Using Osmosis to Determine the Sucrose Concentration of Potatoes:
Pre-lab questions
Please answer these questions in your lab notebook. Turn in the back (yellow) copy when you come to lab.
1) What is the purpose of this lab?
2) What is a concentration gradient?
3) What is diffusion?
4) What is osmosis?
5) Why do we have to be careful about not squeezing the potato cores?
6) Why do we have to calculate the percent change in mass instead of just using the change in mass itself?

Many biological processes require a chemical concentration gradient to operate. One of these that we will learn about is the conversion of ADP to ATP during the light reactions of photosynthesis. Basically, a concentration gradient is present whenever there is a difference in the concentration of a substance on two sides of a barrier. In the absence of a barrier, substances will naturally tend to reach equilibrium, a state in which they exist in equal concntration throughout the area. Thus, concentration gradients require energy to create and maintain.
Imagine a dam. The water is much higher upstream of the dam than downstream. Energy was required to build and maintain the dam, and the water behind the dam is a source of potential energy that may be converted into potential energy, and then into electrical energy when it is allowed to run downhill and turn the generators. The dam is like a concentration gradient because the upstream water level is higher than the level downstream. In the absence of the dam, the water levels upstream and downstream of the area would not differ by such a large amount.
Diffusion is the movement of a substance down its concentration gradient (from higher to lower concentration). Using the dam example, diffusion would be like springing a leak in the dam. The water would move from the upstream (higher concentration) side of the dam to the downstream (lower concentration) side. Conversely, pumping water in the opposite direction
(against the concentration gradient) would require a lot of energy. Another way to visualize diffusion would be to think about a glass of water. If you put a single drop of food coloring in the glass, it would at first be very concentrated in the area where you placed the drop. Over time, the food coloring would diffuse and spread out evenly throughout the glass until it reached equilibrium.
Osmosis is a special type of diffusion in which water moves across a selectively permeable membrane (a barrier that allows water through, but not other molecules). It turns out that biological membranes such as cell membranes are selectively permeable to water but not to many other substances. If you were to pour water into a beaker, and put a selectively permeable membrane across the middle, dividing the beaker into two sides, then stir a bunch of salt into one side only, you would create a concentration gradient. One side of the beaker would have a high salt concentration and the other would have a zero salt concentration. You might expect that salt would diffuse across the membrane from the high-concentration side to the low-concentration side. However, you would be mistaken because the membrane only permits the passage of water molecules. Therefore, the water will pass from the side with a higher water concentration to the side with a lower water concentration. If this seems confusing, consider that a solution of 5% salt in water consists of 5% salt, and 95% water . A 25% salt solution has 25% salt, and 75% water. If these two solutions were separated by a selectively permeable membrane, water would flow from the side with a 5% salt solution (95% water = high concentration of water) to the side with a 25% salt solution (75% water = lower concentration). In this lab, we will use osmosis to determine the concentration of sucrose (sugar) in a white potato.

Overview:
We will soak potato pieces overnight in a series of solutions of different sucrose concentrations. If the potato has a higher sucrose concentration than the solution (i.e., lower water concentration), water will pass through the (selectively permeable) cell membranes and into the potato cells and the potato pieces will gain mass. If the potato has a lower sucrose concentration than the solution (i.e., higher water concentration), water will leave the potato cells, and the potato pieces will lose mass.
Materials: potato cork borer scalpel
7 plastic cups six different sucrose solutions balance marker plastic wrap paper towels
Methods:
Before you begin, set up a table in your lab notebook that looks like this.
Solution Final mass (g)
0.0M
0.2M
0.4M
0.6M
0.8M
1.0M
Initial mass (g) Change in mass (g) Percent change
1) Label 6 of the cups to correspond with the 6 sucrose concentrations (0.0M, 0.2M, 0.4M,
0.6M, 0.8M, 1.0M). Put your initials on each cup.
2) Choose a potato and cut 3 cores for each of the 6 solutions (18 total) with the borer. Select your boring sites carefully to maximize the number of cores you get from each potato.
3) Slice the skins off the ends of the cores carefully, without squeezing the potato cores . Place the trimmed cores into the 7 th cup.
4) Bring your cores and cups to the balance. For each cup, tare the cup (so the balance reads zero), then place 3 cores into the cup and find their combined mass. Record this mass in the initial mass column.
5) Add approximately 100mL (about 1/3 full) of the appropriate sugar solution to each cup.

6) Cover your cups with plastic wrap and let them sit overnight.
Day two:
7) Remove the plastic wrap from your cups, and carefully drain away the sucrose solution into the container provided.
8) Using forceps (not your fingers) , carefully remove each of the cores and set them on paper towels to dry. Make sure you don’t get them mixed up. Do not roll them or squeeze them in any way!!!
9) Determine the mass of each set of cores. Record these data in the final mass column of your notebook. Next, determine the change in mass (and whether positive or negative), and calculate percent change in mass in the last column.

Title:
Your title must be descriptive of the project. “Diffusion Lab” or “Potato Lab” are not sufficient. You must also include the date of the experiment.
Introduction:
Start by talking about diffusion and osmosis. That is, what are they? How do these concepts apply to biological systems? Then state the question you are trying to answer by doing this lab, and how osmosis or diffusion will help you to answer this question. Then, briefly state how you did the lab.
Materials and methods:
In this section, you must provide the reader with a complete set of instructions for doing this lab. The materials must be included in these instructions. Do not just list the materials and then list a step by step (i.e., 1, 2, 3, …) procedure!
The reader should be able to repeat the experiment exactly, just from what you say in this section.
Results:
This section contains only results . This includes your data in a table (called a table), and a graph. Make sure that you use the data to determine the sucrose concentraton and tell what the concentration is. Put the data in a table, with a title. See instructions below.
To make a data table in Microsoft Word:
1) Go to Table > Insert > Table
2) Select the number of rows and columns (in this case, you want 6 columns and 7 rows).
3) Click OK
To make the graph in Excel:
1) Launch Excel
2) Enter your data as shown below
Molarity Sucrose % Change in Mass
0.0
0.2
0.4
0.6
## (don’t put % sign here)
##
##
##
0.8
1.0
##
##
3) With the mouse, click and drag to highlight all the data, including the headings.

4) Click on the chart icon (this is the icon that looks like a little bar graph), or, go to Insert >
Chart.
5) Select as the chart type, XY (scatter), then select the one with a no line connecting the points.
Click next.
6) Click next again. In this window, you can add your title, and your X-axis and Y-axis labels.
You can delete the legend, as long as your X and Y-axes are properly labelled. Your title should begin Figure 1:
7) Click next, and save the chart as a new sheet. This will make your graph full page size.
8) On the graph, double click on one of the numbers on the X-axis. You will get a dialog box.
Select the Scale tab, and set the major unit to 0.1 and the minor unit to 0.02. Next, select the
Patterns tab, and click on the box for “minor tick mark type - cross.” Click OK.
9) On the graph, click on one of the data points. Then go to Chart > Add trendline .
10) Under type, select linear. Click OK.
11) On the graph, double click the trendline you just added. You will get a dialog box. Select the options tab, and click “display equation on chart.” Click OK.
Your graph is now done. You can print it out and attach it to the back of the report. For determining the equilibrium point, you can take the equation from the graph, set Y to zero, and solve for X.
Discussion:
Your discussion should start by restating the reasons for doing this lab. Then you should state your conclusion (what the sugar concentration was). Next, state the relevance of your results. That is, state any reasons why we might want to know this.
The discussion is also where you must discuss any possible sources of error. Be careful here. DO NOT tell me that you might have made a mistake in reading the balance or in calculating your results. The sources of error that are important are related to the procedure you followed. For one example, was there anything you did or did not do that might have altered your masses? Is there a possible problem with the procedure itself that might have affected your results? How might you do the lab differently to address this problem?