Cell Size Lab - DOC

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					Cell Size Lab


       Cells come in a variety of sizes. Some red blood cells are only 8 µm in diameter.
Nerve cells can reach lengths up to 1 meter. Most living cells, however, are between 2
and 200 µm in diameter. Why can’t organisms be just one giant cell?

1. Diffusion limits cell size: cells require a constant supply of glucose and oxygen to
carry out __________________________________ and to produce large amounts of
_______________. These substances, and waste products such as carbon dioxide, move
through the cytoplasm by diffusion. It has been estimated that it takes a molecule of
oxygen only a fraction of a second to diffuse through the cytoplasm from the plasma
membrane to the center of a typical cell that is 20 µm. What would happen if the cell got
        Although diffusion is efficient over short distances, it becomes slow and
inefficient as the distance becomes larger. Hypothetically, a cell with a diameter of 20
cm would have to wait months before receiving molecules that enter the cell. Because of
this time restriction the cell/organisms would die if they were one large cell.

2. A cells DNA limits size: The second reason why cells are small is because they
usually only contain one nucleus. If a cell doesn’t have enough DNA to program its
metabolism, it cannot live. When a cell is larger it requires more enzymes and parts to
function correctly, therefore it needs more instruction. In many large cells there are more
than one nuclei to ensure that all cell activities will be carried out quickly.

3. Surface area-to-volume ratio: The third reason why cells are limited in size is that as a
cell’s size increases, its volume increases much faster than its surface area.
Note: Surface area = length of a side X width of a side X number of sides
        Volume = length X width X height
        Surface area to volume ratio = surface area / volume
        The smallest cell has a volume of 1mm3 and a surface area of 6mm2. If the side
of the cell is doubled to 2 mm, the surface area will increase fourfold. Calculate what
happens when the cell side is 4mm. Volume _________________________
Surface area ____________________________________________.

       If cell size is doubled, the cell would require eight times more nutrients and would
have eight times more waste to excrete. The surface area would only increase by a factor
of 4. Therefore, the plasma membrane would not to have enough surface area through
which oxygen, nutrients, and waste could diffuse. The cell would either starve to death
or be poisoned by the build p of waste products.

        In this lab, the diffusion rate of sodium hydroxide will be observed through 4 agar
cubes of different size. The cubes contain phenolphthalein which turns purple when it
comes in contact with basic solution, sodium hydroxide. Sodium hydroxide will slowly
diffuse into the agar cubes and change its color from clear to purple.

      1. Obtain pieces of agar from your instructor. Using a knife cut the agar into
         cubes following the given measurements below. Plan out your cuts ahead of
         time, it is suggested that you cut out the large cube first. Only one piece of
         agar will be given to your group.

                      Block A:   0.5 cm sides
                      Block B:   1.0 cm sides
                      Block C:   2.0 cm sides
                      Block D:   3.0 cm sides

       2. Return any reusable pieces to instructor and discard remaining agar
       3. Calculate the surface area for each cube and record the answer in table 1.
          Surface area = length of a side X width of a side X number of sides

           Calculate the volume of each cube and record the answer in table 1.
           Volume = length X width X height

           Find and record the surface area to volume ratio for each cube in table 1.
           Surface area to volume ratio = surface area / volume

       4. Place all four cubes in the bottom of a beaker so that they do not touch each
          other or in the sides of the beaker.
       5. Pour in enough sodium hydroxide to cover all four cubes. Allow 20 minutes
          for the diffusion process. GENTLY stir the solution so that diffusion can take
          place evenly over each cube.
       6. Remove the blocks from the solutions, rinse underwater, place on a paper
          towel and slice each cube in half.
       7. Quickly measure to the nearest 0.1 cm across the area with no sodium
          hydroxide on each cube (clear area)
       8. Calculate the surface are, volume, and surface area to volume ratio for the
          portion of the cube with no sodium hydroxide (clear cube) and record it in
          table 2.

Table 1:
  Cube           Side length         Surface area         Volume (cm3)        Surface area to volume
                 (cm)                (cm2)                                    ratio
                                                                              (surface area : volume)




Table 2                         ! Portion of the cube without sodium hydroxide!
  Cube           Original side    Surface area            Volume (cm3) Volume diffused into
                 length of cube (cm )                                           the cube
                 (cm)                                                           (volume of original
                                                                                cube minus the volume
                 (see above)                                                    of the middle cube)




     1. What solution is being diffused into the agar block?

     2. What is the structure and chemical formula of phenolphthalein?

     3. Rank the cubes from the largest surface area to volume ratio to the smallest
        surface area to volume ratio.

     4. Why are cells relatively small in size? List at least three reasons

     5. All of the cells below have approximately the same volume. Calculate the surface
        area for all cubes. Then, circle the letter of the one with the largest surface area.




     6. How would heating the sodium hydroxide solution affect the results in this lab?

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