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Lab 15 Why Cells are Small
glossary
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IntroductIon
In this lab, we will study diffusion in different cell sizes to
explain why cells are so small. You and your partner will make
three model cells of different sizes and measure the extent and
rate of diffusion into each cell. You will then calculate the
surface area to volume ratio for each cell to explain why cell
sizes are limited.
You and your partner will cut three cells: 3 cm square, 2 cm
square, and 1 cm square from a block of agar. The agar has been
impregnated (or prepared with) phenolphthalein, an acid-base
indicator, and treated with a dilute base which turns the agar
pink.
When your cells have been cut, each will be placed in weak acid,
vinegar in this case, which will cause the agar to turn clear as
the acid reacts with it. This will allow you and your partner to
determine how far into the agar the vinegar has diffused.
After reading this introduction, please complete the prelab
questions.
Phenolphthalein is a strong laxative. Be sure to wash your hands
and wipe off your table at the end of this lab.
http://www.biology-online.org/dictionary.aspmailto:[email protected],[email protected]://en.wikipedia.org/wiki/Agarhttp://en.wikipedia.org/wiki/Phenolpthaleinhttp://en.wikipedia.org/wiki/Laxativer
Lab 15 PreLab
Why Cells Are Small
Name:
Pledge:
AUTHORIZED AID: You may discuss questions with classmates or
other knowledgeable persons before
composing your OWN answer or you may use a source and cite it
appropriately. (Total = 6 pts)
1. What makes an elephant larger than a mouse? FORMDROPDOWN
(1)
2. Read about agar on Wikipedia.
a. From where does agar come? (1)
3. Phenolphthalein is a chemical known as an acid-base
indicator. Read about phenolphthalein on Wikipedia. (1)
a. What color does it turn when placed in a base?
b. What color does it turn when placed in an acid?
4. Diffusion means moving from an area of to an area of
. (1)
5. Calculate the volume of a 6 cm cube (it is 6 cm on all
sides). (1) Use this formula:
volume = length x width x height
For further practice, go to the ExploreLearning website and use
the "Prisms and Cylinders" Gizmo to
practice calculating volumes.
6. Calculate the surface area of a 6 cm cube. (1) Use this
formula:
surface area = length x width x number of sides
dwheelerFile AttachmentLab 15 PreLab.doc
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Lab 15 Why Cells are Small
glossary
15. �email us
Procedure
1. BOTH partners will wear goggles while completing this lab.
ONLY the partner handling the agar will wear gloves.
2. Use a plastic knife and a ruler to trim the agar piece
provided into a 3 cm x 3 cm x 3 cm cube, a 2 cm x 2 cm x 2 cm cube,
and a 1 cm x 1 cm x 1 cm cube.
3. CAREFULLY, place each cube into the plastic cup provided. Add
vinegar until the cubes are submerged. Using a plastic spoon, keep
the cubes submerged for 10 minutes. Be careful not to damage the
surface of any of the cubes.
4. As the cubes soak, calculate the surface area, volume, and
surface area to volume ratio for each cube. Record these values in
Data Table 1 on your Answer Sheet.
The materials in this lab can irritate the skin and eyes. Take
precautions to avoid skin or eye contact.
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Lab 14 Answer Sheet
Why Cells Are Small
Names:
Pledge:
Authorized Aid means you worked TOGETHER to complete this lab –
answers were discussed with one another before being typed, both of
you participated in conducting the lab procedures, and any
sources
or pictures are cited appropriately. (18 total points)
Refer to the pdf file for specific instructions.
Formulas
surface area = length x width x number of sides
volume = length x width x height
Total Cube Volume – Volume of Cube which has NOT Changed Color x
100 = Percent of Cube Volume
Total Cube Volume
(Extent of Diffusion)
Data Table 1: Agar Cubes (3 pts)
Cube Size
Surface Area
(cm2)
Volume
(cm3)
Surface Area/Volume (smallest ratio)
3 cm
2 cm
1 cm
Data Table 2: Rate of Diffusion (3 pts)
Cube Size
Depth of Diffusion
(cm)
Time
(min.)
Rate of Diffusion
(cm/min.)
3 cm
2 cm
1 cm
Data Table 3: Extent of Diffusion (3 pts)
Total Volume of Cube
(cm3)
Volume of Cube which has NOT Changed Color (cm3)
Percent volume of Cube which has Changed Color
(Extent of Diffusion)
3 cm
2 cm
1 cm
Summarizing Results
1. Explain why the diffusion of vinegar into the agar cubes
caused the observed color change. (1)
2. What evidence is there that the vinegar diffused into each
cube at the same rate? (1)
3. Which cube has the greatest surface area? (1)
4. Which cube has the most surface area compared to its volume?
(1)
5. According to the last column in Data Table 3, into which cube
did the vinegar diffuse most? (1)
FORMDROPDOWN
6. Examine your data in Data Table 1. (1 pt each)
a. Describe what happens to the surface area as a cell grows
larger.
b. Describe what happens to the volume as a cell grows larger.
c. Describe what happens to the surface area to volume ratio as
a cell grows larger.
7. If each cube represented a living cell, and the vinegar was a
substance needed within the cell, what problem might exist for the
largest cell? (1 pt)
dwheelerFile AttachmentLab 15 Answer Sheet.doc
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Lab 15 Why Cells are Small
glossary
15. �email us
Procedure
5. After 10 minutes, use the spoon to remove the agar cubes and
carefully blot them dry on a paper towel. Cut each of the cubes in
half, rinsing and drying the knife between each cut. Note the color
change from pink to clear indicating the diffusion of vinegar into
each cube.
6. Using a metric ruler, measure the distance (in centimeters)
that vinegar diffused into each cube. Record this data on your
answer sheet in Data Table 2. Also, record the time (in minutes) it
took the vinegar to diffuse and calculate the rate of diffusion for
each cube (in centimeters per minute).
7. Calculate the volume of each cube that did not change color.
Record this result on your answer sheet in Data Table 3.
8. Calculate the extent of diffusion into each cube as a percent
of total volume (see formula on answer sheet). Record these results
in Data Table 3 of your answer sheet.
After cleaning up your lab station (throw away the agar, pour
vinegar down the drain, wash the spoon, knife, ruler, and cup),
remember to wash your hands!After cleaning up your lab station
(throw away the agar, pour vinegar down the drain, wash the spoon,
knife, ruler, and cup), remember to wash your hands!
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Lab 15 Why Cells are Small
glossary
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Homework
Don’t forget to do your homework!
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Lab 15 Homework
Why Cells Are Small
Name:
Pledge:
AUTHORIZED AID: You may discuss questions with classmates or
other knowledgeable persons before
composing your OWN answer or you may use a source and cite it
appropriately.
Formulas
surface area = length x width x number of sides
volume = length x width x height
Total Cube Volume – Volume of Cube which has NOT Changed Color x
100 = Percent of Cube Volume
Total Cube Volume
(Extent of Diffusion)
Questions
1. Calculate the surface area, volume, and surface area to
volume for a cube that is 0.01 cm on each side. Show your work. (3)
2. What predictions about diffusion rate can you make about the
cube in question 1? (1)
3. What is the relationship between diffusion rate and cell
size? (1)
4. Name three important molecules that travel through cells by
diffusion. (1)
5. Why are most all cells less than 0.01 cm in size? (1)
dwheelerFile AttachmentLab 15 Homework.doc
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Lab 15 Why Cells are Small
glossary
15. 5email us
references
Diffusion and Cell Size. Tonawanda: Science Kit and Boreal
Laboratories, 2001.
Holsinger, Rachel and Debbie Wheeler. “Lab 15: Why Cells Are
Small.” Sayre School: Lexington, KY. February 2007.
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