Lab

Observing Movement of Molecules Through a Cell Membrane

(EGG LAB)

Purpose | Hypothesis | Prediction | Materials | Procedure | Observations  

My Data | Analysis | Class Averages | Conclusions | Additional Experiments

 

Purpose: 

 To observe the selective passing of materials into and out of a membrane, and observe the types of materials that are and are not passed through.

Hypothesis: 

The passage of solution through the semipermeable membrane of the egg is facilitated diffusion through transport proteins in the egg membrane.

 Prediction:   

By this hypothesis, the acetic acid solution will dissolve the calcium carbonate in the egg's  outer shell, converting it to bubbles of CO2.  The membrane will then allow much passage of vinegar through the membrane to attain equilibrium solute concentration (because the  vinegar will be a more concentrated solution than the inside of the egg).  There will thus be a gain in egg mass.  There will be little net mass change in the glucose solution, but the two  solutes (acetic acid and glucose) will even out on each side of the membrane.  In the water, there will be a net loss of egg mass, as solutes in the egg pass through the membrane to  even the concentration of solutes in the water.

Materials: 

1) One chicken egg

2) One small beaker

3) One graduated cylinder

4) 100 mL 5% acetic acid solution

5) 100 mL 50% glucose solution

6) 100 mL distilled water

7) 5" x 5" square of plastic wrap

8) Rubber band

9) Ruler

10) About 30 cm of string

11) Balance scale

Procedure: 

 1) Measure transverse and longitudinal circumferences of egg using string and ruler.

2) Measure mass of egg using balance scale.

3) Place egg in beaker.

 4) Measure 100 mL of acetic acid solution with graduate.

5) Pour into beaker.

6) Cover beaker with square of plastic wrap and secure with rubber band.

 7) Store egg for one day in an undisturbed, cool, dry place.

8) Repeat steps 1-2.

9) Repeat steps 3-8 for glucose solution and distilled water.

Observations: 

 Upon placing the egg in the acetic acid solution, we observed small bubbles rapidly forming around the outside of the egg.  When we retrieved the egg one day later, the  surface of the solution was foamy, with yellowish film in some places.  The egg was soft and the now-powdery outer shell flaked at the touch.  Where the egg came in contact with the  bottom surface of the beaker, the shell was intact.

 Upon placement in the glucose solution, the egg floated relatively high in the solution.   When the egg was retrieved one day later, it still was covered with small bubbles.  The appearance of the solution had not changed.  The egg still was floating somewhat, but not  nearly as high as it had been.  The outer shell had thinned to an even greater extent, and there was empty space inside the membrane—it could be gently depressed, like a  paper-maché ball with a punctured balloon inside.  The part of the shell which had not been dissolved by the vinegar still was not.

  Upon placement in the water, the egg did not float.  Small bits of dissolved shell began flaking into the water.  When we removed the egg from the water, small flakes of shell  remained suspended in the water, causing it to become slightly murky at the bottom.  The egg no longer had bubbles on its surface, and the powdery surface was cracked in some  places to reveal the translucent membrane.  The egg rested on the bottom of the beaker, and the empty space within it was gone.  The shell could be completely removed by rubbing,  to reveal a translucent yellow, water balloon-like membrane.  Particles of the shell were abrasive and formed a milk-like fluid when rubbed off with water.  The egg sprung back to  the touch.  When popped by a pin, the yolk and white spilled out.

 

My Data:

Date

Mass (grams)

Longitudinal Circumference (centimeters)

Transverse Circumference (centimeters)

Beginning Fluid (mL)

Ending Fluid (mL)

9/21/99

53.8

15.5

13

N/A

N/A

9/22/99

67.4

16.9

16.3

98

79

9/23/99

55.4

17

14.5

99

111

9/24/99

83.9

18.5

16

99

69

 

 

 

 

 

 

 

 Analysis: 

 

The daily changes in egg mass were as follows: 25% in the vinegar, -18% in the glucose,  and 51% in the water.

 

This graph shows the average percent changes over four categories of egg measurement, averaged over several sets of data.

 

The egg absorbed 19 % of the vinegar, increased the volume of glucose 12%, and absorbed 30% of the water.

 

Changes in circumference (average longitudinal and transverse) were 16% in the vinegar,  -5% in the glucose, and 10% in the water.

Class Averages: 

 

Chang e in Mass

Change in Longit. Circ.

Change in Trans. Circ.

Change in volume fluid in beaker

Day 1-2

19%

5%

8.5%

-26%

Day 2-3

-13%

.5%

-7%

7%

Day 3-4

35%

8%

11%

-29%

 

 

 

 

 In general, students' results correlated closely, except for one group, whose results were either entirely contradictory or significantly "dulled"—that is, everyone else showed a 25%  gain and they showed either a 2% decrease or a 2% gain.

Conclusions: 

 Based on the results we observed, I can conclude that my hypothesis was incorrect.  I now believe that the method of diffusion we observed was osmosis.

  As shown in the diagram to on the next page, osmosis occurs when a semipermeable membrane such as the one surrounding the egg white and yolk or surrounding a cell allows  only water molecules to pass through.  When a semipermeable membrane separates two solutions, the one with the greater solute concentration is deemed the hypertonic solution.   The large solute molecules in that solution (for instance, inside the egg) attract polar water molecules by

Coulombic forces, preventing them from crossing the membrane.  The less concentrated, or  hypotonic solution (in this case, outisde the egg), thus, has more free water molecules, which migrate to the other side of the membrane (inside the egg) to reduce the  concentration gradient.  Small, polar water molecules, thus, can migrate across the membrane, while larger solute molecules cannot.

 When the egg was placed in the vinegar, the acetic acid (HC2H 3O2 ) in the vinegar dissolved (ionized) the calcium carbonate (CaCO3) in the egg shell by the reaction

HC2H3O2 + CaCO3 --> CO2 + CaO + 2 H2O

This formed bubbles of carbon dioxide (CO2) on the outside of the shell, as well as additional water (H2 O) and calcium oxide (CaO).  This allowed fluids to pass through the membrane.  While hypertonic to water, the 5% concentrated vinegar was hypotonic to the  solution inside the shell, and so water molecules from the vinegar diffused into the egg.  The amount of diffusion was moderate, as the hypotonic solution was not pure water; it was  somewhat concentrated.  When the egg was placed in the glucose, it floated, because the glucose was denser than the inside of the egg.  This also meant, interestingly, that the  glucose was hypertonic to the inside of the egg, and some water inside the egg migrated into the glucose solution, attempting to equalize the solute concentration.  When the egg  was placed in water, the inside of the egg was obviously very hypertonic to the pure water, and so its mass and circumference increased greatly as water migrated to the inside of the egg.

 It is also our conclusion that the circumference of the egg and its mass are proportional to each other (except for when the mass decreases, in which case the circumference of the  egg remains about the same, because the membrane does not shrink), and directly linked to the volume of the solution in the beaker (the mass grows by apporoximately as much as  the difference between beginning and ending volumes of fluid).  The transverse and longitudinal circumferences are also proportional to each other.

 As we observed, the part of the shell not dissolved by the vinegar never was dissolved.  This leads us to believe that the increasing wearing away of the powdery dissolved shell  occurred only because of sitting in the other liquids.

 The floating of the egg, which we considered an error, had a legitimate cause—the egg  floated in solutions hypertonic to its inside.  This plays a large role in our analysis, as it explains the large return of water to the glucose solution.

Error Analysis: 

There were many errors we may have incurred in the completion of this lab.  First, obviously,  were human errors sustained in the taking of measurements of liquids and the egg.  Second were imprecise times that the egg sat in each solution; for example, the transition from glucose to distilled water was made during 2 nd period on Thursday instead of 6th, as a result of the test later that day.  Third were unavoidable errors in the actual completion of the  lab, namely, the incomplete contact of the egg with the solution, as a result of either resting on the bottom of the beaker, as was the case with vinegar and distilled water, or floating  partially out of the solution, as was the case with the glucose solution.  This may have affected the rate of diffusion by preventing fluid from diffusing across parts of the membrane  not exposed to the solution.  The final possible error was errors in measurement based on fluid remaining on the egg's surface, i.e., fluid that was measured in mass measurements  but that was not part of the actual mass of the egg.  To improve the consistency of the experiment, we could have made the transitions from solution to solution at precise  intervals, ensured that the egg was in full contact with the solution by using some sort of screen to hold it in the liquid without impeding flow through the membrane, and dried the egg before taking mass measurements.

Additional Experiments: 

Bird eggs are the largest cells, and a cell's size is limited by its relation of mass to the  surface area of its plasma membrane (because of the need for the capacity of the membrane to adequately provide waste disposal and energy intake for the entire  cytoplasm).  Supposedly, the surface area of an egg membrane is directly proportional to its mass raised to the two-thirds power.  This can be expressed by the equation a=x(m2/3),  where a is the area, x is the coefficient of the proportion, and m is the mass.  Below is a table of each of these measurements for our egg at each of its various stages.  The surface area of a sphere, a=4(r2 times pi), when r is the radius of the sphere, and r=(circumference divided by pi)divided by two.  In this case, because the egg is obviously not a sphere, I am  using the average of the longitudinal and transverse circumferences as the circumference of the sphere.  This chart is not fully accurate, because the first day's mass and area include  the cell wall (shell), not only the plasma membrane, and because the average of the two circumferences is not nearly the actual circumference of a sphere of the same volume.  One  could find such a measurement by using the formula for the volume of a sphere, but this would yield merely a mathematical proof and not a practical test.

Date

Mass (grams)

Area (square cm)

  A_

M2/3

 

9/21/99

53.8

64.637

4.536

9/22/99

67.4

87.713

5.296

9/23/99

55.4

78.96

5.433

9/24/99

83.9

94.717

4.492

 

 

 

 

 

 Taking into consideration our imperfections, this proportion does seem feasible, as the coefficients are all within a range of .897, close enough to the perfection level of zero.

Sources:

Campbell, Neil A., Lawrence G. Mitchell, and Jane B. Reece.  Biology: Concepts and

 Connections.  3rd  ed.  San Francisco, CA: Benjamin/Cummings-Addison Wesley            Longman, Inc., 1997

Graphs created using Microsoft Excel©

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