Name: ________________________

Period: ___________  Group#______

LAB: Cell Size ana Maintaining Homeostasis

Backeround: (Highlight or underline key points in this section)

Most cells are very small. Most human cells are about 8~ micrometers in diameter. But there are some exceptions. The width of a nerve cell at its widest point is only a few micrometers, yet it may be more than a meter in length! Some single nerve cells, for example, go from the lower end of your spinal cord to the muscles that move your toes.

When cells reach a certain size, their rate of growth slows down, and stops prior to division. Why are cells so small? Why don't they continue to grow? Can the ability of a cell to diffuse enough materials across the plasma membrane be related to homeostasis in such a way that there are limits on cell size?

An easy way to investigate such questions is to build a model. A model is often thought of as a small copy of something large. In this lab, we wiU use agar as a large model of something small, namely, the cell. And 'a chemical, sodium hydroxide, will represent materials diffusing in and out of the cell.

1.   Problem:

To determine if diffusion is related to homeostasis in such a way that cell size is limited.

2.   Hypothesis:

Write a hypothesis about which cell you believe has the best chance of survival and why.

3.   Procedure:

1. Remove a precut block of Phenolphthalein agar from the large pan and place it on a paper towel at your lab station.

2. Cut 3 different sized cubes of the agar to represent 3 sizes of cells:  3 cm sides, 2 cm sides, and 1 cm sides.  Be careful to make each side of the cubes the same length.

3. Calculate the total surface area and volume of each of your “cells” using the following formulas and enter them in your data table:

Surface Area =  length  x  width  x  number of sides (6)

Volume = length  x  width  x  height

 length of each side (cm) total surface area of all 6 sides (cm2) volume of cell (cm3) cell #1 cell #2 cell #3

4. Use the spool to place the cubes in a beaker, and pour in only enough Sodium Hydroxing (NaOH) to cover them.

(Caution:  NaOH can cause serious burns to your skin!  BE CAREFUL)

5. Record your starting time as you bathe them in the solution for 10 minutes.  Carefully turn the cubes using your spoon and/or knife.  Do not use your hands!

6. Remove them, blot them dry, and slice each cube in half.

7. Measure the colored zones (they show the penetration of NaOH), and record this data for each of the cell models.  Record your results in mm (millimeters).

 millimeters of color penetrated into the “cell” cell #1 cell #2 cell #3

8. Draw three diagrams illustrating the lab.  Draw a “before”, “middle” and “end” for the “average” cube’s diffusion of NaOH.  Be sure to label everything you can in each drawing.

 Before End Middle Leave this box empty Leave this box empty

4.   Analysis and Conclusions:

1. What similarities do you notice in each of the sliced cubes?

2. Based on your results as recorded in Steps 7 & 8 of the procedure, what differences do you notice in the amount of  NaOH was able to diffuse into each cell?

3. Compare the increase in surface area for each “cell” from 1cm to 2cm  to 3 cm on each side.

 Surface area (cm2) 1 cm cell 2 cm cell 3 cm cell

4. Compare the increase in surface area to volume ratio of each “cell” .

 Surface area (cm2)  :  Volume (cm3 ) 1 cm cell 2 cm cell 3 cm cell

5. Describe the change in surface area to volume ratio as the “cell” “grows”.  Example of what you need to compare is:  as a cell grows from a relatively small size like #1,  to a relatively larger size like #3.

6.  As you consider the sizes of the cells and how close to the center of the cell the NaOH gets, and how fast it gets there:

a. Which cell was apparently most in “receiving” materials from the outside?

b. Which cell was the least efficient in “receiving” materials from the outside?

8. Which of the “cells” do you think would be most likely to survive:  #1, #2, or #3?     Why?

9. How is the ability of a cell to diffuse nutrients and wastes across the plasma membrane and then through the cytoplasm inside – related  to its ability to  maintain a stable internal environment?