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 its known as water potential.

• The higher their kinetic energy the more they move and hit the membrane, therefore higher the water potential

Water potential

Why does water move?How does water move?

1.

2.

3.

Downhill

Fresh – salty

Hose, straw

4. Sponge

Pressure potential

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 mol · K

s = -24.22 bars

Typical Water Potential Values

• Outside air (50% humidity): -100 MPa• Outside air (90% humidity): -13 MPa• Leaf Tissue: -1.5 MPa• Stem: -0.7 MPa• Root: -0.4 MPa• Soil water: -0.1 MPa• Hydrated soil (Saturated) +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

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 CellPermeable to: monosaccharides & water

Impermeable to: Disaccharides

Introductory Questions #3 (Lab)1) 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)?

2) What is the ionization constant (i) for sucrose?3) Determine the “C” value for your potato cores. (guidesheets)4) 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) Explain how potential energy is different from kinetic energy. What are some ways we can measure energy?

2) Define each variable in the equation: ∆G = ∆H – T ∆S

3) What is the difference between an exergonic reaction and an endergonic reaction?

4) How is ATP associated with coupled reactions? What purpose does it serve?

5) How is an electron carried from one molecule to the next? Name a molecule that can carry an electron.

6) How is Anabolism different from catabolism? 7) 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) 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)?

2) What is the ionization constant (i) for sucrose?3) Determine the “C” value for your potato cores. (guidesheets)4) 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 constantC= molar concentration of the potatoR= rate constant: 0.0821 L · atm (bar)

mol · KT= temperature (K) 273 + C

• Do the 5 question quiz and print out your results. 1. C 2. D 3. E 4. B 5. A