Explain the laws of transport in biophysics on the basis of flux density and gradient concepts.

Fick's laws: Adolf Fick gives the simplest explanation of diffusion in the 19th century. Today these explanations are known as by his name, Fick's laws. There are two laws: 1. Fick's first law: The molar flux due to diffusion is proportional to the concentration gradient, and 2. Fick's second law: The rate of change of concentration at a point in space is proportional to the second derivative of concentration with space.

According to this law the net diffusion rate of a gas through a membrane is proportional to the difference in partial pressure, proportional to the area of the membrane and inversely proportional to the thickness of the membrane. Combined with the diffusion rate determined from Graham's law, this law provides the means for calculating exchange rates of gases across membranes. In case of gas exchange in lung, the total membrane surface area in the lungs (alveoli) may be on the order of 100 square meters and have a thickness of less than a millionth of a meter, so it is a very effective gas exchange interface.

The transport of solutes by diffusion method is governed by the solute concentration in both the sides of compartment and the thickness of the membrane. In the schematic shown here the flux, J of a solute will be towards the right and it can be estimated by Fick's law: Where, dc/dx is the concentration gradient per unit length, and D is the diffusion constant. 

In general, flux in biology relates to movement of a substance between compartments. ... The movement of molecules across a membrane: in this case, flux is defined by the rate of diffusion or transport of a substance across a permeable membrane.The Rate of FLOW of particles (dn/dt, i.e. the number of particles moving in a specified time) through an area is the FLUX. The symbol for flux is J and the units of flux are mol m^-2 s^-1. 

The number of particles n in a certain volume V is the concentration C (C =n/V) so we can also express the flux equation in terms of the change in concentration with time. The units of dC/dt are mol cm^-3 s^-1.

Fick's First Law states that flux is proportional to the CONCENTRATION GRADIENT, and the proportionality constant D is the DIFFUSION COEFFICIENT.

?Fick's Law again: Flux is directly proportional to gradient.

Now we know that the gradient is represented by dC/dx, but what does "directly proportional" mean? Put simply, it means this:

small gradient --> small diffusion

BIG gradient --> BIG diffusion