Diffusion (Two Experiments)

Diffusion

This concept, along with the concepts of osmosis and osmolarity, is important when discussing movement of molecules across a cell membrane.

Diffusion is the tendency of a substance to spread out evenly in the available space: from an area of high concentration to an area of low concentration.
A solution has one or more substances (solutes) dissolved in another substance (solvent).

Diffusion in Action

To visualize diffusion, imagine a glass of water to which a drop of food coloring is added. Notice how the color spreads out from the point the drop hits the water. This is diffusion of the food coloring molecules.

Diffusion Experiment,1

We will start with the diffusion of one solute.
We draw three boxes and indicate that they are filled with water.
In each box, we draw a membrane with pores vertically down the middle.
In the leftmost box, we draw small solute molecules entirely on the left side of the membrane.
They move randomly: some of them make it through the pores and some bounce back.
The net movement of solute molecules is toward the right side of the box (i.e. to the area of low concentration).
In the middle box, we draw a couple solute molecules on the right and the majority on the left.
Random movement continues, with some molecules on each side making it through the pores and others bouncing back.
The net movement of solute molecules is still towards the right.
In the final box, we draw equal amounts of solute on both sides.
Random movement continues, but note that anytime a solute molecule passes through a pore, a molecule from the other side also passes through: this situation is equilibrium; here, there is no net movement of molecules from one side to the other.

Diffusion Experiment,2

The rate of diffusion of two or more solutes is independent of each other.
Now, we explore diffusion when two or more solutes are present in a solution.
We draw three boxes filled with water.
We draw a membrane with pores vertically down the middle of each box.
In the leftmost box, we draw a few of one type of solute molecule (Solute A) on the left side and more of another type of solute molecule (Solute B) on the right side.
We indicate the random motion of these molecules with some able to make it through the pores in both directions and others bouncing back.
We indicate that the net movement of Solute A is to the right and the net movement of Solute B is to the left.
In the middle box, we draw equal numbers of Solute A molecules on either side of the membrane and draw a few Solute B molecules on the left side of the membrane.
Random movement continues, though now equal numbers of Solute A molecules pass back and forth across the membrane while more Solute B molecules are passing to the left than are passing to the right.
Solute A has reached equilibrium while the net movement of Solute B is still toward the left.
In the final box, we draw equal amounts of Solute A on both sides of the membrane and equal amounts of Solute B on both sides of the membrane: both solutes have now reached equilibrium.

The rate at which Solute A reaches equilibrium is the same no matter if Solute B is present or not, thus...

The rate of diffusion of two or more solutes is independent of each other.

Diffusion (Two Experiments)

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