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.