Name ______Period ______Date ______

Diffusion Activity

Cells rely on diffusion as a means by which some materials pass in or out of the cell. The cell membrane forms a barrier between the inside of the cell and the external environment. It is across this membrane that some materials enter the cell and other materials exit the cell. Movement of materials by diffusion affects the cell’s ability to maintain homeostasis. In this activity you will observe the process of diffusion. You will use iodine, cornstarch, and plastic wrap to model the process of diffusion. You will hypothesize what effects concentration and temperature have upon the diffusion process and then set up an experiment to test your hypothesis.

PRE-LAB

Iodine will be used as an indicator for starch. An indicator is a substance that changes color in the presence of the substance it is used to detect.

1.)  Describe what happened to the iodine when it came in contact with the starch.

______

______

FORM A HYPOTHESIS

Write a hypothesis to explain how CONCENTRATION will affect the extent of diffusion of iodine through a membrane. Fill in the following blanks to form your hypothesis:

The higher the CONCENTRATION of iodine on one side of a membrane, the ______will be the amount of iodine that will move through the membrane because ______.

TEST THE HYPOTHESIS

1.  Fill a beaker about three-fourths full with the cornstarch solution.

2.  Place the plastic wrap inside the beaker so that the plastic is pushed down a little below the surface of the cornstarch solution.

3.  Use a rubber band to secure the plastic wrap tightly around the top of the beaker.

4.  Pour the iodine solution that you are given (A, B or C) in the plastic wrap pouch. (see below)


FORM A PREDICTION

Make a prediction about how the solution will change with the addition of the iodine.

______

MAKE OBSERVATIONS

1.  Observe the color of the cornstarch solution and the color of the iodine solution and record your observations in Table 1.

2.  Wait 20 minutes and watch for color changes in your solution. Use Table 1 to record any color changes you see.

TABLE 1. COLOR CHANGES

Solution / Color at 0 minutes / Color at 20 minutes
Cornstarch solution / Iodine Solution / Cornstarch Solution / Iodine Solution
A
B
C

Identify the constants (controls). Which parts of the experiment are the same for all groups?

______

Identify the independent variables. What parts of the experiment are different for each group?

______

Identify the dependent variable being observed in this experiment.

______

Draw before and after pictures of the setup. Be sure to label where the cornstarch and iodine solutions are located. Use arrows to show the direction that the iodine or cornstarch solution moved through the plastic membrane.

ANALYZE THE RESULTS

Did cornstarch move through the plastic into the iodine, or did iodine move through the plastic into the cornstarch? How can you tell?

______

Based on your observations, which solution contains larger molecules – cornstarch or iodine? Why weren’t the larger molecules able to pass through the membrane?

______

DRAWING CONCLUSIONS

Which iodine solution (A, B, or C) produced the largest color change?

______

Which solution had the highest concentration of iodine? How do you know?

______

What factors affect the movement of different substances through a membrane?

______

Define diffusion.

______

Define osmosis.

______

What is the main difference between osmosis and diffusion?

______

During passive transport, how do molecules tend to move and how is this different from active transport?

______

CONNECT TO THE ESSENTIAL QUESTION

Identify two real-life situations in which diffusion occurs.

______