Osmosis

 

Introduction to Osmosis[1]

 

I. What is happening to these eggs?

 

Most cells are tiny – much too small to see without the help of a microscope. In contrast, an unfertilized chicken egg is a giant cell. You will use a chicken egg to investigate movement of water across the cell membrane that surrounds each cell.

 

1. Why is it important for each cell to be surrounded by a cell membrane that can prevent large molecules from leaving the cell?

 

 

 

 

 

2. The cell membrane allows some small molecules like oxygen and carbon dioxide to cross. Why is it important for oxygen and carbon dioxide to be able to cross the cell membrane?

 

 

 

 

 

In this investigation you will see that water can cross the cell membrane surrounding an egg. You will investigate which way water moves across the membrane, depending on the type of liquid surrounding the egg. When water moves across the cell membrane, the egg changes in size and appearance.

 

Your group will be given two eggs. To begin, record the weight and/or circumference of these eggs in the day 1 row of the table. (Measure the circumference around the widest part, not lengthwise.)

 

Caution: Because these are raw eggs, they may carry salmonella, so you should use gloves when handling the eggs.

 

 

 

Day

Egg 1

 

Egg 2

Weight (grams)

Circumference (cm)

Weight (grams)

Circumference (cm)

1

                 

(with shell)

 

                  

(with shell)

 

Egg put into vinegar

Egg put into vinegar

2

                 (most of shell

                     removed)

 

                  (most of shell

                       removed)

 

Egg put into water

Egg put into corn syrup

3

 

 

 

 

 

 

Put each egg in a container labeled Egg 1 or Egg 2 with enough vinegar to cover the egg. Cover the container. Do you see bubbles forming around the egg? These are bubbles of CO2 which result from the chemical reaction between the acetic acid in the vinegar and the calcium carbonate in the eggshell. This reaction will dissolve most of the eggshell by day 2.

 

Day 2

Observe your eggs. Notice that most of the shell has been dissolved by the acetic acid in the vinegar. Although most of the shell is gone, each egg is still surrounded by a shell membrane outside the cell membrane. The shell membrane has protein fibers that give it much greater strength than the cell membrane. However, the egg without its shell is still fragile, so you will need to handle your eggs very gently and carefully!

 

Dry each egg and measure the weight and/or circumference of each egg. Record your results for day 2 in the table on page 1.

 

3. Did the eggs became heavier/larger ___ or lighter/smaller ___? What do you think happened to cause this change in weight/size?

 

 

 

 

 

Empty the vinegar from the container for egg 1 and replace it with water to cover the egg.

 

Empty the vinegar from the container for egg 2 and replace it with corn syrup to cover the egg. As you pour the corn syrup, notice that it is viscous (thick, sticky).

 

4. What do you think causes the corn syrup to be so viscous?

 

 

 

Day 3

5. Compare and contrast the appearance of the egg that has been in water vs. the egg that has been in corn syrup.

 

 

 

 

6. You should be able to observe a layer of water on top of the corn syrup. Where do you think this water came from?

 

 

Rinse the corn syrup off of egg 2. Dry each egg and measure and record the weight and/or circumference for day 3 in the table on page 1.

 

7. What happened to cause the change in weight/volume of the egg placed in corn syrup?

 

 

 

 

8. Why did the egg placed in water get bigger and heavier? Where did the additional weight/volume come from?

 

II. Understanding Osmosis

 

The cell membrane that surrounds each cell is a selectively permeable membrane. A selectively permeable membrane allows some types of molecules and ions to cross the membrane and prevents other types of molecules and ions from crossing the membrane.

 

Osmosis is the diffusion of water across a selectively permeable membrane.

 

1a. During diffusion, more molecules will move

       a. from regions of higher concentration to regions of lower concentration

       b. from regions of lower concentration to regions of higher concentration

 

1b. The reason is that: 

 

      a. Crowded molecules want to move to an area with more room

      b. Molecules tend to keep moving until they are uniformly distributed and then they stop moving.

      c. The random motion of molecules results in their uniform distribution in the available space.

 

Explanation of Osmosis

 

Na+, Cl-, and the water molecules that are bound to these ions cannot cross the selectively permeable membrane (called a semipermeable membrane in this figure). Only free water molecules (water molecules that are not bound to ions or other solutes) can cross the selectively permeable membrane.

 

During osmosis, diffusion results in movement of free water molecules in both directions across the selectively permeable membrane, but more free water molecules move from the region of higher concentration of free water molecules to the region of lower concentration.

 

2. Why is the concentration of free water molecules lower in water with dissolved salt and higher in pure water?

 

 

 

 

 

 

 

 

3. Why does osmosis result in a net flow of water from the side of the tube that has pure water to the side of the tube that has water with dissolved salt? Include in your explanation the relative concentrations of free water molecules on the two sides of the tube.

 

 

 

 

This figure shows the effects                                              A                         B                            C

of osmosis on animal and plant cells put in three different types of

surrounding fluid.

 

4a. Which animal cell looks like the egg in corn syrup? ___

 

4b. Which animal cell looks like what could have happened to the egg in water, if the egg didn't have a strong shell membrane around it? __

 

 

 

 

Inside the cell membrane, each cell contains cytosol, a watery substance with a high concentration of dissolved molecules and ions.

 

5. Which has a higher concentration of free water molecules –

                                                                        the cytosol inside a cell ___   or pure water___ ?

 

If a cell is surrounded by pure water, will more water diffuse into the cell ___   or  out of the cell __?

Explain your reasoning.

 

 

 

 

 

 

Does this situation match A __ or B __ or C __ in the figure?

 

6. If very salty water has a higher concentration of dissolved substances than cytosol, which has a higher concentration of free water molecules

 –  the cytosol inside a cell ___   or the very salty water ___ ?

 

If a cell is surrounded by very salty water,

will more water diffuse into the cell ___  or  out of the cell ___?

Explain your reasoning.

 

 

 

 

 

 

Does this situation match A __ or B __ or C __ in the figure?

 

7. Most animal cells are surrounded by a layer of water with dissolved substances. For animal cells to function normally, there should be equal amounts of water diffusing into and out of the cell, as shown in figure B on the previous page. Which type of surrounding fluid would result in equal amounts of water diffusing into and out of a cell?

    __ water with a higher concentration of dissolved substances than the cytosol

    __ water with a lower concentration of dissolved substances than the cytosol

    __ water with the same concentration of dissolved substances as the cytosol

 

8. If a person drinks a very large amount of water in a short time, this may result in confusion, seizures, coma, or even death, due to abnormal functioning of nerve cells in the brain. Explain how these problems could result from drinking too much water too rapidly.

 

 

 

 

 

9. Explain why the surrounding fluid has a different effect on animal cells vs. plant cells in figure C on the previous page.

 

 

 

 

 

10. Suppose that an animal's cells had cell walls. What problems would this cause for the animal?

 

 

 

 

 

 

Challenge Questions

What do you think is the reason that a person who is stranded at sea should not drink ocean water? How could drinking salty water harm a person's cells?

 

 

 

 

 

Some archaea (single cell organisms) live in extremely salty water such as the Great Salt Lake or the Dead Sea. Most types of cells would shrivel and die in this very salty water. How do you think these archaea prevent water loss while living in very salty water?

 

 

 

 

[1] By Dr. Ingrid Waldron, Dept of Biology, University of Pennsylvania, © 2014. Teachers are encouraged to copy this Student Handout for classroom use.  A Word file, which can be used to prepare a modified version if desired, and Teacher Preparation Notes with background information and instructional suggestions are available at http://serendip.brynmawr.edu/sci_edu/waldron/#osmosis.