermeant solutes to 0 for extremelypermeant solutes.

Value of is a property of a particular solute and aparticular membrane

Osmotic Water Flow Across a Porous Dialysis Membrane

Induced by Solutes of Different Molecular Size

Flow is expressed as microliter/min caused by a 1M-concentrationdifference of solute across the membrane

The solutions have identical freezing points so the total osmotic pressuresare the same


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

OsmosisPag

Question

Two compartments of equal volume areseparated by a solid partition containing ahole with an area of 2 cm. A stopperinitially occludes the hole. The

compartments have rigid walls and aretotally filled with a solution containingCaCl2 and sucrose in the concentrations

shown in the figure. Simple diffusion is theonly process by which Ca2+ and Cl- cross

the membrane.

Calculate the osmotic pressure (in

osmoles) of the solution (either in #1 or#2) that is hypoosmotic with respect

to the solution on the other side of themembrane before the stopper isremoved from the hole in the partition.

Answer

The solution with thelower osmolarity, theHYPOOSMOTICsolution, will be incompartment #2

There are fewerparticles on thisside of themembrane


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Dr. M. Khachab

GPHY 402

September 7, 20

Membrane Transport

Osmosis

Reference

Chapter 4: Transport of Ions and Molecules Through theCell Membrane inTextbook of Medical Physiology byGuyton and Hall

Chapter 11: Transport of Small Molecules and ElectricalProperties of the Cell Membrane inCell by Alberts,Johnson, Lewis, Raff, Roberts and Walter

010-membrane transport 2

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