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 = Ψ p + Ψ s water potential = pressure potential + Solute (osmotic) potential

Plants & 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 “free” water 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 …Ψ p is zero b/c solution is at atmospheric pressure (unless told otherwise in problem)

RECALL: 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 –L ess “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

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 Water Ψ from Experimental Data Remember: Ψ = Ψ p + Ψ s HOW DO YOU FIND THE SOLUTE POTENTIAL? Solute potential  Ψ s = -iCRT i = ionization CONSTANT (for sucrose it is = 1) C = Molar concentration (determined from graph) R = Pressure CONSTANT ( L bars/ molK) T = Temperature in Kelvin (273 + ºC) Units will all cancel so Ψ s is in bars!

So….. Knowing the Ψ s, you can figure out water potential Ψ. NOTE: Ψ p is zero b/c solution is at atmospheric pressureNOTE: Ψ p is zero b/c solution is at atmospheric pressure Use Ψ = Ψ p + Ψ s 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 2g1RVYhttps:// 2g1RVY

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 = Water flows from “hypo” to “hyper” Or from hi  on left to lo  on right

Water Potential: an artificial model (b) adding pressure with plunger  no net flow of water (c) applying pressure increases water potential solution now has  of 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! WHY IS WATER MOVEMENT AFFECTED???? –This pressure will affect water movement. This is called TURGOR PRESSURE. WHY IS WATER MOVEMENT AFFECTED????

Water relations in plant cells pure water(b) Flaccid cell in pure water  Water potential is into cell  cell becomes turgid

Water relations in plant cells hypertonic(a) Flaccid cell placed in hypertonic 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 bar of pressure = 1 atmosphere