Atoms, ions and molecules are constantly moving, colliding and thus changing their direction. In an ideal gas move the single particles independently of one another. The particle distribution is not influenced by these movements and no predominant or preferred direction exists. The velocity depends on the temperature and increases when the temperature rises. It is therefore also spoken of thermal movements (T). The particle movements within liquids are more restricted but the movements within a solution are subject to the same criteria as those in an ideal gas. Often can the plasma of a cell, too, be regarded as such a solution. Although the description does not fit for all cases (just think of the compartmentalisation).
f(t) = DF (c1 - c2 / x1 - x0)
dm / dt = DF (dc / dx ) [cm2 sec-1]
Phi = - D (dc / dx) [Mol cm-2 sec-1]
Phi = - D (cl - c2 / x1 - x0)
or, when eliminating D:
D = -Phi / (cl - c2 / x1 - x0)
(c1 - c2/ x1 - x0) is called the concentration gradient. D is the diffusion constant that is defined as the amount of a substance (in Mol) that diffuses through a certain area (in square centimetre) at a concentration gradient of 1 (Mol/ cm).
Permeability is the term that describes the diffusion of particles through membranes. It is unimportant whether we regard natural or artificial membranes (like plastic film). We will in a rough approximation assume that the membrane equals a solvent layer of the thickness d in order to obtain a quantitative statement.
From this assumption is the flux through a membrane directly proportional to the diffusion constant. The constant is without exception lower than in water. The net flux through the membrane can therefore be described as:
Phi = - D (c1 - c2 / d) = - D / d (c1 - c2)
Biological membranes are not equally permeable for all substances. They are selectively permeable, i.e. the membranes can be permeated by a substance A but not by a substance B. Strictly speaking are they only permeable for some gases like oxygen, nitrogen, carbon dioxide, etc., because only these can permeate a membrane by free diffusion. As has been shown above, is diffusion strictly subject to the laws of thermodynamics. Molecules are characterized by specific properties like size, ability to be converted into a ion or solubility within a membrane. Such lipophilic (fat soluble) molecules can cross the membrane more easily than hydrophilic ones.
2. The carriers (mostly proteins, but also some antibiotics like valinomycin) bind to the substrate (sugar or amino acid, for example). In contrast to enzymes do they not convert their substrates but transport them through the membrane instead.
3. Several substrates can compete for the same carrier. Their binding properties concerning the carrier may differ and as a consequence differ their transport kinetics, too.
4. The loaded carrier crosses the membrane with an other velocity than the unloaded carrier.
5. Die treibende Kraft der erleichterten Diffusion ist wie bei einfacher Diffusion
6. The maximal transport velocity is dependent on the number of the carrier molecules in the membrane.