Post Lab #2: Diffusion and Osmosis

Further Discussion of Part A and Part B

AP Biology

Ms. Day

10/8/14

PART A: Diffusion and Dialysis Tubing

• What happened to the STARCH?• Iodine moved into bag (high low [

])• Starch too big do NOT move• STARCH + Iodine = BLACK COLOR

• What happened to the GLUCOSE?• Moved out of bag (high low [ ])• Benedict test has POSITIVE result in

presence of glucose • it was cloudy and changed color• Not a lot of glucose present

PART B: Osmosis and Dialysis Tubing

• As sucrose molarity increased, % change in mass also increased.

• WHY???– Solutions were getting more and

more hypertonic IN bag• Water moved INTO bag to maintain

equilibrium

PART B GRAPH: Osmosis and Dialysis Tubing…

What do you expect?

Post Lab #2: Diffusion and Osmosis

Further Discussion of Water Potential

AP Biology

Ms. Day

10/8/14

PART C: Osmosis and Potato Cores

RECALL: Water Potential (Ψ)

• Botanists use this term when predicting the movement of water into/out of plant cells.

• Abbreviated by Ψ (“psi”) Ψ = Ψp + Ψs

water potential = pressure potential + Solute (osmotic)

potential

Plants & Water PotentialPlants & Water Potential (water potential) combines the effects

of 1.) solute concentration 2.) physical pressure placed on cell

= psi = water potential

• What are the units for ? –measured in megapascals (MPa) or

Bars

Solute Potential (Solute Potential ( S)

• Solute potential is also called the osmotic potential because solutes affect the direction of osmosis.

S of any solution at atmospheric pressure ( p = 0) is always negative!

WHY?• Answer = less free water molecules to do

work

Recall: What are Recall: What are ““freefree”” water water molecules?molecules?

• Solutes bind water molecules reducing the number of free water molecules lowers waters ability to do work (ex: move)

Pressure Potential (Pressure Potential (P)

P is the physical pressure on a solution where cell is located.

• P can be negative transpiration in the xylem tissue of a plant (“pulling” pressure)

P can be positive water in living plant cells is under positive pressure by cell wall pushing on cell membrane (“pushing” pressure)

Standard for measuring Standard for measuring

• Pure water is the standard.

• Pure water in an open container has a water potential of zero at one atmosphere of pressure.

Therefore, assume …Therefore, assume …ΨΨp p is zero b/c is zero b/c solution is at atmospheric pressure solution is at atmospheric pressure (unless told otherwise in problem)(unless told otherwise in problem)

RECALL: Water MovementRECALL: Water Movement

• Water will ALWAYS move from an area of HIGHER Ψ to an area of LOWER Ψ

(more positive Ψ more negative Ψ )

– Water diffuses DOWN a water potential gradient

•Higher Ψ – More “free” water molecules; more solutes

•Lower Ψ – Less “free” water molecules; more solutes

PART C:Graphing Potato Core Data

• Graph your data using positive and negative value for % change in Potato Core Mass

• Connect the “dots” between all data points

• The point at which the line CROSSES the X-AXIS represents the molar [ ] of sucrose INSIDE potatoe cell

0.0 0.2 0.3 0.4 0.60.6

Isotonic Solution is at ZERO indicated the amount of solutes in

potato cells (THIS IS YOUR MOLAR [ ] of solutes INSIDE the potato)

PART D: Calculating the PART D: Calculating the Water Water ΨΨ from Experimental from Experimental

DataData• Remember: Ψ = Ψp + Ψs

HOW DO YOU FIND THE SOLUTE POTENTIAL?

• Solute potential Ψs = -iCRT• i = ionization CONSTANT (for sucrose it is = 1)• C = Molar concentration (determined from graph)• R = Pressure CONSTANT (0.0831 L bars/ mol•K)• T = Temperature in Kelvin (273 + ºC)

• Units will all cancel so Ψs is in bars!

So…..So…..• Knowing the Ψs , you can figure

out water potential Ψ.

•NOTE: NOTE: ΨΨp p is zero b/c solution is zero b/c solution is at atmospheric pressureis at atmospheric pressure

• Use Ψ = Ψp + Ψs

• Water potential values are important b/c it allows us to predict the direction of the flow of water.

Bozeman Biology-Review of Water Potential

• https://www.youtube.com/watch?v=nDZud2g1RVY

Water Potential Practice Problems

AP Biology

Ms. Day

10/8/14

Water Potential: an artificial model

• (a) addition of solutes on right side reduces water potential. S = -0.23

• Water flows from “hypo” to “hyper”

• Or from hi on left

to lo on right

Water Potential: an artificial model

• (b) adding +0.23 pressure with plunger no net flow of water

• (c) applying +0.30 pressure increases water potential solution now has of +0.07

• Water moves right to left

• (d) negative pressure or tension using plunger decreases water potential on the left.

• Water moves from right to left

Water Potential: an artificial model

Animal vs. Plant Cells• Animal cells can take in water until it

BURSTS (or lyses)

• Plant cells take in water and exert a PRESSURE on cell due to constrictions by cell wall– Cell wall prevents cells from BURSTING.

Instead, pressure build up! – This pressure will affect water movement.

This is called TURGOR PRESSURE. WHY IS WATER MOVEMENT WHY IS WATER MOVEMENT AFFECTED????AFFECTED????

Water relations in plant cells

• (b) Flaccid cell in pure waterpure water Water potential is into cell cell becomes turgid

Water relations in plant cells

• (a) Flaccid cell placed in hypertonichypertonic solution Water potential is out of cell plasmolysis

Hints & reminders

1. Remember water always moves from [hi] to [lo].

2. Water moves from hypo hypertonic.3. [Solute] is related to osmotic pressure.

Pressure is related to pressure potential.4. Pressure raises water potential.5. When working problems, use zero for

pressure potential in animal cells & open beakers.

6. 1 bar of pressure = 1 atmosphere