Why does diffusion occur

Think of what happens when you take a drink of orange juice from the bottle or carton. Chances are that you perceive the drink as sweet, because the high concentration of sugar in the juice exceeds that of the fluids in your system. However, if you mix the juice with plain water so that the resulting solution contains 10 parts water for every 1 part juice, wait a few minutes, and take another sip, you will perceive the fluid as dilute, because it is now at lower concentration — less concentrated, at any rate, than your body fluids.

Because the molecules of sugar in the juice tend to mix with the water molecules until the concentration of sugar is equal throughout the solution, it is said that diffusion occurs in the direction of equilibrium. Importantly, equilibrium does not mean a cessation of molecule movement, but rather that the movement of the molecules has reached a point of true randomness because all concentration gradients have been eliminated.

While some substances can simply diffuse across cell membranes when the concentration gradient favors this, others are too large to make it between the phospholipid molecules in the membrane, or they carry a net electric charge that opposes their movement.

The plasma membrane is thus a semipermeable membrane : Small, uncharged molecules such as water H2O and carbon dioxide CO2 can simply meander on through, whereas others require help or are unable to cross the membrane outright. Simple diffusion is exactly what it sounds like — the movement of molecules across a membrane down a concentration gradient as if the membrane were, in effect, not there. Conclusion: Passive diffusion of solute into a cell is linearly related to the concentration of solute outside the cell.

Carrier proteins increase the rate of diffusion by allowing more solute to enter the cell. Facilitated diffusion, however, approaches a maximum rate as the carrier proteins become saturated with solute.

What is the slope of this line? What do increases or decreases in the slope mean biologically? Now assume the concentration gradient is a constant. Look at the equation for facilitated diffusion and find the horizontal asymptote.

Try graphing this equation with different values for K. How does this change the concentration at which the carrier proteins are saturated? Sources: Darnell, J. Lodish, and D. After they reach equilibrium, there is still gas molecules travelling between the two regions, just that they are equal in number.

Of course, because my explanation being not very formal and strict, there would be some errors in my explanation, but I hope that you understand it. To avoid this collision, they have a tendency to move away from each other, especially the gaseous molecules, as they have the highest kinetic energy. So they start moving. But the net movement of molecule is from high concentration to low concentration so that equilibrium can be reached. That's why diffusion occurs.

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Diffusion is the movement of particles from regions of high concentration towards regions of lower concentration. Diffusion is the movement of particles move from an area of high concentration to an area of low concentration until equilibrium is reached.

A distinguishing feature of diffusion is that it results in mixing or mass transport without requiring bulk motion. Thus, diffusion should not be confused with convection or advection, which are other transport mechanisms that use bulk motion to move particles from one place to another.

Diffusion : Particles moving from areas of high concentration to areas of low concentration.