Cell Size Lab
Background:
When cells reach a certain size, their rate of growth slows
down. They will eventually stop growing. Each of these cells divides into two
smaller cells, which begin to grow. What causes this? An easy way to
investigate such questions is to build models. A model is often thought of as a
small copy of something larger. Here we will be making a larger model of
something small.
When a cell becomes too large, it is
no longer as efficient as it used to be. It now has a greater volume and it
takes much longer for materials that enter the cell to reach the center of the
cell. This concept will be demonstrated in the lab.
Purpose:
To build a model of a cell to understand why when cells
reach a certain size they stop growing.
Materials:
·
Block
of phenolphthalein agar
·
razor
blade
·
100
ml 4% sodium hydroxide (you MUST wear goggles!!!)
·
plastic
spoon
·
250ml
beaker
·
plastic
table knife
·
millimeter
ruler
·
paper
towel
Procedure:
1. With the razor blade, cut the block
of phenolphthalein agar into cubes. The first cube should be 3cm on each side;
the second, 2 cm on each side; and the third, 1 cm on each side. Measure
carefully and trim away the waste. Examine the cubes. Think of them as giant
models of tiny cells.
Which of your three differentsized cells do
you think would be most likely to survive?
Why do you think so?
Materials used during cell activity and
growth enter the cell from the outside and must travel through the cell to their destination. Waste products go
through the cell surface to the outside. Do you
think the cell with the greatest total surface area will do the best job of
moving materials in and out of the
cell? Why or why not?
2. Calculate the total surface area of
each of your three models using the following formula: Surface area = 6 x
(length x width), that is, the total surface area is 6 times the surface area
of one side.
3. Record the total surface area for
each cube in Table 1.
Which
cell has the greatest surface area?
4. The cells have different
surface areas and they have different volumes (amounts of materials inside).
Calculate the volume of each cube. Volume = length x width x height.
5. Record the volume of each cube in
data Table 1.
Look
back at the prediction you make in #1. Do these new calculations change any of
your earlier ideas? Why or why not?
6. Place the cubes in the beaker. Pour
enough sodium hydroxide solution to cover them. Record your starting time. Use
the plastic spoon to turn the cubes often for the next 4 minutes. Be careful
not to cut or scratch the surface of the cubes. Blot them dry.
7. Slice each cube in half with the
plastic knife.
What differences do you see? Draw a 3cm, 2cm
and 1cm square below and draw what you see. Measure
the distance from the edge that the sodium hydroxide traveled and color code
that on your drawings.
Phenolphthalein Cube Drawings



3 cm cube 
2 cm Cube 
1 cm Cube 
8. Calculate the ratio of surface area
to volume for each cube. To do this, for each cube size, divide its surface
area by its volume. The number obtained can be expressed as a ratio. For
example, a surface area of 24cm^{2} divided by a volume of 3 cm^{3}
= 24/3 = 8/1 = 8:1.
9. Record this ratio on data Table 1.
Which cell model is the most efficient? Why?
How does
this demonstrate why larger cells would want to divide?
Results:
Table 1
Phenolphthalein
– Agar Cubes 

Side
of cube (cm) 



Total
Surface Area (cm^{2}) 



Volume
(cm^{3}) 



Surface
area to Volume Ratio 



Questions:
5.
Jack rabbits found in desert areas have very long ears, while
Arctic Hares have much shorter, rounder ears. Hypothesize how ear shape
(surface area) relates to heat loss.