Diffusion and Osmosis: Investigating the Processes of Diffusion and Osmosis in a Model Membrane System and the Effect of Solute Concentration on Water Potential as it Relates to Living Plant Tissues

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Diffusion and Osmosis: Investigating the Processes of Diffusion and Osmosis in a Model Membrane System and the Effect of Solute Concentration on Water Potential as it Relates to Living Plant Tissues

Mike Karp

Partner: Elizabeth Moran

Mead AP BIO

10/24/04

Abstract

        The purpose of this experiment was to understand the principles of diffusion, osmosis, and water potential. In the first section of this lab, we measured the diffusion of small molecules through dialysis tubing, an example of a selectively permeable membrane. The dialysis tubes were filled with 15% glucose and 1% starch. They were placed into a beaker with H2O and IKI. We analyzed the diffusion of glucose by recording color and presence of glucose in both the bag and the beaker. Next, we studied osmosis by investigating the relationship between solute concentration and the movement of water through a selectively permeable membrane. We were able to do this by filling dialysis tubes with distilled water and different molar concentrations of sucrose. We then placed the tubes into beakers filled with distilled water for 30 minutes. By recording the initial mass, the final mass after taking the tube out of the water, and calculating the mass difference and percent change in mass, we discovered how much osmosis took place. In the final exercise of the lab, we used potato cores placed in different molar concentrations of sucrose in order to determine the water potential of potato cells. This enabled us to explore what is meant by the term “water potential.” We determined the water potential of potato cells by recording and observing the initial mass of four potato cylinders, and then put the cylinders into a sucrose solution that were left overnight. We recorded the final mass the following day, and calculated the mass difference and percent change in mass. The lab was successful because it taught us the principles of diffusion, osmosis, and water potential. Our observations and results concurred with what was expected to happen.

Introduction

        All atoms and molecules have kinetic energy and are constantly in motion. The kinetic energy of the molecules causes them to collide into each other and switch directions. A result of this molecular motion is diffusion.

        Diffusion is the random movement of molecules from an area of higher concentration to an area lower concentration. An example of diffusion is opening a bottle of H2S in a corner of a room. After a few minutes, the opposite side of the room would smell the H2S (the smell of rotten eggs). The reason for this is because the bottle has a higher concentration of H2S molecules than the room does, and the H2S gas diffuses from the area of higher concentration to the area of lower concentration. Eventually, equilibrium will be reached; there will be no net movement of H2S and the concentration of H2S will be equal throughout the room.

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        Osmosis is a special kind of diffusion; it is the diffusion of water through a selectively permeable membrane (a membrane that allows for diffusion of certain solutes and water) from an area of higher water potential to an area of lower water potential. Water potential is the measurement of free energy of water in a solution.

        Diffusion and osmosis do not entirely explain the movement of ions or molecules into and out of cells. Active transport is another property of a living system. The process of active transport uses energy from ATP to move substances through a cell membrane. The ...

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