Have you ever wondered what plants do all night long? Are they sleeping? Do they do anything at all? Students likely understand that plants capture light energy through photosynthesis. However, they may not understand that plants also must perform cellular respiration to release usable energy from the organic molecules produced as a result of photosynthesis. Some students may think that plants only carry out photosynthesis, while others may think plants carry out photosynthesis during the day and respiration during the night.

Today’s task, What Do Plants Do in the Dark?, builds on students’ understanding of the processes of photosynthesis and respiration and will begin to help them clarify ideas about the relationship between these two processes. Students download and analyze data from an investigation that has already been completed and use the thinking tool of patterns to construct an explanation of what plants do all night long. While students need opportunities to develop and use the knowledge and skills of Planning and Conducting Investigations, today’s Daily Do is an example of how students-as-scientists can engage in sensemaking without physically conducting the investigation themselves. 

Ask students, "Have you ever wondered what plants do all night long?" Give students time to independently think about and record ideas about what plants do at night and then ask them to share their ideas with a partner. Invite students to share with the class an idea or a question their partner shared that made them curious.

Share the "Light and Dark" formative assessment probe with your students. If you are completing this Daily Do in a distance-learning scenario, consider sharing this Google Docs version of the Light and Dark formative assessment probe with students. Ask students, “Which friend do you agree with the most?” Provide students independent thinking time to complete the formative assessment probe. Ask them to record their ideas in their science notebook; encourage students to use words, pictures, and/or symbols to communicate their thinking. You might ask students to put the probe away (assure students you’ll come back to their probes later) and then poll them on which friend’s claim they selected. Consider asking students to write their claims on sticky notes and then create bar chart on a classroom wall. You might also use a free digital tool to poll students such as Mentimeter or Socrative.

Ask students, “How could we figure out which friend has the best claim?” If students struggle to come up with ideas, provide them the chemical equations for photosynthesis and respiration.

• Photosynthesis: 6H₂O + 6CO₂ + light energy → C₆H₁₂O₆ + 6O₂
• Cellular Respiration: C₆H₁₂O₆ + 6O₂ → 6H₂O + 6CO₂ + energy

If students continue to struggle, ask them, “What could we measure to determine if photosynthesis and/or cellular respiration are happening?”

Students might share many different ideas about measuring the inputs or outputs of each process. Practically, the gases (O₂ or CO₂) that are consumed or released are easiest to measure. In this task, we are leading up to analyzing data from an investigation in which sensors were used to measure changes in the amounts of these gases.

Say to students, “We may not be in a situation where we can carry out this investigation, but we can analyze published data collected in an investigation similar to the one we are proposing.”

Teacher Guidance: Students are asked to analyze data from Vernier’s Experiment and Sample Data Library, using Vernier’s free software Graphical Analysis 4. These data sets were collected using Vernier probeware; these same experiments conducted by students in the classroom would yield similar results. Watch the Using Vernier Data and Software video for support on downloading and using Vernier data and analysis software.

Provide students with the Photosynthesis and Respiration investigation handout and ask them to read the procedure. You can use slides 1 and 2 in the How Do Plants Get Energy in the Dark? presentation to walk through the investigation setup. Use the following questions to guide a discussion to clarify students’ understanding of the procedure.

• Why is the chamber wrapped in foil to collect Data Set 1?
• Why is the light needed to collect Data Set 2?
• Why is the heat sink needed to collect Data Set 2?
• Why is it important to have fresh, turgid spinach leaves?
• How will we know whether photosynthesis is happening?
• How will we know whether cellular respiration is happening?

Have students complete the hypotheses graphic organizer (Slide 3) based on the claim with which they agreed. If you have a large group of students, you could partner them based on their selected claim. This graphic organizer is adapted from Argument-Driven Inquiry’s instructional resources. Have students share their predictions and reasoning first in small groups and then with the class.

The table provided in the Teacher Guidance for What Do Plants Do in The Dark? summarizes the expected results based on each friend's claim in the "Light and Dark" formative assessment probe.

Tell students to download the data from the experiment and Graphical Analysis 4 software. Have students refer to steps 8, 13, and 14 to analyze data for the datasets and determine the rates of photosynthesis and/or respiration. Have students use the curve fitting tool as directed in the Photosynthesis and Respiration investigation handout and record the rates in the data table on the handout.

In addition to the steps listed in the investigation handout, performing the following steps will make the graphs easier to read and interpret.

1. Click the “View Options” button at the top right. The button looks like a small table.
2. Click “Table.”
3. Click the three dots next to each data set title.
4. Click “Rename Data Set.”
5. Rename Data Set 1 “Dark” and Data Set 2 “Light.”
6. Click the “View Options” button and click “2 Graphs.”
7. For each graph, click the Graph Tools button and toggle on “Graph Legend.”

The final graphs should look like the ones in the screenshot below:

Ask students to work with a partner to answer the following questions and record their thinking in their science notebooks:

• How does the amount of O₂ change with time in the light? In the dark?
• How does the amount of CO₂ change with time in the light? In the dark?
• What patterns do you observe in the data presented in the graphs?
• How are the rates of change changing over time? How might you represent these rates of change mathematically?

Use questions 1-4 from the Photosynthesis and Respiration investigation handout to guide a discussion to help students make sense of the phenomenon.

1. Was either of the rate values for CO₂ a positive number? If so, what is the biological significance of this?
2. Was either of the rate values for O₂ a negative number? If so, what is the biological significance of this?
3. Do you have evidence that cellular respiration occurred in the leaves? What is the evidence?
4. Do you have evidence that photosynthesis occurred in the leaves? What is the evidence?

Teacher Guidance: Question 5 and the extensions are not needed to answer “What do plants do in the dark?”, but they can lead to further sensemaking opportunities.

Have students return to their hypothesis graphic organizers and answer these questions, first as individuals or pairs. Then, have students share with the whole group.

• Do the results support or refute your selected claim?
• Are the results consistent with any other claim(s)?

Say to students, “Let’s revisit the claims we are evaluating. Based on our data analysis, can we determine which claim is the most accurate? How do we know?” Facilitate a discussion to build consensus around the claim that plants carry out respiration but not photosynthesis in the dark.

Students might still be wondering why plants carry out respiration at all. Ask them, "Should we investigate this question next?" (This question will be investigated in a forthcoming Daily Do.)