Study of Osmosis in Living Plant Tissue
Objective: To study osmosis in living plant tissue. Biological principles: Water potential is the tendency for water molecules to enter or leave a system by osmosis through a differentially permeable membrane, while osmosis is the process in which water molecules move passively from a region of higher water potential to a region of lower water potential through a differentially permeable membrane. Therefore, potato cells, with their differentially permeable membrane, are selected as the medium for studying osmosis in this experiment.
In this experiment, strips are cut from the same potato and placed in sucrose solutions with different concentrations to see if there are any changes in term of weight. Different concentration of sucrose solution is the independent variable while the volume and length of each potato strip and the amount of sucrose solution used are the control variables which should be kept constant. The dependent variable is the percentage change in weight of the potato strips. In addition, it is assumed that the nutrient content of the potato is evenly distributed and all the potato cells are fresh and alive.
Apparatus and materials: Refer to the lab menu Procedure: Refer to lab menu Discussion: The potato strips put in distilled water became heavier. This was because distilled water was hypotonic to the potato tissue, so that there was a net movement of water into the cell by osmosis. The weight of potato strips put in 10% sucrose solution increased only slightly because 10% sucrose solution was also hypotonic to the potato tissue, however the water potential difference between the sucrose solution and potato tissue was small. The potato strips put in 20% sucrose solution was hypertonic to the potato tissue.
Therefore, more water molecules leaving the tissue resulted in a net movement of water out of the cell by osmosis. The accuracy of the result might be reduced by the errors made during the experiment. For example, the impermeable potato skin might not be totally peeled off which reduced the surface area and hence the efficiency of osmosis taking place on the potato strips. Besides, evaporation of water from the beaker set-up decreased water potential of the solution. Moreover, the reading of the potato weight from the electronic balance might not be precise enough to show the small difference in weight between different strips.
Several improvements can be made to reduce the influence of the errors. First, it should be ensured that all skin is peeled off from the potato before immersion. Secondary, a plastic food wrap should be used to cover the beaker in order to prevent water vapour from escaping from the set-up. Thirdly, for the error caused by electronic balance, it is suggested to use a more accurate balance with more decimal places so as to measure the weight more accurately. Conclusion: Living plant tissues are differentially permeable. When living plant tissues are placed in solutions with different water potential, osmosis takes place.