This activity best suits students with the ability to use a graphing calculator and a CBL interface. They should understand the basic concepts as presented in lecture and through the use of at least one other diffusion and osmosis laboratory.

Science often advances with the introduction of new technologies. Solving technological problems often results in new scientific knowledge. New technologies often extend the current levels of scientific understanding and introduce new areas of research. Cells have particular structures that underlie their functions. Every cell is surrounded by a membrane that separates it from the outside world. Inside the cell is a concentrated mixture of thousands of different molecules which form a variety of specialized structures that carry out such cell functions as energy production, transport of molecules, waste disposal, synthesis of new molecules, and the storage of genetic material.

Graphing Calculator Linear Regression knowledge Spreadsheet Graphing

1. Potatoes need to be bored with a 1/2 - 1 inch piece of metal pipe. Standard cork borers will not be large enough. 2. Rinse out the potatoes several times with tap water to remove all starch. 3. Let the sucrose solutions equilibrate for at least 5 minutes. 4. Connect the distilled water potato to Channel 1, the 0.5M sucrose to Channel 2, and the 1.0M sucrose to Channel 3. This ensures that the data is entered into the spreadsheet in that order so no sorting is needed. 5. Make sure that the stopper is firmly seated in the potato and that the solution is in the pipette but not the tygon tubing before turning the blue valve and beginning the data collection.

Students can be given data to enter into a graphing calculator or graphical analysis program.

WATER POTENTIAL IN POTATO CELLS-OSMOTIC PRESSURE Introduction: Our focus in this lab will be on the diffusion of materials across a potato cell plasma membrane. Specifically we will examine the diffusion of water across the membrane-a process called osmosis. Understanding osmosis is central to understanding a number of physiological phenomena including the translocation of materials within plants, adaptations of unicellular and multicellular organisms living in freshwater and marine environments and the functioning of the human kidney. A typical cellular membrane is permeable to water. Many solutes will not pass through the membrane as freely as water. It is the presence of these impassable solutes that creates some interesting osmotic situations. When a membrane separates two water solutions, the relative concentration of solute (and thus the relative concentration of water) will mostly determine whether or not osmosis occurs. Let us consider a case in which a plant cell is placed in pure water. Now a cell contains much water and in that water one finds a number of dissolved particles that cannot exit through the membrane. Thus water diffuses into the cell by osmosis. If we assume for a moment that no solutes whatsoever can leave, then water should continue to enter the cell until there is virtually no more water outside the cell. After all, the concentration of water in 100% water will always be higher than the concentration of water in the cell. But, after a few moments the rigid cell wall puts a halt to the influx of water. In other words, the tendency of water to diffuse in is balanced by the pressure being built up within the cell and a dynamic equilibrium is reached! Osmosis stops. The tendency of water to leave one place in favor of another place is called water potential (symbolized by the Greek letter psi). Water potential has two main components-osmotic potential which depends on the amount of solute present and pressure which depends on the pressure exerted. We express the relationship as: water potential equals the sum of the osmotic potential and the pressure potential. Water will move from a region of higher water potential (higher free energy) to a region of lower water potential (lower free energy). Let us return to our hypothetical cell. Initially the osmotic potential outside the cell was high (in other words, there was very little, actually no solute). Initially the osmotic potential inside the cell was much lower (there was more solute). The pressure inside and outside the cell was similar. But due to the high external osmotic potential, the water potential outside the cell was higher than the water potential inside the cell. As water moves into the cell by osmosis, the pressure potential inside the cell becomes higher. Even though the osmotic potential inside the cell can never equal that outside the cell, the high internal pressure potential offsets the high external osmotic potential. After a certain amount of water had entered the cell the water potential inside equaled the water potential outside the cell and there was no net movement of water-which is why osmosis stopped. Purpose: To determine the water potential of potato cells. Materials: TI-83 or TI-83 Plus w/CHEMBIO 3 potatoes CBL w/link cable Large cork borer 3 Biogas probes w/DIN adapters Scalpel Sucrose Solutions (0.0, 0.5, 1.0 M) 3 250-ml beakers 3 rubber stoppers w/glass tube Tygon tubing Procedure: 1. Refer to the diagram below as you set up the apparatus. Tygon tubing to Biogas Sensor Inverted Pipette One Hole Rubber Stopper Sucrose Solution Potato 2. With a twisting motion, insert the cork borer completely into the long end of a potato. Insert the sharp edge of a scalpel into the potato, through the back of the cork borer. Twist both the cork borer and the scalpel

TI-83(Plus) Calculator CBL Interface