Passive Transport: Diffusion & Osmosis

Learning Objectives for this lab:

To gain an understanding of how important and powerful passive transport can be.
To gain familiarity with the notion of diffusion and concentration gradients.
To understand how osmosis works.
To learn how both diffusion and osmosis may affect cells.

Contents of this lab:

1. Introduction to Passive Transport (below)

2. Introduction to Diffusion

Diffusion Exercises: submission form available off BlackBoard

	Exercise 1: Diffusion of dye through a solid
	Exercise 2: Diffusion through an artificial membrane
	Exercise 3: More diffusion through an artificial membrane

3. Introduction to Osmosis

Osmosis Exercise: submission form available off BlackBoard

	Exercise 4: Osmosis through egg membranes
	the other exercise shown in your video is not being done this week... you'll do that in the cell lab.

4. Appendix: Key Terms

Note: There are two submission forms. Both are available from BlackBoard. The diffusion exercises are due by Monday night, 3/11, at midnight. The osmosis exercises are due by Wednesday night March 13 at midnight. You get extra time on osmosis because the eggs take time to set up.

Introduction to Passive Transport:

All molecules have some amount of energy that enables them to move. When molecules move with this intrinsic energy, they do so according to the rules of diffusion or osmosis. Basically, molecules can move to spread out more evenly... that means that they move from where they are more abundant (in greater concentration) to where they are less abundant (in lower concentration). All sorts of molecules move by diffusion, but only water moves by osmosis (which is actually a special example of diffusion). Keep in mind that passive transport occurs without supplying any extra outside influence or energy. For example, if one puts a teaspoon of salt into a glass of water, all the water will taste salty, even without stirring the water.

You have already learned about passive transport in lecture (Lesson 5). Basically, the word "transport" refers to how molecules can cross cell membranes, to either enter the cell or leave the cell. When you add the word "passive" in front of transport, that means that we are now talking about molecules entering or leaving the cell due to passive means-- either diffusion or osmosis. Solutes, if small enough, may be able to cross a cell membrane by diffusion. Water can cross a membrane by osmosis. Keep in mind that in order to figure out whether something will enter or leave a cell, all you have to do is look at the concentration or osmotic gradient on that item. We can't make things go any which way across the membrane if transport is passive... they can only go in the direction of their gradient.

It is your task now to try to understand how diffusion and osmosis work across cell membranes. To do that, you will need to complete these exercises.