Diffusion and Osmosis: Lab Report on Molecular Transport

Introduction to Diffusion

The core purpose of the first experiment was to study the principle of diffusion. The experiment aimed at analyzing the movement of molecules from a region of higher concentration toward a region of lower concentration. Potassium permanganate crystals were dissolved in water to observe the process of diffusion. The observations and results attained from the experiment indicated molecular movement driven by molecular collisions, which occur more frequently in areas of higher concentration.

Molecules present in a liquid or a gas move constantly in random directions. During their movement, they collide with each other and bounce in various directions. This random movement ultimately results in the uniform distribution of molecules throughout the available space. The tendency of molecules to spread out evenly is referred to as diffusion. The process of diffusion does not require any cellular energy; molecules diffuse due to their own natural movement, which is called Brownian motion. Since diffusion relies only on kinetic energy derived from the environment, it is classified as passive transport (Lauren et al., 2010).

The movement of molecules always occurs from a region of higher concentration toward a region of lower concentration. The difference in concentration between two regions before diffusion occurs is called the concentration gradient. When a substance is present at higher concentration on one side of a membrane, the molecules of that substance tend to move across the membrane down the concentration gradient. Figure 1 illustrates the principle of diffusion, showing molecules migrating gradually from a high-concentration area toward a low-concentration area.

Figure 1: Diffusion of molecules from a higher-concentration area to a lower-concentration area.

The experiment aimed at observing the process of diffusion and the net movement of molecules from a region of higher concentration toward lower concentration, using potassium permanganate crystals dissolved in water. The hypothesis was that permanganate crystals would diffuse in water and change its color. Observations indicated the transfer of kinetic energy among molecules of the solute and solvent, showing how molecules spread evenly throughout the water. The beaker containing potassium permanganate was observed for several minutes without disturbance in order to follow the random motion of molecules and their diffusion into water.

Diffusion Experiment: Results and Observations

Potassium permanganate crystals are dark purple in color. When placed in water, the purple color of the permanganate began to spread gradually. The water was observed continuously without disturbance in order to track the gradual diffusion of permanganate crystals into water. After some time, the crystals spread evenly throughout the water, turning it a uniformly light purple color.

Figure 2(a) shows the initial stage when potassium permanganate crystals were placed in water; the crystals can be seen concentrated on one side of the beaker in dark purple. Figure 2(b) represents the intermediate stage in which potassium permanganate crystals have diffused into the water, imparting a light purple color, while some concentrated crystals remain visible at the bottom of the beaker — the stage at which molecules had begun moving down their concentration gradient. Figure 2(c) shows the final stage, in which permanganate molecules have spread evenly across the entire volume of water in the beaker, and no concentration gradient remains.

Figure 2(a): High concentration gradient existing between KMnO₄ crystals and water.
Figure 2(b): Movement of KMnO₄ molecules into water (diffusion in progress).
Figure 2(c): Equilibrium reached — no concentration gradient exists.

Diffusion Experiment: Discussion and Conclusion

In the experiment, the crystals of potassium permanganate contained tightly packed, highly concentrated molecules. The surrounding water contained no permanganate molecules. As a result, permanganate molecules moved from the crystals into the water until they distributed themselves evenly throughout (Mitchelmore & Roberts, 1985).

Each permanganate molecule transfers its kinetic energy to adjacent molecules, causing shifts in the velocities of both. The net result is that each molecule appears to move in a random fashion. The entire population of molecules, initially concentrated in a small area, moves collectively from the region of high concentration to the region of low concentration. A net movement of permanganate molecules occurs, carrying them away from the crystal and into the water. This confirms that diffusion took place and that the concentration gradient drove the flow of molecules from a region of higher concentration toward lower concentration.

The experiment successfully demonstrated the principle of diffusion. Potassium permanganate crystals turned the entire volume of water a light purple color, indicating the even distribution of molecules throughout the solution. This result confirmed the hypothesis that permanganate would diffuse in water and change its color. The experiment also showed that diffusion occurs whenever a concentration gradient exists between two regions, and that molecules will move from the region of higher concentration toward the region of lower concentration until equilibrium is reached.

Introduction to Osmosis

Osmosis is a special case of diffusion, defined as the net movement of water through a semi-permeable membrane (Mitchelmore & Roberts, 1985). This means that osmosis involves the flow of water or any fluid from an area of higher water concentration toward an area of lower water concentration.

Osmosis depends greatly on the concentration of solute present in a solution (Science Encyclopedia, 2011). Hypertonic solutions have a higher concentration of solute than the solvent. Hypotonic solutions have a lower concentration of solute relative to the solvent, while isotonic solutions have equal solute and solvent concentrations.

The objective of this experiment was to study the impact of solution concentration on osmosis by observing the mass of decalcified eggs after placing them in four different solutions with varying concentrations of glucose and water. The solutions used were water, 0.5M, 1.5M, and 2.0M glucose solutions. The hypothesis was that the size of the egg would remain the same in water, increase in 0.5M solution, decrease in 1.5M solution, and decrease in 2.0M solution. It was observed that the mass of the egg placed in each of the four beakers varied in accordance with the concentration of glucose present in the solution.

References

Jeff, Sack. "Osmosis and Diffusion." American Biology Teacher, 67.5 (2005), pp. 3–11.

Lauren, M., Victor, S., & Lindsay, M. "Exploring Osmosis and Diffusion in Cells: A Guided-Inquiry Activity for Biology Classes, Developed through the Lesson-Study Process." Science Teacher, 77.8 (2010), pp. 55–60.

Mitchelmore, J., & Roberts, M. Biology for CXC. London: Thomas Nelson and Sons Ltd., 1985.

Science Encyclopedia. "Cell Membrane Transport," 2011.