Experiment 2: Exploring Cell Size
Have you ever wondered why cells don't grow past a certain size? There is a size limit for
cells that they cannot surpass. Once they reach this value, cells divide and form two
smaller, daughter cells. Why do they do that? You will look at the importance of cell size
in this experiment to help you understand. Materials:
(1) 125 mL Nutrient Agar Bottle
10 mL Bromothymol Blue
2 mL Saturated (15%) Sodium Bicarbonate Solution,
NaHCO3
Plastic Wrap
3 Pipettes
Acetic Acid (Vinegar), CH3COOH
(1) 250 mL Beaker Rectangular Mold
Ruler
*Kitchen Knife
*Microwave
*Hot Pad or Towel
*You must provide Note: This experiment requires preparation 24 hours in advance.
Procedure
Part 1: Agar Preparation
1. Remove or loosen the cap on the agar bottle and place it in the microwave.
2. Heat the bottle in 10 second increments for approximately one to two minutes.
While in the microwave, watch the solution for boil-over. Agar solutions can get
very hot very quickly, so be certain to watch the bottle at all times. If it begins to
boil-over, immediately stop the microwave, and allow the agar to cool down
before proceeding.
3. After heating for approximately one minute, check on your agar bottle. To do
this, remove the bottle with a hot pad, screw the lid back onto the bottle, and
swirl the solution. If the solution is not completely liquefied, remove the lid and
place the agar bottle back into the microwave for 10 second intervals, swirling in
between, until it is completely liquefied.
4. After the agar is liquefied, let the solution sit for a minute to cool down.
5. Once the agar solution has cooled slightly, measure 40 mL into the 250 mL
beaker.
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All Rights Reserved 6. Add 10 mL of the bromothymol blue solution to the liquefied agar in the beaker.
7. Add two mL sodium bicarbonate solution to the beaker solution. Pipette the
solution up and down to mix. This will tint the mixture and create a pH change.
8. Once the solution is mixed, pour the solution into the rectangular mold. Cover
the mold with plastic wrap.
9. Let the mold sit at room temperature for 24 hours to give the agar time to set. Note: After the 24 hours, the liquid agar should have firmed up to a Jello®-like
consistency.
Part 2: Assessing Cell Size
1. Put on safety gloves, safety glasses, and an apron. Check to be sure the agar has
solidified. If it has not, let it sit for another 12 hours.
2. Invert the rectangular mold, and gently allow the agar block fall onto the
underpad.
3. From this block, safely cut out a 1.0 cm x 1.0 cm x 6.0 cm block.
Note: It is helpful to measure the 6.0 cm side of the block first.
4. From the remaining agar, safely cut out a 1.0 cm x 1.0 cm x 1.0 cm cube. Set the
block aside.
5. From the remaining agar, safely cut out a 1.0 cm x 2.0 cm x 2.0 cm cube. Set this
block with the others.
6. Once all three blocks have been cut, dispose of the scraps of remaining agar. Do
not dispose of the blocks you just cut.
7. Use a ruler to calculate the surface area, volume, and surface area to volume
ratio for each block. Refer to the example calculations provided after the
procedure for help. Record the calculations in Table 2.
8. Fill the 250 mL beaker with 150 mL of vinegar. Gently place all three blocks into
the vinegar solution.
9. Let the blocks rest in the vinegar for seven minutes. Observe as the blocks begin
to change color (from blue to clear). Record how long it takes for each block to
completely change colors (from blue to clear). Note, use any of the blocks that do
not experience total diffusion to measure the distance of diffusion in Step 11.
© 2013 eScience Labs, LLC.
All Rights Reserved 10. After seven minutes, remove the blocks from the vinegar solution. Pour the
remaining vinegar solution down the drain.
11. Gently blot the blocks dry and then safely cut them in half. For each block,
measure the distance the vinegar diffused into the gelatin cube, as detected by
the color change. Do this by measuring from the outer edge of the block to the
blue rim inside the cube. Record that value in Table 2. Sample Calculations (refer to these examples to complete Table 2)
Surface Area: can be calculated with the following equation: Length x Width = Area. To
find the surface area of a cube, calculate the area of one side and multiply that by the
total number of sides.
Problem:
If an equilateral cube is structured so that every side is 3 cm. long, what is the total
surface area?
Given:
Length = 3 cm
Width = 3 cm
Total Number of Sides = 6 Solution:
1. Solve for the area of each individual side:
Length x Width = 3 cm x 3 cm = 9 cm2
2. Multiply the area of one side by the total number of sides in the shape.
6 sides x 9 cm2 = 54 cm2
Note: If the 3D structure you are measuring does not contain equilateral dimensions,
© 2013 eScience Labs, LLC.
All Rights Reserved you must determine the surface area of each side and add them up individually . Volume can be calculated with the following equation: Length x Width x Height = Volume.
Problem: Suppose you are working with the same cube as above. What is the total volume?
Given:
Length = 3 cm
Width=3 cm
Height = 3 cm Solution:
1. Plug your variables into the equation to solve for volume:
© 2013 eScience Labs, LLC.
All Rights Reserved 3 cm x 3 cm x 3 cm = 27 cm3
Note: To determine surface area to volume ratio, divide the surface area by the volume. Table 2: Results from Surface Area to Volume Experiment
Block Dimensions Surface Area
(cm2) Volume (cm 3) 1 cm x 1 cm x 1 cm
1 cm x 2 cm x 2 cm
1 cm x 1 cm x 6 cm
© 2013 eScience Labs, LLC.
All Rights Reserved Time Required for
Complete Color Change Distance of
Diffusion Post-Lab Questions 1. How did the surface area effect the diffusion of the block? What about the volume?
What about the surface area to volume ratio? Which of these had the greatest affect on
the diffusion of the block? 2. How does this experiment demonstrate the need for larger cells to divide? 3. Determine the surface area, volume, and surface area to volume ratio for the following
three blocks. Then, circle the one you believe would be the most efficient as a cellular
morphology, and write a summary stating why.
1.5 cm x 1.5 cm x 1.5 cm 0.5 cm x 0.5 cm x 6.0
m © 2013 eScience Labs, LLC.
All Rights Reserved 3.0 cm x 2.0 cm x 2.0
cm

 

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