Name:__________________________________________________________ Per:______Date:_______

Diffusion in Agar Cubes Pre-Lab

1.Which of the following shapes is a cube?  Circle it.

2. Using the formula below, find the surface area of the following shapes.  Show your work.  All measurements are in centimeters.

Surface Area = Area (L x W) x # of sides

3. What is the formula for volume?

4. Using the formula above, find the volume of the following shapes.  Again, all measurements are in centimeters.

5. What is the surface area to volume ratio of cube A (SA:V)?  ___:___

     SA     V

6. Express the above ratio in its simplest form. _______________

7. What is the surface area to volume ratio of cube B (SA:V)? )  ___:___

     SA     V 

8. Express the above ratio in its simplest form. _______________

9. Which of the two ratios is the smallest? __________________

Diffusion In Agar Cubes

Is Bigger Better?   OR   Is Smaller Smarter?

Adapted from Flinn Scientific Publication No. 10110

Introduction:  

Diffusion is one of the very important processes by which substances such as nutrients, water, oxygen, and cellular wasted are transported between living cells and their environment.  This activity will help you explore the relationship between diffusion and cell size by experimenting with model “cells.”

In this experiment, you will use agar cubes to which the indicator phenolphthalein has been added.  Phenolphthalein is an acid/base indicator that turns pink in the presence of a base such as NaOH.  Thus the surface of the agar cubes will turn pink immediately when put into a NaOH solution.  The NaOH will continue to diffuse through the cube and gradually turn the inside of the cube pink.  The guiding question for this lab is thus:

Investigative Question: What determines the efficiency of diffusion throughout the model “cells”?  

Use this question to help formulate a hypothesis:

Materials:  

Agar

A ruler

0.1M NaOH per group 200mL

Knife

Large beaker

Spoon/Tongs

Procedure:  

1. Each group will cut three agar cubes:  A 3cm cube, a 2cm cube, and a 1cm cube.  CUT AS ACCURATELY AS POSSIBLE.  (This may be already completed for you.)
2. Pour 200mL of 0.1M sodium hydroxide solution into your 400mL beaker.
3. Immerse your 3 cubes in the sodium hydroxide solution, noting the time.
4. Let the cubes soak for approximately 10 minutes.
5. Periodically, gently stir the solution, or turn the cubes over.
6. After 10 minutes, use a spoon or tongs to remove the cubes from the sodium hydroxide solution.
7. Blot the with a paper towel.
8. Promptly cut each cube in half and measure the depth to which the pink color has penetrated.  Sketch each block’s cross-section.
9. Record your measurements and sketch each cube in the table found on the next page.
10. Do the following calculations for each cube and complete the following data table:

Calculating % diffusion in each cube:

• Calculate total volume of each cube (volume = L x W x H)
• Calculate volume that did not turn pink. (That is, calculate total volume of the small portion of the cube that did not turn pink – use the same formula L x W x H)
• Calculate volume diffused = total volume – volume not pink.
• Calculate % diffusion = Volume diffused /total volume x 100

Calculate the surface area of each cube and the surface area to volume ratio:

• Calculate the surface area of a cube = L x W x # of sides
• Calculate surface area/volume ratio.

Conclusion Questions: In your logbook answer the following questions. Be sure to tape data table on top of your hypothesis and conclusion questions.

1. What was the investigative question you were trying to answer as a result of this lab?
2. What was your hypothesis?
3. Was your hypothesis supported or not supported (support your results with data from the lab)?
4. What is the claim you can make from your investigation (the answer to the investigative question)?
5. In terms of maximizing diffusion, what was the most effective size cube that you tested?
6. Why was that size most effective at maximizing diffusion?  What are the important factors that affect how materials diffuse into cells or tissues?
7. If a large surface area is helpful to cells, why do cells not grow to be very large?
8. You have three cubes, A, B, and C.  They have surface to volume ratios of 3:1, 5:2, and 4:1 respectively.  Which of these cubes is going to be the most effective at maximizing diffusion, how do you know this?
9. How does your body adapt surface area-to-volume ratios to help exchange gases?
10. Why can’t certain cells, like bacteria, get to be the size of a small fish?
11. What are the advantages of large organisms being multicellular?

DATA TABLE:  Diffusion In Agar Cubes                             Name:_____________________________

DATA TABLE: Diffusion In Agar Cubes                         Name:______________________________

Preparation of agar

1. Mix 20g of agar with one liter of distilled or deionized water.  Recipe based on 15 groups of students working in pairs.
2. Heat almost to a boil.  Stir frequently until solution is clear
3. Remove from heat. AS the agar mixture cools add 10 mL of 1% phenolphthalein solution (1g phenolphthalein in 100 mL 95% ethyl alcohol) and stir. If mixture is pink, add a few drops of dilute hydrochloric acid until the pink color disappears.
4. Pour agar into a shallow tray to a depth of 3cm and allow it to set (overnight).  A tray measuring 12cm x 2cm that is at least 3cm deep will accommodate one liter of agar mixture.