Diffusion, Osmosis and the Cell Membrane An introduction to the function of the cell membrane and mechanisms of passive transport by Bob Birch, Fontbonne Academy, Milton, MA. Ybelise Escoto, Chelsea High School, Chelsea, MA. Bryan Henry, Chelsea High School, Chelsea, MA.

Introduction Using both wet labs and simulations t his unit will introduce students to the: role of the cell membrane as the primary mediator of transport of material into and out of the cell mechanisms of diffusion and osmosis the relationships of temperature and molecular mass to rates of diffusion role of phospholipids in the structure and function of the cell membrane

Goals Master the use of the virtual molecular program. Describe the components and their arrangement of the cell membrane. Define diffusion, osmosis, selectively permeable membrane, and equilibrium. Predict the effects of temperature and mass on diffusion rates. Distinguish between diffusion and osmosis. Explain the role of the cell membrane in passive and active transport. Analyze the effects of concentration on osmosis. Demonstrate the formation of micelles and lipid bilayers.

Audience and Adjustments This unit has been designed primarily for standard level Biology students With additional material the unit can be adapted for more advanced students With some modification the unit can be adapted for younger students or situations with limited computer resources

Placement in the Curriculum This unit is designed to be used to supplement classroom and lab units at the point when cell structure and function are being introduced to students. Specifically this unit will be used when the role of the cell membrane in the transport of material into and out of the cell (passive and active transport) are discussed. This unit is aligned with the Massachusetts Department of Education frameworks for Biology (section 2.5)

Time and Instructional Activities This unit includes four lecture type lessons and three lab units. Two of the labs are combinations of wet labs and simulations. The third lab is a simulation lab only.

Lessons 1 and 2: Simple Diffusion Students begin to explore the role of the cell membrane as the primary mediator of movement of material into and out of the cell. Students compare the properties of passive and active transport including the role of concentration gradients and energy requirements. The wet lab introduces students to the relationship between rates of diffusion and temperature. Simulab 1 reinforces the wet lab and challenges students to design an experiment to test the relationship between molecular mass and rates of diffusion.

Sample Data from SimuLab 1: Diffusion Temperature vs Diffusion time. As temperature increases the rate of diffusion increases.

Sample Data from SimuLab 1 Diffusion Diffusion time vs Particle mass. As mass increases the diffusion time decreases. Advanced student can use this to explore the relationship among mass, velocity, and kinetic energy.

SimuLab 1: Diffusion, Sample Data

Lessons 3 and 4: Osmosis Students study the properties of osmosis as a special form of diffusion involving water moving across a selectively permeable membrane. Students conduct the traditional thistle tube osmosis wet lab predicting the direction of movement of sugar, water, and starch, and relate their observations to osmosis. The wet lab is followed by SimuLab 2 in which student can explore the relationship between cellular protein concentration and the influx of water into the cell and the relationship between osmosis and cell size.

SimuLab 2: Osmosis, sample data Varying concentrations of “proteins” in the cell from 0 to 150 molecules As concentration increases the maximum radius of the cell increases as osmosis brings water into the cell.

SimuLab 2: Osmosis, Sample data Initial concentration of protein in cell vs the maximum radius of the cell. As protein concentration increases the maximum size the cell reaches increases.

Lesson 6 and 7: Membrane Structure Students begin to study the molecular structure of the cell membrane and relate this structure to the mechanisms of passive and active transport. SimuLab 3 provides students with a simple visualization of the formation of lipid bilayers.