Love in the Time of Cholera - Osmosis in action

Author(s): Amy Trusso, Brad Stohr, Stephen Floor, Greg Friedland

Lesson Overview

Grade level(s):

High School (9-12), Grade 10

Subjects(s):

Biology/Life Science

Topic:

Osmosis, Cholera

Big ideas(s):

How does cholera cause disease? Osmosis leads to loss of water from the body.

How is cholera treated? Use osmosis to pull water back into the body.

Vocabulary words:

osmosis, semipermeable membrane, electrolyte, bacteria, bacterial toxin, cholera,

What you need:

computer, projector, hand-out (osmosis worksheet), dialysis tubing with 1" width (Carolina Scientific), 50-ml conical tubes, water, 80% sucrose solution, scissors, rubber bands, short plastic tubes (2-3 mm diameter bore, 3-4 cm in length), plastic pipets

Grouping:

groups of 4 students each

Setting:

laboratory

Time needed:

class time is 1 hour and 15 minutes

Author Name(s): 
Amy Trusso, Brad Stohr, Stephen Floor, Greg Friedland
Summary: 

Students will first learn about the cause of cholera, and propose treatment options to save a hypothetical patient. They will then learn about the osmotic basis of the disease by using a simple dialysis tube/sucrose model for cholera diarrhea. Finally, they will discuss how osmosis can be harnessed to effectively treat the disease and how this treatment has saved millions of lives.

Learning goals/objectives for students: 

- understand the key concepts of osmosis

- understand how osmosis contributes to cholera diarrhea

- understand how knowledge of a basic scientific concept (osmosis) can be used to advance medicine and treat patients

- develop critical thinking skills

Content background for instructor: 

The attached PowerPoint lecture slides contain talking points for each slide in the Notes section of the program. To see these notes, just download the file and open in PowerPoint.

Note that the description of cholera and osmosis in this lecture requires a number of simplifying assumptions for clarity. The human intestine is obviously much more complicated than dialysis tubing, but the two are compared in this lesson to allow students to work directly with a very simple model of cholera diarrhea. In reality, the mechanism of cholera diarrhea and the success of oral rehydration therapy is significantly more complicated than presented here.

Please e-mail Brad Stohr (bradley.stohr@ucsf.edu) with any questions, comments, or corrections regarding this lesson.

Getting ready: 

make 80% sucrose solution, cut dialysis tubing in 15-cm segments, prepare PowerPoint slides

Lesson Implementation / Outline

Introduction: 

The lesson will begin with a patient presentation. The scene will first be set with a short description of living in a poor community in a developing nation, with a focus on the lack of sewage system. A patient suffering with profuse diarrhea will be introduced, and the students will be guided towards a diagnosis of cholera. Working in small groups, students will be asked to develop a treatment plan for the patient using a set of materials available (see PowerPoint list).

Activity: 

1) PowerPoint presentation about poverty in developing countries, with a focus on lack of sewage systems.

2) Introduction of patient and discussion of cholera diagnosis.

3) Student groups consider treatment options using the PowerPoint list of available materials. After 5-10 minutes of discussion, students are asked to pick which items they would use to save the patients life. Surprisingly, antibiotics won't really help patient survival. While antibiotics will kill the bacteria, the pateint's body will ultimately fight off the bacteria successfully without drugs. The key need in the acute treatment of the disease is to get the patient rehydrated, and antibiotics won't help with that. Aspirin, Pepsi, and antiviral medication would also not be expected to help. The correct answer is water plus salt and sugar as will be discussed. The hope is that students will not know the answer up front, but will instead learn and understand the right answer through the lab exercise below.

4) Lab exercise (see attached PowerPoint description, as well as the attached photo and diagram of the osmosis apparatus). 1) Hand out supplies for lab and demonstrate how the dialysis apparatus is constructed. Each group will construct two dialysis apparatuses (as described in the attached PowerPoint). 2) Wet two 15-cm segments of dialysis tube in water and tie off one end of each. 3) Use the plastic pipet to fill the one of the tubes with water and one with 80% sucrose. 4) Insert a plastic tube into the top of each dialysis tube (the end that is not tied off) and secure the dialysis tubing to the plastic tubes using rubber bands. While securing the rubber bands, keep as much of the water or 80% sucrose in the tube as possible. 5) Place each dialysis tube in a 50-ml conical tube and fill up each conical tube with water so that the water level is above the rubber bands (see attached photo and diagram). If necessary, use the plastic pipet to add additional water or 80% sucrose to each dialysis tube. 6) Wait. Eventually (usually 15-20 minutes depending on the set-up), osmosis will cause water to flow into the dialysis tube containing sucrose, and the water will rise up into the tube on top and spill over. This is our simple model for cholera diarrhea. Nothing should happen to the control dialysis tube containing water. Once students have observed the osmosis, they should work on the questions on the attached hand-out. Instructors should circulate to help the students answer the questions.

5) Students will discuss the questions on the hand-out as a class, with PowerPoint diagrams showing how osmosis caused the movement of water into the dialysis tube and how a similar process occurs during cholera infection.

6) As a class, students will brainstorm ways to reverse the osmosis and help the patient. One idea that students may hit upon is to give the patient a salt solution intravenously. Based on our simple osmosis model, IV administration of salt would draw fluid out of the intestine and back into the patient's body (since it is equivalent to adding salt to the water in the 50-ml conical tube). This is a great answer, and IV fluids are indeed a common treatment for cholera. One major downside of this treatment is cost, since it requires a large supply of clean needles, sterile salt solution, trained personnel, etc . The general hope is that, from the lab experiment, the students will understand how getting salt back into the patient will help reverse the diarrhea and rehydrate the patient. This is the key point that should be emphasized in the discussion.

7) Finally, the discussion should return to the original PowerPoint list the students used to consider treatment strategies. The correct choice from that list is water containing salt and sugar. Students are unlikely to hit upon this answer even after the lab experiment since it is counterintuitive. If osmosis into the intestine is the problem, how can more salt and sugar in the intestine be the answer? Wouldn't this just lead to more diarrhea? This can be presented as a paradox, with the teacher then providing the unexpected solution. Basically, the intestinal wall contains a glucose-sodium transporter which will move salt (sodium) from the intestine into the body, but the key point from a treatment standpoint is that the transporter only works if sugar (glucose) is also present. The reason is that this is a cotransporter, which must move salt and sugar together from the intestine into the body. It will not transport sugar or salt alone, but must have both present to function. This explains why the treatment works. The patient ingests a salt/sugar solution, and the glucose and sodium are then transported from the intestine into the body. Osmosis then leads to the movement of water back into the patient, successfully rehydrating the patient. This is the basis of oral rehydration therapy. Just like IV fluids, it gets salt and water back into the patient. The advantage of oral rehydration therapy is that it is cheap (since the salt/sugar packets can be shipped dry), and it doesn't require any expensive equipment or personnel.

Checking for student understanding: 

During class discussions, students will be expected to share ideas and move the discussion forward. The ability of the students to answer the questions is one way to gauge student understanding. Secondly, the instructors will circulate during the small group exercises to answer questions, stimulate discussion, and gauge understanding on an individual basis.

Wrap-up / Closure: 

The lesson ends with a discussion of how oral rehydration therapy has saved millions of lives by harnessing the power of osmosis to get much-needed liquid back into cholera patients.

AttachmentSize
Cholera Mechanism Hand-Out.doc22.5 KB
dialysis tubing set-up.jpg266.59 KB
Tubing_setup.JPG2.97 MB
Cholera intro.ppt4.55 MB
NGSS Topics
High School Life Sciences: 
NGSS Disciplinary Core Ideas
NGSS Science and Engineering Practices
NGSS Crosscutting Concepts
NGSS Crosscutting Concepts: