Biology, Eighth Edition (Raven)

Chapter 39: Plant Nutrition and Soils

How Osmosis Works

Why is diffusion involving water a special case? Water is necessary for life on earth. Not only is water directly involved in many key biochemical reactions, virtually all the molecular processes of life occur in an aqueous environment. Why is water so important? It is a small, weakly polar molecule. These properties mean that water molecules can pass through cellular membranes but also form solutions with ions and polar molecules. Water is the medium in which the biochemical processes of life occur. This animation shows how the concentration of molecules in solution in water can cause the movement of water across a membrane, otherwise known as osmosis. Preventing the loss or gain of too much water through osmosis is often an important challenge for cells.

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


1.	Osmosis is best defined as the movement of

		A)	molecules from an area of high concentration to an area of lower concentration.
		B)	molecules from an area of low concentration to an area of higher concentration.
		C)	water molecules across a membrane from an area of low water to an area of higher concentration.
		D)	water molecules across a membrane from an area of high water concentration to an area of lower concentration.
		E)	water molecules inside a container.

2.	Which of the following will pass through a cell membrane most easily?

		A)	small polar molecules
		B)	small nonpolar molecules
		C)	large polar molecules
		D)	large nonpolar molecules
		E)	large neutral molecules

3.	A red blood cell placed in a hypertonic medium will

		A)	expand.
		B)	burst.
		C)	shrink.
		D)	have no change in shape.
		E)	become a white blood cell.

4.	A 5% urea solution is hypotonic to a 10% urea solution.

		A)	True
		B)	False

5.	If a cell is placed in an isotonic medium, there will be no net movement of water.

		A)	True
		B)	False