Lab 3: Diffusion and Osmosis

```Major Lab: Osmosis
Prelab Due 10/27(A) or 10/28 (B)
Postlab Due 11/2 (A) or 11/3 (B)
In this experiment gummy bears will be used to investigate the relationship between solute concentration
and the movement of water through a selectively permeable membrane by the process of osmosis. Gummy
bears will model living organisms.
Ursus sucrosidae*, commonly known as the gummy bear, is a large single-celled aquatic protist.
Microbiologists are currently studying this organism to determine the appropriate environment for Ursus
sucrosidae, based on its osmotic tendencies. If the bear takes in too much water or releases too much water
(indicated by a percent change in mass), it will die. The best environment for Ursus sucrosidae will be the one
in which it experiences the least change, and thus, is best able to maintain homeostasis. Possible
environments for Ursus sucrosidae include fresh water, 10% salt solution, 20% salt solution, and 30% salt
solution.
*Not a real organism.
In PRE-LAB section of LAB NOTEBOOK:
Purpose
General hypothesis (include term “osmosis”)
Independent Variable
Dependent Variable
Control
Constants
Materials
fresh water
gummy bears
salt water solutions
four plastic cups
balance
Procedure
Part 1:
1) Label four cups freshwater, 10% salt, 20% salt, 30% salt.
2) Pour ~50ml of each solution into appropriately labeled cup.
3) Obtain 4 gummy bears. Determine the mass of each bear and record the initial mass in the data
table. Be sure bear massed was placed in appropriate cup of solution.
Part 2:
1) Remove the gummy bears from each cup. Be careful not to mix them up.
2) Determine the final mass and record in Data Table: Osmosis Results
3) Calculate the percent change using the % change equation.
Percent change = (final mass – initial mass)
100
Data Table: Osmosis Results
Initial mass (g)
Solute
(g/mL)
Final Mass (g)
% Change
(+ if bear got
bigger, - if bear
got smaller)
Class Average %
Change
(graph this data)
Fresh water
(0.0 g/mL)
10% salt water
(0.1 g/mL)
20% salt water
(0.2 g/mL)
30% salt water
(0.3 g/mL)
POST LAB
Analyzing and Interpreting Results:
1. Complete data tables. Include qualitative data.
2. Use equation to calculate % change. Show work (numbers plugged into formula.)
3. Graph osmosis results for the class average. Since some percent change is positive and some is negative,
the 0 axis line should be in the middle of the graph.
Questions and Answers: (WRITE THE QUESTION AND THEN THE ANSWER IN YOUR LAB NOTEBOOK. Ex: Q1:
Determine the concentration… A1: *****…)
1. Determine the concentration of sugar within a gummy bear. To find this, draw a straight line on the
graph that best fits your data. The point at which this line crosses the x axis represents the point at
which no water would move in or out of the bear (0% change).
2. Which solution(s) caused water to move into the bear by osmosis? Why?
3. Which solution(s) caused water to move out of the bear by osmosis? Why?
4. To avoid the fate of the bloated gummy bear, some unicellular freshwater protists have a special
organelle to help remove water that has entered the cell by osmosis. What is this organelle called and
what is an example of a protist that has one?
5. Fish eggs are water permeable. What would happen to a freshwater fish egg placed in salt water?
Why?
Discussion: (follow lab instructions – worth 20 points!)
Experimental Error:
Going Further:
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