Photosynthesis - Analyzing Data (Day 3)

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Students will be able to analyze evidence that light and air are the ingredients for photosynthesis.

Big Idea

Plants make their own food.

Note to Teachers

The Leaf Treatment Image Sets (see an example below) used in this lesson were downloaded from SEP Lessons, University of California San Francisco Science and Health Education Partnership.    



10 minutes

I start this lesson by asking the students to refresh their memories around the question, "What do plants need in order to perform photosynthesis?" By now, I expect the response to include all of these - sunlight and air/carbon dioxide and water.

I then ask, "How do you think we (humanity) came to know this answer?" or "What kinds of experiments do you think were carried out to figure it out?", and have the students Think-Pair-Share at their tables. This sets the stage for playing the video where that information is given. 


40 minutes

I tell the students that today we will be looking at evidence of photosynthesis by analyzing and drawing conclusions from the work of other scientists (NGSS Practice 4: Analyze and Interpret Data). Although we will not be performing the experiment ourselves, we will be able to make up our own minds by looking at their results. 

I ask the students to bring out the foldable they created for macromolecules. Once they have it in front of them, I ask, "Where do you find glucose?", followed by, "Now I need some examples of polysaccharides." My goal is to get students to realize that starch is a polysaccharide made up of multiple glucose molecules. For the students that need to be able to visualize this, I show the following image:

Then I tell students that plants use starch to store glucose (like in potatoes), so the presence of starch provides indirect evidence that a plant is performing photosynthesis. The presence of starch can be detected using iodine (which will turn starch a purplish-black).

Note to teachers: My students have worked with indicators before, including determining the presence of starch using iodine in a "white powders" lab. If your students have not had a similar experience, I would suggest explaining that an indicator is a substance that undergoes an observable change in the presence (or absence) of a particular chemical. Even better if you also prepare a small amount of cornstarch solution and use iodine (make sure it is not the decolorized version) to show students the color change. In fact, the day I taught this lesson I did just that, but asked the question "What will happen when I add iodine to this cornstarch solution?", eliciting the response "It will turn purple." from the class. 

I then tell the class that a group of scientists at UCSF set out to detect photosynthesis. I have their results but nothing else. My challenge: "Using only the image sets and your knowledge of the ingredients needed for photosynthesis (air, water and light), complete an experimental design sheet, from question to conclusions." Completing the experimental design sheet puts the students in the role of the researchers and gives them practice in formulating a testable question, identifying variables and controls (NGSS Practice 3), analyzing and interpreting data (NGSS Practice 4) and constructing explanations (NGSS Practice 6). It also provides opportunities to formulate evidence based on data - Modified Inquiry (NGSS Practice 7), as they create explanations and discuss their observations.

I note that the images show two side-by-side experiments, and that I want them to focus on the evidence present in both. The experiments were run simultaneously, which does not mean two variables in one experiment, but rather two experiments run at the same time.

I hand out the Experimental Design - Analyzing Data, and assign picture sets. I don't make copies of the picture sets, but rather post them for students to have access to them on Edmodo (saves time and color toner).

There are a total of 10 picture sets. The picture sets are assigned randomly by having students count off and then finding a partner with their same number. I use this type of partner assignment for this lesson in order to give students a break from working with the same partner they are working on the WISE activities. In this way, I ensure that everyone has an opportunity to work/learn with different people all the time. 

As students are working, and in order to promote the development of the scientific thinking necessary to construct a scientific explanation based on the evidence obtained from their data sets (NGSS Practice 4), I circulate the room guiding the discussions with questions like:

  • What were the variables? Which leaf is the control?
  • What do these investigations tell you about photosynthesis? What do they not tell you?
  • Is there any evidence that light (carbon dioxide, water) is important to photosynthesis? Where?



5 minutes

To bring this lesson to a close, I have students write down a post-it Exit Ticket. They can choose between "My AHA moment", "I am still thinking about", "My learning stopped when". These exit tickets are placed on the poster I created for this purpose. I use these exit tickets to have a snapshot of how the lesson went.

I also collect their work (SW 1SW 2SW 3SW 4SW 5) to analyze it to determine areas of need. Collecting the student work, and reading through it, led me to the discovery that students needed explicit instruction in making observations, support in writing a procedure (although this might just have been lack of time) and more practice in writing conclusions. The process will take time as they perfect their abilities in the science and engineering practices in upcoming lessons. However, the work also showed me that the concept of photosynthesis taught is well understood.