Biology, Eighth Edition (Raven)

Chapter 5: Membranes

Osmosis

Why does the concentration of a solute cause the movement of water through osmosis? Water is unusual in that it can cross a non-polar membrane but also react and form solutions with ions and polar molecules that cannot cross the same membrane. When water molecules react with a solute they are no longer free to cross the membrane. The more solute, the fewer molecules left that are free to diffuse across the membrane. Therefore a highly concentrated solution (one with lots of solute) has a low effective concentration of water. Therefore water will tend to diffuse across a membrane from areas of low solute concentration to areas of high solute concentration – until the concentration of the solution is equal on both sides. This is only true if the solute cannot cross the membrane.

View the animation below, then complete the quiz to test your knowledge of the concept.


1.	When a membrane that is permeable to both a solute and water separates water with a high osmolarity from water with a low osmolarity...

		A)	the water will only move through the membrane in one direction while the solute will move through the membrane in the other.
		B)	the solute will pass through the membrane in both directions until equilibrium is reached on both sides of the membrane for the solute.
		C)	both the water and the solute will pass through the membrane in both directions until equilibrium is reached on both sides of the membrane for both water and the solute.
		D)	the motion of the molecules of both the solute and water will cease when equilibrium is reached.
		E)	none of the above

2.	When a membrane that is only permeable to water separates water with a high osmolarity from water with a low osmolarity...

		A)	the water will diffuse through the membrane until there is an equal volume of solute on both sides.
		B)	the water will diffuse through the membrane until there is an equal volume of water on both sides.
		C)	the water will diffuse through the membrane until there is an equal concentration of water on both sides.
		D)	the water will diffuse through the membrane until there is an equal concentration of solute on both sides.
		E)	the water will diffuse through the membrane until there is an equal concentration of solute and water on both sides.

3.	Water is more concentrated in...

		A)	the solution on the side of the membrane with the least solute.
		B)	the solution on the side of the membrane with the most solute.
		C)	the solution on the one side of the membrane that has reached equilibrium.
		D)	the solution on either side of the membrane that has reached equilibrium.
		E)	the solution on neither side of the membrane due to the constant motion of the molecules.

4.	Water molecules will no longer pass through the semipermeable membrane once equilibrium is reached.

		A)	True
		B)	False

5.	Osmosis can be described as both the diffusion of a liquid from an area of high concentration of that liquid to an area of a low concentration of that liquid, and, as the diffusion of a liquid from a low concentration of solutes to an area of high concentration of solutes.

		A)	True
		B)	False