Instructor’s Copy
Lab Worksheet – Let Me Through
Time agar is placed in NaOH: _____________
Part I Questions:
1. If the cubes were actually cells in your body, what could the NaOH solution represent?
Blood or blood plasma.
2. What happened to the liquid when you poured it over the blocks of agar?
It should turn bright pink.
3. What could be coming out of the agar that is causing the color change you observed?
Phenolphthalein.
4. What physical process is operating when the substance in the agar moves out of the agar into the
liquid?
Diffusion.
5. Is there any other substance which could be moving from one location to another in the beaker?
Yes, NaOH can be moving into the agar. Water can also be moving into the agar.
6. Based on what you have observed, what do you infer is happening inside each block of agar?
The NaOH is moving into the agar, causing the phenolphthalein to turn pink.
Data Table 1: Observations of agar after soaking in Sodium Hydroxide
Cube
Observations
3 cm Observations will vary but, in general, the penetration should be complete or
close to complete in the 1 cm cube and should decrease more for each larger
block.
2 cm
1 cm
Data Table 2: Calculations – Actual answers vary but this is a sample
Cube
Depth not
Penetrated
Surface Area
Volume
3 cm
Depth of
Penetration
of NaOH
.4 cm
.6 cm
54 cm2
27 cm3
Surface Area
to Volume
Ratio
2:1
2 cm
.5 cm
.3 cm
24 cm2
8 cm3
3:1
1 cm
.5 cm
0
6 cm2
1 cm3
6:1
Part II Questions:
7. List the cubes in order from largest to smallest. List the cubes in order from largest surface areato-volume-ratio to smallest ratio. Are these two lists the same?
3 cm > 2 cm > 1 cm
6:1 (1 cm) > 3:1 (2 cm) > 2:1 (1 cm)
They are in reverse order.
8. What can you conclude about the size of a cube (or cell) and its surface area-to-volume ratio
based on your answer to question #7?
The bigger the cell, the smaller the surface area-to-volume ratio.
9. How did the blocks compare in the depth of penetration of the NaOH?
The smallest block had complete penetration. The bigger the block, the less penetration.
10. If the agar blocks were living cells and the NaOH were blood plasma containing dissolved food.
Which block would have the most efficient penetration of the dissolved food to its center?
The smallest block (1 cm).
11. What is the relationship between the surface area-to-volume ratio of a cell and the efficient
penetration of liquid to its center?
The larger the surface area-to-volume ratio, the larger the amount of penetration.
12. What can you infer happens to the surface area-to-volume ratio of a cell as the cell grows?
The ratio gets smaller.
13. How do you think the surface area-to-volume ratio of each of two “daughter cells” would
compare to their parent cell?
The ratio would be greater in the daughter cells.
14. Why is it beneficial for an organism to have many small cells instead of a few large cells?
Small cells would get food and nutrients through the cell faster and could remove
wastes faster.