Diffusion Lab

Overview

The phenolphthalein in the agar cubes reacts with the NaOH, changing the color of the cube to pink. After the cubes

are exposed to NaOH, you will be able to see how far the NaOH diffused based upon the change in color which it

caused. This will allow you to determine the relationship between diffusion and the surface area and volume of the

cubes.

Aim

There are two main points to this lab. One is to determine how surface area and volume relate to one another and the

other is to examine how the rate of diffusion varies with the ratio of surface area to volume.

Materials

three cubes of 3% agar-phenolphthalein (1 cm, 2 cm, and 3 cm on a side), 4% NaOH solution, a ruler, a razor, a

plastic spoon, a paper towel, and a beaker

Procedure

1. Calculate the surface area and volume of each agar cube and record these values in a table.

2. Carefully fill the beaker with 4% NaOH so that the cubes will be completely submerged when placed in the beaker.

3. Place the three agar cubes in the beaker. After 15 minutes, remove them and place them on a paper towel.

4. Using the paper towel, blot the cubes dry.

5. Use the razor blade to carefully cut each cube in half.

6. Measure the distance across the portion of the cube which did not change color.

7. Calculate the surface area and volume of this portion of the cube and record these values in a table.

Observations/Data

You will need three tables with space for 15 sets of data (class data). The Table Should have the following Headings:

Cube Size; Surface Area; Volume; Surface Area/Volume Ratio; Size of uncoloured portion of cub; Surface Area of

Uncoloured Portion; Volume of Uncoloured Portion

Calculate the mean, range, and Standard deviation for each column of data.

Graph the Data

Do a t-test to compare the results of the largest and smallest agar cubes

The rate of diffusion into the cubes is the volume of the colored area divided by the time it took. Calculate the rate of

diffusion for each of the cubes.

Discussion : Make sure the following questions are addressed within your discussion

1. If the length of the side of a cube is increased, what happens to the surface area to volume ratio of the cube?

2. Address the question as to why cells are so small (hint: a large cell would have a large surface area for

diffusion, but what about the volume? What is the SA function...

3. Discuss why many cell organelles have folded membranes as opposed to flat membranes.

4. In which of the cubes was the rate of diffusion greatest and why? ________________

Conclusion: Look at your handout to figure out what to include here

References: Don’t forget this part!

Diffusion Lab- Tips & Hints

Introduction Make sure you reference include citations for your references. See references for more details. Also in your

workbooks!

Materials Make sure you include the changes

Procedure Write this out yourself and say what you actually did. Remember though don’t use we, I… write it as a set of

instructions

Observations/Data

You will need three tables with space for 15 sets of data (class data). The Table Should have the following Headings:

Cube Size; Surface Area; Volume; Surface Area/Volume Ratio; Size of uncoloured portion of cub; Surface Area of

Uncoloured Portion; Volume of Uncoloured Portion

Cube Size

(side lengths)

Surface Area Volume SA/Vol ratio Dimensions

uncoloured

portion

SA of

Uncoloured

Volume of

Uncoloured

cm +- 0.05cm cm2 +- ? cm3 +- ? ***no units! cm +- 0.05cm cm2 +- ? cm3 +- ?

Uncertainties in measurements need to make sense for the calculations. You can estimate an uncertainty that makes sense

for the data column!

Calculate the mean, range, and Standard deviation for each column of data. The Standard deviation is the

“uncertainty” of your mean.

Graph the Data Do Two graphs for each set of data.

Bar graph of mean cube size vs SA/Vol ratio with SD error bars as well as a line graph of Group # vs SA/Vol ratio

with a key for the three sized lines

Bar graph of mean cube size vs rate of diffusion

Do a t-test to compare the results of the largest and smallest agar cubes We are going to compare the medium and

large cubes and we are comparing their rates of diffusion IE> you need to calculate the rate of diffusion as well

as the mean and standard deviation of the rates of diffusion

The rate of diffusion into the cubes is the volume of the colour area divided by the time it took. Calculate the rate of

diffusion for each of the cubes.

References: Don’t forget this part!

Use the following link to see more about how to cite and reference. Basically, any information in your paper that you

are summarizing from a resource must be cites within the paper by using a bracket after the sentence or paragraph.

Then, at the end of the lab, you will have an alphabetical list of references.

http://www.lib.uoguelph.ca/assistance/writing_services/components/documents/APA.pdf

Recap from Today: When you measure something, there is always an uncertainty in your measurement. In Biology we

use a standard of +- ½ a unit. If multiple measurements are taken, and a mean is calculated, the uncertainty of the mean

is the standard deviation. On the other hand, if you are using a measurement with uncertainty in a calculation then you

must +or- the uncertainties when adding or subtracting OR calculate the percentages etc. when multiplying or dividing.