Exercise 4
Cell Function: Diffusion and Osmosis
•Procedure 1: skip
•Procedure 2: Effect of Cell size on Speed of diffusion
•Procedure 3: skip
•Procedure 4: Osmosis
•Procedure 5: Plasmolysis in red onion skin cells
Procedure 2: Set up agar cubes experiment
(groups of four)
Cut cubes in following order:
3x3x3
2x2x2
1x1x1
Immerse each cube in a beaker with vinegar and start your stopwatch
Take the time that it took for the entire cube to turn yellow
1
Review of cell transport
Build a concept map for the cell transport using the following terms:
Cell transport
Crossing the membrane
Not crossing the membrane (via vesicles)
Endocytosis
Exocytosis
Passive transport
Active transport
Movement down concentration gradient
Movement against concentration gradient
Energy not required
Energy required
membrane protein
Diffusion
Osmosis
Facilitated diffusion
water
Part 2: Effect of cell size on
the speed of diffusion
Cells volume = cytoplasm
requirement of nutrients
or wastes removed
Cells surface = cell membrane
area through which
nutrients and wastes are
transported into or out of cell
Why are most cells microscopic?
Volume increases FASTER than the surface
Larger cell has problems with exchange of nutrients and wastes
2
Agar cubes represent different size of cells
Bromothymol Blue changes color in the presence of an acid
Surface area =
Volume =
SA/V ratio =
Acid =
B blue =
Time form turning completely yellow=
Cell Dimensions
(cm)
Surface Area
Volume
Surface Area to
Total time for
(cm2)
(cm3)
Volume Ratio
diffusion
(cm2/cm3)
(min)
A
1X1X1
6
1
B
2X2X2
24
8
C
3x3x3
54
27
Part 4: Osmosis
How does osmosis affect the weight of cells?
Tap water
10% salt
water
Calculate change in mass: final weight – initial weight= gain or loss
Which solution is hypotonic to potato tissue? Which is hypertonic?
3
Part 2: Plasmolysis in red onion skin cells
How does osmosis affect onion cells?
2 slides with onion skin
10% salt water
Distilled water
Which solution is hypotonic to the cells?
Which is hypertonic?
Plan of Attack
• Procedure 2. take time and fill out table and answer questions
• Procedure 3. Osmosis
one couple does potato 10% salt water
one couple does potato tap water
while you wait 30 min, do procedure 4
• Procedure 4. Plasmolysis in red onion
one couple does onion skin with distilled water
one couple does onion skin with 10% salt water
4