How Can I Do The Cell Size Lab If I Don’t Have Agar?
I love the Cell Size Lab that uses agar and an indicator. It just an awesome lab to help my students see how things diffuse into the cell. But, this year, I decided to move the cell size discussion to just before we discuss mitosis and I forgot to order my agar 😳 So now what do I do?
So this was my lesson for today with my 9th grade Biology class. I have to preface this with the fact that it has not been above freezing here in Alabama for the past week and our science labs do not have any heat (or air conditioning either but that’s a long story!). In light of the frigid temps both outside and inside, I opted to do this lab as a demonstration today. So….how do I keep them engaged if they aren’t actually doing the lab themselves? Good question…. lol
Here’s what I did….
I started the class with an anticipation guide with 5 questions about cell size, volume, etc. Then, I did a visual demonstration of “Do these 2 things have the same volume or not?” Before class, I took 2 sheets of old-school transparency film and made a tall skinny cylinder with one of the rolling it longways (portrait orientation or as my students call it “hot dog” fold) and taping it where the edges meet. I did the same to the other piece of transparency film except I made it shorter and rolled it the other direction (landscape orientation or, according to my kids “hamburger” lol). I started with the tall cylinder and filled it with macaroni to the top. Then, I had them theorize if they thought the shorter cylinder would hold the same amount or not. I put the larger cylinder over the tall one and pulled the tall one out. Most of my students thought that, since the sheets had the same dimensions 8.5 x 11 inches that they would also hold the same volume of macaroni. This was a good opportunity to address their misconceptions and open the door to discuss surface area.
The demonstration was also a good lead in to our demo with cubes of potato (since I didn’t have agar). Since iodine reacts with starch in potato and turns a bluish- black color, I figured that I could use this to show how diffusion occurs when potato cubes of different sizes are put into an iodine solution. Same principle as the agar lab right? So, I cut potato into 1 cm cubes, 2 cm cubes, and 3 cm cubes (Practical hint: I cut them the night before when I was cooking supper and just put them into sandwich bags with water covering them. The water kept them from turning dark. Nice trick so you don’t have to cut them right before class 🙂 ). We were able to talk about surface area as we compared the cubes. I also explained that an efficient cell should be able to diffuse nutrients into and wastes out of the entire cell. I had them develop a hypothesis about which cube “cell” they thought would be the most effective and what they would see as evidence if it was… They had to write it as an If, then statement “If the 1cm cube cell is the most effective, then the whole cube will be dark from the iodine when cut it open.” Then, I put them into the solution so they could “soak” in the iodine solution.
We watched the short Bozeman science video on cell size (https://www.youtube.com/watch?v=wuXSEOKNxN8 ) because he does the same demo (except with bark) and adds a really good explanation (my students love Mr Anderson!).
After the video… sketch notes!!
I use my document camera and we color together. We did the top half of our sketch notes, color coding the surface area title and arrows and circling the important terms to highlight them (Side note: We skipped the formulas until we had talked about the concepts of surface area and volume. I covered those when we did the calculations at the bottom. I had them draw a square around the formula with the same color they did the titles and arrows in from Surface Area and Volume at the top). We did the same thing in a different color to the volume side. The center circle was last because it is the foundation for why surface area and volume are important to cell size.
The bottom of the sketch notes examined the math behind the surface area to volume ratio. We worked through the formulas so they could see where all of the numbers came from with each “cell”. I had them predict which cell should be most efficient. Then, I took the potato cubes out of the iodine solution and we cut them open under my document camera.
Here’s what they looked like:
I was pleasantly surprised that the diffusion if the iodine was so evident after a relatively short time they were in the iodine solution (They stayed in about 20 minutes total). This is a really easy substitute if you don’t have agar or have time to prepare the full lab.
We used our results from the cubes to color that part of our sketch notes. Then, they went back to their original hypothesis and evaluated whether it should be adjusted. We also talked about what might occur if we left the cubes in there longer or even overnight and what our results might look like if our 1 cm cube was a .5 cm cube. This was such a good discussion point and a fantastic place to have students justify their reasoning with evidence from our results!
To wrap up our lesson, I always put their typed note sheet (that I give them with their sketch notes) up on my document camera and we highlight the important things together. This helps them work on their reading skills of finding the main idea. It also helps model for them how to read through science text and pull out the important ideas. It is a fantastic tie-in to the graphic sketch notes so that they can see how the text is translated into pictures. Having two sources of notes really seems to help many of my students who struggle with just written text. For their exit slip today, they revisited their anticipation guide and completed the “after” column.
Overall, I thought it was a successful lesson, especially since I had to do it as a demonstration instead of having the students in the lab performing it themselves. The students (all 30 of them) were actively engaged which is a miracle, especially on the first day back from Winter Break! 🙂 My principal actually came into my classroom twice during the block because he wanted to see what happened to the potato cubes in the iodine (he had his own hypothesis lol). I will definitely use this as a demonstration next year.
Get the Cell Size Sketch Notes complete with Teacher’s Guide and Student Notes in my TPT store here: https://www.teacherspayteachers.com/Product/Cell-Size-Sketch-Notes-Doodle-Notes-WTeachers-Guide-Student-Notes-3558606
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