Laboratory Review ❮ 241
Key Skills
- Explain the relationship between genetic sequences and evolutionary relatedness.
- Draw and/or analyze a cladogram that shows evolutionary relationships.
Investigation 4: Diffusion and Osmosis
This investigation draws on information covered in Chapter 6, Cells. If you feel uncom-
fortable with this material, take a few moments to flip back to Chapter 6 and scan through
the information about diffusion, osmosis, and cell transport. In summary, osmosis occurs
from an area of high water potential to an area of low water potential. In a given solution,
the higher the solute concentration, the lower that solution’s water potential.
Part 1: Surface Area and Cell Size
Basic Setup for Part 1
Besides turning your fingers blue, this lab will demonstrate the relationship between sur-
face area and volume and how this ratio affects diffusion rates in a cell. Your cell model is
a block of agar that contains an indicator dye that changes color when the pH drops. You’re
given a chunk of this blue agar to carve into three different block sizes, each with differing
surface area–to–volume ratios (SA:V). Each block is dropped into a solution, and as the
liquid diffuses into the agar, the pH causes a change in agar color. This enables you to easily
track the amount of time it takes for diffusion to be completed.
Results for Part 1
It’s all about a large surface area–to–volume ratio. The block with the biggest SA:V ratio
wins the race. For example, say you have three sizes of agar blocks:
block 1 =1 cm ×1 cm ×1 cm
block 2 =2 cm ×2 cm ×2 cm
block 3 =1 cm ×1 cm ×8 cm
You should be able to calculate both the volume (cm^3 ) and the surface area (cm^2 ) for
each of these blocks. (Remember: formulas will be provided for you on the AP exam!)
Human
A
B
C
D
Figure 19.1 Cladogram for percent similarity between gene X in humans and other
species. The closer two species reside on a clade, the closer they are genetically related.
BIG IDEA 2
Cellular
Processes:
Energy,
Communication,
and Homeostasis
BLOCK VOLUME (cm^3 ) SURFACE AREA (cm^2 ) SA:V
11 6 6
2 8 24 3
3 8 34 4.25
Notice that even though blocks 2 and 3 have the same volume, their surface areas are
different. This results in diffusion taking longer in block 2 than it does in block 3.
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