Cellular Transport
DIFFUSION AND OSMOSIS


Diffusion and Osmosis are processes of cellular transport. Through these processes and in conjunction with kinetic energy, molecules are able to pass across the cell membrane. Diffusion is the movement of molecules from an area of higher concentration to an area of lesser concentration across a permeable membrane. For example, if you placed an opened bottle of ammonia in the front of a room, the higher concentration of molecules in the bottle will disperse into the lower concentration of water molecules in the air of the room. Eventually, the aroma of the ammonia will be detected in the rear of the room.

Osmosis is the diffusion of water molecules across a semipermeable membrane. For example, in taking an extended hot bath, the water molecules will diffuse across the epidermal cell membranes since the fluid in the body has less water and more salt molecules than the bath water. Thus, you will feel bloated.

Osmotic solutions (in relationship to the fluids of the body) can either be hypertonic, hypotonic, or isotonic. Hypertonic solutions have a greater concentration of solutes (NaCl in this lab experiment) than solvent (water, H2O). A hypertonic solution that will be used in this lab exercise is 10% NaCl solution. Hypotonic solutions have a greater concentration of solvent compared to the concentration of solute. For example, distilled water, which has more water molecules and fewer solute molecules, will be used in this lab exercise as a hypotonic solution. Isotonic solutions have equal solute to solvent ratio. Therefore, an example of an isotonic solution would be a physiologic saline solution (0.85% NaCl).

When blood cells are placed in these different osmotic solutions, it causes the blood cells to either shrink, burst, or stay the same. Blood cells placed in an isotonic solution will stay the same since there is an equal amount of solvent/solute both inside and outside the cell. However, when blood cells are placed in a hypotonic solution, the cells will burst because there is a lesser amount of water inside the cell and a greater amount of water outside the cell thus causing the water to flow in the direction of the concentration gradient. Water rushes inside the cell (to establish equilibrium), but there is no mechanism that tells the cell equilibrium has been established. Therefore water continues to rush into the cell until the cell eventually bursts. This bursting process is called hemolysis (hema=blood; lysis=to burst). Blood cells placed in a hypertonic solution will have a reverse reaction to the hypotonic solution. Water will rush out of the cell thus causing the cell to shrink. This shrinking process is scientifically termed crenation.


EXPERIMENT 1 - DIFFUSION

  1. Place 30-35ml of tap water into a 50ml beaker.
  2. Place the beaker on your lab table then place very few crystals of Potassium Permanganate (KMnO4) into the water. DO NOT STIR OR SHAKE THE WATER.
  3. Set the beaker aside making note every 5 minutes of what is happening to the crystals.


EXPERIMENT 2 - OSMOSIS

A DEMONSTRATION WILL BE MADE OF MOLECULES MOVING ACROSS A SELECTIVELY PERMEABLE MEMBRANE. THE DEMONSTRATION WILL CONSIST OF A THISTLE TUBE CONTAINING MOLASSES PLACED IN DISTILLED WATER. YOUR LAB INSTRUCTOR WILL EXPLAIN THIS SET-UP AND THE PROCESS INVOLVED.


EXPERIMENT 3 - OSMOSIS IN BLOOD CELLS

You will need four clean microscope slides and four cover slips.

From the supply table or container, obtain the following:

LABEL TEST TUBES #1 - # 4.

IN TEST TUBE # 1:

  1. With a ruler, mark off the tube at 3 cm.
  2. fill to the mark with 0.9% NaCl and add 3 drops of blood.
  3. Place rubber stopper securely in opening of test tube and shake well.

IN TEST TUBE # 2:

  1. Mark off the tube at 3 cm.
  2. fill to the mark with 10% NaCl and add 3 drops of blood.
  3. Place rubber stopper securely into test tube opening. Shake.

IN TEST TUBE # 3:

  1. Mark off the tube at 1cm and 3cm.
  2. Fill to the first mark with 0.9% NaCl and to the second mark with distilled water.
  3. Add 3 drops of blood, close with rubber stopper, shake.

IN TEST TUBE # 4:

  1. Mark off the tube at 1/2cm and 3cm.
  2. fill to the first mark with 0.9% NaCl and to the second mark with distilled water.
  3. Add 3 drops of blood, close with rubber stopper, and shake.

Note the clarity of each test tube.

EXPERIMENT 4 - APPEARANCE OF CELLS

Prepare a wet mount from each of the tubes, check the appearance of the cells, and in table 1.1 give the correct scientific word to describe this appearance.

**Dispose of contents of test tubes and slides as directed by your lab instructor.

Table 1.1 - Osmosis Data

TUBE #

APPEARANCE

SCIENTIFIC TERM

1    
2    
3    
4