Relationship Between Molecular Size/ Solute Permeability and the Movement of Water

Data

Abstract

The purpose of our experiment is to explore the relationship between molecular size/ solute permeability and the movement of water into and out of plant cells. To do this we will determine the concentration at which the solute and the cellular concentration of the solute in the cell are equal: the iso-osmotic concentration or the point where 50% of the cells are plasmolyzed. In this experiment we tested the isosmotic point, the point of 50% cell plasmolysis, of Elodea leaf cells at different concentrations of NaCl and KCl to determine which solute has the higher isosmotic point. It is hypothesized that KCl will have a higher isosmotic point then NaCl; therefore, a higher concentration of KCl will be required to plasmolyze half of the cells than concentration of NaCl. One drop of each solute at each concentration being tested was added to a sample of Elodea leaf and let sit. After three minutes, each slide was placed under a light microscope, focused, and analyzed. The number of plasmolyzed cells per 100 cells were counted and recorded (Table 1.0 and 2.0). The percent treatment mean and standard deviation were then calculated for each concentration, and all data was plotted. After much analysis, our hypothesis was seen to be correct, for the isosmotic point of KCl was plotted to be higher than that of NaCl. Our results, therefore, suggest the conclusion that there is naturally more KCl present in Elodea Leaf cells than NaCl.

Introduction

Cells must regulate what molecules enter and exit across the cell membrane. The cell membrane is selectively permeable. Water and some small molecules (depending on their properties) are able to diffuse passively across the membrane. Other molecules require transport through channels or pumps, often requiring an input of energy. The cell membrane is composed of a phospholipid bilayer which provides structure to organelles and cells. Water is critical to life on planet Earth. A cell is composed of 70% water. Water can move in and out of cells through passive diffusion in a process called osmosis. Osmosis is passive movement of water from a region of high solute concentration to lower solute concentration. When placed in a hypertonic environment, water flows out of the cell and the cell shrinks. When placed in a hypotonic environment, water flows into the cell and the cell swells. When using the term hyper- or hypotonic, the reference frame is the cell itself. If a cell is in a hypertonic environment, the solute concentration outside of the cell is higher than the solute concentration within the cell. If the cell is in a hypotonic environment, the solute concentration inside the cell is higher than outside. In an isotonic environment, the solute concentration within the cell and outside the cell is equal. This is the iso-osmotic point. Plant cells are surrounded by a rigid cell wall. When placed in a hypertonic environment, water flows out of the plant cell and the plasma membrane shrinks and pulls away from the cell wall. This phenomenon is referred to as plasmolysis.

Materials and Methods

Materials:

Elodea Leaf

Micro Forceps

Glass slides

Light Microscope

Cover Slips

Kimwipes

Droppers

Pipettor (P20, P200, P1000)

Pipettor Tips

Cell Counters

1 M KCl

1 M NaCl

Eppendorf Tubes (1.5 mL)

Distilled Water

Gloves

Methods:

• 1 mL dilution- calculate the volume needed of the 2 M concentration of KCl to make desired concentration of KCl (C₁V₁=C₂V₂)
  • Determine the amount of water that needs to be added
• Experiment the plasmolysis of the Elodea cells with water. This is the negative control group; positive control group- 2M concentration of KCl
• Dilutions of 0.2, 0.5, 1.0, and 1.5 M KCl
  • Use a P1000 pipettor to pour the necessary amount of water previously calculated and add in the 2M concentration of KCl respectively
• Obtain three samples of Elodea leaf and separate them onto three individual glass slides
• Put one drop of one of the dilutions onto each sample of Elodea leaf and cover it with a cover slip
  • Make sure this is done quickly so the Elodea leaf does not dry out!
• After three minutes, observe the plasmolysis of the cells of each sample under a microscope and take a picture
  • Three replicates for each treatment
• Repeat this process with the remaining four concentrations of KCl
• Collecting data – roughly count the number of plasmolyzed cells under each concentration
• Perform the experiment a second time with NaCl
  • Use the new data to compare the isosmotic point of both solutes

Results

The graph represents the mean (average) percentage of plasmolyzed cells at varying concentrations of NaCl and KCl. Three replicates were used for each average point plotted (n=3). The error bars represent percent ±STDEV (Standard Deviation). The isosmotic point occurs at 50% cell plasmolysis which is at a concentration of 0.6 M for NaCl and 0.75 M for KCl. Results are statistically significant with a p value less than 0.05.

Conclusion

The results obtained show that the isosmotic point of KCl is higher than that of NaCl. This can be seen by the graph since 50% cell plasmolysis occurs at a concentration of 0.6 M for NaCl and 0.75 M for KCl. A group of Elodea leaf cells require a higher concentration of KCl to become 50% plasmolyzed because there is more KCl already inside the cells than there is NaCl. Therefore, the isosmotic point of Elodea leaf cells is different for different solutes because it depends on how much of that solute is already in the cell. This experiment was designed to calculate the Iso-osmotic point in various solutions and explain the direction of water flow into and out of a plant cell under various conditions. Our data highlights the selective permeable membrane of plant cells and the homeostatic regulation of how much water is allowed to enter and/ or exit the cell based on environmental conditions.

References

• Alberts, B., Bray, D., Lewis, J., Raff, M., Roberts, K., and Watson, James D. (1994) Molecular Biology of the Cell (3rd edition). P 90, 478-485.
• Bennethum, Todd M. (1992) Membrane Permeability: A Quantitative Approach. Page 212 in Tested Studies for Laboratory Teaching, Volume 14 (Cory A. Goldman, Editor). Proceedings of the 14th Conference of the Association for Biology Laboratory Education (ABLE), 240 pages.