Using Osmosis to Determine the Sucrose Concentration of Potatoes
Pre-lab questions – Turn in before beginning your data collection.
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?
Introduction:
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 concentration 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.
Procedure:
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 “Sticks” – 2 or 3 per cup
scalpel
seven plastic cups
six different sucrose solutions
balance (use balance at the front of the room)
marker
plastic wrap
paper towels
Methods:
Day One:
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.
1) Choose a potato and cut 2-3 sticks for each of the 6 solutions (12-18 total) with the scalpel.
Select your potato sites carefully to maximize the number of sticks you get from each potato.
2) Slice the skins off the ends of the sticks carefully, without squeezing the potato cores.
Place the trimmed sticks into the 7th cup.
3) Bring your sticks and cups to the balance. For each cup, tare the cup (so the balance reads
zero), then place 2-3 cores into the cup and find their combined mass. Record this mass in
the initial mass column.
4) Add approximately 75mL (about 1/3 full) of the appropriate sugar solution to each cup.
5) Cover your cups with plastic wrap and let them sit overnight.
Day two:
6) Remove the plastic wrap from your cups, and carefully drain away the sucrose solution into
the container provided.
7) Using forceps (not your fingers), carefully remove each of the sticks 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!!!
8) Determine the mass of each set of sticks. Record these data in the final mass column of your
Individual Data Table.
9) Calculate the change in mass (and include whether positive or negative)
10) Calculate the percent change in mass in the last column.
Individual Data:
Solution
Initial Mass (g)
Final Mass (g)
Change in Mass
(g)
Percent Change
in Mass(g)
0.0 M
0.2 M
0.4 M
0.6 M
0.8 M
1.0 M
Class Data:
Group
0.0 M
0.2 M
0.4 M
0.6 M
0.8 M
1.0 M
Average
Honors Enrichment Project
We will be completing a full lab report on this lab, including data tables, graphs, calculations, data
analysis and conclusions. We will be completing the report one piece at a time – with each segment
due as outlined below.
Due Date
Assignment Due
Due: Methods
Due: Data Tables
Due: Calculations
Due: Introduction
Due: Conclusion
Due: Title Page
Due: Final Lab Report