RHS - Biology
___
Name ___________________________ Per.
Why are cells so small and why do cells divide?
Background:
When cells grow to a certain size their growth rate slows until they stop growing. They
have reached their size limit. What determines this size limit anyway? Why doesn’t the cell
just keep getting bigger and bigger and stronger and stronger as a single unit?
We’ve already discussed the concept of diffusion (the spreading of molecules from areas
of high concentration to areas of low concentration). Materials needed for cell activity and
growth must enter the cell by diffusion. Cell wastes must be eliminated by diffusion. It
seems as though it would be easier for a larger cell to get the good stuff (nutrients) in and the bad
stuff (waste) out because it has a larger surface area for diffusion to take place across the cell
membrane. However, that is not the only factor contributing to cell success. The surface area
must be compared to the volume of the cell (the area inside the cell membrane) in order to
determine how efficiently materials can pass in and out of the cell.
In this lab you will be making model cells of different sizes, allowing diffusion to occur,
and calculating surface area to volume ratios (surface : volume) in order to explain one reason
why cells are so small and why cells divide.
Pre-Lab Questions:
1) Make a prediction about the following. Record your thinking below.
a) Which agar block will take up more liquid within a given period of time a larger block or smaller block? Explain
b) Will liquid penetrate farther into a small agar block or into a larger agar
block?
Procedure:
1) Calculate the surface to volume ratios of each block and fill in the data chart.
2) Cut 4 agar blocks into regular cubes of 3 cm, 2 cm, 1 cm, and 0.5 cm on each side.
2) Place the agar blocks in a beaker so they are not touching.
3) Cover them with vinegar solution.
4) Let them sit for 10 minutes.
5) Pour off the vinegar into a waste container (not down the sink)!
6) Rinse the agar blocks with tap water.
7) Slice the agar blocks in half and make drawings for your data/results section.
8) Determine the volume of the colored area. Do this by measuring 1 side of the
colored area … this number is the L, W, and H. Multiply L x W x H to find the
volume. Do this for each agar block.
9) Determine the volume of the uncolored area. Do this by subtracting the volume of
the colored area from the total volume of your cube. Do this for each agar block.
10) Determine how much diffusion took place in the different agar blocks. Do this by
calculating the percentage of each cube into which the HCl solution has diffused.
Divide the volume of the uncolored area by the volume of the total agar block.
Calculate and determine the total percentage of diffusion for each of your agar
blocks.
I love diffusion ‘cause I couldn’t survive without it!
RHS - Biology
___
Name ___________________________ Per.
Data/Results
Table 1
cube size
(cm)
surface area =
L x W x # of sides
(cm2)
volume =
LxWxH
(cm3)
surface : volume
whole # : 1 ratio
3
2
1
0.5
:1
:1
:1
:1
Drawings - Use a ruler to draw the cross section (cut in half) of each cube in color and
full size after removal from the acid.
Table 2
cube size
(cm)
Volume of colored
area
Volume of uncolored
area
% Diffusion (Volume
of uncolored area ÷
Volume of total cube)
3
2
1
0.5
Conclusion:
1) Why are we doing this lab?_______________________________________________
2) What do the agar blocks represent?______________________
3) What does the vinegar represent? ________________
4) What evidence is there that HCl diffuses into the blocks? _______________________
5) Was there any evidence that something was diffusing out of the blocks? ___________
6) Which block had the greatest surface area? _____________________
7) Which block had the greatest surface : volume ratio? ________________
8) What happens to the surface : volume ratio as a cell grows? _________________
9) What happens to the surface : volume ratio when cells divide? _________________
10) Why are cells so small, or in other words, why do cells divide? (Discuss diffusion %)
I love diffusion ‘cause I couldn’t survive without it!