Teaching osmosis?
Osmosis is one of my favorite topics to teach during the cells unit because it’s so easily visualized. There are quite a few ways to easily see osmosis in action! Here’s a round-up of six labs and the pros and cons of each:
1. EGG OSMOSIS
The “naked egg” lab is very popular in biology. Students really get a kick out of seeing the eggshell dissolve! In this lab, students dissolve the shell of an egg by soaking it in vinegar (this takes about 3 days, so its best to set up on a Friday). Once the shell is gone, students carefully transfer their naked eggs to hypotonic distilled water, and a hypertonic solution like corn syrup or molasses. Students compare the mass of the egg before and after soaking and figure out which way the water moved while attempting to reach equilibrium.
PROS: Students love this lab! Also, eggs are round like red blood cells, so they can visualize what happens to cells in different hypertonic or hypotonic solutions.
CONS: It can get expensive to purchase enough eggs for all of your classes. Also, expect some to break along the way, so soak some extras just in case. Overall, it feels a little wasteful of perfectly good eggs. Maybe do this one as a demonstration, and then choose one of the options below for students to do in groups.
2. PURPLE ONION SKIN
Of all the osmosis labs, this one might be my favorite because I’m partial to getting out the microscopes. In this lab, students get a small piece of onion skin and make a wet mount slide using fresh water. They will see nice rectangular purple onion cells. Next, they will swap out the fresh water for salt water, and watch the cytoplasm in each cell shrivel up. If your students already know how to use microscopes, this one is a hit! Here is a full blog post with more details and pictures.
Note: I used to use elodea for this lab, but it became difficult to find at pet stores since it is invasive in many states. Purple onion is easy and cheaper! You can also find a lab write-up here.
PROS: Inexpensive, and fool proof. It is impossible to mess up! Unlike other options, this lab uses real plant cells. Also, it is the only version that doesn’t require soaking things overnight so you can get it done in one class period.
CONS: Your classroom will smell like onion for a day.
3. DIALYSIS TUBING
This version is a little fancier, and is a great option if you have honors or AP students. In this lab, students will get dialysis tubing and fill them with varying concentrations of sugar water solution. They will measure the initial mass of the tubes, and then soak the dialysis tubing overnight in distilled water. The following day they will measure the new mass, and see how water moved across the dialysis tubing membrane. You can find a free version of this lab from Amy Brown on TpT.
PROS: Students collect quantitative data, and it feels more “scientific” than other options.
CONS: Dialysis tubing is expensive, and if students don’t tie the string tight enough they can leak.
4. GUMMY BEARS
I am the first to admit gummy bears are not my favorite option, but I know many teachers who love doing it this way. In this version, students soak gummy bears in tap water, distilled water, and salt water overnight. They measure the change in the size of the gummy bear using rulers.
PROS: A large bag of gummy bears is only a few dollars, and students always love working with candy.
CONS: I don’t love this lab because depending on the brand you buy, the gummy bears can begin to dissolve and fall apart. Also, since they are an irregular shape it is difficult to calculate the change in volume.
5. WATER BEADS
An alternative to gummy bears is water beads, or Orbeez. You set it up the same way by soaking them in fresh water and salt water. If your kid already has some at home, use them instead of gummy bears. I like them better for a few reasons:
PROS: Water beads won’t fall apart like gummy bears, even after soaking multiple days. Since they are round, you can have students measure the diameter, and calculate the volume of the sphere. Also, you can dry them out afterward, place them in a Tupperware, and re-use them the following year.
CONS: Orbeez are smaller than gummy bears, so if your students struggle taking small, accurate measurements that might be a point of struggle. (Note: Beware of cheaper off-brands you can find on Amazon, because they WILL fall part unlike Orbeez).
6. BABY CARROTS
Last but not least are baby carrots! In this lab, each lab group gets 2 baby carrots. Just like other labs, they measure the mass before and after soaking them in fresh and salt water. Students will also notice that the baby carrot soaked in salt water becomes flimsy and bendable overnight. If you have already covered organelles, this can lead to a discussion about vacuoles and how plant cells become limp and flimsy when they lose water. You can find a lab write-up on my website!
PROS: Inexpensive materials, and uses real plant cells.
CONS: You won’t see the size of the carrot change, only the flexible vs stiff texture.
Alright, which is your favorite? Choose one (or two) and have a blast!
Rock on,
