lab assignment 4 diffusion and osmosis

Optional Lab Activities

Osmosis and diffusion, lab objectives.

At the conclusion of the lab, the student should be able to:

• define the following terms: diffusion, osmosis, equilibrium, tonicity, turgor pressure, plasmolysis
• describe what drives simple diffusion (why do the molecules move?)
• list the factors that may affect the speed of simple diffusion
• list which molecules, in general, can freely diffuse across the plasma membrane of a cell
• describe what drives osmosis (why do water molecules move?)
• explain why water moves out of a cell when the cell is placed in a hypertonic solution
• explain why water moves into a cell when the cell is placed in a hypotonic solution
• describe what physically happens to a cell if water leaves the cell
• describe what physically happens to a cell if water enters the cell

Introduction

Understanding the concepts of diffusion and osmosis is critical for conceptualizing how substances move across cell membranes. Diffusion can occur across a semipermeable membrane; however diffusion also occurs where no barrier (or membrane) is present. A number of factors can affect the rate of diffusion, including temperature, molecular weight, concentration gradient, electrical charge, and distance. Water can also move by the same mechanism. This diffusion of water is called osmosis .

In this lab you will explore the processes of diffusion and osmosis. We will examine the effects of movement across membranes in dialysis tubing, by definition, a semi-permeable membrane made of cellulose. We will also examine these principles in living plant cells.

Part 1. Diffusion Across a Semi-Permeable Membrane: Dialysis

• Cut a piece of dialysis tubing, approximately 10 cm.
• Soak the dialysis tubing for about 5 minutes prior to using.
• Tie off one end of the tubing with dental floss.
• Use a pipette and fill the bag with a 1% starch solution leaving enough room to tie the other end of the tubing.
• Tie the other end of the tubing closed with dental floss.
• Fill a 250 mL beaker with distilled water.
• Add Lugol’s iodine to the distilled water in the beaker until the water is a uniform pale yellow color.
• Place the dialysis tubing bag in the beaker.
• The movement of starch
• The movement of iodine
• The color of the solution in the bag after 30 minutes
• The color of the solution in the beaker after 30 minutes
• Add the dialysis bag to the beaker and allow the experiment to run for 30 minutes. Record the colors of both the dialysis bag and the beaker.

Lab Questions

• Is there evidence of the diffusion of starch molecules? If so, in which direction did starch molecules diffuse?
• Is there evidence of the diffusion of iodine molecules? If so, in which direction did iodine molecules diffuse.
• What can you say about the permeability of the dialysis membrane? (What particles could move through and what particles could not?)
• What is the difference between a semi-permeable and a selectively permeable membrane

Part 2. Plasmolysis—Observing Osmosis in a Living System, Elodea

If a plant cell is immersed in a solution that has a higher solute concentration than that of the cell, water will leave/enter (circle one) the cell. The loss of water from the cell will cause the cell to lose the pressure exerted by the fluid in the plant cell’s vacuole, which is called turgor pressure. Macroscopically, you can see the effects of loss of turgor in wilted houseplants or limp lettuce. Microscopically, increased loss of water and loss of turgor become visible as a withdrawal of the protoplast from the cell wall (plasmolysis) and as a decrease in the size of the vacuole (Figure 1).

• Obtain a leaf from the tip of an Elodea Place it in a drop of water on a slide, cover it with a coverslip, and examine the material first at scanning, then low power objective and then at high power objective.
• Locate a region of health. Note the location of the chloroplasts.  Sketch a few cells. For the next step, DO NOT move the slide .
• While touching one corner of the coverslip with a piece of Kimwipe to draw off the water, add a drop of 40% salt solution to the opposite corner of the coverslip. Do this simultaneously.  Be sure that the salt solution moves under the coverslip. Wait about 5 minutes, then examine as before. Sketch these cells next to your sketch of cells in step two, note the location of the chloroplasts. Label it 40% salt solution .
• What happened to the cells in the salt solution?
• Assuming that the cells have not been killed, what should happen if the salt solution were to be replaced by water?
• Are plant cells normally hypertonic, hypotonic, or isotonic to their environment? Why?
• Can plant cells burst? Explain.

Overall Conclusions

• Review your hypothesis for each experiment. Was your original hypothesis supported or rejected for each experiment. Explain why or why not. This should be based on the best information collected from the experiment. Explain how you arrived at this conclusion.
• If it was incorrect, give the correct answer, again based on the best information collected from the experiment.

Sources of Error

• Identify and explain two things that people may have done incorrectly that would have caused them to get different answers from the rest of the class. Be  specific .
• Biology 101 Labs. Authored by : Lynette Hauser. Provided by : Tidewater Community College. Located at : http://www.tcc.edu/ . License : CC BY: Attribution
• BIOL 211 - Majors Cellular [or Animal or Plant]. Authored by : Carey Schroyer and Diane Forson. Provided by : Open Course Library. Located at : http://opencourselibrary.org/biol-211-majors-cellular-or-animal-or-plant/ . License : CC BY: Attribution

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Module 4: Diffusion and Osmosis

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Introduction:

The cell membrane plays the dual roles of protecting the living cell by acting as a barrier to the outside world, yet at the same time it must allow the passage of food and waste products into and out of the cell for metabolism to proceed. How does the cell carry out these seemingly paradoxical roles? To understand this process you need to understand the makeup of the cell membrane and an important phenomenon known as diffusion.

Diffusion is the movement of a substance from an area of high concentration to an area of low concentration due to random molecular motion. All atoms and molecules possess kinetic energy, which is the energy of movement. It is this kinetic energy that makes each atom or molecule vibrate and move around. (In fact, you can quantify the kinetic energy of the atoms/molecules in a substance by measuring its temperature.) The moving atoms bounce off each other, like bumper cars in a carnival ride. The movement of particles due to this energy is called Brownian motion. As these atoms/molecules bounce off each other, the result is the movement of these particles from an area of high concentration to an area of low concentration. This is diffusion. The rate of diffusion is influenced by both temperature (how fast the particles move) and size (how big they are).

Part 1: Brownian Motion:

In this part of the lab, you will use a microscope to observe Brownian motion in carmine red powder, which is a dye obtained from the pulverized guts of female cochineal beetles.

• Glass Slide
• Carmine Red Powder
• Obtain a microscope slide and place a drop of tap water on it.
• Using a toothpick, carefully add a very minuscule quantity of carmine red powder to the drop of water and add a coverslip.
• Observe under scanning, low, and then high power.

Lab Questions:

1. Describe the activity of the carmine red particles in water.

2. If the slide were warmed up, would the rate of motion of the molecules speed up, slow down, or remain the same? Why?

Part 2: Diffusion across a Semipermeable Membrane

Because of its structure, the cell membrane is a semipermeable membrane. This means that SOME substances can easily diffuse through it, like oxygen, or carbon dioxide. Other substances, like glucose or sodium ions, are unable to pass through the cell membrane unless they are specifically transported via proteins embedded in the membrane itself. Whether or not a substance is able to diffuse through a cell membrane depends on the characteristics of the substance and characteristics of the membrane. In this lab, we will make dialysis tubing “cells” and explore the effect of size on a molecule’s ability to diffuse through a “cell membrane.”

The following information might be useful in understanding and interpreting your results in this lab:

• Atomic formula: C 20 H 14 O 4
• Atomic mass: 318.32 g/mol
• Color in acidic solution: Clear
• Color in basic solution: Pink
• Atomic formula: I or I2
• Atomic mass: 126 g/mol
• Atomic formula: (C 6 H 10 O 5 )n
• Atomic mass: HUGE!
• Color in Iodine: Bluish
• Atomic formula: NaOH
• Atomic mass: 40.1 g/mol
• Acid/Base: Base
• 2 pieces of dialysis tubing
• Phenolphthalein
• Starch solution
• Using a wax pencil, label one beaker #1. Label the other beaker #2.
• Fill beaker #1 with 300 ml of tap water, then add 10 drops of 1 M NaOH. Do not spill the NaOH—it is very caustic!
• Fill beaker #2 with 300 ml of tap water, then add iodine drops drop by drop until the solution is bright yellow.
• Now prepare your 2 dialysis tubing “bags.” Seal one end of each dialysis tube by carefully folding the end “hotdog style” 2 times, then “hamburger style” 1 time. Tie the folded portion of the tube securely with string. It is critical that your tubing is tightly sealed, to prevent leaks.
• Add 10 ml of water and three drops of phenolphthalein to one of your dialysis tube bags. Seal the other end of the bag by carefully folding and tying as before.
• Thoroughly rinse the bag containing phenolphthalein, then place it in into the beaker containing the NaOH.
• Add 10 ml of starch solution to the other dialysis tube. Again seal the bag tightly and rinse as above. Place this bag containing the starch solution into beaker #2.
• Let diffusion occur between the bags and the solutions in the beakers.
• After 10 minutes, observe the color changes in the two bags and the external solutions. Draw a picture of each system below.

Record the colors (below) and label contents inside and outside the bags (above):

1. Which substance diffused across the membrane in beaker #1? How do you know?

2. Which substance diffused across the membrane in beaker #2? How do you know?

3. Why might some ions and molecules pass through the dialysis bag while others might not?

Part 3: Osmosis and the Cell Membrane

Osmosis is the movement of water across a semipermeable membrane (such as the cell membrane). The tonicity of a solution involves comparing the concentration of a cell’s cytoplasm to the concentration of its environment. Ultimately, the tonicity of a solution can be determined by examining the effect a solution has on a cell within the solution.

By definition, a hypertonic solution is one that causes a cell to shrink. Though it certainly is more complex than this, for our purposes in this class, we can assume that a hypertonic solution is more concentrated with solutes than the cytoplasm. This will cause water from the cytoplasm to leave the cell, causing the cell to shrink. If a cell shrinks when placed in a solution, then the solution is hypertonic to the cell.

If a solution is hypotonic to a cell, then the cell will swell when placed in the hypotonic solution. In this case, you can imagine that the solution is less concentrated than the cell’s cytoplasm, causing water from the solution to flow into the cell. The cell swells!

Finally, an isotonic solution is one that causes no change in the cell. You can imagine that the solution and the cell have equal concentrations, so there is no net movement of water molecules into or out of the cell.

In this exercise, you will observe osmosis by exposing a plant cell to salt water.

Prediction:

What do you think will happen to the cell in this environment? Draw a picture of your hypothesis.

• Elodea Leaf
• Microscope Slide
• 5% NaCl solution
• Remove a leaf from an Elodea plant using the forceps.
• Make a wet mount of the leaf. Use the pond water to make your wet mount.
• Observe the Elodea cells under the compound microscope at high power (400X) and draw a typical cell below.
• Next, add several drops of 5% salt solution to the edge of the coverslip to allow the salt to diffuse under the coverslip. Observe what happens to the cells (this may require you to search around along the edges of the leaf). Look for cells that have been visibly altered.

Draw a typical cell in both pond and salt water and label the cell membrane and the cell wall.

What do you see occurring to the cell membrane when the cell was exposed to salt water? Why does this happen?

Describe the terms hypertonic, hypotonic, and isotonic.

How would your observations change if NaCl could easily pass through the cell membrane and into the cell?

Part 4: Experimental Design

You and your group will design an experiment to determine the relative molecular weights of methylene blue and potassium permanganate. You may use a petri dish of agar, which is a jello-like medium made from a polysaccharide found in the cell walls of red algae. You will also have access to a cork borer and a small plastic ruler.

• 1 Petri Dish of Agar
• Methylene Blue
• Potassium Permanganate

Your experiment design should include all of the following portions:

• Experimental design
• Conclusions
• Further Questions/Other Comments

Licenses and Attributions

CC licensed content, Original

• Biology Labs. Authored by: Wendy Riggs . Provided by: College of the Redwoods. Located at: http://www.redwoods.edu . License: CC BY: Attribution

Public domain content

• Osmotic pressure on blood cells diagram. Authored by: LadyofHats. Located at: https://commons.wikimedia.org/wiki/File:Osmotic_pressure_on_blood_cells_diagram.svg . License: Public Domain: No Known Copyright