Teaching Cases in Large Lecture Sections

Eric RibbensWestern Illinois University

So:

• You are convinced: cases (a story that sets up a problem to be solved) are a good way to teach science.

• But how do you teach a case to a class of 150 students? (or 800?)

I use a mix of strategies:

• Emphasize goals for the course, for each day• “Flipping” content and problem-solving• Teaching to the test: practice practice practice• LOTS of cases (1 a week)

• But especially, clickers

I teach large intro science courses

• Plant Biology (140 to 180 science majors)• Nonmajors Biology (147 students this spring)– Students resent having to take the class– Overwhelming majority dislike science

• I use clickers, I require attendance, and my students are thriving!

My class last week

Clickers:

• Students like them• Especially benefit shy, nontraditional, minority

students• Keep students alert and focused on the topic• Help students recognize what I want them to

know• Easily give quizzes and track student progress• Help me evaluate where they are at!

Student Comments:

• Clicker questions are great. They allow us to apply what we are learning, and if we get it wrong you are here to explain to us why it is wrong.

• I also want to say that using the clickers was a great idea.

1: Why do I use clickers? Students learn.

Before Clickers• Class average 61 to 67%• Students sleeping in class• Attendance rate by

midsemester 65%?

After Clickers• Class average last semester:

72%• No one sleeps in my class

anymore• I started this semester with

143 students. 12 have withdrawn from the university.

• In class March 6? 117.

Student comments:

• I really enjoyed clickers because it gave me a chance to test my knowledge in class.

• The clicker questions helped me to understand the material a lot better as well.

Clickers and cases

• Clickers can be used in traditional lecture formats, of course.

• But they are great for “interrupted” or progressive disclosure cases

• Become a dialogue and a guide to thinking through the problem

• And this mix fits well with my course goal to be able to apply scientific concepts

Student comment:

• I really enjoyed clicker questions. I’ve never had a class that utilizes this tool before, but it is fabulous. It’s a great way to help us learn more and to “check” us throughout the semester.

Student comment:

• I found the clicker questions useful because you got realtime feedback on your understanding of the material. It was also nice to get to discuss our answers with others to try to gain a better understanding of the material.

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The Case of the Exploding Fish

• I was lounging in my office when a client called.

• “Do fish explode?”

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Do fish explode?

• I was stumped. Do fish explode? And why did she want to know? I made a list.– Why would fish explode?– Why don’t they explode?– How do fish handle the very different freshwater

and saltwater environments?

It was time for some research.

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Principle

• Diffusion: A chemical will spread out and become completely the same concentration wherever it can go (unless some other force is acting on it).

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What about across a membrane?

• If it can cross the membrane, it will attempt to diffuse

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? So what do you predict about the green dots in the diagram?

• A: They cannot cross membranes• B: They do not obey the law of diffusion like

other chemicals

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So a semipermeable membrane …

• Will allow some chemicals to diffuse across it, and prevent other chemicals from diffusing across it

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A good example: kidney dialysis

• Salts and urea can cross the membrane

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Another tricky piece of diffusion

• The chemical tries to achieve equal concentration OF IT RELATIVE TO THE TOTAL SOLUTION

• So if there are equal amounts of water on each side of the membrane, but on one side the solution is only water while on the other side it is a mixture, water will still keep moving to the mixture side

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Water diffusing

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In my office …

• I sat back and tried to think. What did I know?• Diffusion:– What it is– What happens across membranes– And kidney dialysis. A far cry from exploding fish, I

thought. It was time for some questions.

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? A solution is 90% water on the left, and 80% water on the right, of a membrane that only water can cross. Assuming no

other forces are operating, which direction will water move?

• A: Water will move from left to right.• B: Water will move from right to left.• C: It depends.• D: It is impossible to tell.

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? Remember it is important to think about the concentration of the substance that can move. Consider this question: The solution on the left is 10% salt. On the right it is 5% salt. Salt cannot cross the membrane, but water can. Which way will water move?A: Water will move from left to right.B: Water will move from right to left.C: It depends.D: It is impossible to tell.

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Consider this experiment:

• You place an animal cell in a container of distilled water. What will happen? Make a hypothesis with your neighbor.

• (not this kind of cell)

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? What do you predict?

• A: Water will not move.• B: Water will move from the cell into the

beaker holding the experiment.• C: Water will move into the cell for a while.• D: Water will move into the cell and the cell

will explode!

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Water will move into the cell and the cell will explode!

• Why?• Start: concentration of water is higher outside

the cell than inside, so water moves in• As time passes: the concentration of water

inside the cell increases, but it never gets as high as the distilled water

• The water moving in increases the cell’s volume, until it can’t handle the volume and explodes

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Now consider this:

• You place a plant cell in the same solution. The animal cell has exploded, but the plant cell never does. Why not? (Group work!)

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? The plant cell didn’t explode because …

• A: Plants are tougher than animals• B: Plants have a cell wall that pushes back

against the increased cell volume• C: Plants don’t let water cross membranes• D: We have no idea

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Ok. I was on to something here.

• Animal cells explode when placed in fresh water. But there are many animals that live in fresh water. We don’t find fish that have exploded! Why not?

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? Why don’t freshwater fish explode?

• A: They really do, but underwater where we don’t see it.

• B: They don’t explode because they have cell walls.

• C: They don’t explode because they have kidneys to get rid of excess water.

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Freshwater Fish

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How about a saltwater fish?

• Freshwater: water is continually entering the fish

• Saltwater: water is …

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How about a saltwater fish?

• Freshwater: water is continually entering the fish

• Saltwater: water is continually exiting the fish

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Or another strategy:

• What else could a fish do to reduce or stop water from leaving its body? Think about it!

• Hint: the ONLY way to keep water from moving in or out is to have the concentration of water the same inside and out

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The only way to prevent water from moving out is …

• Make the inside of the fish as solute-concentrated as the outside!

• How? Urea. Sharks and other cartiliginous fish build up urea in their muscle tissues. This means the fish fluid more concentrated, similar to ocean water.

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? So the next time you see this you will think:

A: Bleah, that meat is soaked with fish peeB: A great example of osmotic balanceC: Um, I’m supposed to know something about this from bio 100

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Conclusion

• I called the client. “No, they don’t explode. Well, at least not if they have kidneys.”

• She sounded disappointed, and hung up, leaving me to wonder:

• WHY did she want a fish to explode?