Osmosis and Diffusion- Starch Baggie (Monica Platten)
Osmosis and Diffusion- Starch Baggie Lab
Principle(s) Investigated: Diffusion and Osmosis
Standards : Biology/Life Sciences
1a. Students know cells are enclosed within semipermeable membranes that regulate their interaction with their surroundings.
Materials:
2 Celery sticks
Salt water solution (enough to make water cloudy)
Corn Starch (Market)
Water
Beakers
Plastic Baggie (A plastic produce baggie works best)
Iodine (I2K2)
Graduated Cylinder
Procedure:
- Fill a plastic baggie with 14 grams of corn starch and 100mL of water, mix and seal bag.
- Fill a 600 mL beaker with 200mL of water and add 5mL of iodine.
- Place the baggie in the cup so that the cornstarch mixture is submerged in the iodine water mixture.
- Wait fifteen minutes and record your observations in the data table
- While you are waiting, answer the questions.
This lab was modified from the following website:
Student prior knowledge:
Students will know the structure of the cellular membrane and it's semi-permeability. Students should be introduced to the terms hypotonic, hypertonic, isotonic, solute, solvent.
Explanation:
Introduce lab with spraying air freshener in the back of the room (make sure you are not near a student). See if students can smell the type of freshener. Have students raise their hands as soon as he/she can smell the freshner. You can also show food coloring dropping in a beaker of water under the document reader.
The students will learn about semipermeable membranes, osmosis and diffusion. The osmosis portion of this experiment should be something the teacher has done prior to the lab. I placed a celery stick in a cup of blue dyed hypertonic solution (salt water) water to show osmosis in the celery. The point of this lab is to show that the molecules that make up a solid are constantly in motion. They move from areas of higher concentration to areas of lower concentration by the process of diffusion. Some molecules are also able to diffuse into and out of living cells, providing a source of nutrients and allowing for the export of cell products or wastes.
A cell is surrounded by a cell membrane made mostly of lipids and proteins. The membrane is selectively permeable because it allows some materials, but not others to move across the membrane. Small ions and molecules of oxygen (O2), carbon dioxide (CO2), water (H2O) can move across freely. Other molecules, especially those that do not mix well with lipids (oily or fatty substances) and large macromolecules, must move through special pores or channels in the cell membrane made up of proteins.
The movement of water through a selectively permeable membrane is a special type of diffusion called osmosis. Water moves from an area of lesser solute concentration (hypotonic) to an area of greater solute concentration (hypertonic).
While the students are waiting for the results of their lab they are to answer the questions on the lab worksheet (which is attached below). This will help them get started on seeing the relationship between the solute in the bag and in the beaker.
At the end of the lab the students will see that the plastic baggie is semi-permeable to iodine. This is shown by the starch indicator that turns purple/black when it comes in contact with iodine. The baggie is not semipermeable to starch because the starch does not move out of the bag into the beaker. This example of diffusion shows how iodine is going from an area of high concentration to an area of low concentration (into the baggie) and it will constantly have a flow in and out of the baggie but at equal levels.
After the lab I will show how I placed a celery stick in salt water (hypertonic solution) and have my control celery in fresh water.The saltwater celery is now rubbery and more flexible when compared to the freshwater celery. The cells have lost water due to the hypertonic state.
Questions & Answers:
1. What would happen if you drink sea water to stay alive?
You would actually not be doing your body a favor. Instead of quenching your thirst, the salt in the sea water would create an hypertonic environment in your body. The cell membranes may start to rupture based on the amount of water ingested. Your body would become dehydrated.
2. Paramecium live in pond water environments which is hypotonic to the cell, how is it that they are able to do so?
The bodies of every animal is adapted to live in their environment. So something that is considered a hypertonic or hypotonic environment to humans may be not be for them. These animals have special adaptations for osmoregulation, the Paramecium has a plasma membrane that is much less permeable to water than the membranes of other animals. It is also equipped with a contractile vacuole that functions as a pump to force water out of the cell as fast as it enters by osmosis.
3. Why would you be unable to put saltwater fish into freshwater?
Salt water fish deal with a great amount of osmotic pressure for the salt water they live in. They are constantly drinking water and pushing salt out of their bodies. When placed in fresh water they continue to push out salt and therefore lose to much salt from their bodies and gain too much water.
Applications to Everyday Life:
Produce Section at the Market
Water is sprayed regularly on produce to help balance out evapotranspiration. The produce constantly loses water and therefore water needs to be replaced so as to keep turgidity. Since a small amount is sprayed there is a net diffusion of water into the plant cells and therefore the plants are kept fresh.
Water purification.
Water can be purified through a process called “reverse osmosis.” Energy is used to force the reverse process of forcing water to pass through a membrane while the solutes does not. Energy is needed because usually water will diffuse from an area of little solutes to one of more concentrated solutes.
Drink Water to Maintain Homeostasis
The body requires water for its proper function. Everyday cells within our body keep an isotonic relationship in our body. Our body does constant exchanges using osmosis to get nutrients needed to different organs. Our kidneys use osmosis to expel or retain the necessary amount of water in our body.
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