1. Pre-Lab #2: Diffusion and Osmosis Introduction to Water Potential, Diffusion and Osmosis
AP Biology
Ms. Day

2. Water Balance of Cells with Walls
Cell walls
Can help maintain water balance
Cell walls are in:
Plants***
Prokaryotes
Fungi
Some protists

3. Plasmolysis If a plant cell is turgid If a plant cell is flaccid
It is in a hypotonic environment
It is very firm
A healthy state in most plants
If a plant cell is flaccid
It is in an isotonic or hypertonic environment
Cells are limp or wilted
Plasmolysis
plasma membrane pulls away from cell wall in hypertonic solutions
causes cell w/ walls to wilt; can die!

4. Water balance in cells with walls
Plant cell. Plant cells
are turgid (firm) and
generally healthiest in
a hypotonic environ-
ment, where the
uptake of water is
eventually balanced
by the elastic wall
pushing back on the
cell.
(b)
H2O
Turgid
(normal)
Flaccid
Plasmolyzed
Figure 7.13

5. How Other Organisms Deal with Osmotic Pressure
Bacteria and plants have cell walls that prevent them from over-expanding.
In plants the pressure exerted on the cell wall is called tugor pressure.
A protist like paramecium has contractile vacuoles
pump out incoming water to prevent them from bursting/lysing.
Salt water fish pump salt out of their specialized gills so they do not dehydrate.
Animal cells are bathed in blood. Kidneys keep the blood isotonic by remove excess salt/water in urine.

6. Lab 2: Diffusion & Osmosis
Concepts
semi-permeable membrane
diffusion
osmosis
solutions
hypotonic
hypertonic
isotonic
water potential
Solute potential
pressure potential

7. Lab Report…see rubric DUE-10/30/2018
You will be completing a FORMAL LAB REPORT on this lab
1 printed out copy PER GROUP
1 group grade
You will also turn in PEER EVALUATIONS for each group member and yourself.

8. PART A and B: What is Dialysis Tubing?
Perforated tubing  acts like a cell membrane
It is semi-permeable!
Glucose, ions and H2O can move in/out
Sucrose or starch CAN’T move in or out!

9. Molarity You will be using 0.0 M through 1.0 M sucrose solutions
Means concentration or amount of
Measured in:
# of mols of substance/1 L of H20
mol/L
You will be using 0.0 M through 1.0 M sucrose solutions
The higher the M number, the more sucrose in the solution (more sugar!)
0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M & 1.0 M

10. PART B: Osmosis and Dialysis Tubing (Skipping Part A)
Tie dialysis tubing at one end
Pour 20 mL of different M sucrose solutions
Obtain mass (initial) in grams
Place in distilled H2O (0.0 M sucrose)
Wait 30 minutes
Obtain mass (final) in grams

11. Which way will the water move?
Hint:

12. PART C: Osmosis and Potato Cores
Potatoes are PLANT cells so they have cell walls and tugor pressure!
How much SUCROSE is in each cell?

13. PART C: Osmosis and Potato Cores
Get 4 potato cores PER cup
Obtain mass (initial) of cores
Pour 50 mL of different M sucrose solutions into labeled beakers
Wait 24 hours (1440 minutes)
Obtain mass (final) in grams

14. Tips/Advice Do NOT let bags or cores sit on a paper towel
Capillary action will draw WATER out…experimental error!!! Changes data/mass
Leave bags/cores in a labeled weigh boat
Dab cores and bags dry before massing (but NOT too much…see above)
Take pictures for your formal lab report (qualitative data)

15. Hypotheses Create a hypothesis for:
If…(IV)…then…(DV will change this way)…because…(prior knowledge)
Create a hypothesis for:
PART B: Dialysis Tubes and Osmosis
PART C: Potato Cores and Osmosis
You will be making and turning in these hypothesis TODAY in your new lab groups!

16. PART 2: Osmosis & Potato Cores
Why is % mass change necessary?
Change in mass (%) = (final mass – initial mass) x 100%
initial mass

17. Ψ = Ψp + Ψs water = pressure + solute
Water Potential (Ψ)
Botanists use this term when predicting movement of water into/out of plant cells.
Abbreviated by Ψ (“psi”)
Ψ = Ψp Ψs water = pressure solute
potential potential potential
(aka-osmotic pressure)

18. Water Movement Water diffuses DOWN a water potential gradient
Water will ALWAYS move from an area of HIGHER Ψ to an area of LOWER Ψ
(larger number Ψ  smaller number Ψ )
Water diffuses DOWN a water potential gradient
Ψ, Ψs and Ψp are measured in MPa or in barrs

19. (always a negative value)
Solute Potential (Ψs)
(always a negative value)
Represents the solute [ ] in a solution
Adding more solutes  less free water  LOWER (more negative) water Ψ

20. Pressure Potential (Ψp)
(can be + or - value)
Physical pressure on either side of membrane
Increasing pressure increases water Ψ
NOTE: Atmospheric (atm) pressure is defined as being = O (zero) in a open container; This is what you will use in Lab #2!!!

21. Bozeman Biology & Water Potential
Watch until minute 5:17