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Investigating Osmosis Effects on Plant Cells

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              Investigating Osmosis in Plant Cells
All atoms and molecules are constantly in motion. The kinetic energy of this movement, particularly in
liquids and gases, eventually leads molecules to distribute themselves randomly and uniformly throughout
their container. This phenomenon is called diffusion. The general rule for movement of particles is:

       “Particles move from areas of higher concentration to areas of lower concentration.”




                                          Over time, the dissolved molecules in
                                  this beaker of water will be evenly distributed. Note
                                 that they are still in motion, but there is no net change
                                      in concentration once equilibrium is reached.


Osmosis is the diffusion of water across a selectively permeable membrane such as those found in living
cells. Plasma membranes are made of phospholipid molecules. Since phospholipids have both a
hydrophobic and hydrophilic part, they arrange themselves as a double-layer sheet, giving the cell a
barrier that very few molecules can pass through without help. Water can pass through the plasma
membrane through special protein channels called aquaporins. These channels are not gated, so water
effectively has free movement across the plasma membrane, in and out of the cell.

As a type of diffusion, osmosis follows the same principle of movement from higher to lower
concentration. However, it helps to also think of osmosis as a relationship between water and the
dissolved solutes in a solution.

Consider the “U tube” experiment as a demonstration of osmosis. A U-shaped tube is constructed with a
selectively permeable membrane at the bottom. This membrane is permeable only to water. The left side
of the U-tube is filled with a low-salt water solution. The right side is filled with a high-salt water
solution.

             Left Side                                Right Side
            10% NaCl                                  30% NaCl
            90% H2O                                   70% H2O

  This side of the tube has a lower      This side of the tube has a higher
    concentration of solute (the           concentration of solute (the
    dissolved salt), but a higher           dissolved salt), but a lower
      concentration of water!                 concentration of water!


           What direction would you expect water to move in this situation?

           In this apparatus, why don’t the solute molecules diffuse?
 Biology Teaching Resources                                http://www.aurumscience.com/biology.html 1
             Left Side                                                                  Right Side
            20% NaCl                                                                    20% NaCl
            80% H2O                                                                     80% H2O

This side of the tube has experienced                                       This side of the tube has experienced
       a net loss of water, the                                                   a net gain of water. The
   concentration of NaCl is now                                              concentration of NaCl is now less,
      greater, and H2O is less.                                                      and H2O is greater.


For osmosis, it is useful to consider a slight variation on the diffusion rule:

  “Water moves from areas of high solute concentration to areas of low solute concentration.”

The behavior of water molecules during osmosis has a significant impact on living cells. Consider the
example of the human red blood cell. A cell is dropped into one of three types of solution:

       A isotonic solution, such as Ringer’s solution, that contains the same concentration of solute
        (…and the same concentration of water) as the cell.

       A hypotonic solution, such as distilled water, that contains a lower concentration of solute
        (…and a higher concentration of water) than the cell.

       A hypertonic solution, such as ocean saltwater, that contains a higher concentration of solute
        (…and a lower concentration of water) than the cell.

In each of these situations, will there be a net movement of water into the cell or out of the cell?

Solution Tonicity               Effect on Red Blood Cell
Isotonic Solution

Hypotonic Solution

Hypertonic Solution

The diagrams below show the physical effects of these different solutions on the red blood cells.

        Isotonic Solution                    Hypotonic Solution                    Hypertonic Solution




   No net movement of water.            Net movement of water into cell,     Net movement of water out of the
                                              causing it to burst.           cell, causing it to shrivel and die.

This lab will examine how osmosis affects plant cells when placed in different types of solutions.
Procedure 1: Quantitative Measurement of Osmosis in Carrots

Purpose:
To identify what type of solution would be considered isotonic for a carrot cell.

Hypothesis:
What percent concentration salt solution would create an isotonic environment for a carrot cell?
(e.g. 0%, 0.5%, 1%, or 5%?)

Materials:
   Four carrot sticks
   Distilled water
   Sodium Chloride (NaCl)
   Four test tubes

Procedure
   1. Label four test tubes #1-4.

   2. Record the salt concentrations of each stock solution available.

      Table 1: Salt concentration of each beaker
     Beaker 1      Beaker 2       Beaker 3       Beaker 4



   3. Take a baby carrot and cut it in half so that it will fit in a test tube easily.

   4. Record the mass and length of each carrot piece.

       Table 2: Initial recorded mass and length of carrot pieces
                             Carrot 1      Carrot 2      Carrot 3           Carrot 4
   Initial Mass (g)

   Initial Length (mm)

   5. Place each carrot piece into a test tube. Make sure the carrots are completely immersed.

   6. Allow the carrots to sit in the test tube overnight.

   7. Record the final mass and length of each carrot piece.


       Table 3: Final recorded mass and length of carrot pieces
                                 Carrot 1      Carrot 2       Carrot 3            Carrot 4
   Final Mass (g)

   Change in Mass (g)
   Final Length (mm)

   Change in length (mm)


          Which carrots experienced a net gain of water?

          Which carrots experienced a net loss of water?

Analysis

Create a graph to show the change in size of each carrot. Place a scale for mass on the left side of the
graph, and a scale for length on the right side of the graph.

Graph 1: Effect of Different Concentration Salt Solutions on Carrot Mass




Conclusion

Based on your results, describe what happened to the carrot cells in each beaker in terms of net water
movement. How would this physically affect the carrot cells? Would the effects be the same as with red
blood cells? What is different about plant cells such as those in carrots and animal cells such as ours?
Identify each solution as hypotonic, isotonic, and hypertonic.

				
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Description: A biology lab where students investigate the effects of different salt concentrations on the size and mass of plant cells.