Laboratory Experiment 1: Diffusion and Osmosis for AP Biology
If you feel uncomfortable with this material, take a few moments to review and scan the information about diffusion, osmosis, and cell transport.
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This experiment is designed to examine the diffusion rate of small particles through selectively permeable membranes—dialysis tubing is the membrane of choice for the experiment.
Basic Setup for Part 1 of the Experiment
In the first part of this experiment, the student places a solution of glucose and starch into a bag of dialysis tubing. This dialysis bag is then placed into a beaker of distilled water. Every experimenter's favorite step follows: the 30-minute wait. After this time, both the bag and the beaker are examined to determine if starch and/or glucose are present.
Results Obtained from Part 1
The bottom line is that glucose leaves the bag, the starch sits still, and water enters the bag. Why do they move in a particular direction? Because of the concentration gradients present at the beginning of the experiment. These two substances move down their concentration gradient from a place of higher concentration to a place of lower concentration. The starch does not move anywhere during all this activity. Why not? Because the starch molecules are too large for the pores of the dialysis bag. The water and glucose molecules are able to move through the bag because of their small size. The rate of diffusion is found to be inversely proportional to size. The smaller you are, the higher the rate of diffusion.
Basic Setup for Part 2 of the Experiment
The second part of the experiment deals with osmosis, the diffusion of water down its concentration gradient. It is important that you know the following three terms. Isotonic solutions have identical solute concentrations. A hypertonic solution has a solute concentration higher than that of a neighboring solution. A hypotonic solution is one that has a lower solute concentration than a neighboring solution.
The bottom line here is that osmosis moves water from hypotonic to hypertonic. In this part of the experiment, six dialysis bags are filled with various concentrations of solute, weighed, and then dropped into a beaker full of distilled water. Once again, good things come to those who wait, and after 30 minutes of waiting, the bags are removed and examined.
Results Obtained from Part 2
The higher the original concentration in the dialysis bag, the more water that moves into the bag during the 30 minutes. The water is driven from a region of lower solute concentration to a region with higher solute concentration (or from high water concentration to low water concentration). If a dialysis bag with a solute concentration of 0.2 M were placed into a beaker having a solute concentration of 0.4 M, the water would flow out of the dialysis bag and into the beaker.
Other Important Concepts from Experiment 1
Another concept covered in this experiment is water potential (ψ)—the force that drives water to move in a given direction. It is important to recognize that solute concentration is only one part of this potential force. Another factor is the pressure potential of the solution. Increased pressure potential translates into increased water movement. Water moves from a region of higher water potential to a region of lower water potential. Water will continue to pass from one region to another until the net water potential difference between the two regions has equilibrated at zero.
In this part of the experiment, each student takes four cut pieces of potato, weighs them, places them into a 250-mL beaker filled with water, and lets them sit overnight. The next day, the potatoes are removed and weighed to determine what changes have occurred. Potatoes placed in distilled water absorbed water. As the solute concentration of the solution in which the potatoes rested overnight is increased, the amount of water that flows out of the potatoes increases as well:
ψ = ψsolute + ψpressure
The last main concept covered in this experiment is plasmolysis—the shriveling of the cytoplasm of a cell in response to loss of water to hypertonic surroundings. This causes the plasma membrane to separate from the cell wall. When a cell is placed into a hypertonic environment, diffusion of water from the cell to that environment will cause this plasmolytic response. Just remember that it can happen when a cell is hypotonic to its surroundings.
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