Osmosis
Today’s Lab:
A. Two experiments:
1. Sucrose Osmosis
2. Osmosis in green algae
B. Based on these experiments, write a complete
lab report. Include all sections of a scientific
paper (i.e. title, introduction, methods, results,
discussion, and conclusion). It will be 4-5 pgs.
C. Due Monday, February 19, 2007
Sucrose Osmosis –
Osmosis in a non-biological membrane
Purpose: to determine an unknown concentration of
sucrose
Hypothesis (two choices):
Sucrose Osmosis: Getting Started
a. Form 4 groups with 4 people in each and 1 with 3 people:
Group 1 – 0.2M sucrose
Group 3 – 0.6M sucrose
Group 2 – 0.4M sucrose
Group 4 – 0.8M sucrose
Group 5 – 1.0M sucrose
b. Each group will prepare 2 bags using: 2 - 6” pieces of dialysis
tubing.
c. Soak each piece of tubing in DI water to soften it. Tie knot at
one end of tube with string. Fill tubing with ~ 30ml of
UNKNOWN Sucrose solution. Tie other end of each bag with a
string. Attach a paperclip to end of one bag. This is Bag 1. The
other is Bag 2.
Sucrose Osmosis: Continued
d. Pat dry each bag with paper towels. Weigh each bag separately.
Turn on balance. Open side door and place a weighboat on the
balance. Press the ‘Tare’ button (indicated by a “T”). Once zero
is displayed, open door and place Bag 1 on the weighboat. Close
side door. Record starting mass (in grams), in your notebook for
Bag 1, to two decimal places (ex. 23.36 g). Repeat for Bag 2.
e. Place both bags into your group’s assigned sucrose solution.
Record the start time. Let stand for 1 hour.
f. After 1 hour, remove both bags from the beaker. Pat dry with
paper towels. Re-weigh each bag, remembering to tare the
balance with the weighboat on it. Record the ending mass (in
grams) in your notebook for each bag, to two decimal places.
g. Enter all values for both bags on the Excel spreadsheet as
indicated.
Osmosis in Green Algae –
Osmosis in a biological membrane
Purpose:
to determine if two intertidal algae are euryhaline
for a large salinity gradient
Hypothesis:
Osmosis in Green Algae
a. In same groups:
Group 1 – 0 ppt
Group 2 – 15 ppt
Group 3 – 35 ppt
Group 4 – 44 ppt
Group 5 – 65 ppt
b. Each group places 1 ball of Valonia sp. (bubble-shaped alga) and 23 strands of Cladophora sp. (hair-like alga), in individual petridishes. Fill petri-dish with saltwater from the 34 ppt beaker to
cover algae.
c. Using a dissecting microscope, make careful, initial observations of
each algae under the average ocean salinity (i.e. 34 ppt). Describe
characteristics, including color, shape, and size. Draw an
illustration in your notebook of each sample. Do not leave algae
samples under the microscope lights for more than 10 minutes
at a time.
Osmosis in Green Algae
d.
Pour off the water in your sample dishes. Replace with your
group’s assigned salinity. Record start time. Let stand for 1
hour. Do NOT leave the microscope light on.
e.
After 1 hour, using a dissecting microscope, make careful,
final observations of each algae. Again describe
characteristics, such as color, shape, and size. Draw an
illustration in your notebook of each sample.
f.
Record your qualitative observations for your samples on the
table on the board at the front of the room. Copy this table
with all initial and final observations into your notebook.
Salinity
- Water is a universal solvent. (i.e. dissolves substances)
Salinity:
- Total amount of salt dissolved in seawater.
Salinity refers to the number of grams of inorganic salts
(i.e. NaCl) dissolved in one kilogram (1000 g) of water.
Unit: g/kg = parts per thousand (ppt)
What is the average salinity of the oceans?
Diffusion
- Movement of molecules or ions from a region of
high concentration to one of low concentration,
until they are evenly distributed.
e.g. an open
bottle of
perfume in a
room.
Diffusion
Figure 4.12
Video
Diffusion
Diffusion across a biological membrane:
- Passive transport
Diffusion against a concentration gradient; from low
concentration to high concentration:
- Active transport
Osmosis
Definition:
Diffusion of WATER molecules across semi-permeable
membranes (e.g. cell membranes) until water
concentrations are equal on both sides of the membrane.
Movement is from a higher [H2O] to lower [H2O]
Osmosis
Osmosis
Why is osmosis important for life in the ocean?
Osmosis is the physical process where WATER passes
through a semi-permeable membrane that separates 2 fluids
with different SALT concentrations.
WATER is moving from an area of:
Higher [water] and lower [salt]
higher [salt]
Video
lower [water] and
Osmosis Terms:
Hyperosmotic solution (hypertonic)
a solution with a greater solute concentration
(i.e. more salt ions) than another
Hypoosmotic solution (hypotonic)
a solution with a lesser solute concentration
(i.e. fewer salt ions) than another
Isoosmotic solutions (isotonic):
solutions of equal solute concentrations (i.e.
same number of salt ions)
Osmosis
Isoosmotic
Hyperosmotic
Hypoosmotic
Figure 4.13
Hyperosmotic
Related Terms
Euryhaline:
Organism that is able to withstand large changes in salinity
- no change in cell morphology (e.g. color, shape, or size)
Stenohaline:
Organism that is not able to withstand changes in salinity
- changes in cell morphology (e.g. color, shape, or size)
Related Terms

Osmoregulators:
Organisms that can maintain a constant internal salinity
despite external changes in salinity. e.g. fish

Osmoconformers:
Organisms that change their internal salinity along with
the external environment. e.g. invertebrates
Osmoregulators
Figure 4.14
Writing A Scientific Paper

Title:
Used to get readers attention – concise and focused

Intro:
Sets the stage for the study, and hooks reader
Orients the reader – go from general to specific
Explains the importance of the study – purpose and
hypotheses

Methods:
Includes info so that study can be repeated
What measurements were made and why
Writing A Scientific Paper

Results:
Summarize and illustrate your findings
Do not interpret data, just report
Number Figures and Legends

Discussion:
Interpret your results
Don’t over explain
Convey confidence and authority

Conclusion:
What would you differently and why?
Sucrose Osmosis: Data presentation
a. You will record the data in a table in Excel.
[Sucrose]
0.2M
0.4M
0.6M
0.8M
1.0M
Initial
Weight (t=0)
g
Bag #
Final
Weight
(t=1hr) g
Abs.
Weight
change % Weight
g
change
1
2
1
2
1
2
1
2
1
2
b. To calculate Absolute Weight change (g):
Final Weight (t=1hr) – Initial Weight (t=0)
c. To calculate % Weight Change:
Absolute Weight Change x 100
Initial Weight
Sucrose Osmosis: Data presentation
d. Follow the instructions on the handout to construct a graph. You will
use this graph to determine the concentration of the Unknown Solution.
Include the graph in your report.
20
15
y = -22.755x + 16.655
2
R = 0.9381
% Weight Change
10
5
0
0.0
0.2
0.4
0.6
-5
-10
Sucrose Concentration (M)
0.8
1.0
1.2
Sucrose Osmosis:
Unknown Solution concentration
e. To calculate the concentration of the Unknown Solution,
use the equation generated from the trendline on the
graph, in the example, y = -22.755x + 16.655.
f. Set y=0 and solve the equation for x. The result will be
the concentration of the Unknown Solution. It will be a
positive number between 0 M and 1.0M.
Osmosis in Green Algae

Observations