The salinity and osmolarity of the environment place significant constraints upon organisms, requiring a range of regulatory responses. It is largely through their effects on the intra-cellular environment that salinity and osmolarity produce their effects. In today’s lab you will measure the responses of cells to
solutions that have a range of osmolarities. Two helpful rules when considering the effects of osmolarity on cells are: 1.  When a cell is exposed to a given solution, water and permeable solutes will move across the cell membrane and the system will arrive at a new equilibrium. At equilibrium, the total concentration of solute particles in the cell and the solution will be equal, i.e. the osmolarities will be the same. (This does not mean that particles stop moving, only that there is no net movement across the cell membrane.) 2.  In general osmolarities of cells and solutions are stated before they are put together because at equilibrium, the osmolarities of these two solutions will be the same. Tonicity is defined according to a cell’s behavior in those solutions of varying osmolarity. The
tonicity of a solution refers to whether a cell will shrink, expand or keep the same volume in that solution. If the cell volume remains unchanged when the cell is placed in that solution, the solution is referred to as isotonic. If the cell volume shrinks in the solution, the solution is referred to as hypertonic. And, if the cell swells, the solution is hypotonic. Tonicity is applied specifically to the cell whereas the terms isosmotic, hyperosmotic and hyposmotic are used to compare solutions.
In this laboratory session you will study how one type of cell, the red blood cell (erythrocyte), behaves when placed into solutions of different osmolarities. If you were to make direct observations of changes in cell volume, you would be able to infer the direction of water flow (into or out of the cell) when the cells are placed into each solution; this would also allow you to determine the concentration of the solution within the cell itself. Such direct observations are difficult to make, so in this lab you will determine how many cells undergo hemolysis by using a colorimeter. This device measures how much
light is transmitted through a solution. When the cells are whole in solution and centrifuged, they go to the bottom of the tube and the liquid above them is clear and therefore readily transmits light. If they burst, their membranes will also go to the bottom of the tube during centrifugation, but the hemoglobin that is released into the solution does not go to the bottom being distribute throughout the solution, causing less light to be transmitted. Therefore, the greater the absorbance of the solution, the greater the degree of cell hemolysis. B. Benchmarks:
1.  Explain the relationship between conductivity and salinity 

2.  Explain a calibration procedure for the conductivity probes 

3.  Define salinity, conductivity, osmolarity, osmolality and absorbance. 

4.  Effectively use several items of laboratory equipment. Specifically: 

a) pipette to accurately dispense a set volume of solution
b) mix solutions with a vortex
c) centrifuge solutions in order to separate them into different components
d) measure the absorbance of a solution with a spectrophotometer, analyzing and converting the absorbance data into % hemolysis of red blood cells e) Explain the relationship between extracellular solution concentration and cell volume, or between
extracellular solution concentration and amount of hemolysis.