Water on the Move: Osmosis (#2 of 3)

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Students will be able to understand the hierarchical nature of multicellular organisms. Furthermore, students will be able to model the process of osmosis and predict and explain its outcomes given the conditions of the environment.

Big Idea

The structure of the cell membrane in addition to specific internal and external conditions dictate how water diffuses sometimes with drastic results!

Learner Goals

Photo Credit: "Oncorhynchus nerka" by Timothy Knepp of the Fish and Wildlife Service. - US Fish and Wildlife Service. Licensed under Public Domain via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Oncorhynchus_nerka.jpg#/media/File:Oncorhynchus_nerka.jpg                                          

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...

1) Understand that multicellular organisms have a hierarchical structural organization, in which any one system is made up of numerous parts and is itself a component of the next level. (LS1.A)

2) Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms. (HS-LS1-2)

3) Generate a logical conclusion that is supported by evidence from the investigation and/or provide a scientific reason to explain the trend in data given a description of and the results from a scientific investigation. (WA-INQC-1)

I hope you get some value from my work!

Anticipatory Set ("Hook")

10 minutes

Please click here to link to the previous lesson in the series.

Compare and contrast: There is a concept of the "student:teacher ratio" deftly explained by Doug Lemov in his book Teach Like a Champion. The basic premise is that in the classroom there ought to be "heavy lifting" happening; in essence cognitive exertion to master a skill or knowledge. All too often, the teacher variable in the ratio is very high permitting students to expend little to no mental effort during class. It needs to be at least equal and preferably higher on the student end (provided that the teacher has done the requisite heavy lifting of master planning ahead of time). 

I use the technique that Lemov coined "Everybody Writes" to stimulate independent processing. Essentially each student is prompted to write out his response to a prompt given a quiet atmosphere and a set time limit.

Lemov cites no fewer than three benefits to such an approach to getting students in increase their side of the equation or ratio. Giving students the time to process quietly and write out their thinking "increases the quality of the ideas discussed... expands the number of students likely to participate... (increases) their readiness to do so." (Lemov, p. 141)

In the context of today's lesson, students are asked to write out the similarities (2) and differences (2) between salt-adapted animal cells that are placed in a freshwater tank and fresh-water animal cells that are placed in a saltwater tank.

Compare: Students should correctly note that there is a semipermeable membrane and a flow of water in both scenarios.

Contrast: Students should correctly note that the salt-adapted cell will have more salt and less water inside (when compared to the outside) thus it will expand and possibly burst in the freshwater tank. Conversely, the freshwater-adapted cell will have less salt and more water inside (when compared to the outside) thus it will shrink in the saltwater tank.

Instructional Input/Student Activities

45 minutes
Teaching Challenge: How can I develop my students’ ability to apply unifying ideas to make connections across science content (among and between physics, chemistry, biology, earth and space science)?

1) Osmosis Lecture (Part #2 of 2): 

Slides 28-30: I use these slides to explain the microscopic forces at work that drive osmosis toward areas of higher solute concentration. Essentially there are "bound" water molecules that are attracted to the solute via hydrogen bonding. The ratio of bound water molecules increases with solute concentration leaving a relatively small number of "free" water molecules that may diffuse across the membrane. Conversely, more water is free in solutions of lower concentration giving rise to more "movers". Thus, the rule of thumb that I teach is "water follows the solutes".

2) Practice Test* (Check For Understanding):  As a means of assessing whether students understand the concepts and skills related to osmosis (and diffusion) I use the “Pepper” technique. Essentially this is a randomly generated Q&A requiring a student to answer a simpler style of question (e.g. list, define, recall, describe). I have student name (index) cards that I use on a daily basis to solicit student responses in a completely unbiased fashion.

*Credit: The practice test was created by C. Stephen Murray (2003-2012) and has permitted the use of his work and is credited according to copyright laws.

As a class we will progress through the 20 questions featured on the practice test hoping to go 20 for 20!

3) End-Of-Course Exam (EOC): Writing Conclusions

Let's recap what we've done so far. Students have observed the effect of osmosis on potato cells and learned about the concept of osmosis, including the four step problem solving process. Now we will turn our attention to how a proper conclusion is written by way of a contrived osmosis scenario: A saltwater aquarium with varying concentrations of salt water.

In my state (WA), students are required to pass the Biology EOC in order to graduate from a public high school. This exam features seven different types of short answer prompts in addition to a number of multiple choice questions. As a means of preparation, I will review the basic gist of the "conclusion" styled prompt.

The following EOC Prep: Conclusion PowerPoint and video (podcast) review the salient details that students ought to know. Namely, a proper conclusion includes the following attributes:

  • Conclusive statement
  • Supporting data (at least two)
  • Explanatory language
  • Scientific Explanation

 Students are then shown three anchor papers: "Meets standard" (2/2 points); "Close to standard" (1/2); "Well below standard" (0/2)

Using these as examples, I point out what the 2 score includes and what the 1 and 0 scores lack. Students will complete a conclusion-oriented exercise for homework tonight.

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

5 minutes

"Cold Call" Technique: Define osmosis, concentration, and concentration gradient

Please refer to this link for a more explicit description of "cold call".

Please click here to link to the next lesson in the series.

Lesson Extension & Follow-Up Activities

EOC Short Answer Prompt (Conclusions): Complete Saltwater Fish Conclusion for homework. Students are encouraged to refer to the EOC podcast or PPT for assistance.