Discussion Topic 2 Osmosis Introduction Osmosis is the net movement of solvent across a semipermeable membrane in the direction of an increasing solute gradient In biological systems the solvent is always water but there are many different solutes such as inorganic ions including sodium and potassium or organic compounds such as glucose Osmosis is a special case of diffusion Diffusion can be simply defined as the net movement of molecules down their concentration gradient from higher to lower and does not necessarily require a membrane Diffusion can be measured by calculating flux In biological systems flux can be defined as the diffusion or transport of molecules across a semipermeable membrane such as a lipid bilayer Several factors influence the rate of flux of a molecule across a semipermeable membrane Some of the key variables influencing the rate of flux are a Concentration difference across the membrane C1 C2 b Membrane thickness x c Temperature d Size charge molecular weight of the diffusing molecule e Lipid solubility of the diffusing molecule if the membrane is a lipid bilayer A relationship referred to as the Fick diffusion equation can be used to calculate flux and it takes into account many of the variables listed above Typical units for diffusion in biology are mols cm 2 s 1 This is micromoles per square centimeter per second The solution of the Fick diffusion equation is called the coefficient of diffusion which is a proportionality constant that takes into account the molecular velocity of the diffusing substances in the mixture e g water and glucose and the concentration gradient for each molecule A critical factor when measuring diffusion is the concentration gradient C1 C2 i e the change in concentration of a solute over some distance The concentration gradient provides the driving force free energy that powers diffusion In biology we often have to consider the rate at which solutes move across the cell membrane Biologists use the variation of Fick s Law given below J PA C I C 2 P the permeability coefficient of the solute across the membrane A membrane area cm2 C1 C2 the concentration gradient across the membrane J or Flux is a measure of the amount of substance that moves through a given surface area per unit of time The permeability coefficient P is a measure of how readily the molecule under consideration diffuses through the membrane For a given solute this depends on both the properties of the membrane and the solute Purpose The purpose of this activity is to directly observe the impact of osmosis on a model cell flexible dialysis tubing filled with solution and to use data collected during the activity to determine the concentration of an unknown sucrose solution Dialysis tubing is an artificial semi permeable membrane typically made of cross linked cellulose fibers Dialysis tubing is characterized by its molecular weight cutoff MWCO MWCO can range from molecular weights of 1000 daltons to 100 000 daltons Note that the MWCO is not discrete but is an estimate of the size of molecules that can pass through the membrane By observing the movement of solvent under different conditions you will better understand the impact osmosis can have on cells exposed to environments with different solute concentrations keeping in mind that organisms can exist in environments ranging from pure upland mountain streams to inland seas such as the Great Salt Lakes in Utah that are more concentrated brines than seawater in the ocean Learning Outcomes By the end of section you will be able to Define diffusion and explain osmosis as a special case of diffusion Predict how the solute concentration of the environment will affect a cell and explain how osmosis accounts for this effect Relate the steepness of the concentration gradient to the extent or rate of osmosis Use data regarding change in mass to determine the concentration of an unknown solution Pre lab Questions Please upload your answers to Canvas by the time your section begins Also the blank space below each question is formatted for this document and is not necessarily representative of the space needed for your answers 1 Read through the entire procedure Which of the variables in the flux equation given above J PA C I C 2 is being manipulated The variables manipulated are C1 C2 which is the concentration gradient across the membrane is manipulated The dialysis tubes are filled with solutions of unknown concentrations and are placed in beakers with sucrose solutions of known concentrations The concentrations will be manipulated between the solution inside the dialysis tubes and solutions in the beakers 2 What do you expect to observe during the lab How might your data be used to determine the solute concentration of the unknown solution in the dialysis tubes I expect to observe changes in the mass of the dialysis tubes after they are placed in the sucrose solutions for 20min If the sucrose concentration inside the tube is lower than the beaker the water solution will move out of the tube making the mass decrease Conversely if the sucrose concentration inside the tube is higher water will move into the tube causing an increase in mass Forceps Paper towels 2 100ml beakers filled with sucrose solutions of 2 of the following concentrations Shared with the other student pair at your table Supply of dialysis tubes filled with the sucrose solution of unknown concentration stored in an isotonic solution Large beaker for disposal of solutions Materials At your station you will find o 0 0M sucrose o 0 25M sucrose o 0 50M sucrose o 0 75M sucrose Scale Plastic weigh boats Available from your TA Graph paper Extra paper towels Procedure 1 Use forceps to obtain a dialysis tube filled with a sucrose solution of unknown concentration from the weighing station at your table Use paper towels to gently blot it dry The goal is to limit any drops of solution on the outside of the tubes from adding mass without excessive manipulation of the tubes 2 Find the mass of the dialysis tube This should be recorded as the initial mass 3 Use forceps to place the dialysis tube into one of the labeled beakers at your station 4 Repeat steps 1 3 for the remaining beaker at your station 5 Allow the dialysis tubes to sit in the beakers for 20 minutes 6 Remove the dialysis tubes from the beakers use forceps and gently blot dry 7 Use the scale to determine the final mass of each dialysis tube record these data 8 Obtain data for the two sucrose