Observing Osmosis

By Laura Hospitál on Aug 31, 2015

In basic biology courses, students learn about the movement of water into or out of the cell by means of osmosis. 
 
Osmosis is the movement of water from a more dilute solution to a less dilute solution through a semi-permeable membrane (the cell membrane).  Student learn the terms hypotonic, hypertonic, and isotonic.  (See below)
 
These terms are often difficult for students to remember as cells shrink in hypertonic solutions and grow in hypotonic solutions which may seem counterintuitive but makes sense as the terms relate to the solutions and not to the cells themselves.  I have found this lab helps students to better remember and comprehend these terms and concepts.
 
I like this lab better than a lab that I originally tried using grapes as there is a greater increase or decrease in the size of the egg.  Students can both measure the increase or decrease in the size of the egg and feel it as well.  Occasionally the egg in the hypotonic solution even breaks. 
 
Soaking the eggs in vinegar causes the eggshell to dissolve and the white of the egg becomes rubbery. (due to a chemical reaction)  Water can therefore flow into and out of the egg. 
 

Vocabulary 

Isotonic solution: (iso = same as normal)  The concentration of dissolved substances in the solution is greater than the concentration of dissolved substances in the cells.  (Boric acid)
 
Hypertonic solution (hyper = mor than normal  The concentration of dissolved substances int he solution is greater than the concentration of dissolved substances int he cells.  (Corn Syrup)
 
Hypotonic solution (hypo= less than normal) The concentration of dissolved substances in the solution is less than the concentration of dissolved substances in the cells.  (Distilled water)
 
Osmosis:  the movement of water through a cell membrane from an area of lower solute concentration to an area of higher solute concentration
 

Preparation:

2 Days prior to the lab -  Place eggs in a large container and cover with vinegar for 48 hours
 
If time allows, students can do this preparation and observe the eggs each day.  I usually soak the eggs at home and bring them to school
 

Materials

  • 3 eggs per group
  • vinegar
  • distilled water  (hypotonic solution)
  • corn syrup (hypertonic solution)
  • boric acid (isotonic solution)
  • Goggle and Aprons 
  • gloves (Latex-free for those with latex allergies)
  • (3) 100 mL beakers for each lab group
  • Braille paper (for labels)
  • Bold-lined paper (for labels)
  • double sided tape (for labels)
  • accurate balance (electronic if available)
 

Procedure

Prior to completing the procedure a hypothesis will be formulated.  

Problem:  

How do hypotonic, hypertonic, and isotonic solutions affect the water movement in a cell?  

Hypothesis:

Students should formulate a hypothesis describing expected change in mass when placed in each solution.  

Define and discuss: 

isotonic solution: (iso = same as normal)  The concentration of dissolved substances in the solution is greater than the concentration of dissolved substances in the cells.  (Boric acid)   hypertonic solution (hyper = mor than normal  The concentration of dissolved substances in the solution is greater than the concentration of dissolved substances int he cells.  (Corn Syrup)   hypotonic solution (hypo= less than normal) The concentration of dissolved substances in the solution is less than the concentration of dissolved substances in the cells.  (Distilled water)   Osmosis:  the movement of water through a cell membrane from an area of lower solute concentration to an area of higher solute concentration  

Procedure

  1. Label the 3 beakers: isotonic, hypertonic, and hypotonic
  2. Pour 50 mL of each solution into the respective beakers
    • isotonic - boric acid
    • hypertonic solution - corn syrup
    • hypotonic solution - distilled water
  3. Gently wipe off one egg and weigh the egg.
  4. Record the measurement on the chart in the first column (Original Mass)
  5. Place the egg in the hypertonic solution (corn syrup)
  6. Repeat steps 3-5 for the hypotonic solution (distilled water)
  7. Repeat steps 3-5 the isotonic solution (boric acid)
  8. Allow eggs to sit in the solutions for 2 days.
  9. Remove eggs and gentrly dry off
  10. Take the mass of each egg and record it in the 2nd column (final mass)
  11. Calculate the difference between the initial and final mass of each egg and record it in the final column  (Difference)
  12. Compare and contrast the before and after masses.
  13. Make comments in the Other Observations section of the lab procedure

observing osmosis chart

 

Conclusion:  

Did the change in mass for each egg support your initial hypothesis?  

 

Variations

As mentioned in the intro, this lab may be completed with grapes.  In this case the following are used as the 3 solutions.
  • isotonic solution:  grape juice
  • hypotonic solution:  distilled water
  • hypertonic solution: almost saturated sugar solution
However, I don't recommend this procedure due to the size of the grapes compared with the eggs.  

NGSS Standards:

Middle School: Structure, Function, and Information Processing
LS1.A: Structure and Function
All living things are made up of cells, which is the smallest unit that can be said to be alive. An organism
may consist of one single cell (unicellular) or many different numbers and types of cells (multicellular).
(MS-LS1-1)
Within cells, special structures are responsible for particular functions, and the cell membrane forms the
boundary that controls what enters and leaves the cell.
(MS-LS1-2)
 
osmosis collage
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