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

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

3. PART 1: What is Dialysis Tubing?
Perforated tubing  acts like a cell membrane
It is semi-permeable!
Glucose, ions and H2O can move in/out
Sucrose CAN’T move in or out!

4. PART 1: Osmosis and Dialysis Tubing
Tie dialysis tubing at one end
Pour 25 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

5. Which way will the water move?
Hint:

6. PART 2: Osmosis and Potato Cores
Potatoes are PLANT cells
How much SUCROSE is in each cell?

7. PART 2: Osmosis and Potato Cores
Get 4 potato cores PER cup
Obtain mass (initial) of cores Pour 100 mL of different M sucrose solutions into labeled beakers
Wait 24 hours (1440 minutes)
Obtain mass (final) in grams

8. 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)

9. Hypotheses Create a hypothesis for:
If…(IV)…then…(DV will change this way)…because…(prior knowledge)
Create a hypothesis for:
PART 1: Dialysis Tubes and Osmosis
PART 2: Potato Cores and Osmosis
You will turn this in at the BEGINNING of the period BEFORE the lab. It is your PRE-LAB work for this lab.
You will also have a 30 pt PRE-LAB Quiz before we start the lab.

10. PART 2: Osmosis & Potato Cores
Why is % mass change necessary?

11. Ψ = Ψp + Ψs water = pressure + solute
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 = pressure solute
potential potential potential
(aka-osmotic pressure)

12. Water Movement Ψ, Ψs and Ψp are measured in MPa or in barrs
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
Ψ, Ψs and Ψp are measured in MPa or in barrs

13. (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

14. 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)  open containers have this; This is what you will use in Lab #2!!!

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