Diffusion Lab
Target Concepts: Understanding diffusion and how it works in the human body.
Standards Addressed: B-2.5
Materials:
Dialysis Tubing
Starch Solution (Starch in water)
Iodine Solution (Iodine in water)
300ml Beaker (1 per table)
Paper towels
Optional: smocks and gloves
Pre-Macmod
A
Molecules
B
Semi-permeable
Membrane
The molecules (circles) in diagram A are in a solution separated by a semipermeable membrane. These molecules are able to move through the membrane.
1. On Diagram A, draw arrows to indicate which way the molecules will flow
through the membrane.
2. On Diagram B, draw how the molecules will look after the molecules have moved
through the membrane.
3. Explain your answers. Try and use the following words in your answer.
1. Diffusion
2. High/Low Concentration
Introduction:
Explain to the students they will be performing a lab today. Talk about each
component they will be in contact with. For example, discuss starch, what it is and
have a picture of what it looks like. Also, discuss iodine, what it is and what it looks
likes. It’s helpful to have these pictures on the same slide. The students can notice
the size differences of the molecules. Explain to the students that in the presence of
iodine, starch turns black.
Each group should have a list of directions. Let the students collect the
equipment needed and get started on the lab. While doing the lab, they must be
filling out and answering questions along the way. During the lab they should be
able to see the reaction occurring inside the dialysis tubing but not outside in the
beaker. Walk around to each group and ask them leading questions as to why it is
only occurring in the tube and not outside the tube. Refer back to your power point,
which shows the size differences in iodine and starch molecules. This can help
explain why iodine was able to move through the membrane and starch was not
(selectively permeable). Also, introduce the idea of concentrations and discuss the
concentration gradients by asking leading questions. Have the students clean up
and take out their notebooks.
Post-Discussion:
Talk about what happened inside and outside the tube as a class. Introduce
the idea about the different concentrations within the beaker and the tubing.
Discuss how molecules move from high concentrations to low concentrations
(concentration gradient). Also talk about the movement of iodine into the tube
without the starch moving out of the tube. Explain possible reasons for this (i.e. size
of molecule) and discuss how the tubing is selectively permeable. Relate how the
tubing is like a cell membrane.
Post Mac Mod
The semi-permeable membrane is permeable to both types of molecules (dark and
light circles).
1. On diagram A draw arrows to show the direction each type (black and white)
of molecule is moving.
2. On Diagram B show how the molecules will look after the molecules have
moved through the membrane.
3. Explain your answers. Use the following words in your answer.
a. Diffusion
b. High/Low concentration
c. Equal