Biophysics I. 2015/2016
Lecture 15.
Related literature:
Related multimedia material:
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Topics of the lecture:
1.Definition of osmosis
2. Osmotic pressure
2.1. Vant’t Hoff’s law
2.2. Practical significance of the osmotic pressure
3. Osmosis in the medical practice
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Osmosis
(1) Definition of osmosis
We will study situations in which a „filter” limits the free diffusion of one of the components.
Two containers (filled withdifferent gases) separated by different types of walls:
type of the wall
no
yes, non-permeable
yes, semi-permeable
matter transport
free diffusion
no
restricted diffusion: OSMOSIS
SEMIPERMEABLE – „filter”:
allows smaller slovent molecules to pass
through, but not the larger solute
molecules (e.g. sugar ↔ water)
PORE SIZE  SELECTIVITY
Semipermeable membranes: animal skin pellicles, walls of living cells, ceramic plate with holes,
cellophane.
OSMOSIS: unidirectional matter flow, which takes place by means of diffusion, conditions:
semipermeable wall + concentration difference (from the perspective of osmosis, the dissolved
substance’s qualities are irrelevant).
Biophysics I. 2015/2016
For example: Small-sized, semi-permeable bag filled with sugar solution placed in a pure water filled
container. After several hours the bad swelling, the water surrounding it remains pure, the solution
int he bag is diluted.
The above experiment in a U-shape tupe:
- concetration difference
- semipermeable membrane: allows
solvent to pass through but not the solute
solvent
flow
throught
the
semipermeable membrane
- the volume of the solvent + solute
mixture
increases:
HYDROSTATIC
PRESSURE (ph)
- solvent flow slows down
- dynamic equilibrium
OSMOTIC EQUILIBRIUM
(2) Osmotic pressure
The pressure that inhibits the net solvent flow.
𝑷𝒉 = 𝑷𝒐𝒔𝒎𝒐𝒕𝒊𝒄
𝑷𝒐𝒔𝒎𝒐𝒕𝒊𝒄 = 𝝆𝒈𝒉
2.1. Vant’t Hoff’s law
Container divided by a semi-permeable wall into two equal
sections of volume „V”. Due to the equal volumes the
amount of the blue substances is equal on both sides:
nblue1=nblue2.
Using the state equation of an
Thermodynamics): pV=nRT. Therefore:
p1=
𝑛𝑟𝑒𝑑 +𝑛𝑏𝑙𝑢𝑒1
𝑅𝑇
𝑉
és p2=
𝑝1 − 𝑝2 = 𝑝𝑜𝑠𝑚𝑜𝑡𝑖𝑐 =
𝑝𝑜𝑠𝑚𝑜𝑡𝑖𝑐 𝑉 = 𝑛𝑅𝑇
𝑛𝑏𝑙𝑢𝑒2
𝑅𝑇
𝑉
𝑛𝑟𝑒𝑑
𝑅𝑇
𝑉

ideal
gas
(see
Biophysics I. 2015/2016
𝑛
𝑉
𝑝𝑜𝑠𝑚𝑜𝑡𝑖𝑐 = 𝑅𝑇 because
𝑛𝑟𝑒𝑑
𝑉
= cred , therefore 𝒑𝒐𝒔𝒎𝒐𝒕𝒊𝒄 = 𝒄𝑹𝑻
The osmotic pressure is linearly proportional to the concentration: 𝑝𝑜𝑠𝑚𝑜𝑡𝑖𝑐 ~𝑐.
Solution with higher density becomes diluted.
Problem (1)
What is the osmotic pressure of 0.05 mol/dm3 glucose solution at 20°C?
𝑃𝑜𝑠𝑚𝑜𝑡𝑖𝑐 = 𝑐𝑅𝑇
𝑇 = 20 + 273 = 293 𝐾; 𝑅 = 8.314 𝑃𝑜𝑠𝑚𝑜𝑡𝑖𝑐 = 0.05𝑥8.314𝑥293 = 121.8 𝑃𝑎
2.2. Practical significance of the osmotic pressure
Classifying solutions on the basis of osmotic pressure
Role of osmosis in the life of plant cells
Biophysics I. 2015/2016
(3) Osmosis in the medical practice
INJECTION, INFUSION: pharmaceutics are dissolved in physiological saline solution  isotonic
environment (compared to the body fluid)
TREATMENT OF OEDEMAS, INFLAMED AREAS: dextran-solution/bitter salt (MgSO4-solution) 
hypertonic environment is created (compared to the swollen areas) induces water outflow from
the swollen areas  reduced swelling
TREATMENT OF CONSTIPATION - LAXATIVE SALTS: laxative salts are not absorbed by the large
intestine  hypertonic environment is created in the large intestine  water influx into the large
intestine  dilution of colonic content, facilitated excretion
HAEMODIALYSIS: treatment of patient with severe kidney disease. Aim: remove soluble chemicals
(accumulated in the blood due to the kidney failure) toxic for the body; removing of protein
products, toxins, other waste products exit with water, while essential plasma proteins, cellular
elements of blood remain in the blood. Average time of treatment is: 4-8 h, meanwhile dialysis
solution has to be changed frequently. Ion-concentrations and metallic-ion-contaminations in the
solution should be checked continuously.
essential element: long semipermeable membrane (cellophane),
surrounded by dialyis solution
carefully
adjusted
to
body
temperature. The blood cirulation is
connected to the cellophane-coil
and blood is made to flow through it
then reenters the vein of the
patient.
Schematic diagram of haemodialysis („artificial kidney” instrument).