FACTORS AFFECTING RATE OF DIFFUSION
Rate of diffusion of
substances through the cell membrane is affected by the following factors:
1. Permeability of
the Cell Membrane
Rate of diffusion is
directly proportional to the permeability of cell membrane. Since the cell membrane is
selectively permeable,
only limited number of substances can diffuse through the
membrane.
2. Temperature
Rate of diffusion is
directly proportional to the body temperature. Increase in temperature
increases the
rate of diffusion. This is because of
the thermal motion of molecules during increased temperature.
3. Concentration Gradient
or Electrical Gradient of the Substance across the Cell Membrane
Rate of diffusion is
directly proportional to the concentration gradient or electrical gradient of
the diffusing
substances across the
cell membrane. However, facilitated
diffusion has some limitation beyond certain
level of concentration gradient.
4. Solubility of
the Substance
Diffusion rate is directly proportional
to the solubility of substances, particularly the lipid-soluble
substances.
Since oxygen is highly soluble in lipids, it diffuses very rapidly through the
lipid layer.
5. Thickness of
the Cell Membrane
Rate of diffusion is
inversely proportional to the thickness of the cell membrane. If the cell
membrane is thick, diffusion
of the substances is very slow.
6. Size of the
Molecules
Rate of diffusion is
inversely proportional to the size of the molecules. Thus, the substances with
smaller
molecules diffuse rapidly than the
substances with larger molecules.
7. Size of the
Ions
Generally, rate of diffusion is
inversely proportional to the size of the ions. Smaller ions can
pass through the
membrane more easily than larger ions with the same charge.
However, it is not applicable always. For instance, sodium
ions are smaller in size than potassium ions. Still, sodium ions cannot pass
through the membrane as easily as potassium ions because sodium ions have got the
tendency to gather water molecules around them. This makes it
difficult for sodium ions to diffuse through the membrane.
8. Charge of the
Ions
Rate of diffusion is
inversely proportional to the charge of the ions. Greater the charge of the ions,
lesser is the rate of diffusion. For example, diffusion of
calcium (Ca++) ions
is slower than the sodium (Na+) ions.
SPECIAL TYPES OF PASSIVE TRANSPORT
In addition to
diffusion, there are some special types of passive transport, viz.
1. Bulk flow
2. Filtration
3. Osmosis.
BULK FLOW
Bulk flow is the
diffusion of large quantity of substances from a region of high pressure to the region
of low pressure. It is due to the pressure gradient of the substance across the cell
membrane.
Best example for bulk
flow is the exchange of gases across the respiratory membrane in lungs. Partial
pressure of oxygen is greater
in the alveolar air than in the alveolar capillary blood. So, oxygen moves
from alveolar air into the blood through the respiratory membrane. Partial
pressure of carbon dioxide is more in blood than in the alveoli. So, it moves from the
blood into the alveoli through the respiratory membrane .
FILTRATION
Movement of water and
solutes from an area of high hydrostatic pressure to an area of low hydrostatic
pressure is called
filtration. Hydrostatic pressure is developed by the weight of the fluid.
Filtration process is seen
at arterial end of the capillaries, where movement of
fluid occurs along with dissolved substances from blood into the interstitial
fluid. It also occurs in glomeruli of kidneys.
OSMOSIS
Osmosis is the special
type of diffusion. It is defined as the movement of water or any other solvent
from
an area of lower concentration to an
area of higher concentration of a solute, through a semipermeable
membrane. The
semipermeable membrane permits
the passage of only water or other solvents but
not the solutes. Osmosis can occur
whenever there is a difference in the solute concentration on either side of
the membrane. Osmosis depends upon osmotic pressure.
Osmotic Pressure
Osmotic pressure is the pressure
created by the solutes in a fluid. During osmosis, when water or any
other solvent
moves from the area of lower concentration to the area of higher concentration,
the solutes in the area of higher concentration get dissolved in the solvent.
This creates a pressure which is known as osmotic pressure. Normally, the
osmotic pressure prevents further movement of water or other solvent during
osmosis.
Reverse Osmotic
Pressure
Reverse osmosis is a process
in which water or other solvent flows in reverse direction (from the area of
higher concentration to the area of
lower concentration of the solute), if an external pressure is
applied on the
area of higher concentration.
Colloidal Osmotic
Pressure and Oncotic Pressure
The osmotic pressure exerted by the
colloidal substances in the body is called the colloidal osmotic pressure. And,
the osmotic pressure exerted by the colloidal substances.
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