I'm excited to share with you an EASY PEASY way for students to see osmosis in plant cells! In the past, I always used elodea leaves for this lab. Elodea can be hard to find at pet stores and is a little temperamental to keep alive. This year I decided to use onion skin from a purple onion and we got awesome results!
I used this lab BEFORE I taught any vocabulary such as osmosis, equilibrium, hypertonic, hypotonic, or isotonic. I wanted students to visually see what happens to cells in fresh water vs. salt water before I threw any vocabulary at them. Students were really excited to see the cells change within a matter of 60 seconds. Here are a few tips when doing this lab for the first time:
1. You cannot use the dry layers of the onion skin. You need to use the very top of the purple fleshy layer. It can be a little bit tricky to get a specimen that is thin enough, so I decided to do it myself and hand each kid a piece. I took metal tweezers, gently pushed them under the purple layer, and slid the tweezers out so a small flap of onion skin was loose. I peeled it off, handed it to each kid, and they set up their own wet mounts. No dye needed!
2. Have students make drawings using fresh water first. After they finished their drawing, they switched to salt water. To do this they do not need a new piece of onion, just leave it directly on the slide. Add a drop or two of salt water directly to the slide, no need to pre-soak the onion. Make sure your salt water solution is pretty saturated.
3. Tell students to wait at least 2 minutes before drawing the salt water image, because sometimes it takes a little time for the cytoplasm to shrivel up. Below are images of the onion cells in fresh water (left) and salt water (right) on 100x magnification. We had a discussion on whether or not the cell wall shriveled as well. Students automatically said yes, because the cell wall is almost transparent and harder to see. Once I told them to switch to high power (400x) they were able to see the cell wall more clearly and realize that the cell walls were still intact, while the membrane and cytoplasm shriveled.
Tomorrow we are going to follow up with the discussion of what happened and why. Students will take notes on osmosis and we will relate it to real world situations such as: Why can't I drink salt water if I'm stranded on a boat in the ocean? Why is my contact lens solution saline instead of pure water? Why do grocery stores spray the produce with water? If you want a quick worksheet to use as a formative assessment to follow this lesson, check out my tonicity and osmosis worksheet in my TpT store HERE.
I hope your students enjoy the lab as much as mine did! Other than having my classroom smell like onion for a day, it was a total win! Want more ideas? Check out this blog post that has 10 resources for teaching cell membranes!
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As fun as prepared slides are, students always LOVE looking at living organisms under the microscope. I generally order mixed protist specimens from Wards or Carolina Biological, but this year I didn't get an order put in on time (if your district is like mine it often takes months to get things ordered and delivered...) Since I didn't have anything for my students to look at, I decided to make a hay infusion. It turned out great for what I needed.
Overall, here are the pros and cons of doing a hay infusion:
Before using the hay infusion, have your students practice using the microscope with prepared slides. If students are comfortable with how to focus and scan, it makes it much easier when they are looking for things that are swimming around. We began the class with learning how to set up a wet mount slide using an elodea leaf. Once they got the hang of it, they cleaned their slide and took a sample of the pond water.
Here is a video I took under 40x magnification:
and 100x magnification:
Although there weren't a variety of species to see, students were still pretty excited to see them swimming around. I wasn't able to identify which type of protists we had- if you had honors or AP students it might be fun to give them a protist dichotomous key and see if they can figure out which species they find. Overall it was a success and didn't cost me a cent!
Why the nucleus isn't King of the castle
Ever since middle school when students learn about cells, they are taught that nucleus is the control center of the cell. They hear that the nucleus is "the brain" and in charge of all cell functions. When teachers do the cell-as-a-factory analogy, the nucleus ends up being the boss. This is not technically true... while the nucleus houses all the information the cell needs to complete different tasks, it isn't in charge of when that information is used. We need to make sure students understand why cells do the things they do, and it all comes down to cell signaling.
Cells complete cellular processes when the cell membrane gets a signal from the outside environment. Once the signal is received, then the cell will respond by using the genetic information in the nucleus to carry out the task. That task will generally keep going until the signal is terminated. Here are a few examples:
How to get students thinking:
This can be a tricky concept to introduce to students. High school students don't usually understand how the cell operates as a whole and communicates with the outside environment. A great way to introduce the topic is by posing them these questions: "Are identical twins truly identical? Is it possible for one twin to get cancer while the other does not get cancer?" Most students will say yes, this is possible. But if they have the same genes, how can this be? Our cells are not pre-programed to behave based on our DNA. Genes are only regulated based on signals from the environment. Many students also get confused when we talk about "the environment," because they are so used to hearing this term used in ecology. Make sure students understand that the cell has its own environment within the body.
The moral of the story:
Do you want your students to read an article on this topic? Check out this close reading article I wrote available in my TpT store. It is a 3 page article with reading comprehension questions at the end for students to answer. The article covers an overview of: proteins and the central dogma, the lipid bilayer, and epigenetics. It does not cover the details of the types of cell signaling. I believe it is written at a level where most high school students can fully understand the concept of cell signaling and the cell membrane.
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Would you like lab ideas for teaching about the cell membrane? Check out this blog post!