Comparative Analysis of Diffusion and Osmosis Rates Under Varying Conditions

Abstract

One of the main tasks of a cell is to balance the water and molecules in the cell. This experiment studied the effects of diffusion and osmosis under varying conditions. The diffusion experiment examined two dyes of different molecular weights at different temperatures. The diffusion rates were measured and the results were recorded during 15-minute intervals for 45 minutes. The osmosis experiment examined the diffusion rates of molasses bags submerged in water of varying temperatures. The results were recorded during 5-minute intervals for 20 minutes.

The diffusion rates of were compared and contrasted. The results showed that small molecules diffused at a faster rate and at higher temperatures than large molecules. They also showed that water diffused at a faster rate at higher temperatures.

Introduction

Diffusion is the passage of solute molecules from an area of high concentration to an area of low concentration (Fowler 2005-2006, p. 111). An example is ammonia diffusing throughout a room. A solute is one of two components in a chemical solution. The solute is the substance dissolved in the solution. The solvent, the other component, is any liquid in which the solute can be dissolved (Mosby's, 2002 p.1602).

The rate of diffusion is affected by temperature and molecular size. Diffusion requires little or no energy because molecules are always randomly moving; this is due to their kinetic energy. The rate of kinetic energy, energy utilized by an object by virtue of its motion, is dependent on temperature (Mosby's, 2002 p.957). Temperature measures the movement of molecules in a solution or solid. Molecules will move faster at higher temperatures, which consequently, increases the rate of kinetic energy. Equally important, molecular speed is also dependent on the size of the molecule. Small molecules are faster than larger ones.

Diffusion occurs only when there is an imbalance in the areas of concentration. The difference in concentration of molecules over a distance is known as a concentration gradient (Enger, Ross, and Bailey 2005, p.70). Furthermore, net movement relates to the movement of molecules in one direction minus the movement of molecules in the other direction. Finally, if movement in one direction equals movement in the other direction then dynamic equilibrium occurs (Enger, Ross, and Bailey 2005, p.69).

In addition to solutions there are two other mixtures involved in diffusion. The physical appearance of colloids is best described as "cloudy" or "milky". The solute particles are larger than solutions but they do not settle out (Marieb 2004, p.32). An example is agar. A suspension contains large visible solute particles that will settle out unless some means - mixing, shaking or, in the body, circulation - is used to keep them in suspension (Marieb 2004, p.32). An example is oil and vinegar. The commonality of these three mixtures is their composition of two or more physically intermixed components (Marieb 2004, p.31).

The diffusion of water molecules across a selectively permeable membrane involves a special form of diffusion called osmosis. Water molecules will always move from a hypotonic region (low solute, high solvent) to a hypertonic region (high solute, low solvent). In osmosis, a selectively permeable membrane allows passage of water molecules but prevents the passage of all other molecules. By contrast, a permeable membrane allows passage of most molecules whereas an impermeable membrane prevents passage of most molecules. An example of a selectively permeable membrane is dialysis tubing, which removes toxins from the blood of dialysis patients while returning cleansed blood. The rate of osmosis depends on such variables as the concentration of the solute, the temperature of the solution, and the osmotic pressure exerted on the membrane separating the solution from the solvent.

The purpose of this experiment was to measure the rate of diffusion and osmosis under varying conditions. For the diffusion demonstration, the smaller solute molecule will diffuse at a faster rate than the larger solute molecule. Diffusion will also occur at a faster rate at the higher temperatures. The water Petri dishes were the positive control group and the solutes will completely diffuse. In the osmosis demonstration, the water will diffuse into the molasses bag at a faster rate at the higher temperatures.

Methods and Materials

Diffusion Demonstration:

Materials:

Review Fowler (2005-2006, pg. 111) for the list of materials used in the experiment.

Procedure:

Worked in a group of three. Wrote group name and date on the lids of the agar filled Petri dishes. Then drew 2 circles at opposite ends of the lids. Labeled one circle "P" for Potassium Permanganate and the other "M" for Methylene Blue. Wrote 37oC on one lid, 20oC on another lid, and on the third, "refrigerator".

Placed the three agar filled Petri dishes in their proper locations - in the incubator for 37oC, on the laboratory table for 20oC and in the laboratory refrigerator. Wrote "P" on top of empty Petri dish and "M" on bottom dish. Filled the top and bottom dishes with water and placed them on the laboratory counter (it was very important that dish remained stationary throughout experiment). Waited 10 minutes for Petri dishes to adjust to their environments.

This part was done upside down. Dipped the wooden stick in the Potassium Permanganate crystals, held it so the end of the stick with the dye on it was up, held the Petri dish upside down, and touched the dye covered stick to one side of each of the three agar Petri dishes. Tried to pick up the same amount of crystals each time.

Dipped a different wooden stick in the Methylene Blue crystals, held it so the end of the stick with the dye on it was up, held the Petri dish upside down, and touched the dye covered stick to one side of each of the three agar Petri dishes. Tried to pick up the same amount of Methylene Blue crystals, as done with the Potassium Permanganate crystals.

This part was done right side up. Tried to pick up the same amount of Methylene Blue and Potassium Permanganate crystals and added them to the appropriately labeled water dish.

Placed lids on the agar filled Petri dishes with the dyes positioned in the center of the appropriately labeled circles. Then placed the 37oC dish in the incubator, the 20oC dish on the laboratory table and the "refrigerator" Petri dish in the refrigerator.

Each Petri dish