Laboratory Review ❮ 243
What if you wanted to determine the actual water potential (molarity) of the potato?
It’s easy, once you remember that if the water potential of the solution equals that of the
cells, there is no net change in weight. If you graph your percent changes in weight, you
can estimate the potato’s water potential (see Figure 19.2).
The point where the line crosses the x-axis indicates the molarity when there would be
no net change in weight. Therefore, 0.5 M is the approximate molarity (or, in this case,
water potential) of your potato!
One final thought about your data: Although we often focus on quantitative (numeri-
cal) measurements in our labs, qualitative observations are still very important. When you
removed your potato cores from the solution, did they feel different? Floppy and bendy?
That would suggest that the cells lost water because they were in a hypertonic solution (the
higher molarities). Was the potato core rigid, and would it snap if you bent it? That would
suggest that it was in a hypotonic (low molarity) solution and water flowed into the cells,
increasing their turgor pressure.
Key Concepts (for All Three Procedures)
- A high surface area–to–volume ratio increases diffusion rates.
- A cell that relies on diffusion would evolve to have a high SA:V ratio.
- Water diffuses from an area of high to low water potential.
Investigation 5: Photosynthesis
Photosynthesis is a complex (and possibly confusing) metabolic process through which
autotrophs generate glucose by first converting the sun’s light energy into the chemical
form that all living cells use (ATP). If you wish to design a lab to track the rate of pho-
tosynthesis, how would you do it? Measure glucose production? Carbon dioxide use?
Maybe focus on the light reaction’s reduction of the electron carrier NAD+into its
reduced form (NADH) as in the DPIP lab? Well, some of the best experiments are the
simplest, so let’s watch little pieces of a leaf float in water as oxygen is produced as a
by-product of photosynthesis.
Basic Setup
This is a cool way to measure photosynthetic rates, based on the amount of oxygen pro-
duced. Little leaf disks are put in a large syringe with some slightly soapy water (this helps
Percent change
in weight
Sucrose
concentration
0.2 0.4 0.6 0.8 1.0 (M)
0
Figure 19.2 Potato core change in weight
KEY IDEA
BIG IDEA 2
Cellular
Processes:
Energy,
Communication,
and Homeostasis
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