Photosynthesis, Part 2-Elodea Oxygen Bubbles Lab

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Students will examine the process of photosynthesis using elodea plants.

Big Idea

Seeing is believing...plants release oxygen gas.


5 minutes

What are the 2 products of photosynthesis and how many molecules of each are produced?

Ask this question to assess how well students understand the content from the previous lesson, Photosynthesis, Part 1. Look for students to correctly identify both glucose and oxygen as products of photosynthesis.  If students are not readily responding with the correct answers, take a few minutes to spiral back and conduct a brief review of the photosynthesis reaction. Based on how little or how well students responded to the initial question, decide if you will use the chemical equation or the photosynthesis diagram as the "anchor document" for the review.     I prefer the photosynthesis diagram because it shows the movement of the reactants and products.  Using whole group participation, identify and number the three reactants and the two products. 

Once it’s apparent that students have a basic understanding of the reaction, ask students, “Where does the carbon in the glucose molecule come from?”  This is a higher order question which allows you to identify the depth of their knowledge which helps you scaffold the review.  When conducting this type of review at the start of class, I like to question, observe, respond (and review) then move to a higher ordered question.  I continue with this cycle until I possess a clear picture of how well students comprehend the previously taught content.


Introduce New Material

15 minutes

Start the lesson with a video clip, Introduction to Plants. This video clip allows students to see plants from around the world, while at the same time learning more about the aspects of plants that make them essential for life.  

Select one of the statements from the video to discuss with students after the clip finishes.  For example,  “What did it mean in the video when it was stated, plants feed the world”?  The academic dialogue that will ensue serves a platform for introducing the lesson and it also can be a great opportunity to review the ecological concepts that were previously taught regarding the role of plants as producers.  Academic discourse is one of my favorite ways to ensure that students use speech as a way to deepen content comprehension.

After the video clip, introduce new content related to the light dependent and light independent cycles of photosynthesis.  This is the second “chunk” of content related to photosynthesis.  

In Photosynthesis, Part 1 , students modeled the process of photosynthesis at a high level.  In this lesson, students will receive a second “chunk” by drilling deeper into the content to learn what occurs in each of the two cycles of photosynthesis.  

“Chunking”can be effective tool for the student and teacher.   I will often “chunk” a lesson into smaller pieces over within a class period or even over a day or two to allow students to learn smaller sections of complex content.  By "chunking" a lesson, I have a better idea of where the content understanding has ended versus teaching a lot of information and not being sure where student understanding ended.

Explain the two cycles of photosynthesis and the products produced from both the light and dark cycles.  I use a visual image to help students visualize the two cycles.  When possible, I like to use both words and pictures to ensure that different student learning styles are being met.  Be sure to address the misconception that plants create energy.  Emphasize that photosynthesis is the process plants use to make food molecules called glucose.  Without teaching respiration at this point, briefly mention that the glucose that is produced during photosynthesis is converted to energy in another process called cellular respiration that will be taught later.

To clear up misconceptions about the presence of mitochondria in plants, ask students if they think plants have both chloroplasts and mitochondria. Listen to a few comments and write them on the board without making any corrections to what they have said.  After 3-4 comments are noted in the board, explain the correct response and address any misconceptions noted in the comments from students as you teach why plants need both organelles.   Remind students that plants make their food but it has to be converted into energy, much like the process used to convert our food into energy. Based on the information shared about the conversion of glucose into energy, some of them will be able to make the connection and explain the role of mitochondria in plants.



Guided Practice

10 minutes

Ask students, “If plants do in fact, release oxygen gas, how can we prove it?  Allow them “brainstorm” and share ways in a whole class discussion without correcting any of their thoughts. Inform students that they will investigate whether plants, in fact, do release oxygen gas.

Show students a brief clip Elodea O2 Bubbles. Do not provide any information to the students before viewing.  Ask students to explain what they saw.  Look for students to identify that the plant released oxygen gas.  Ask students to explain how they know that oxygen production occurred.  Without adding any feedback or redirection, listen to students’ comments. This type of active listening on your part allows you to identify misconceptions that you will can address before class ends.

Inform students that they will participate in a Photosynthesis in Elodea to observe whether plants, in fact, produce oxygen.  Begin by distributing lab instructions to students.  As a way to provide differentiated instructions to the various learner types, provide a verbal summary of the lab materials and procedures, and also project the instructions using a LCD projector.   For students with reading challenges, consider using Kurzweil software to assist them in reading the lab.  I like using the Kurzweil software because it allows struggling readers to read materials with computer assistance.  The program also defines any terms that they do not know in the reading material.

Demonstrate how to measure 0.5 g of baking soda using either a digital scale or balance.  Modeling is an effective way to not only tell, but show students how to complete a task. Because it is important for students to be able to use a balance or digital scale correctly, take time to model correct usage before releasing students to work independently. Check for understanding by allowing them to measure a pencil. Have each group of two measure the mass of the pencil and compare their measurements to see if both, either or neither were able to use the scale correctly.    Walk around to observe and listen for correct or incorrect usage of the equipment. 

The measurement activity is also a good opportunity to review previously learned concepts on accuracy and precision from the Scientific Method lesson.  Ask students to identify how the lab might be affected if they do not accurately measure the correct amount of baking soda.

Lastly, before releasing students to work in their small groups, reinforce the safety rules.  Make it a practice to regularly review safety rules before beginning labs. Because most students find labs fun and engaging,  I’ve found that some students get overly excited when they learn they will be performing a lab.  Reviewing the behavior expectations up front often times keeps issues from arising.  Inform students that the lamps get pretty hot so encourage them to exercise care around the lamps to avoid burns.


Independent Practice

30 minutes

In preparation for the lab, purchase and then pre-cut sprigs of elodea.  Just make sure that they are kept in cool water until used.  Instruct students to work in groups of 2-3 students to conduct the Photosynthesis in Elodea lab.  Have at least 7-9 lab stations set up in the room with lamps. Students will have to utilize keen  observation skills to count the number of bubbles that form in the different settings.

Use a timer on the LCD projector to help students manage their time so that they can work efficiently to complete the lab.  There are a lot of free timers from which to choose on the internet.  It’s also a good idea to periodically call out the remaining time.  




5 minutes

Quickwrite Prompt: What factors do you think affect the rate of photosynthesis and why?

Students will respond to the writing prompt in 5 sentences or less within a time frame of 5 minutes.  Giving them an allotted time and limit on how much they can write will give them a chance to practice the skill of writing concise responses.

When you read over the responses, look for responses that clearly show that students know that the availability of any of the three reactants can affect the rate of photosynthesis.  Also, look for standard writing conventions like correct grammar, correct vocabulary, etc.  Based on the responses, determine if you will need to spend more time reviewing key concepts before moving forward.