Reflection: Student Ownership Diffusion Lab (Day #3 of 3) - Section 4: Closure: What did we learn? Where do we go from here?


This lab is always a favorite one for me because the logic underlying the conclusion is pretty simple to grasp however the real reason that starch will not diffuse through the membrane is not intuitive. I have tried to very intentionally and repeatedly teach and reinforce student argumentation & forming conclusions this year (see Science Practice #6: "Explanations and Designing Solutions").

In two ways, students showed their understanding of the concepts represented in the lab. First, the young man featured in the video was able to articulate that starch did not diffuse through the dialysis tubing (claim). He stated his premise: "Starch and iodine, when mixed together, turn purple/black". He stated further that there was no such color change in the water surrounding the dialysis tube "system"; it was yellow upon adding iodine. Being curious about this color relationship, he cut open the top of the tube, added iodine, and discovered the expected color change (evidence). He reasoned that since there was a color change inside the tube (starch present) and no corresponding change on the outside, it is logical to conclude that starch (placed on the inside originally) did not escape.

In another example, students were asked to create a visual model of the microscopic membrane relative to the sizes of both glucose and starch. It is clear to see that the way the featured student's model was conceived is in line with the empirical evidence. That is, the glucose molecules are smaller than the pores in the membrane, therefore they can diffuse unlike starch. 

  Hands-on Learning
  Student Ownership: Hands-on Learning
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Diffusion Lab (Day #3 of 3)

Unit 2: 2) Cells ("Form and Function")
Lesson 13 of 20

Objective: 1. Students will be able to investigate the effect of molecule size on diffusion through a semi-permeable membrane. 2. Students will be able to collect evidence and use it to develop a claim and compose an argument about the permeability of an artificial membrane. 3. Students will be able to use a scientific model to share and expand one’s thinking about the nature and function of cell membranes.

Big Idea: Cell membranes are semi-permeable. Diffusion of substances across the membrane is driven largely by the size of the solute particle.

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