Osmosis Lab

8/22/14-8/27/14

BW 8/22/14

• What would be more cruel, to put a saltwater fish in freshwater or a freshwater fish in saltwater? Explain your reasoning.

Freshwater fish in freshwater

Freshwater fish in salt water

Saltwater fish in saltwater

Saltwater fish in fresh water

Osmosis Lab

8/22/14-8/27/14

Lab Overview

• In this lab you will…

– Investigate the process of osmosis in a model membrane system

– Investigate the effect of solute concentration on water potential as it relates to living plant tissues

Background Info

• Diffusion- the random movement of molecules from an area of higher concentration to an area of lower concentration

• Osmosis- diffusion of WATER across selectively permeable membrane

Selective Permeability

Nota Bene!

• In Osmosis, Water follows Solute!!!!!

• Isotonic- when 2 solutions have the same concentration of solutes

• Hypertonic- one with less solvent AND more solute

• Hypotonic- one with more solvent AND less solute.

• Net Movement- direction in which the water is moving between substances

Isotonic Hypotonic Hypertonic

Water potential is a concept that helps to describe the tendency of water to move from one area to another, particularly into or out of cells.

oWater molecules move randomly. oWhen water is enclosed by a membrane some of the moving water molecules will hit the membrane, exerting pressure on it. oThis pressure is known as water potential.

It is measured in units of pressure, can be measured in kPa, MPa, bar. Pure water has a water potential of zero. A solution will have a lower concentration of water molecules so it will have a negative water potential.

Water Potential • We look at water movement in terms of water potential. (ψ psi)

• Two factors: –Solute concentration and pressure

• Pure water ψ =0

• The addition of solute lowers the water potential. (negative number)

Water potential determines the rate

and direction of osmosis.

Pressure potential is important in plant cells because they are surrounded by a cell wall which, is strong and rigid.

When water enters a plant cell, its volume increases and the living part of the cell presses on the cell wall.

The cell wall gives very little and so pressure starts to build up inside the cell.

This has the tendency to stop more water entering the cell and also stops the cell from bursting.

When a plant cell is fully inflated with water, it is called turgid. Pressure potential is called turgor pressure in plants)

ψp

ψp

ψp

ψp

Water potential (ψ ) =

pressure potential (ψp ) + solute (osmotic) potential (ψs)

Pressure potential (ψp): In a plant cell, pressure exerted by the

rigid cell wall that limits further water uptake

Solute potential (ψs): The effect of solute concentration. Pure water at atmospheric pressure has a solute potential of zero. As solute is added, the value for solute potential becomes more negative. This causes water potential to decrease also. *As solute is added, the water potential of a solution drops, and water will tend to move into the solution.

Water potential (ψ) = pressure potential (ψp ) + solute potential (ψs)

(osmotic)

This is an open container, so the ψp = 0

This makes the ψ = ψs

The ψs =-0.23, so ψ is -0.23MPa, and water moves into the solution.

Water moves from a place of high water potential to a place of low water potential.

Can a solution with a molarity of 0.2 be in equilibrium with a solution with a molarity of 0.4? YES! Pressure Two solutions will be at equilibrium when the water potential is the same in both solutions. This does not mean that their solute concentrations must be the same, because in plant cells the pressure exerted by the rigid cell wall is a significant factor in determining the net movement of water.

BW 8/25/14 • Which side (A or B) has lower water potential (ψ)?

• What will be the direction of net water flow?

• How could we stop net water flow?

A B

ψ = ψp + ψs

Solute (osmotic) potential (ψs )= –iCRT i = The number of particles the molecule will make in water; for

NaCl this would be 2; for sucrose or glucose, this number is 1

C = Molar concentration

R = Pressure constant = 0.0831 liter bar/mole K

T = Temperature in degrees Kelvin (273 + °C) of solution

Example Problem: The molar concentration of a sugar solution in an open beaker has been determined to be 0.3M. Calculate the solute potential at 27°C degrees. Round your answer to the nearest hundredth. What is the water potential?

Answer: -7.48

Solute potential = -iCRT = -(1) (0.3 mole/1) (0.0831 liter bar/mole K) (300

K) = -7.48 bar

Water potential = -7.48 + 0, so water potential = -7.48

{ψ = ψp + ψs} & {ψs = –iCRT}

• What is the solute potential of a 0.1M NaCl solution at 25°C?

• If the concentration of NaCl inside a plant cell is 0.15M, what must be the turgor pressure for there to be no net water movement if the cell is placed in the solution above?

Dialysis Tubes

• Our dialysis tubes are SEMI-PERMEABLE

– Permeable to water

– Impermeable to sucrose

Standardization

• If we fill dialysis tubes with different solutions and submerge them in tap water for equal amounts of time, which solution has the higher concentration?

– 100g of Solution A that gains 2g

– 10g of Solution B that gains 1g

• How can we quantify the difference?

% change in mass of dialysis tubes

BW 8/26/14 1. If a dialysis bag filled with 10ml of a 1M KCl

solution gains 3g after sitting in distilled water for 20 minutes, approximately how many grams would you expect a dialysis bag with 10ml of 0.5M KCl to gain after sitting in distilled water for 20 minutes?

2. Calculate the water potential of a 0.25M NaCl solution in a beaker at room temperature (25°C) with 2 bar of pressure (in addition to atmospheric pressure) being applied. 1. R = 0.0831 Lbar/MoleK

Struggling with water potential?, check out bozeman: https://www.youtube.com/watch?v=nDZud2g1RVY

2 Questions to investigate

A. 5 colored solutions of 5 different molarities (0, 0.2, 0.4, 0.6, 0.8M). Which is which?

B. 2 potato types (sweet and white). What are the total intracellular water potentials of these related root vegetables? Which is higher, and why?

• Part B is only possible with accurate results from Part A.

Materials – Part A

• 5 colored sucrose solutions

• Tap water

• Plastic Beakers

• Dialysis tubing (permeable to water NOT sucrose)

• Transfer pipettes

• Balances (digital scales, centigram accuracy)

Dialysis Tubes

• Our dialysis tubes are SEMI-PERMEABLE

– Permeable to water

– Impermeable to sucrose

% change in mass of dialysis tubes

Once you have determined the identity of the solutions in Part A…

• You will use the colored solutions to determine the total intracellular water potential of potatoes. (½ the class will do white potatoes, ½ will do sweet potatoes)

Determining ψtotal of potatoes

• Determine the sucrose concentration in which no net water movement (AMB %Δ mass of potatoes) occurs between the sucrose solution and the potato cells.

• ψsol’n of zero net water movement = ψpotatoes

• However, none of our solutions will give exactly zero net water movement, so we’ll graph our results and interpolate.

Interpolation Practice

• Graph (sketch) this data. • Approximately how

many bacteria were there at hour 5?

• Just because your data doesn’t include your answer, doesn’t mean you can’t use your data to find your answer.

Hour # of bacteria

0 0

1 10

2 40

6 360

10 1000

12 1440

16 2560

X-intercept?

Part B – approach

• Put potato cores in beakers of each solution, let sit overnight.

• Determine percent change.

• Graph percent change (y-axis) versus solution concentration (x-axis).

• Interpolate for X-intercept will give you the concentration that would give zero percent change (no net water movement)

• Use that concentration to determine ψtotal of both the solution and the potato cores.

How can we standardize and

control?

In lab groups, design experiment

• Through procedures for Parts A & B

• In Comp Books

• Use lab template

• When you are finished, get approved

• Tomorrow you will implement experiment

• Thursday we will analyze results

BW 8/27/14 1. What is the formula for % change?

2. Why are there 5 different transfer pipettes for

the 5 different colored solutions?

3. What are some ways to avoid being wasteful of

materials?

Struggling with water potential?, check out bozeman: https://www.youtube.com/watch?v=nDZud2g1RVY

Get procedures approved, then implement

• Use pre-cut dialysis tubes first, then cut your own from roll (15cm is good)

• While you let dialysis bags sit, prepare potato cores (3 cores - 1 cm3).

• As soon as you are done collecting data for Part A, reset and begin part B.

BW 8/29/14 1. According to your preliminary data, what is

the order of the colored solutions (from

highest molarity (0.8M) to lowest (0.0M)?

2. What are the 4 unique properties of water?

3. Hand in chapter 5 RGs!

Struggling with water potential?, check out bozeman: https://www.youtube.com/watch?v=nDZud2g1RVY

Osmosis Lab - Finishing Up

• Analysis – Graphs of potato data – Water potential calculations & final results

• Conclusion – standard conclusion, plus – Discuss 2013 data versus 2014 data and suggest sources of error

in 2014 data – Explain calculations & their relationship to water potential of

potato cells – Conclude about sweet vs. white potatoes – Discuss/answer this question: “Of the total calculated water

potential, what are the relative contributions of solute potential and pressure potential? Suggest reasonable values for each in both potato types and defend your answer with outside research.”

Remember our two questions that we started with when writing conclusion

A. 5 colored solutions of 5 different molarities (0, 0.2, 0.4, 0.6, 0.8M). Which is which?

B. 2 potato types (sweet and white). What are the total intracellular water potentials of these related root vegetables? Which is higher, and why?

• Part B is only possible with accurate results from Part A.

Lab Due – 9/12/14

• Before the end of the day!