Cell Size _ Agar Diffusion

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					                                           Diffusion & Cell Size Lab
Background
       The absorption of nutrients, excretion of cellular wastes, and the exchange of respiratory gasses are life processes
which depend upon the efficient transport of substances into, out of, and throughout living cells. The process of diffusion
can be easily visualized by adding a drop of blue food coloring to a glass of water. Initially, the food coloring remains in a
small area in the water, dying it a dark blue. Over time, the molecules of food coloring collide with each other, and with
molecules of water, and the food coloring eventually disperses throughout the entire glass of water, resulting in a light
blue color in the water. Much like the drop of blue dye diffuses through the glass of water, many important substances
move into and out of cells by diffusion. Diffusion is the movement of a substance through a concentration gradient from
high to low concentration. It is an example of passive transport because it requires no energy on the part of the cell. For
this reason, diffusion is one of the most common and efficient means by which substances are transported between cells
and their environment.
       The cell membrane is the selectively permeable barrier whose total surface area is important in regulating the
substances that diffuse into or out of the cell. However, as a cell grows in size, its volume increases at a greater rate than
its surface area. Consequently, the surface area of the growing cell soon becomes inefficient for effective diffusion
throughout the cell. This relationship between surface area and the volume of a cell can be expressed as a ratio; and the
need for an effectively large surface area to volume ratio is considered to be the most significant factor in triggering a cell
to divide, and therefore, determining cell size.
OBJECTIVES

   Determine the extent and rate of diffusion into three different size agar cubes.
   Calculate the surface area to volume ratio for each agar cube.
   Observe the relationship between cell size and extent of diffusion in the agar cubes.
                                                                                                          Figure 2
   Understand the necessity for microscopic cell sizes.
MATERIALS

        3 cm x 3 cm x 6 cm phenolphthalein agar block
        Plastic knife                                 Plastic ruler
        Plastic cup                                   Diffusion medium

PROCEDURE

1.   Obtain a 3 cm x 3 cm x 6 cm agar block. Using a plastic knife, trim this piece to a cube 3 cm x 3 cm x 3 cm. Repeat
                                                    3         3
     this procedure to make two more cubes 2 cm and 1 cm (Figure 1).
2.   Place the three cubes carefully into a plastic cup. Add diffusion medium until the cup is approximately half full. Be
     sure the cubes are completely submerged. Using a plastic spoon, keep the cubes submerged for 10 minutes, turning
     them occasionally. Be careful not to scratch any surface of the cubes.
3.   As the cubes soak, calculate the surface area, volume, and surface area to volume ratio for each cube. Record these
     values in Data Table 1. Use the following formulas:

           surface area = length x width x number of sides                  volume = length x width x height

4.   After 10 minutes, use a spoon to remove the agar cubes and carefully blot them dry on a paper towel. Then, cut the
     cubes in half. Note the color change from red or pink to clear that indicates the diffusion of diffusion medium
     (vinegar) into the cube.
5.   Using a metric ruler, measure the distance in millimeters that the diffusion medium diffused into each cube by
     measuring the width of the clear area (Figure 2). Record the data in Data Table 2. Next, record the total time of
     diffusion. Finally, calculate and record the rate of diffusion for each cube as millimeters per minute.
6.   Calculate the percentage of cell reached by diffusion for each cube.
             a. Measure one side of the colored area of each cube. Calculate the volume of the colored area for each
                  cube and record in Data Table 3.
             b. Determine the volume of the cleared area of the cubes by subtracting the volume of the colored area from
                  the total volume. Record in Data Table 3.
             c. Finally, calculate the percentage of each cube into which the diffusion medium diffused. Divide the
                  volume of the cleared area by the total volume and multiply by 100. Record in Data Table 3.

7.   Clean up your area. Dispose of agar cubes in the garbage. Return diffusion medium to a waste beaker designated
     by your instructor. Rinse and return cup, knife and ruler neatly to your lab table.
                  Diffusion & Cell Size Data Tables
                               Data Table 1: Surface Area to Volume Ratio


 Cube Size         Surface Area (mm2)              Volume (mm3)             Surface Area : Volume




                                     Data Table 2: Rate of Diffusion


            Cube Size    Depth of Diffusion (mm)        Time (min)           Rate of Diffusion




                                  Data Table 3: Efficiency of Diffusion

                                    Volume of
                Total Volume                        Volume of clear    Percentage of cube reached
Cube Size                          colored area
               of cube (mm3)                          area (mm3)               by diffusion
                                      (mm3)
NAME __________________________________________ PERIOD ____
                                               ASSESSMENT
1.    The agar you used to make your cubes contained phenolphthalein and had a pH of greater than 9. Explain
      how the use of a pH indicator allowed you to visualize the extent of diffusion into the cubes.




2.    According to Data Table 2, into which cube did the diffusion medium diffuse the deepest?


3.    Into which cube did the diffusion medium diffuse the most by volume?



4.    Examine your data in Data Table 2 for a relationship between cube size and the rate of diffusion into the
      cube. Make a generalized statement about the relationship between cell size and the rate of diffusion.



5.    Examine your data in Data Table 1. Describe what happens to the surface area, the volume, and the ratio
      between the two values as a cell grows larger.




6.    If each cube represented a living cell and the diffusion medium a substance needed within the cell, what
      problem might exist for the largest cell?




7.    According to the results of your investigation, describe the characteristics of cell size, surface area, and
      surface area to volume ratio which best meet the diffusion needs of living cells.




8.    Compare the surface area to volume ratios between one 27cm cube and 27- 1cm cubes. Show work.

      Cube Size                   Surface Area                          Volume                  SA/V Ratio

     1- 27cm cube


     27 – 1cm cube

				
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