To Gain a Better Understanding of Diffusion and Osmosis

Lab #2

Name:______

Membrane Function – Diffusion,

Osmosis, and Endocytosis

Introduction

This lab will introduce you to cellular membranes, diffusion and osmosis (especially as they relate cell membranes) and receptor-mediated endocytosis.

PURPOSE

To gain a better understanding of diffusion and osmosis.

To understand these terms: diffusion, osmosis, diffusion or concentration gradient, hyposmotic, hyperosmotic, isosmotic, hypotonic, hypertonic, isotonic, selectively permeable, semipermeable

APPARATi and Materials

DIFFUSION

dialysis slide filled with water, starch solution or glucose solution

beakers filled with water

beakers filled with water and I2KI (Lugol's solution)

stir plates

magnetic stir bars

test tubes

boiling water bath

Benedict’s solution

water

15% glucose solution

1% starch solution

OSMOSIS

Elodea leaf

microscope depression slides

water

NaCl solutions - 0.05M, 0.1M, 0.2M, 0.3M, 0.4M, and 0.5M

sucrose solutions - 0.05M, 0.1M, 0.2M, 0.3M, 0.4M, and 0.5M

microscope

RECEPTOR-MEDIATED ENDOCYTOSIS

actively growing amoeba proteus

microscope slides

petroleum jelly

0.001M alcian blue

0.01M sodium azide

microscope

General information

1.Work in groups (the size of the groups will be determined by the size of the class and by the

amount of equipment available).

2.Remember how to use and care for the microscopes from Lab#1.

Procedures

A. Demonstration of Diffusion - Diffusion across a Semi-Permeable Membrane

1. A demonstration dialysis slide and a beaker of solution with the following contents will have been

set-up before class:

Based on an understanding of the dialysis membrane, what are your predictions regarding the

movement of water molecules and solute particles when the dialysis slide is placed into the

beaker?

2.Record the colour of each solution at the beginning of the experiment. Wait until the colour

changes begin to occur (usually 30 minutes). Record the results in the table provided in the results section.

3.Add 5 mls of Benedict’s reagent to 9 separate tubes. For tubes 1-3, add 8 drops of water (tube #1), starch solution (tube #2), or glucose solution (tube #3). For the remaining tubes, add 8-10 drops from each of the beaker solutions into a separate tube. Record the colour of the solution. Boil the samples for 1-2 minutes. Record the colour of each tube in the table provided.

B. Osmosis in a Plant Cell – NaCl-induced Cell Swelling or Shrinking

1. Obtain leaves from an Elodea plant. Assemble 7 depression slides (labeled with 0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5) with one leaf in each slide. Place drops of salt solution on eachdepression slide, such that each slide has a different concentration of salt. Cover with a coverslip, and examine the material first at low power (100X) and then at high power (400X).
2. Locate a region of typical cells and make a sketch in the boxes of Table 3 (below). Note especially the location of the chloroplasts. (Don’t forget to include total magnification.) Compare all the salt solutions.
3. Repeat 1 and 2 using the sucrose solutions provided.

C. Receptor Mediated Endocytosis

1. Carefully make a ring of petroleum jelly on a microscope slide. Add a few drops of amoeba to the inside of the ring. Observe the amoeba under low magnification on the microscope. While looking under the microscope, add 10 l of 0.001M alcian blue dye. Record the results below.

2.Set up a second amoeba slide. Add 10 ul of 0.01M sodium azide (careful!!) to the amoeba and let sit for 5 minutes before adding the dye. Repeat the rest of step 1.

data sheet Name: ______

A. Diffusion Experiment Results:

Table 1: Your Pre- and Post- Colour Observations from the Dialysis Slide Experiment

Table 2: Benedict’s Test Results of Beaker Contents

Is there evidence of the diffusion of starch molecules?

Is there evidence of the diffusion of iodine molecules?

Is there any evidence for the diffusion of glucose molecules?

What can you say about the permeability of the dialysis slide? (What particles could move through and what particles could not?) And what conclusions can you draw about your predictions? (Were you correct or incorrect with your predictions?)

In this experiment, what was the independent variable? What was the controlled variable? What was the dependent variable? What were the negative and positive controls?

B. Osmosis Experiment Results

Table 3. Sketch your Elodea leaf cells here. Be sure to note your magnification and label your drawings!

At what concentration of salt did the plant cells swell? At what concentration of salt did the plant cells shrink? At what concentration of salt did the plant cells stay the same? Which concentration of salt is isotonic? Hypertonic? Hypotonic?

What concentration of sucrosewas required to maintain the cell shape (the isosmotic solution)?Is it the same concentration of salt required to maintain the cell shape? Why?

Assuming that the cells have not been killed, what should happen if the salt solution were to be replaced by water?

Based on your observations, explain what happens on a cellular level when plants wilt from lack of proper watering.

Can plant cells burst? Explain.

D. Receptor-Mediated Endocytosis Results

What happened to the blue dye? Did the amoeba take up the dye? Did the dye stay inside the amoeba? How did sodium azide affect the amoeba? What does sodium azide do? Explain your results.

TO TURN IN: This lab report is due on Thursday (7/6). Turn in the answers to all of the questions posed in the text above. Include appropriately labeled drawings where requested.