Cell Boundaries and Human Health

Why is water balance so important in cells? Among the poor and especially in developing countries, diarrhea is a major killer.

Why is water balance so important in cells? 4 billion cases of diarrhea yearly 2.2 million deaths -many under the age of 5 That’s 4 deaths every minute

How do diarrheal diseases spread? The Story of Cholera

A very different type of life than what we are used to A very different type of life than what we are used to. Most notably for today’s purposes, there is no sewage system. (Mathare Valley, several miles outside Nairobi) http://picasaweb.google.com/dmeyer1usa/AfricaTrip2007/photo#5132143011588101730

As a result, raw sewage literally flowing in the streets As a result, raw sewage literally flowing in the streets. Imagine you are a doctor in a clinic here. One day a sick young woman enters your clinic… http://abandoned-orphaned.typepad.com/photos/uncategorized/2007/08/01/matharesewagerut.jpg

Why is there an increase in diarrheal diseases after natural disasters?

Pakistan - Flooding

Haiti - Earthquake

Indonesia - Tsunami

Philippines - Typhoon What happens next? What are the biggest concerns of the medical community?

What jumps out at you. Bean-shaped things with tails. Diagnosis What jumps out at you? Bean-shaped things with tails. Diagnosis? Cholera. Infection with cholera is often fatal if untreated. http://upload.wikimedia.org/wikipedia/commons/thumb/d/db/Vibrio_cholerae_01.jpg/800px-Vibrio_cholerae_01.jpg http://microbewiki.kenyon.edu/images/a/a8/V_cholerae.jpg

Can you tell from this picture what the biggest problem with cholera is? Diarrhea…note the holes in the cots and the buckets under the holes. These are called “cholera cots”. One patient can have as much as 10-20 liters of diarrhea per day. This type of fluid loss can cause fatal dehydration. Now the big question: how will you as the doctor treat this patient? http://www.msf.org/source/countries/africa/angola/2006/cholera_tent.jpg

To understand why diarrheal disease can be so deadly – you have first have to understand how healthy cells manage water balance

Since diarrhea is the big problem with this disease, we must understand it’s origins. Digestive system. What happens to the food you eat and the water you drink? It is absorbed by the body, and that absorption is done in the intestine. Diarrhea is a function of the water absorption (or lack thereof) in the intestine. If your body loses water or doesn’t absorb enough, the result is diarrhea. Let’s look at a slice of the human body… http://www.medicalook.com/systems_images/Digestive_System.jpg

Cell Membrane

Moving in & out of cells by OSMOSIS Water molecules move from an area of high concentration to an area of low concentration through a semipermeable* membrane Does not require extra energy * Also called “selectively permeable”

Effects of Osmosis on Blood Cells

Which Way Will Water Move? Water Balance in Cells Water Outside Cell Water Inside Cell Which Way Will Water Move? Same Water flow is equal in and out of cell Less Water More Water Net water flow out of cell Net water flow into cell

Effect of Osmosis on Blood Cells

Intestinal Cells Body Intestinal Lumen Salt Salt Salt Critical that the students understand this schematic: skin, body, and intestine. Cholera invades the intestine in an infected patient (after the patient is exposed through contaminated water, food, etc.). Most bacteria in your system actually help you (~100,000,000,000 of them). But this bacteria is different, because it release a toxin (cholera toxin) that attacks the cells lining the intestine. As a result, these cells excrete salt into the intestine. This will prove to be critical in the formation of diarrhea, and we are now going to see how using a simple model system. Salt Salt

Salt Salt Salt Salt Salt So because of the cholera infection, you have a lot of salt in the intestine. This is how the bacteria causes so much trouble. How is this going to cause diarrhea? Let’s do an experiment to find out…

Our Model System The Body Conical Tube Skin Dialysis Tube Intestine Dialysis tubing will mimic intestine, and sucrose will be used instead of salt (since salt is so small that it would be able to diffuse through our dialysis tubing).

One Dialysis Tube per Group Control One set-up/Class Experimental One set-up/Lab Group Conical Tube Demonstration of set-up. Each group will set up two dialysis tubes, one containing water and one containing 80% sucrose. One: demonstration. Two: groups set up two tubes. Three: consider questions on next page while experiment is running. = water = salt

Hand-out for students to read and answer while experiment is running. Water Salt Solution Hand-out for students to read and answer while experiment is running.

What happened to the dialysis tube with Salt? Why? After the students have set up the dialysis tubing, seen the osmosis in action, and considered the questions on the handout, the last step is to go over the questions in front of the class.

Osmosis Water Salt Solution Water Water Water Osmosis is diffusion of water down a concentration gradient. So why does this happen? Water Osmosis

How does your experiment explain cholera diarrhea?

Salt Salt Salt Salt Salt Net Flow of Water Net Flow of Water Because of the salt secretion into the intestine, where is the water concentration higher? In the body. So which way will the water flow? From the body into the intestine. This is one root cause of cholera diarrhea. Osmotic movement of water into the intestine. The patient’s intestine will fill up just like our dialysis tube with sucrose. This is what will kills the patient, because the water in the patient’s body will be lost as diarrhea. So now the big question: how can you reverse this?

How would you treat the patient using osmosis? See the Activity notes in the SEP lesson plan for a description of how to approach this question with students, since the answer is somewhat counterintuitive.

Salt Sugar Salt Salt Sugar Sugar Salt Sugar Give your patient water containing salt and sugar. Note that this is completely counterintuitive based on the lab experiment, since one would expect the additional salt and sugar to in the intestine to increase osmosis into the intestine, thereby worsening the diarrhea. The key is that the intestinal wall has a sodium-glucose transporter that will move the salt and sugar from the intestine into the body. This transporter must transport salt and sugar at the same time, so it only works if both salt and sugar are present. This explains why oral rehydration therapy must contain both. As a result, the salt and sugar are transported together into the body, and water follows by osmosis.

Salt Sugar Salt Salt Sugar Sugar Salt Sugar Salt Sugar Salt Salt Sugar Net Flow of Water Net Flow of Water Salt Sugar Salt Sugar Now that the salt and sugar have been absorbed by the body, where is the water concentration highest? In the intestine. So which way will the water move? Back into the patient. This osmotic movement of water saves the patient’s life. So osmosis was killing the patient, and now we have exploited osmosis to save the patient. So what does the treatment actually look like? Salt Salt Sugar Sugar

How do diarrheal diseases spread? The Story of Cholera

Gatorade could save the patient (although it’s got more sugar than is optimal). There is actually a scientific study (funded by Gatorade) which shows that it works well for diarrhea treatment. But in practice, this is the real lifesaver…

Oral rehydration therapy Oral rehydration therapy. All it takes is salt, sugar, and water to save millions of lives, by exploiting the power of osmosis to rehydrate patients. Still have 1.8 million deaths per year from diarrhea, so the problem is not solved. But millions more would have died without the little packet shown above.