Membrane transport, diffusion, osmosis, passive transport through cell membrane. Lectures from Biophysics.

1. The properties of biological
fluids and features of transport
through cell membrane
Membrane transport,
diffusion, osmosis
Juliana Knocikova

2. The cell - one of the most basic units of life
• Is defined as a open system – possibility to
communicate with extracellular space by transfer of:
energy
informations
• Providing for the metabolism and also for the
communication between the cells
• There are millions of different types of cells
The one-celled organism
amoeba proteus
A single-celled bacteria
of the type: E. coli
A human red blood cell

3. The cell
Basic structural and function unit in life organism
• largeness ⇒ 2 - 120 μm
• shape ⇒ varied

4. Structure:
The cell : ⇒ cytoplasm
⇒ plasma cell membrane

5. Plasma cell membrane ⇒ (thickness 2 – 7 nm)
Electrically polarized (positive charged outside and negative charged
inside)
Semipermeable
• Isolate inside of the cell from the extracellulary space
• Ensure and enable to regulate transport of the water and substances inside and outside
the cell
• Conclusive importance for:
⇒ electrical potential genesis (resting and action membrane
potential)
⇒ excitability,
⇒ immunity,
⇒ reproduction

6. Semipermeability
• Come through:
– easy : water, small ions as K+
, Cl-
– with difficulty : Na+
, ...
– Substances dissolvable in fats
• Do not come through:
- large ions
-polar molecules like proteins

7. Structure of the membrane:
-it is a collage of many different proteins embedded in
the fluid matrix of the lipid bilayer
Main components of the lipid bilayer:
phospholipids
cholesterol

8. - hydrophilic polar heads (electricaly charged)
– interaction with water, they are oriented into the surrounding
of solution
- non-polar hydrophobic ends
- no interaction with water, they are oriented into the interior of
the biomembrane
• Phospholipids – molecules with baculiform shape:
⇒ Fluid mosaic model – model of cell membrane
Membrane structure
– relatively stable because of hydrophobic interactions
Membrane.swf

9. Proteins:
Peripheral proteins – work especially as enzymes
Integral proteins – basic structural unit of the membrane channels

10. Transport mechanisms
paracellular – slotted connection between cells ⇒ gap
junction (structures of the cell membrane – connexones are slotted
and create channel)
transcellular
– through the cell membrane
- passive ⇒ filtration
diffusion,
facilitated diffusion,
diffusion through the channels,
osmosis
- active ⇒ primary transport
secondary transport
Exocytosis and endocytosis

11. Exocytosis and endocytosis – active transport
Exocytosis – is the process by which the cell leads secretory vesicles
towards the cell membrane. These membrane-bound vesicles contain
soluble substances to be secreted to the extracellular environment.
Endocytosis - is a process whereby cell absorbs material from the outside
by engulfing it with its cell membrane. It is used by all cells of the body
because most important substances are large polar molecules, and thus
cannot pass through the hydrophobic plasma membrane. The function of
endocytosis is the opposite of exocytosis.
Fagocytosis – it is the type of endocytosis when an entire cell is
engulfed
Pinocytosis - it is when the external fluid is engulfed

13. Filtration is the process of using a filter to mechanically
separate a mixture of solid and fluids. Depending on the
application, the solid, the fluid, if they may be isolated.
Examples of filtration include a coffee filter which
separates the coffee grounds from the brewed coffee.
To separate a mixture of chemical compounds, a solvent
is chosen which dissolves one component, while not
dissolving the other. By dissolving the mixture in the
chosen solvent, one component will go into the solution
and pass through the filter, while the other will be
retained. This is one of the most important techniques
enabling transport of substances in live organism.
Passive transport mechanisms - Filtration

14. Diffusion
- spontaneous movement of substance from positions
with higher contcentrations to those with lower
ones – direct of concentration gradient
Movement substance speed in solution depends on
temperature and largeness of molecules
Result of diffusion in solution
- transfer of soluble substances from the space
with higher concentration to the space with lower
concentration, until the concentrations are equal
Passive transport mechanisms

15. Fick’s law
dx
dc
S..Dvd −=
vd - speed of the diffusion
dc/dx - concentration gradient
S - largeness of the diffusion space
D - diffusion coefficient (temperature,
locomotion and largeness of the molecules,
interactions ...)

16. Diffusion – important function for the substance transport
in life organism
It enables to interfuse of compounds soluble in fats
(hormones, ethanol, glycerol, urea) through lipid bilayer
without difficulties, small molecules of O2, CO2 without join
the to the protein channels.
Diffusion - spontaneous, passive movement of substances
soluted in fats through the cell membrane determinating
direction of the concentration gradient.
Cell membrane is an important barrier because of its
semipermeability.

17. One of the most
important transport
mechanisms:
transport O2
Form the extracellular space
into the cell
transport CO2
From the cell into the
extracellular space.
Working of both these actions is
the basic theorem for running of all
physiological processes– not only in
the cell domain but also for the
Oxygen transport and blood flow

18. Facilitated diffusion
- Selective passive transport through the membrane is facilitated
by the creation of bond between substance and a special kind of
protein - carrier.
- carriers are able to establish contact with transferred substance
on the one side of the membrane, transfer it through the
membrane space and to empty it on the other side of the
membrane
- no supply of energy is needed and facilitated diffusion runs in
accordance with direction of concentration gradient
- velocity of facilitated diffusion depends on concentration and
amount of open protein channels
-especially transport of ions, glucose and amino acids
-Enables transport of substances which can not move
by simple diffusion mechanism

19. Characteristics of Facilitated
Diffusion & Active Transport - both
require the use of carriers that are
specific to particular substances (that
is, each type of carrier can 'carry' one
type of substance) and both can
exhibit saturation (movement across a
membrane is limited by number of
carriers & the speed with which they
move materials; see graph below).
Facilitated diffusion transport – overshoots much faster than
simple diffusion, but only up to the certain concentration.
If the certain concentration is reached, diffusion is saturated and
does not continue (either if the concentration of transmitted
substance is increasing)
filtration and facilitated diffusion.swf

20. Transport through the protein channels
Opened protein channels
Plasmatic membrane is permeable for water and
amount of ions and substances. They pass through
the membrane by special pores – proteins located
in membrane.
Inside the protein, there is a channel filled with
water. Small molecules may move from the one
side to the another side of the membrane. Protein
channels have highly selective permeability for
different substances.

21. - Semipermeability of the protein channel - only
some specific ions or molecules may pass through it.
Selectivity depends on largeness, shape of the protein channel,
electrical charge on its surface.

22. -
Gating
- is connected with changes of protein channel molecule.
∀ ⇒ Voltage gating
changes of the channel protein are realized by the change of electrical voltage
Example:
closed natrium channel is opening by depolarization
(largeness 0,3x0,5nm) Large negative charge caries the natrium ions inside to the
channel.
∀ ⇒ Ligand gating
Join of the ligand (molecule) results in changes of the channel protein and opening
(or closing) of the channel.

23. Osmosis
- occurs when solution and solvent are separated by a
semipermeable membrane
- defined as a motion of solvent molecules (water) in
the direction of their concentration gradient – against
concentration gradient of the dissolved substance
- from the space with lower osmotic pressure
(lower concentration of the soluted substance) to
another space with higher osmotic pressure (higher
concentration of the soluted substance).

24. R - universal gas constant
T - absolute temperature
c - molar concentration (mol.m-3
)
i - Van´t Hoff´s coefficient
Van´t Hoff’s law
i.c.T.Rπ =

25. Osmotic pressure
Osmosis results in change of the
pressure inside the space with
higher concentration of the active
substances. This change is
stopped by certain value (in the
case of balance of pressure and
concentration gradient)
Changes of osmotic pressure result in changes of the cell
volume.
Life organism is enable to protect total osmotic balance using
osmoregulation mechanisms.

26. Solutions
Hypertonic solutions are those in which
more solute (and hence lower water
potential) is present. Water is fading from
the red blood cells, their volume is
reduced – and results in haemolysis
(solution of NaCl with higher
concentration than 0,9 %).
Hypotonic solutions are those with less
solute (again read as higher water
potential). Erythrocytes absorb water
from the solution, enlarge their volume
and swell. Destruction of their structure is
called haemolysis (solution of NaCl with
lower concentration than 0,9 % ).
Isotonic solutions have equal
(iso-) concentrations of
substances. Water potentials are
thus equal, although there will
still be equal amounts of water
movement in and out of the cell,
the net flow is zero. Erythrocytes
in physiological solution (0,9 %
NaCl) keep their characteristic
shape.

27. Red blood cells in different solutions
Isotonic solution
Hypotonic solution
Hypertonic solution
0,9 % NaCl
< 0,9 % NaCl
> 0,9 % NaCl

28. The surface tension
The forces between liquid molecules are responsible for the phenomenon
known as surface tension. The molecules of the surface do not have other
like molecules on all sides of them and consequently they cohere more
strongly to those directly associated with them on the surface. This forms a
surface "film" which makes it more difficult to move an object through the
surface than to move it when it is completely submersed.
Fluid surface tension is the tangential force that keeps a fluid together at
the air/fluid interface. The surface tension is intermolecular force of
attraction between adjacent molecules, expressed in force per unit
width, as milliNewtons/meter (mN/m).
σ = F / l

29. Why is the surface tension important?
Surfactants greatly reduce the surface
tension of solvents, water and water-
based solutions, inks, fountain solutions,
adhesives and other coating
formulations. To reduce the surface
tension, the surfactant molecules have to
migrate to the interface, and this takes
some finite amount of time. Given
enough time, the formulation will
eventually reach equilibrium (static)
surface tension. This takes several
seconds or even several minutes
depending on the type of surfactant and
the concentration. If you do not allow
enough time for the solution to reach
equilibrium you are then operating in the
dynamic zone, and the critical
measurement parameter is dynamic
surface tension.R
2σP=La place law

30. Surfactant
Plain water Water & surfactant
add surfactant
Surface
tension

31. Changes of the surface tension

32. Viscosity
Viscosity is a measure of the resistance of a fluid to
deform under shear stress. It is commonly perceived as
"thickness", or resistance to flow. Viscosity describes a
fluid's internal resistance to flow and may be thought as a
measure of fluid friction.
It is expressed in Pa.s.
Thus, water is "thin", having a lower viscosity, while
vegetable oil is "thick" having a higher viscosity. All real
fluids (except superfluids) have some resistance to shear
stress, but a fluid which has no resistance to shear stress
is known as an ideal fluid or inviscid fluid.
.
η
r
Δv
SF =

33. Viscosity
Example of the viscosity of milk and
water. Liquids with higher viscosities will
not make such a splash when poured at the
same velocity.
In many situations, we are
concerned with the ratio of the
viscous force to the inertial
force, the latter characterised by
the fluid density ρ. This ratio is
characterised by the kinematic
viscosity, defined as follows:

34. Laminar and turbulent flow - comparison
Reynold’s number
Concepts of flow:

35. Laminar flow – substances flow in
paralel layers.
Velocity of the flow is not the same –
lower at the boundaries, the
highest at the centre of tube (blood
vessel).
Flow in Normal Artery:
Flow in Significantly Narrowed Artery:
Turbulent flow – disorder movement
of substances.
Velocity of the flow is equal and
higher under comparison with laminar
flow.
Concepts of flow:

36. TURBULENT FLOW
during blood flow in a
stenosed, or narrowed,
carotid artery.

37. Thank you
for attention
© 2007 Juliana Knocikova