Osmosis (Jacob Bear)

Title: Demonstrating Osmosis with Eggs and Celery

Principle(s) Investigated: Diffusion, osmosis.

Standards : Biology/Life Sciences 1a. Students know cells are enclosed within semipermeable membranes that regulate their interaction with their surroundings.

Materials:

Eggs (1 per table or group, plus a few extras), vinegar, salt, distilled water.

A picture/diagram illustrating osmosis. I got mine from the following source: http://serendip.brynmawr.edu/sci_edu/waldron/.

A worksheet with questions and explanations on one side, lab/prediction questions on the other. Mine (attached) was adapted from a worksheet on the following website: http://serendip.brynmawr.edu/sci_edu/waldron/.

Procedure:

Approximately 3 days before the activity, place chicken eggs in vinegar to dissolve the shells. (One egg per group or table).

Soak eggs for 24 hours, then gently rub with fingers under cool running water to remove the rest of the shell.

Promptly place eggs in distilled water and let soak until the activity takes place. Give a detailed explanation of the procedure and include diagrams if possible.

Fold the worksheets in half and tape or staple shut, so that the diagram and explanation are invisible.

On the day of the demonstration, place a 6” cut stalk of celery on the demonstration table. Fill a 1 liter beaker with approximately 700 ml water. Stir in 50 g of salt, and place beaker on the demonstration table. Fill an identical beaker with 700 ml water and place on demonstration table. Distribute the sealed worksheets. Then follow the steps below:

1. Have a volunteer weigh the celery and tell the class to write down the weight on their worksheets

2. Put celery into the first beaker (salt water).

3. Pass out the eggs that have been soaking in distilled water. Put out a few “regular” eggs for comparison

4. Ask, “Draw or write on this sheet (don’t open it!) whatever you’ve observed and what you think is going on”

5. Tell the class to answer questions 1-2 on their worksheets

6. Invite the class to share/discuss possible reasons for the things they’ve seen so far

7. Ask the volunteer to look at the celery and see if there’s a visible difference

8. weigh celery again, have the class record this on their worksheets

9. Place the celery into the 2^nd beaker (water)

10. Tell the class to break the seal and open the worksheets

11. Discuss the activity and diagrams

12. Complete the inside of the worksheets

13. Predict what will happen to celery/cilantro, and complete the questions on the outside of the worksheet

14. Weigh celery and see if it’s been restored to its original size and weight

15. Discuss the further questions and applications if time allows such as: How this process could be reversed (active transport), the reasons saltwater fish couldn’t survive in freshwater and vice-versa, implications for water purification, etc.

Student prior knowledge: This requires an understanding of the basic structure of cells, especially the idea that a cell membrane separates the cell from its environment; students should also already know that energy and most matter will flow from an area of higher pressure/concentration to an area of lower concentration.

Explanation:

This is a hands-on demonstration of osmosis, and the class is given examples of osmosis both causing water flowing into a cell and causing water to flow out of a cell.

The distilled water is free of solute, and therefore there will be a net movement of distilled water from the outside of the egg’s membrane to the inside. Since the eggs may need a day or more to take on water and swell to double their size, they are prepared in advance.

The celery, on the other hand, will have a lower concentration of solutes in its cells than exists in the salt solution, and will therefore lose water through osmosis. This process only requires about 10 minutes in the salt solution for water to flow out of its cell membranes, allowing a detectable change in weight and possibly a visible change in turgidity.

This means you can give “before and after” demonstrations of water passing through a cell membrane via osmosis. The worksheets and discussions are timed to allow 10 minutes or so for osmosis to take place.

If there is extra class time, you can reverse the celery demonstration by placing it in the plain water flask, allowing water to pass through the membranes back into the cells, restoring turgidity.

Questions & Answers:

1. What are some reasons for an organism to actively move water or solutes in the opposite direction of osmosis? A: Many biological processes create osmotic pressure and then harness the resulting activity to perform work. Examples: the electron transport chain in photosynthesis and the Krebbs cycle; action potentials in neurons.

2. It is thought that many of the earliest life forms emerged in a saline environment. How might this have created conditions inside the cell that would be conducive to life? A: Since there was a high concentration of solutes outside the cell membrane, it would be possible to maintain a higher concentration of solutes within the membrane as well, and this would allow the accumulation of organic molecules such as RNA and amino acids, thought by many to be the precursors of life.

3. Why are most salt water-dwelling organisms unable to live in fresh water? A. Fresh water is “fresh” in that it usually contains a much lower concentration of solutes. This means there is far more osmotic pressure in the environment. Organisms adapted to this environment have mechanisms for dealing with it, but salt water-dwelling organisms would be in danger of having their cells engorged as the net flow of water was directed inside their cells. This could reduce the concentration of many substances inside the cell, and possibly impair the ability of certain essential chemical reactions to occur. In a worst-case scenario, the cells would burst as the water pressure (which is difference from osmotic pressure) exceeded the strength of the cell membrane.

Applications to Everyday Life:

Spraying water on produce in the supermarket. In the produce sections of most supermarkets, water is sprayed on the produce, especially salad greens, on a regular basis. This keeps the produce from becoming wilted. Plants give off moisture through evapotranspiration as well as through natural diffusion. This lowers the water content inside the plant cells, and therefore increased the concentration of dissolved solutes. If left unchecked, the plants lose turgidity. However, by frequently coating the produce with a thin layer of water, there is moisture with a low concentration of solute outside plant cells. This results in a net flow of water from outside the cell to inside the cell, and therefore balances the water loss and keeps the produce fresh.

Water purification. Water is often purified through a filtration process called “reverse osmosis.” Under normal circumstances, fresh water (relatively free of solutes) would flow across a membrane towards salty or “contaminated” water (containing more solutes). However, energy can be expended to reverse the process, forcing water to pass through a membrane while the solutes remain behind.

Helium balloons losing their buoyancy. A few days after the party, the balloons tend to hover just a short distance above the ground, or they may sink to the floor completely. Rubber and plastic are also membranes, and gases will pass across them from an area of greater concentration to an area of lesser. There’s a higher concentration of helium inside the balloon than outside. It will pass across the membrane (the balloon) into the air, while oxygen and other gases move towards the inside of the balloon.

Photographs:

Videos: Include links to videos posted on the web that relate to your activity. These can be videos you have made or ones others have made.