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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. 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!
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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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One comment I frequently hear from biology teachers is "My students keep mixing up mitosis and meiosis." I had this problem for many years (the first 5 years of teaching to be exact). During my cells unit I would teach both mitosis and meiosis. I would begin by teaching them both separately, and then had worksheets and activities that compared the two. But when I would give the unit test, it was clear the students still confused the two. I needed to do something differently.
After teaching middle school for 5 years, I switched to a high school near my house. When we got to the cells unit one of my colleagues suggested only teaching mitosis, and waiting to teach meiosis until we got to the genetics unit. Light bulbs kept going off in my head. The more I thought about it, the more sense it made.
So I tried it. At the end of my cells unit (after teaching organelles, membranes, and cellular energy) I would teach mitosis. When I would test them just on mitosis they would score well, because they didn't have both processes in their head to get confused. Then, after Christmas break when we got to genetics, I would teach meiosis. It made so much sense because:
By the time I quizzed the students on meiosis they were experts on cell division. If your school gives you some freedom with the order of your curriculum, try teaching it this way! You won't regret it.
BONUS! If you want a fun way to make sure students understand the differences between mitosis and meiosis, try this FREE bingo game in my TpT store! Bingo is a great way to review scientific vocabulary. In this game you will call out the definitions and students will cover up the words on their bingo cards. You can download this product free in my TpT store HERE. Enjoy!
Every teacher has that one unit they don't like to teach. For me, it was cells. Having taught every grade from 6th to 12th, it seemed like no matter how hard I tried, the same thing happened every year. I taught organelles, students memorized them for a test, and then completely forgot about them. Later when I taught mitosis and would ask "Hey, remember centrioles?" I would get blank stares. But what was even more frustrating was the fact that students just didn't get how organelles worked together. I tried everything I found on the internet. I tried the "cell is like a factory" analogies. I tried to have students make cell models out of clay or food. I had them make posters. I even had them write me a "tour through the cell" book (inspired by the magic school bus). And guess what? None of it really worked. Sure, students would come in with really cool jello models and beatiful posters, but if I asked "How do the endoplasmic reticulum and golgi work together?"..... more blank stares.
After 9 years of teaching cells, I was ready to cry. Then one day I was venting to a professor at a local university and he said he had his students group their flashcards together and lay them out like dominoes. Inspiration hit! Why couldn't I have my students link them together like puzzle pieces? I immediately got to work.
I made a list of all the organelles my students needed to know. I decided to make 2 versions of the activity since I teach multiple levels of biology. In the first version (picture on left), I linked two organelles together, and students would have to write out the relationship between them on the connecting puzzle piece. To make it harder for my honors students, I would have them figure out which organelles go together on their own (picture on right).
For the second version, I had students lay them out on butcher paper and connect as many puzzle pieces as possible. They called me over to check before gluing. What was great about this activity was that every group had a different final product, but all of them had correct answers. By the end of the activity (it took them 2 class periods) they had a much greater understanding of which organelles worked directly together and why. (Insert happy dance here). Students also took pictures of their posters and used them as a study tool before the test. So if you were as frustrated as me, kiss your cell factory goodbye and check out my lesson plan HERE.
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Lately there is a big push for STEM in the classroom. Data has projected that STEM related jobs will increase to 9 million by the year 2022 (www.bls.gov). As teachers we need to not just teach science, but let students truly experience it first hand.
Every year when I teach cells, students do a good job memorizing what the organelles do but have a hard time understanding how the organelles actually work together. I wanted my students to really visualize cell processes and how the cell functions as a whole. I came across a website from MIT that allowed students to create animated videos. I decided I was going to have my students create a video for a specific cellular process. This project can be scary for many students that aren't tech-saavy (although most students are better with technology than we are!) To ease their minds, I let students work in pairs- one student could do a lot of the research and the other student could do more of the video building. Next I came up with a list of 15 different cellular processes (endocytosis, mitosis, DNA replication, etc.) that they could pick from. I have class sizes around 30 students so each group had a different topic for their video. This project could be used for any topic, not just cells!
Here are a few tips that will make the project run smoother:
1. Before you assign the project, play around with the website yourself. It was also helpful for me to watch youtube tutorials (like the one HERE) as I was learning. If you are familiar with the website then it is easier for you to help students when they hit road blocks... which they will.
2. Students will need to create a login for their video. I told students to use their school ID number as their login and their school password. Many students have multiple usernames for their emails and social media accounts, so I didn't want them to forget their login. Also, when students shared their videos with me I could see whose video it was based on their ID number.
3. Before students begin, have them map out what they want their video to look like. I gave them a storyboard timeline worksheet (see image 2 below) and made them draw out their cellular process and write captions. I had to check and approve their worksheet before they could begin working on the video. It was a good way to check in with them and give them feedback to ensure they weren't missing anything.
4. Allow students to look around at videos that are already made. On the scratch homepage you can search for videos that other people have shared. If you find a video you like, you can click See Inside (see image 3 below) and see how they actually built the video. I made it clear that students could only look there for ideas, but couldn't copy what other people made.
5. It will take time, and get ready for the whining. If I had a dollar for every time I heard "can't we just make a powerpoint instead?" I would be going to a steakhouse for dinner tonight courtesy of my students. One student even said "Come on Mrs, we've been making powerpoints since we came out of the womb!" That is exactly why I didn't let them make a powerpoint. In the end (I gave them a week), they came up with some awesome videos. The great thing about this site is they don't have to be at school to work on it, just anywhere with an internet connection. If they don't finish in the assigned class time, they can work on it at home.
6. Chances are you will have a group or two that just can't figure out the website and how to make things move and work. As a last resort for these groups, I showed them how to make it "powerpoint-like." When you click on the "backdrops" tab, you can create multiple backdrops, which is essentially like powerpoint slides (see image 4 below). Then all they have to do is add a script that when the space bar is clicked, it moves to the next backdrop.
7. When students are finished, they need to click SHARE before the video goes live (see image 5 below). Once they clicked share, I had them copy and paste the URL into an email and send it to me for grading. I made it clear to students that the majority of their grade would be based on the video content, not the animations. For example, if the mitosis group had awesome visuals but forgot to tell me about what mitosis is, why cells divide, and which cells undergo mitosis then they wouldn't get a great grade. That lowered the stress level for students who struggled with the animations.
Even though both teacher and student felt frustration at times, I'm so glad I had my students create these videos. Below is a sample from one of my students. Enjoy!
This lab is one of my top sellers in my Teachers Pay Teachers store. It is easy to set up and doesn't require a lot of materials. However, I frequently get questions about the lab so I'm hoping this blog post will be useful to those teachers out there who about to set up this lab.
In this lab, students will be testing whether or not aquatic plants do photosynthesis in the dark or light, and also testing if they do cellular respiration during the dark or light. The plant I usually use for this experiment is called elodea, which is available at any local pet store in the fish area. One nugget of information you will need to know- pet stores call it anacharis, not elodea. It is usually sold in bunches of 4-5 stems for a few bucks. Two big bunches should get you through the day. If they don't have elodea, any other aquatic fish tank plant will work fine, but make sure it is a tall skinny plant that will fit down into your test tubes.
One reason this lab is great is because it can be used in multiple places in your curriculum:
~ Cells unit: When you are teaching cells, chances are you will be talking about chloroplasts and mitochondria. Along with these organelles you will be discussing photosynthesis and cellular respiration. This lab fits in great because it shows that plants not only do photosynthesis, but cellular respiration as well.
~ Ecology unit: During my ecology unit, we cover the 3 major biogeochemical cycles (water, carbon, and nitrogen). What better way to talk about the carbon cycle than to demonstrate the relationship between plants, animals, and gas exchange?
A little background....
This lab uses the chemical bromothymol blue. This chemical is used as a pH indicator. When the pH is above a 7 (basic) it is blue, but when the pH drops below 7 (acidic) it starts to turn yellow.
Image below is courtesy PureySmart on Wikimedia Commons.
Before beginning the lab, I like to demonstrate to the students how bromothymol blue works. I get 2 erlenmeyer flasks (beakers will work just fine too) and fill them 3/4 of the way full with water. Add enough bromothymol blue for the water to be visibly blue. (In a beaker of 200mL of water, I add about 4mL of bromothymol blue). Call up a student, and have them blow through a straw into the beaker. As they blow (it will take 3-4 big breaths) the water will slowly change from blue to yellow. This is because when the carbon dioxide in our breath reacts with the water it forms carbonic acid, lowering the pH.
Inquiry, Inquiry, Inquiry
When I do this lab, I do not tell students how to set up the experiment. I split the class into lab groups, and assign each group one of the following questions:
1. Do plants to photosynthesis in the dark?
2. Do plants do photosynthesis in the light?
3. Do plants do cellular respiration in the dark?
4. Do plants do cellular respiration in the light?
Obviously the group that gets assigned "do plants do photosynthesis in the light" will know the answer, but they will still have to set up a controlled experiment that can demonstrate it. I give each group a big white board and have them set it up like the image below. They will have to fill it out based on the specific question they are assigned. If you don't have whiteboards, butcher paper works great too. Students will know what materials they have to work with because they are listed on their lab worksheet (available in my TpT store).
As we walk around the room and discuss experimental design, students will begin to see that each group will set up their test tubes the same way, the only difference being if their tubes get left in the light or wrapped in foil and put in the dark for 24 hours.
Two notes: I get asked how much bromothymol blue to add to the test tubes. I have each group add 1mL to each tube. If you would like to add more or less that is fine, as they add the same amount to each test tube for consistency. Also- make sure to fill the test tubes to the top and cap them tightly, or use parafilm to cover the tops. We want the gas to stay in the water, not escape.
When students come in the following day they will pick up their test tube rack and fill out their data tables on what happened. They will see that the elodea did photosynthesis in the light, and cellular respiration in the dark.** (see note below)
**One thing you will have to discuss with your students: Plants are doing cellular respiration in the day time as well, but since photosynthesis is also occurring the indicator stayed blue.
A great extension activity is to add aquatic animals to this experiment and see how the added respiration affects the color change. If you can get your hands on some small snails, they will fit great into the test tubes. I had trouble finding snails in Arizona, so I went to my local pet store and picked up two feeder goldfish. I filled up two large erlenmeyer flasks with water and bromothymol blue, and turned one yellow. I added elodea and a goldfish to each flask. Next, I asked my students what will happen when we leave these in the light for 24 hours. The next day we came in and saw both flasks were a shade of bluish green (somewhere in the middle of where the two flasks began). If you don't add a ton of bromothymol blue, and only leave the fish in for 24 hours the fish will not be harmed.
Hopefully you are ready to start this experiment! If you have any questions, drop them in the comments below!
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Becca of Science Rocks
Hi, I'm Becca! I've been teaching science for 10 years at both the middle and high school levels.