Activity Overview:
Many diseases are the result of structural changes in the cell membrane; some of these changes may result from genetic disorders (mutations), some are caused by toxins, produced by certain bacteria and others by the actions of viruses.
Students apply their knowledge to the mystery of red blood cells (erythrocytes) responding to changes in the salinity of their surrounding environment. Their main focus is on water movement in response to the changing concentration of ions in the outer environments (the erythrocyte case), and why the "health" of cell membranes is so important for cellular (organism) health --the case of Cystic Fibrosis.
Learning Objectives:
Students will:
Students will:
1. Summarize as a class their enhanced model of the Erythrocyte Case that has incorporated their understanding of the molecular structure of water and aquatic solutions, diffusion and the origin of osmotic pressure. They apply this knowledge to the behavior of the erythrocyte in solutions of various salinity.
Macro to Micro Connection:
Students connect their micro model with problems in proper membrane functioning, such as Cystic Fibrosis and Cholera.
Conceptual Prologue:
Students have seen ionic compounds such as salt broken into ions and surrounded by water. Usually these ions cannot make it through the oily interior of a cell membrane. But ions with their watery shells, if they are the right size, can get into cells through little waterways or through ion chennels made of proteins designed just for them. Sometimes the watery shell needs one or more of the attached water molecules to be stripped off just before the ion enters the channel.
Small ions can usually be pumped out, but they are also attracted to macromolecules and contribute to osmotic pressure inside. Small ions like Na+ get actively pumped out.
Active Transport: Cells have to pull some materials across the gradient using specialized protein pumps set in the membrane. This takes energy. A sign of aging is when cells lose their turgor, indicating a water loss.
Cystic fibrosis is a genetic defect that effects 1 out of 1000 births. It works as if salt is added to the inside of the cell. As a result of the defect, the proteins that should form a set of chloride pores are badly shaped, and do not make it from their point of origin to the membrane for insertion in the membrane. Consequently, chloride is not pumped out of the cell; instead it is retained inside the cell. Water follows the salt concentration, and the mucus membranes of many critical organs become dry and sticky.
WATER PORE: (an "aquapore")
- passive transportationMeet a pore designed for water ...only. This membrane pore allows water to freely and passively, using no extra energy, move across membranes.
SALT ION PORE - active transportation Salt ions cannot move freely across the membrane. The oily, hydrophobic inner core of the membrane keeps the charged ions out. (There are pumps that the cell uses to obtain the concentrations of Na and Cl that it needs.)
As of December 2001, we know the structure of an aquapore and understand the molecular basis of its specificity.
See Nature
On the right you can see a membrane chloride pore designed to move cloride ions out of the cell. These ions do not diffuse passively, but rather actively, using energy. The pore uses ATP for energy to move the ion. This is the very chloride pore damaged in cystic fibrosis.
Activity Design and Execution:
Major Science Concepts active transport, membrane disease Assumed Previous Knowledge: ions, solutions,membrane Time: 1 50 minute class Materials: The Cholera Story; The Oral Rehydration Story; Membranes Advanced preparation (if any)
STEPS
1. Summarize as a class all the ideas of their enhanced model of the Erythrocyte Case, incorporating their understanding of the molecular structure of water and aquatic solutions, diffusion and osmosis and the origin of osmotic pressure.
2. Ask students to read and discuss case studies of illnesses, cystic fibrosis and cholera. Students will explore further the idea that certain diseases are the result of various damages of the structure and function of cell membrane, some (cholera) caused by a bacterial toxin, or resulted from genetically determined deficiencies in the membrane structure (CF). Their focus is on water and ions movement in response to the structural changes in the membrane. This chart about membranes may help.
3. Adjust their models to explain macroscopic phenomena of CF and cholera.
4. Take a web tour previewing the idea of active transport mechanisms in cells.
5. Take a post-test.INDEX
