NAME______________________________________
Cell Size Lab
Question:
Date_______________________
Per_________
What is the effect of the surface area to volume ratio on the rate of diffusion?
Background: Most cells are very small. Very few are macroscopic, and those that are usually are the eggs of the
lower vertebrate or invertebrate organisms. The cells that make up the bulk of the tissue of all organisms are
microscopic. What keeps a single-celled organism, such as an amoeba, from growing as large as a bowling ball?
Why are large multicellular organisms made up of billions-trillions of microscopic cells instead of hundreds of
thousands of larger cells? What conditions signal or trigger cellular processes to initiate cell division? This short
investigation is designed to help you understand one of the most important relationships in all of biology:
the surface area to volume ratio of a cell.
In this laboratory you will use different sized (1, 2, and 3 cm) blocks of agar, which will represent different sized
cells. By measuring the rate of diffusion of a basic solution into the "cells" (blocks of agar), it will be possible to
determine the effect of the cell size on the surface area to volume ratio of the cell. This in turn will enable you to
make inferences concerning the adaptive value of small cells.
The agar cells contain the indicator dye phenolphthalein. This indicator turns pink in the presence of a base, and
clear in the presence of an acid. When the pink agar blocks are placed in an acidic solution, some of the acid
diffuses into the agar, and turns the agar milky-clear. This will allow us to measure the rate of diffusion of the
acidic solution into the agar cells.
Predictions: You will use three sizes of agar cells: 1cm, 2cm, and 3cm.
a. Into which size “cell” will the acid diffuse the most rapidly? ______________________________________
Why?_______________________________________________________________________________________
____________________________________________________________________________________________
b. Which cell size will have the largest percentage of it's volume affected by the acid? (Remember, volume is the
total space that the cell takes up.)_________________________________________________________________
Since you are working with an acid in this lab, be especially careful not to spill.
Safety glasses must be worn the entire period.
Materials: Working in pairs, secure the following and follow the procedure outlined below.
1 250ml glass beaker
metric ruler
1 agar block (3 cm x 5 cm)
safety glasses
1 plastic spoon
1 plastic knife
150 ml HCl (hydrochloric acid)
paper towels
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Procedure: (l=length, w=width, h=height)
1. Using the plastic knife and the 3 x 5 cm block of agar supplied by your teacher, cut out a 3cm cube, a 2cm
cube, and a 1cm cube. These should look like three different sized dice. All three dice are to be cut out of the
big block you were given. (You only get one chance, so plan carefully before cutting.) There will be some agar
left over.
2. Calculate the following for all three of your cells:
the total surface area (l x w x 6 sides) in cm2
the volume (l x w x h) in cm3
Record this information in the data table.
3. Determine the surface area:volume ratio for each of the three agar cell sizes. To do this, divide the total
surface area of the cell by the volume of the cell. Record this in the data table as a ratio __:__ (surface
area:volume).
4. Very carefully go to the supply table and obtain about 150 ml of HCl acid solution in your glass beaker. Be
very careful not to spill. If you get any on your hands, immediately wash your hands gently with water.
Exact timing is required for the following steps.
DECIDE WHO IS GOING TO KEEP THE TIME AND WHO IS GOING TO HANDLE THE CELLS BEFORE YOU START THE
NEXT PART OF THE PROCEDURE.
5. Place all of your cells in the HCl solution. Lower them into the solution with the spoon. Gently swirl and roll
the cells over with the spoon once or twice. Be careful not to cut the agar with the spoon.
6. As soon as the 1cm cell is completely white/clear, lift all of the cells out of the acid with the spoon. Place
them on the paper towel, and quickly pat them dry.
Record the time (to the nearest second) that the cells were in the acid.
Time in acid: _____________
7. Then, carefully slice each cube in half with the plastic ruler (wipe off the ruler before each cut). Measure (to
the nearest .1cm) the width of the whitish band around the perimeter for each cell size. If the block still has
pink in the center you must measure the white width on two sides and add them together to report the
White Width. Record this width in the data table under column "WHITE width."
8. Draw a picture of the cross section of the three agar cells. Make your drawings to scale, showing the exact
portion of the cell that remains pink, if any. (Use your ruler!)
3cm cube
2cm cube
1cm cube
9. Calculate the rate of diffusion for all three cells. A rate, in this case, is distance divided by time. Therefore to
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calculate the rate of diffusion of the ACID, you will need to divide the white width by the time recorded in # 6
above. Your units will be in cm/minute. Record your rates in the column of the data table under "Diffusion
Rate."
10. Add a title and appropriate units to the data table below.
Title:
Cell Size
Surface Area
Volume
SA/Vol ratio
White Width
Diffusion Rate
1cm
2cm
3cm
11. Measure (to the nearest .1cm) the central area of the cubes that is still pink (not white). This is the part of the
“cell” to which the acid has not yet diffused. Record this width in the space below:
distance across central pink area of 1cm cell________
distance across central pink area of 2cm cell________
distance across central pink area of 3cm cell________
At this point you can clean up: return all materials to the location where you got them, but throw
out the used agar blocks in the AGAR BUCKET. Rinse off the knife and glass sheet, carefully
return the acid to the “USED ACID” container. Rinse and dry beaker, ruler, glass sheet, and
spoon. Get new paper towels out.
Keep your safety glasses on until all students are finished and cleaned up.
12. Calculate the volume of cube that is still pink for all cell sizes. Assume that the agar blocks are perfect cubes.
volume of pink in 1 cm cell ________
volume of pink in 2 cm cell ________
volume of pink in 3 cm cell ________
13. Calculate the percentage of each cell that was not affected by the acid (remained pink) for all cell sizes:
(volume pink divided by total cell volume)
percentage not affected (pink) in 1 cm cell ________
percentage not affected (pink) in 2 cm cell ________
percentage not affected (pink) in 3 cm cell ________
14. Calculate the percentage of each cell that was affected by the acid (turned white) for all cell sizes: (volume
white divided by total cell volume OR 100% - percentage not affected)
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percentage affected (white) in 1 cm cell ________
percentage affected (white) in 2 cm cell ________
percentage affected (white) in 3 cm cell ________
ANALYSIS QUESTIONS:
1. List the cells in order of surface are to volume ratio (SA/V ratio), from largest ratio to smallest ratio.
____________________________________________________________________________________________
2. What happens to the SA/V ratio as the cell size increases? __________________________________________
____________________________________________________________________________________________
3. Examine your calculations from procedure step #14. Which cell had more "cytoplasm" affected by the acid?
___________________ Why? __________________________________________________________________
____________________________________________________________________________________________
4. Think of the agar blocks as if they were cells consisting of cytoplasm surrounded by a cell membrane. Which
one has the greatest amount of cytoplasm for the amount of membrane? _____________________________
5. Which one has the greatest amount of membrane for the amount of cytoplasm? _______________________
6. Compare the rates of diffusion for your three cells. Describe and explain any pattern that you observe in the
diffusion rate data. _________________________________________________________________________
____________________________________________________________________________________________
7. Assume that the solution that you placed your agar cells in was a food/nutrient solution for the cells. In which
cell would the nutrient solution penetrate to the entire volume of the cytoplasm the fastest? Cite
experimental data to support your answer. ______________________________________________________
____________________________________________________________________________________________
8. Based on the discussion in the background for this lab and the data that you have collected; explain why cells
are most generally quite small. In your discussion make sure that you use your surface are to volume ratio
(SA/V ratio) to support your explanation. ________________________________________________________
____________________________________________________________________________________________
____________________________________________________________________________________________
9. Consider the following two cubic organisms, both of which have the same interior volume. Organism A is
made of one cell, and has the exterior dimensions of 3 cm x 3 cm x 3 cm. Organism B is a multicellular
organism made of 27 cells, each of which is 1 cm x 1 cm x 1 cm. (see drawings)
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Calculate the total surface area to total volume ratio for each organism in number 9 above.
____________________________________________________________________________________________
____________________________________________________________________________________________
10. How do your calculations demonstrate, as we said in the introduction to this exercise, why large organisms
are made of millions of small cells? Make sure that you include a discussion of the importance of the surface
area to volume ratios in cells. _________________________________________________________________
____________________________________________________________________________________________
____________________________________________________________________________________________
11. In an old movie called The Blob, an amoeba like creates oozes over and consumes entire buildings. From a
scientific perspective, why is this unrealistic. ________________________________________________________
____________________________________________________________________________________________
____________________________________________________________________________________________
12. How do the size of elephant and mouse cells compare? ____________________________________________
____________________________________________________________________________________________
13. Another reason that cells stay small is due to DNA Overloading. What does this mean? _________________
____________________________________________________________________________________________
____________________________________________________________________________________________
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