Kinds of passive transport
Diffusion - the basic mechanism of passive transport. Diffusion is a spontaneous process of penetration of substance of area of greater in area of its smaller concentration as a result of thermal chaotic movement of molecules. Simple diffusion can occur through lipid double layer or through an protein interstice in a water environment.
If
concentration of particles inside of a cell (Сin)
above, than in environments (Сex)
that inside of a membrane is created a gradient of concentration
.
In
the equation
on
a membrane it is possible to consider a gradient of concentration a
constant.
,
then
(6)
where 1 -
thickness of a membrane
- concentration
of substance inside of a membrane at its borders, which are defined
by concentration
in
a water phase and by coefficient of distributions k
of the substances between a membrane and surrounding water phase. The
parity is in most cases carried out:
(7)
The density of a stream through a membrane is equal:
(8)
Size Р is called coefficient of permeability of a membrane:
(9).
The equation (9) is known under the name of the law of Fick for passive carry of substances (diffusion) through a membrane. The coefficient of permeability costing in the right part of this equation, is very important size; from its formula it is visible, if the more factor of diffusion, then it is more factor of permeability.
In biological membranes there is one more kind of diffusion - the facilitated diffusion. It occurs at participation of molecules-carriers. For example, the antibiotic is a carrier of ions of potassium. Features of a chemical structure allow antibiotic to form a complex with ions potassium. Molecules of antibiotic can grasp ions of potassium from a surrounding solution. Molecules transfer these ions through a membrane, and then give ions to a solution on other party of a membrane. Carry of substance occurs to participation of a carrier more intensively, than in case of simple diffusion.
The facilitated diffusion possesses property of saturation (fig. 8): at increase in concentration on the one side of membranes the density of a stream of substance increases only up to some limit, and then when all molecules-carriers are already borrowed, remains constant.
The third kind of passive transport is the filtration - movement of a solution through interstice in a membrane owing to presence of a gradient of hydraulic pressure. At last, last, fourth type of passive transport is osmosis.
O
smosis
is
a movement of molecules of water through semipermeable a membrane
from area of smaller concentration of the dissolved substance in area
of greater concentration of the dissolved substance. Osmosis - simple
diffusion of water from places with its greater maintenance in places
with the smaller maintenance.
Osmosis plays a greater role in many biological phenomena.
Fig. 8. Dependence of density of a stream of substance through a biological membrane in a cell from a gradient of concentration of this substance at simple (1) and at the facilitated (2) diffusions.
Osmotic pressure in direct proportional to concentration of the dissolved substance which are not passing through a membrane.
Oncotic pressure is part of the general osmotic pressure which is caused by high-molecular substances - protein. Water goes from a lymph in blood under action of a difference oncotic pressure of blood of the person and a fabric liquid, a lymph.
Abnormal osmosis is a process of carry of water at simultaneous presence of osmotic and electric gradients.
Negative abnormal osmosis - occurs at carry of water against osmotic, hydrostatic and electric gradients simultaneously. For example, in osmotic work of kidneys.
Resultant carry of a liquid occurs at positive abnormal osmosis on an osmotic gradient, or with additional acceleration, or with delay due to an electric gradient. In all cases abnormal osmosisа carry of water is carried out on the general electrochemical gradient.
Active transport of substance.
Carry of molecules and ions against an electrochemical gradient, which the cell carries out due to energy of metabolic processes is called active transport of substance. Thus the cell makes work which is called concentration or osmotic. It consists in accumulation by a cell of the substances which are present at an environment in small quantities, and deducing in an environment of substances which in this environment are present at high concentration.
The size of this work is defined from the equation:
A= mRT
ln
Where m quantity of molecules of the substance transferred through a membrane from area with active concentration С2 in area with concentration С1. If the membrane is electric polarized, work against forces of an electric field is considered.
A = mRT
ln
,
where n - valency of ions.
There are two kinds of active transport: carry of ions and carry of organic substances. Carry of ions is distinctly expressed in various sector cells - in salivary and sudoriserous glands, especially in glands a stomach, which allocate a lot of ions of hydrogen and chlorine. Active carry of ions potassium and sodium through cellular membranes plays the important role in generating bioelectric potentials and carrying out of excitation. Active transport promotes carry of organic substances, mainly sugars and amino acids. Oxidation of food stuffs, and then hydrolysis ATP (adenosinetriphosphat) with release free energy cause against gradient processes in cells, one of which is active transport of substances.