Diffusion Lab (Day #3 of 3)

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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.

Learner Goals

Note: I recommend that you first check out this resource in order to get the most out of this lesson!

In high school I took several drafting classes and, for a while, I had hoped to become an architect. With respect to planning instruction and teaching, I feel that I can still live out the detailed approach to building something intricate and complex even though the product is a lesson rather than a certain "built environment".

The lesson-planning document that I uploaded to this section is a comprehensive overview of how I approach lesson planning. This template includes the "Big Three" aspects of the NGSS standards: Disciplinary Core Ideas, Crosscutting Concepts, and Science Practices. Of course, there are many other worthy learning goals, skills, instructional strategies, and assessments that can be integrated into a class session. I don't feel compelled to check every box but, rather, use it as a guide to consider various options and tailor the lesson in light of these.

With regard to this particular lesson, students will be able to:

1. Investigate the effect of molecule size on diffusion through a semi-permeable membrane.

2. Collect evidence and use it to develop a claim and compose an argument about the permeability of an artificial membrane.

3. Use a scientific model to share and expand one's thinking about the nature and function of cell membranes.

I hope you get some value from my work! Please find the more intricate details of this lesson plan in the uploaded document!

[Note: Refer to the planning document for specific details throughout the three day investigation]

Anticipatory Set ("Hook")

10 minutes

Continued from Day #2...

Collecting Evidence: Teams are shown the YouTube demo that demonstrates how to determine whether a given unknown solution contains starch, glucose, or both.

Refer to the teacher version of the Diffusion Lab Design PPT and slides #19-20.

Instructional Input/Student Activities

20 minutes

Teaching Challenge: Use a scientific model to share and expand one's thinking about the nature and function of cell membranes.

The heart of this, and any other, investigation is to lead students to the point where they can pose a meaningful scientific question, make a reasonable and testable prediction, and devise a method for collecting evidence with which to answer the original question and compose a thoughtful argument about the related phenomenon.

-Carbohydrate Testing: The half of my eight teams that investigated whether glucose is permeable to the dialysis membrane used the Benedict's Solution indicator and the other half investigating the permeability of starch used IKI as the indicator. At the end of the experiment, the evidence (the color change in the water surrounding the dialysis tubing) collected will be shared out and analyzed with respect to the question of permeability.

-Clean up: Disposal of waste chemicals and the dialysis bags is achieved by teams as well as washing and storing of glassware; key elements of being good lab citizens.

Closure: What did we learn? Where do we go from here?

25 minutes

Data Analysis and Communicating Scientific Thinking:

Using the class Diffusion Lab Data table, I prompt students to share out their data (from Groups #1-8) and record it in the Excel form (projected on the classroom screen) for all to see. Quickly, students see the trends among and between all groups. By the way, all students correctly inferred that the glucose did diffuse based on the evidence collected (Benedict's solution turned orange when added to water surrounding dialysis tubing and heated appropriately). Here, a student reflects on the evidence collected and makes the argument against his earlier prediction. Two more pieces of evidence against the diffusion of starch: Sample #1 & Sample #2

From here, students are guided toward making a claim about diffusion, citing evidence, then explaining the reason (rule) that drives diffusion. This follows the “Claim-Evidence-Reasoning” (CER) model used by many teachers.

Claim: The dialysis membrane is permeable to glucose.

Evidence: The Benedict's Test turned orange in the presence of the water outside of the tubing.

Reasoning: The glucose was originally inside the tubing and, 24 hours later, was detected outside the tubing. Therefore, glucose was small enough to diffuse through the pores in the dialysis tubing.

Here, two students made sense of this conclusion visually: Microscopic Diffusion

You will notice how they drew the glucose particles small enough so that they will pass  (diffuse) through the membrane but the starch particles are too large, therefore they are not permeable to the membrane. Great thinking students!

Lesson Extension & Follow-Up Activities

Homework: Students ought to complete the Diffusion Lab conclusion for class tomorrow.