242 ❯ STEP 4. Review the Knowledge You Need to Score High
Key Concept- A high SA:V ratio is important for any cell that relies on a high diffusion rate. If you
were a tiny bacterium, your health and well-being would be dependent on quickly get-
ting good stuff in (glucose for cellular respiration!) and bad stuff out (metabolic waste).
The linings of your small intestine and lungs have many folds in order to create the high-
est surface area possible in the smallest amount of space and thereby facilitate diffusion
of food monomers or oxygen molecules.
Part 2: Modeling Diffusion and Osmosis
Basic Setup for Part 2
Now you get to create a model of a cell using dialysis tubing. Just like a real cell, the tubing
is selectively permeable to water and some solutes. The point of this lab investigation is to
use different solutions to model how water potential influences osmosis.Results for Part 2
Say you filled your dialysis bag with a 1 Molar (1 M) sucrose solution, weighed it, and
placed it in a beaker of 1 M NaCl solution. After 30 minutes, you weigh the bag again
and—voilà!—it got lighter! That means it lost some water, right? Can you use water poten-
tial to show why that makes sense? Recall that you can calculate the solute potential for a
solution with this equation:
ΨS=iCRT, where i =ionization constant and C =molarity
The molarities are equal for both of these solutions, so the ionization constant is the
deciding factor. NaCl ionizes and sucrose does not! So for NaCl, i =2, whereas for sucrose
i=1. Therefore, the NaCl solution has a higher solute potential; or, in other words, it is a
hypertonic solution compared to your dialysis tubing “cell.” Therefore, water will diffuse
out of the bag into the surrounding NaCl solution.Key Concepts- Osmosis occurs from an area of high water potential to low water potential.
- A cell’s environment allows you to make predictions about molecular movement through
cell membranes.
Part 3: Observing Osmosis in Living Cells
Basic Setup for Part 3
Here you get to work with pretty, color-coded (unlabeled) sucrose solutions ranging from
0.0 M up to 1.0 M, and use potato cores to figure out the relative concentrations of these
solutions. To take it one step further, you can then calculate percent change in weight of
your potato cores and determine the water potential of the potato tissue. Remember that
the bigger the difference in water potential between a cell and the solution, the bigger the
movement of water (either into or out of the cells).Results for Part 3
Once you calculate the potatoes’ percent change in weight for each of the unknown solu-
tions, you can arrange them from most negative to most positive. A supernegative percent
change in weight indicates a significant loss of water; a highly hypertonic solution increases
water loss from cells. The more negative the number, the higher the molarity of the solu-
tion! The same is true for superpositive percent change in weight. That means the potato
cores gained a lot of water, which happens in a hypotonic solution. The greater the weight
gain, the lower the molarity of the solution.KEY IDEAKEY IDEA