Download Fall 2016 Lab #1- Osmosis, Diffusion

Bio 120 LAB #1- OSMOSIS, DIFFUSION, AND
MEMBRANE PERMEABILITY
Lab Objectives
After completing this lab topic, you should be able to:
1. describe the effects of a semi-permeable membrane on diffusion and
osmosis.
2. predict the direction and relative rates of diffusion of molecules of
different sizes and concentrations.
Diffusion of Molecules through a Selectively Permeable Membrane
In biological systems, substances frequently move between solutions and through
membranes in a predictable manner. This passive, directional movement of molecules is
diffusion. The direction of movement depends on concentration gradients, temperature,
and pressure. Specifically, molecules move from an area of higher concentration, higher
pressure, and/or higher heat to areas of lower concentration, lower heat, and/or lower
pressure.
PROCEDURE #1- Osmosis and Diffusion in a Model Cell
Dialysis tubing can be used to construct a model cell’s semipermeable membrane. If two
solutions composed of different solutes are separated by this membrane, some of the
solutes may readily pass through the pores of the membrane, while others may not.
Working in pairs, you will investigate the selective permeability of dialysis tubing. You
will test the permeability of the membrane to starch (a polymer of glucose) and iodine
potassium iodide (I2KI). You will place a solution of starch into your model cell, and
then place the cell in a solution of I2KI. After completion of this exercise, you should be
able to determine which of the molecules diffused through the membrane, as well as
assessing whether there was any net movement of water through the membrane.
Before beginning this experiment, do you predict that the starch, the iodine, or both will
diffuse through the membrane? Will the cell gain or lose water?
Draw a diagram of the starting conditions for your experiment.
What is the original color of the starch solution? What is the original color of the iodine
solution?
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What is the final color of the solution inside the model cell? What is the final color of the
solution surrounding the model cell? Explain why these color changes occurred.
Would you expect the model cell swell or shrink during the experiment? Why?
Osmotic Activity in Plant Cells
Water passes through cell membranes from areas of higher water concentration to areas
of lower water concentration. This movement of water is osmosis. When cells are
placed in an environment in which the concentration of water is less than inside the cell,
water will flow out of the cell and into the environment.
PROCEDURE #2- Osmosis and Diffusion in Plant Cells
The effects of water loss in plant cells can be observed by watching the shrinking of the
cell membrane away from the cell wall under a microscope. The data collected from
these observations will consist of a series of drawing and descriptions.
Draw a diagram of a plant cell with its cell membrane and cells wall. What would you
expect to happen to the cell membrane when the cell is placed in an environment with a
lower concentration of water (hypertonic environment)?
What would you expect to happen when the cell is placed in an environment with a
higher concentration of water (hypotonic environment)?
Our method for measuring the movement of water is to weigh plant tissue before and
after being placed in a solution to quantify the mass of water loss. Working in a group of
three or four, you will place pieces of potato into a series of solutions and record weight
changes. For each solution type, you will do three replicates.
Record your results in the table below. Record weight in grams.
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Solution
Replicate
Initial
weight (gm)
Weight at
15 min (gm)
% change
at 15 min
Weight at 30
min (gm)
% change
at 30 min
0% salt
0% salt
0% salt
1% salt
1% salt
1% salt
9% salt
9% salt
9% salt
In this experiment, what is the independent variable? What is the dependent variable?
What is the level of treatment?
Does an increase in weight correspond to movement of water into or out of the potato
cells?
Does the concentration of the external environment influence the direction of water
movement?
Does the concentration of the external environment influence the RATE at which water is
gained or lost?
Does the concentration of the external environment influence how much weight is lost or
gained?
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Name____________________________
Bio 120 Homework #1- due Friday 9/2 at 9:00
1. Answer the following questions about the model cell procedure (Procedure #1):
Final chemical composition of solution inside the cell (1 pt)
Why did only one environment color change occur? (2 pts)
2. Answer the following questions about Procedure #2:
What was the independent variable? (1 pt)
What was the dependent variable? (1 pt)
What is the level of treatment? (1 pt)
Does the concentration of the external environment influence the direction of
water movement? Explain your answer. (1 pt)
3. Construct a graph of the average % weight change after 30 minutes in the three
different environments (0%, 1%, and 9% salt). Make sure your graph includes
titles and appropriate units. (3 points)
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