Biology Lab - Estimating Osmolarity by Change in Volume of Potato Tuber Tissue

1. Abstract

        In this experiment we are trying to find at what osmolarity of sucrose solution is isotonic with a potato cylinder.  If diffusion will not occur between the potato and sucrose solution at the same osmolarity then there will be a low percentage change in weight and volume between initial and final volume of the potato cylinder.  We used calipers to measure the diameter and length of each potato cylinder pre and post incubation in their given sucrose solution.  The cylinders were made by using a cork borer and were undamaged and at least five cm in length.  Cylinders were incubated for an hour stirring each beaker every 15 minutes.  We found that the inside of a potato was isotonic with a 0.2 sucrose solution which had the lowest percentage in weight and  volume change.  

2. Introduction

        Scientist always need to determine proper water content for normal processes to happen in plants.  Without this, all vital functions will not happen properly from having too much water or by having too little (Morgan & Carter, 2011).  Tonicity is defined as the ability of a solution to cause a cell to lose or gain water (Freeman et al, 2014).   There are three classes of tonicity: Isotonic, hypertonic, and hypotonic (Sperelakis, 2011).  An isotonic solution has the same osmolarity inside and outside of a cell or object, so the object doesn't swell or shrink  (Sperelakis, 2011).   A hypertonic solution, the solution has a higher amount of solutes and water tends to come out of the cell or object by diffusion (Sperelakis, 2011).  A hypotonic solution is the exact opposite with a higher solute concentration on the inside of a cell or object than the outside solution so the object tends to swell (Sperelakis, 2011).    In this experiment we will find the osmolarity of a potato cylinder by measuring its change in volume.  If diffusion will not occur between the potato and sucrose solution at the same osmolarity then there will be a low percentage change in weight and volume between initial and final volume of the potato cylinder.

3. Materials and Methods

        We started by obtaining 100mL of distilled water and 100mL of each sucrose solutions and labeled each beaker appropriately.  Using a cork borer we obtained seven cylinders of potato making sure they were at least 5cm in length and undamaged.  We then cut the potato cylinders to a uniform length of 5cm and then placed them into a covered Petri dish.  Using the caliper we measured the length and diameter of a cylinder to the nearest 0.1mm and then transferred it to the 0M beaker and recorded the length and time.  We then repeated these steps for the other six potato cylinders placing each one in the appropriate incubating solution 0.1M to 0.6M.  Allow the potato cylinders to incubate for 1 hr, making sure to swirl each beaker every 15 minutes.  At the end of the hour record the time each cylinder is removed from its solution and do so in chronological order.  Blot each cylinder dry and use the caliper to measure the length and diameter to the nearest 0.1mm.  When finished be sure to record the data. for each potato cylinder.    

4. Results

        The potato cylinders were 5cm and length and all of them were undamaged and similar in size.  Initial volume for these potato's from smallest osmolarity to the highest were 1046.62, 975.56, 1153.18,1070.77, 1084.46, 1054.63, and 1183.21 respectively (table 1).  Final volume from least osmolarity to highest osmolarity in sucrose solution was 1110.17, 1108.32, 1097.62, 1182.07,927.24, 895.02,  and 778.79 respectively (table 1).  These initial and final volumes were found by the following calculation (Volume of Cylinder= 3.14( diameter/2)2 x length).  Graph 2 shows the relationship between final and initial volumes at varying osmolarities of sucrose solutions (graph 2).   It appears that the least change in volume in a potato was seen at a sucrose osmolarity of 0.2M (graph 3).  Initial and final lengths and diameters of the seven potato cylinders can be found in table and can be compared in graph 1 (table 1)(graph1).

Table 1:  Sucrose Osmolarity and its Effect on Potato Volume