This unit’s Experiment exercises will deal with Diffusion and Osmosis. Read through the introductory material located below and complete the questions found in the Unit 3 Experiment Answer Sheet.

How to Proceed

• Read through the Experiment Exercise Introductions below.
• Open the Unit 3 Experiment Answer Sheet and complete the following Experiment exercises this unit:
  • Experiment 3 Exercise 1 – Diffusion (~1 hr)
  • Experiment 3 Exercise 2 – Osmosis (~1.5 hrs)
• Save your completed Unit 3 Experiment Answer Sheet and submit it no later than Sunday midnight (CT).

Diffusion – Introduction

This unit we are learning about the structure and function of cells. The plasma membrane, for example, is an important structure of all cells and it is responsible for regulating the passage of materials into and out of the cell. Plasma membranes are differentially (selectively) permeable, meaning some substances are allowed to enter and exit the cell, while the movement of other materials is either carefully regulated or blocked. Two ways in which materials can move freely across the cell membrane are diffusion and osmosis.

Diffusion is the movement of solutes (material dissolved in liquid) from an area of high concentration to an area of low concentration. If these areas are separated by a membrane, that membrane may or may not be permeable to the solute. The membrane is always permeable to water though and the movement of water across a membrane is a special form of diffusion called osmosis.

In our first exercise, we will examine diffusion of solutes through a semipermeable membrane and the factors that affect their movement. You’ll want to be sure to review our online lecture this unit on Cell Structure and pp 83 – 86 in your book. View the following two animations BEFORE starting this exercise:

McGraw-Hill. 2006. How Diffusion Works
http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html (Links to an external site.)

McGraw-Hill. 2006. How Osmosis Works
http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html (Links to an external site.)

When you are ready to begin, open the Unit 3 Experiment Answer Sheet and answer the questions associated with the first exercise.

Osmosis – Introduction

In our second exercise this unit, we will to take a closer look at osmosis; the movement of water across a membrane. The direction water moves depends on the relative concentration of solute molecules on either side of the membrane (in this case, these solutes are not able to cross the membrane). Furthermore, the presence or absence of cell walls (e.g., in plant cells) influences how cells respond to osmotic fluctuations in their environment. This exercise will examine the forces that determine whether water moves into or out of a cell.

We will be using the following website in this exercise. Be sure you are able to access and use this website before starting.

The Biology Place. No Date. Osmosis: Movement of Water across Membranes
http://www.phschool.com/science/biology_place/biocoach/biomembrane1/osmosis.html  (Links to an external site.)

Open the Unit 3 Experiment Answer Sheet and complete the questions for this exercise.