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.

We will be using dialysis tubing to simulate a semipermeable membrane. This tubing allows small molecules (e.g., water, ions, glucose) to pass while preventing large molecules (e.g., macromolecules like proteins, starch, glycogen) from moving across.  Be sure you have read over the suggested material before starting this exercise and that you have reviewed the following video:

Diffusion and Osmosis (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 (Links to an external site.)

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

WEEK 3 EXPERIMENT ANSWER SHEET Please submit to the Week 3 Experiment dropbox no later than Sunday midnight.

SUMMARY OF ACTIVITIES FOR WEEK 1 EXPERIMENT ASSIGNMENT

· Experiment 3 Exercise 1 – Diffusion: Movement of Solutes across a Membrane

· Experiment 3 Exercise 2 – Osmosis: Movement of Water across a Membrane

Experiment 3 Exercise 1: Diffusion – Movement of Solutes across a Membrane

We will be using dialysis tubing to simulate a semipermeable membrane. This tubing allows small molecules (e.g., water, ions, glucose) to pass while preventing large molecules (e.g., macromolecules like proteins, starch, glycogen) from moving across. Be sure you have read over the suggested material before starting this exercise and that you have reviewed the following video: https://www.youtube.com/watch?v=zuNMVzTeCtw

Experimental Design

A. The dialysis bag we will use is permeable to water and small molecules (e.g., less than 500 g/mol) and impermeable to large molecules (e.g., more than 500 g/mol).

B. The dialysis bag is filled with a mixture of glucose (molecular weight = 180 g/mol) and protein (molecular weight = 10,000 g/mol) dissolved in water. A small subsample of the dialysis bag contents is saved and will be used in Step 4.

C. The dialysis bag is then placed into a beaker of water. A small subsample of beaker water is also saved and is to be used in Step 4 as well.

The presence or absence of glucose and protein will be determined using indicators. Indicators change colors in the presence certain materials. The two tests that we’ll use are the Benedict’s test for simple sugars (e.g., glucose) and the Biuret test for the presence of proteins.

· If glucose is present, the Benedict’s indicator will turn green. If no glucose is present, the solution will be blue.

· If protein is present, the Biuret indicator will turn violet. If the solution remains clear, then no protein is present.

D. The subsample of dialysis bag solution and the beaker water are tested for the presence of glucose and protein. See Table 1 below for the results.

E. The dialysis bag is then left in the beaker of water for 60 minutes.

F. At the end of 60 minutes, the dialysis bag solution and the beaker water are again tested for the presence of glucose and protein. See Table 1 below for the results.

Table 1. Results of testing of the dialysis bag and beaker contents at the beginning and end of the Experiment.

	Test for Glucose	Test for Protein
	Beginning	End	Beginning	End
Dialysis Bag	Green	Green	Violet	Violet
Beaker	Blue	Green	Clear	Clear

Questions

1. Summarize the results regarding the presence (+) or absence (-) of glucose and protein in the dialysis bag and beaker in Table 2 below (4 pts):

Table 2.

	Glucose	Protein
	Beginning	End	Beginning	End
Dialysis Bag
Beaker

2. Explain the movement or lack of movement of protein and glucose across the dialysis bag membrane (4 pts)

3. Which solution, that in the bag or that in the beaker, is hypotonic compared with the protein solution (2 pts)?

4. What factors affect the movement of molecules across a semipermeable membrane? Which factor plays the greatest role in biological systems (4 pts)?

5. Briefly explain what active transport is and how it differs from passive transport, especially in terms of concentration gradients (4 pts).

 

Experiment 3 Exercise 2: Osmosis – The Movement of Water across a Membrane

Before starting, let’s see what you know about the terms hypotonic, isotonic and hypertonic. Examine the diagrams below. Note that the small green circles represent dissolved solutes like salt, glucose, and amino acids. You can assume that the additional space surrounding the solutes is water and that the tan area is INSIDE the cell.

Question

1. Define each term below in terms of solute concentration outside compared to the inside of the cell. You do not need to explain which direction water will move (3 pts).

a. Hypotonic –

b. Isotonic –

c. Hypertonic –

Procedure

A. Open the following website to get started:

The Biology Place. No Date. Osmosis: Movement of Water across Membranes http://www.phschool.com/science/biology_place/biocoach/biomembrane1/osmosis.html

B. Read over the information presented and then Click on 

C. Then, Click on . Read through the information presented and be sure to click on Animate beneath the illustration.

Questions

2. What concentration of salt is isotonic to animal cells (1 pts)?

3. When cells are in isotonic solution, is there movement of water into or out of the cell? If so, describe this movement (3 pts).

 

Procedure (continued)

D. Click on .

E. Read through the information presented and be sure to click on Animate beneath the illustration. When ready, answer the following question.

Question

4. Describe the net movement of water molecules when cells are placed in a hypotonic solution. Explain why water moves this way (3 pts).

 

Procedure (continued)

F. Click on 

G. Read through the information presented and be sure to click on Animate beneath each of the illustrations. Answer the following questions. Your answers should incorporate the terminology used in the animations.

Questions

5. What happens to an animal cell when placed in a hypotonic solution (2 pts)?

6. What happens to plant cells when placed in a hypotonic solution? What accounts for the difference in outcomes between animal cells and plant cells (3 pts)?

 

Procedure (continued)

H. Click on 

I. Then, Click on . Read through the information presented and be sure to click on Animate beneath the illustration. Answer the following question.

Question

7. Describe the net movement of water molecules when cells are placed in a hypertonic solution. Explain why water moves this way (3 pts).

 

Procedure (continued)

J. Click on 

K. Read through the information presented and be sure to click on Animate beneath the illustration. Answer the following questions.

Questions

8. Compare and contrast what happens to plant and animal cells when placed in a hypertonic solution. Be sure to use proper terminology (4 pts).

9. Based on what you learned in this exercise, explain why salt might make a good weed killer (3 pts).

 

Week 3 Experiment Grading Rubric

Component	Expectation	Points
Experiment 3 Exercise 1	Interpretation of results and demonstrated understanding of diffusion (Table 2 and Questions 1-5).	18 pts
Experiment 3 Exercise 2	Demonstrates understanding of isotonic solutions and no net movement of water (Questions 1-3).	7 pts
	Demonstrates understanding of hypotonic solutions and the movement of water (Questions 4-6).	8 pts
	Demonstrates understanding of hypertonic solutions and the movement of water (Questions 7-9).	10 pts
TOTAL		43 pts

 

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