Diffusion Osmosis Lab
Osmosis
• Osmosis = the movement of water molecules from a
region of higher water potential to a region of lower
water potential through a partially permeable
membrane. Osmosis is considered in terms of water
potential and solute potential.
Water Potential
• Water potential = a measure of the kinetic
energy of water molecules. Here, water
molecules are constantly moving in a
random fashion. Some of them collide with
cell membrane, cell wall, creating a
pressure on known as water potential.
• The higher their kinetic energy the more
they move and hit the membrane, therefore
higher the water potential
Water potential
How does water move?
Why does water move?
1. Downhill
Pressure potential
2. Hose, straw
3. Fresh – salty
4. Sponge
Osmotic/solute potential
Matric potential
Components of Water Potential
1. Pressure potential: pushing (positive pressure, like the hose)
sucking (negative pressure, like a straw)
Major factor moving water through plants
2. Osmotic, or Solute potential: reduction in water potential due
to the presence of dissolved solutes
Dissolved substances dilute pure water, so
salty water has lower water potential (lower concentration) than pure water
Water Potential
• Unit of measurement: megapascals (Mpa)
1 MegaPascal = 10 atm = 145.1 psi
• Water potential for pure water: 0
• Anything that lowers the “free energy”of
water lowers it potential.
-dissolved solutes
Water potential = pressure potential + solute potential
Clarifying Water Potential Values
• (2) Factors to consider:
p = pressure potential (outside & inside)
s = solute potential
system = p + s
SO……
 p results in
“+” value
 p results in “-” value
Water Potential Values
• High water potential (+Value):
- less solute
- more water
- (hypotonic)
• Zero (0) Value:
- Pure water
• Low water potential (-Value):
- More solute
- less water
- (hypertonic)
****Water will move across a membrane in the direction of the lower water potential****
Analysis of the Data Collected
• Mass Difference: Final – initial (absolute diff.)
• % Change in Mass: Final – initial x 100
initial
**Why do we use the % change in mass instead of
simply the straight difference?
• Plot your data on the graph.
• Determine the molar solute concentration of the
potato cores. How???
***Where your line crosses the “0” mark
Calculating Solute Potential
• Variables involved: i, C, R, T
i = ionization constant: NaCl = 2.0 (Na+ & Cl-)
**for sucrose it will be 1.0 (it doesn’t ionize)
C = Molar concentration of your potato (graph)
R= rate constant: 0.0821 L · atm (bar)
mol · K
T = Temperature: K
Calculating the Solute Potential
(s)
• s = - iCRT
• Sample Calc.
A 1.0 M sugar solution @ 22° C under
standard atmospheric conditions:
s = -(1)(1.0mol)(0.0821 L · bar )(295K)
L
s = -24.22 bars
mol · K
Typical Water Potential Values
•
•
•
•
•
•
•
Outside air (50% humidity):
Outside air (90% humidity):
Leaf Tissue:
Stem:
Root:
Soil water:
Hydrated soil (Saturated)
-100 MPa
-13 MPa
-1.5 MPa
-0.7 MPa
-0.4 MPa
-0.1 MPa
+2 - +5 MPa
** When the soil is extremely dry what happens to the water
potential and water movement into the plant?
**Does the value become more negative or more positive?
Water Potential in Plants
•The end!
Water Balance (pg. 117-118)
• Osmoregulation~ control
of water balance
• Hypertonic~ higher
concentration of solutes
• Hypotonic~ lower
concentration of solutes
• Isotonic~ equal
concentrations of solutes
•
•
•
•
Cells with Walls:
Turgid (very firm)
Flaccid (limp)
Plasmolysis~ plasma
membrane pulls away from
cell wall
Dialysis Tubing Experiment
An Artificial Cell
Permeable to: monosaccharides & water
Impermeable to: Disaccharides
Introductory Questions #3 (Lab)
1)
2)
3)
4)
Suppose you have an artificial cell that was permeable to
monosaccharides and impermeable to disaccharides. What
would happen to the cell if it had 0.80 M maltose and 0.85 M
fructose in it and was placed in a solution containing 0.45 M
glucose, 0.65 M fructose, and 0.40 M sucrose.
a) Which direction would the water flow?
b) Which area has a higher water potential?
c) What would happen to the concentration of the maltose
inside the cell (increase, decrease, remain the same)?
What is the ionization constant (i) for sucrose?
Determine the “C” value for your potato cores. (guidesheets)
Graph your results:
% change in mass vs. sucrose molarity within the beakers
(guidesheets)
Key Sections you need for your
Lab
• Title
• Intro/Background: Defining water potential
Importance & Sig. Of the lab
• Hypothesis & reason for your prediction
• ID Experimental Variables
• Materials (diagram & visual of set up optional)
• Procedure
• Data: tables charts & graphs
• Analysis- be thorough
• Conclusion & Evaluation: error & improvements
Introductory Questions # (lab)
1)
2)
3)
4)
5)
6)
7)
Explain how potential energy is different from kinetic energy.
What are some ways we can measure energy?
Define each variable in the equation:
∆G = ∆H – T ∆S
What is the difference between an exergonic reaction and an
endergonic reaction?
How is ATP associated with coupled reactions? What purpose
does it serve?
How is an electron carried from one molecule to the next?
Name a molecule that can carry an electron.
How is Anabolism different from catabolism?
Briefly explain how the first two laws of thermodynamics apply
to a living organism
Lab #3: Water Potential & Osmosis
• Read the Handout provided
• Go to my web site and click on:
“Review of 12 AP Labs”
• Click on the hyperlink shown on the
worksheet
• Choose Lab #1: Diffusion & Osmosis
• Go through the tutorial and READ each
section.
What to have Ready for Tomorrow
• Bring in a Large Potato (one per lab table)
• Have the Pre-lab finished w/quiz
• Write a statement of purpose or reason for doing this
lab
• Materials list - review the handout
• Hypothesis:
• Data Table – review website or others for an idea.
**Need (2): Individual data & class Data
Introductory Questions #3 (Lab)
1)
2)
3)
4)
Suppose you have an artificial cell that was permeable to
monosaccharides and impermeable to disaccharides. What
would happen to the cell if it had 0.80 M maltose and 0.85 M
fructose in it and was placed in a solution containing 0.45 M
glucose, 0.65 M fructose, and 0.40 M sucrose.
a) Which direction would the water flow?
b) Which area has a higher water potential?
c) What would happen to the concentration of the maltose
inside the cell (increase, decrease, remain the same)?
What is the ionization constant (i) for sucrose?
Determine the “C” value for your potato cores. (guidesheets)
Graph your results:
% change in mass vs. sucrose molarity within the beakers
(guidesheets)
Questions to answer from the
Website- Prelab
• How many concepts are there?
• How many Exercises are there for
designing the experiment?
• After looking at the Analysis & Results portion
of tutorial define each of following terms: i =
C=
R=
T=
• Do the 5 question quiz and print out your
results.
Answers for the Website- Prelab
• How many concepts are there?
8
• How many Exercises are there for
designing the experiment?
5
• After looking at the Analysis & Results portion of tutorial define
each of following terms:
i = ionization constant
C= molar concentration of the potato
R= rate constant: 0.0821 L · atm (bar)
mol · K
T= temperature (K) 273 + C
• Do the 5 question quiz and print out your results.
1. C
2. D
3. E
4. B
5. A