In this lesson, students design an experiment to provide evidence that plants go through the process of cellular respiration by measuring the acidity level in a jar of germinating pea plants. Trapped inside a closed jar, as carbon dioxide is produced during cellular respiration, it dissolves in water producing an acidic environment that can be measured using bromthymol blue.
This lesson is specifically designed to address the following NGSS and Common Core Standards:
MS-LS1-7 Develop a model to describe how food is rearranged through chemical reactions forming new molecules that support growth and/or release energy as this matter moves through an organism
CCSS.ELA-LITERACY.WHST.6-8.9 Draw evidence from informational texts to support analysis, reflection, and research.
CCSS.ELA-LITERACY.WHST.6-8.1 Write arguments focused on discipline-specific content.
CCSS.ELA-LITERACY.WHST.6-8.2 Write informative/explanatory texts, including the narration of historical events, scientific procedures/ experiments, or technical processes.
Science and Engineering Practices:
In this lesson, students design and conduct their own experiment to provide evidence that plants go through cellular respiration. Thus, they plan an investigation collaboratively, and in the design, they identify independent and dependent variables and controls (SP3). After carrying out the investigation, the students analyze the data to provide evidence for phenomena (cellular respiration) (SP4). Last, the students construct a written argument supported by evidence and scientific reasoning to support or refute their hypothesis (SP7).
The focus of this lesson is Energy and Matter. The NGSS asks that students consider the following:
As students recognize that bonds are broken and reformed during cellular respiration and that energy is released in the process, students can track the flow of energy and matter through the system of peas in the jar.
Begin class by asking, "What are you going to learn today?" Students should respond by referring to the Essential Question, "How do cells contribute to the function of living organisms?" This EQ can be referenced both on my front board as well as on their Cells Unit Plan.
Have students get out their Cells Unit Plan. Explain that their focus of this particular lesson is Skill 4: I can identify important materials and processes that are required for cells to function. Have students read the skill and self-assess where they are at with their learning. Students rank themselves on a scale of 1 to 4 (4 being mastery).
As the unit moves forward, I have the students continually self-assess on each skill. Check out the student's unit plan below to see how students update their mastery level with each lesson.
On the front board or chart paper, I draw a light bulb with the question, "What is required to produce reliable data when designing an experiment?" Ask the students to brainstorm with a partner, or small group, what they believe the important factors are when designing a valid experiment. After groups brainstorm, call on each group to share one consideration they came up with and add it as a "light ray" coming off of the light bulb. After going around to each group. go around again so that all developed ideas are added to the light bulb.
This resource is a representation of what my students typically brainstorm. My students have already been exposed to the scientific method and experimental design criteria. If there are any important criteria that the students forget, be sure to add it to the graphic organizer on the board. Leave this graphic organizer on the board or chart paper for the remainder of the lesson.
In a previous lesson, my students were introduced to the cellular processes of photosynthesis and cellular respiration. (I have included the Photosynthesis and Cellular Respiration Notes Page in the resource bin.) I go through a quick review of the process of cellular respiration by discussing the following questions as a class:
Then, I explain that one of the products of respiration that the students identified is carbon dioxide. I explain that when carbon dioxide dissolves in water, it turns into an acid. So, the more CO2 that is formed and contained in a system with water, the more acidic something will be.
I then explain that bromthymol blue is a pH indicator. (My students have gone through a unit on chemical reactions including pH so this has a lot of meaning for them. If your students have not, it will warrant an explanation here.) When bromthymol blue is in the presence of a base, it is blue. When it is in the presence of a neutral substance, it is green. When it is in the presence of an acid, it is yellow. The more acidic a substance is, the more yellow bromthymol blue will turn!
After providing the background, I explain to the students that they will be designing an experiment to provide evidence that plants go through photosynthesis. On a lab table, I set out all of the available materials:
1. As a group, determine a general plan for how you could test if plants go through cellular respiration. Discuss your general plan with Mrs. Roehm.
**When groups discuss their plan with you, they will typically come up with the idea that they will place peas in a jar with bromthymol blue and record the color of the test tube each day. At this point, I encourage the students to really look at the brainstormed list of factors that allow for a valid experiment completed in the mini lesson. I emphasize that they should spend time and have deep discussions when completing the next step of the procedure. In addition, I tell them that as they plan for setting up their experiment that I would like the test tube to begin basic (blue). I further explain that bromthymol blue, when first poured from the bottle, will be neutral (green).
2. After determining your general plan, complete the information on the front of the lab page. While there is not a set number of constants you have to identify, remember that when designing an experiment, you will be assessed on how well you were able to control the variables to produce reliable data. Have thoughtful discussion with your group about all of the factors you must keep constant.
3. Write a procedure for your experiment.
4. Carry out your procedure.
Each day for 4 or 5 days, have the students record data. At the completion of experiment, have students write the conclusion on the lab sheet.
The front of the student lab document gives insight into the students' understanding of experimental design. In this work sample, the student identifies the variables, includes a control group, and lists a large number of constants. For me, the awareness of the constants is the key to a student understanding how to design an experiment with valid data. In order to make sure that there is only one variable being tested, it is key that the students control all other variables. This student has enough awareness to recognize that the amount of peas (plastic/real), bromthymol blue, paper towel, and water on the towel must be kept constant. In addition, he notes that the size of the jar and test tube should be the same. When I set out the materials for this experiment, there are students that will grab different sized jars and test tubes. These students have not yet mastered the idea of controlling variables. Last, the student explains that the jars should be located in the same place in the room and that data should be collected on the same days for each jar.
Notice in the data that the color turns from blue to turquoise to green to yellow in the jar with peas. The student indicates that the amount of carbon dioxide in the jar is increasing. The jar without peas remains blue the entire time.
Below are jars at different days in the process. The jar on the right is the control containing plastic beads (blue). The middle is after 2 days (green) and the jar on the right is after 5 days (yellow).
In the conclusion, the students provide evidence to support the claim of whether their hypothesis was supported or refuted. In my class, we use a format using the acronym "ABCDE" in writing conclusions.
A - Assertion - The student states that the hypothesis was supported. Notice that they also included what their prediction was and did not just write "The hypothesis was supported."
B - Background - The student explains what the students did in the lab so that the reader has an idea of how the evidence that the author is about to describe was obtained.
C- Citation - This student cites three pieces of reference. She cites two different texts by title and compares two pieces of data from her data table.
D - Discussion - The student explains how the fact the test tube turned yellow compared to the control staying blue proves that her hypothesis was supported.
E - End - The student includes a conclusion sentence.
*For some more background in the ABCDE paragraph, I have included the Quick Guide to the ABCDE Paragraph as a resource.
To close the lesson, I ask students to complete the following with their table group:
1. Have a group discussion about the question, "What does the yellow test tube provide evidence of?". Remind students to utilize their best discussion strategies as they cite evidence both from the text and from the lab!
2. Remind students that one of the NGSS Crosscutting Concepts that we have been working with is "Energy and Matter". Review with the students the meaning of this concept:
Energy and Matter: Flows, Cycles, and Conservation: Tracking energy and matter flows, into, out of, and within systems helps one understand their system’s behavior.
***Haven't taught your students the crosscutting concepts yet? This is a great opportunity! While my students are familiar with this, you could introduce the idea that in science it is important to be able to to track energy and matter as it flows into and out of systems. Then, they still could complete the next step of the closure.
After reviewing the concept, I ask the groups to draw a diagram that tracks the matter and energy in the system (jar, peas, test tube).
Here the student notes that energy came from the sun and then that energy was released when bonds were broken and reformed. Also, the student shows that glucose and oxygen go into the pea and new molecules of water and carbon dioxide are released.