 Dear students, in this topic we shall discuss the processes of diffusion and membrane flux. You know that molecules have thermal energy due to which they are in continuous motion. This motion is called thermal motion. One important effect of this thermal motion is the diffusion of substance. We define diffusion as the random thermal motion of molecules that results in their dispersion from the regions of higher concentration to the regions of lower concentration. These students, in cells, most of the traffic of substances takes place by diffusion. When a substance is more concentrated on one side of the membrane, there is a tendency for its movement from that side to the other side, where it is present in low concentration. However, membrane must be permeable to such movement. If the membrane is not permeable and a substance is more concentrated on one side of the membrane, then the substance cannot diffuse from the membrane. Therefore, this condition is necessary for diffusion that the membrane must be permeable to that substance. Dear students, we shall explain diffusion by taking an example of uptake of oxygen by the cell. You know that oxygen diffuses into the cell in dissolved form. When oxygen enters the cell, it is readily taken up by the cell because of cellular respiration. Due to this utilization, oxygen concentration decreases in the cell. In contrast, there is an extra cellular fluid in which oxygen is generated due to continuous circulation. There, oxygen concentration is higher. So, inside the cell, concentration of oxygen is low. Outside the cell, concentration of oxygen is high. So, in both the levels of oxygen, we get a concentration gradient across the plasma membrane. So, oxygen has a tendency to move from extra cellular fluid into the cell. Dear students, diffusion is a passive process. It occurs without the expense of chemical energy of ATP. It is the concentration gradient itself that acts as a source of potential energy and drives diffusion. However, rate of diffusion varies due to the permeability differences of various substances. Dear students, the rate of diffusion of a solute is calculated by Fick diffusion equation. This equation defines that the rate of diffusion that is delta QS over delta T depends on the concentration gradient of the substance which is represented by delta CS over delta X. The rate of diffusion also depends on the diffusion coefficient of the substance DS which varies with the type and molecular weight of the substance and it also depends on the cross-sectional area A through which the substance moves through the membrane. Dear students, the amount of solute that passes through a unit area of the membrane every second in one direction is known as membrane flux. Membrane flux is represented by J and J is equal to delta QS over delta T. If a diffusible solute occurs on both sides of a membrane, it exhibits a flux in each direction. Flux in one direction is independent of the flux in opposite direction. If influx and efflux i.e. flux into the cell and flux out of the cell are equal, then the net flux is zero. However, if flux in one direction is greater than the other, then there is a net flux and net flux is equal to the difference between the two fluxes.