Activity 3.2GL: Observing Plants’ Mass Changes, Part 1 (30 min)

Target Student Performance

Students harvest their radish plants and dry down the plants and the paper towel or gel in preparation for Activity 3.4.

Resources You Provide

Tubes of radish plants from the Pre-Lesson (1 per student)
Digital scale (1 per group)
Small containers to collect gel from individual tubes to mass.
Small paper bags or envelopes for drying plants (1 per plant)
Sunny windowsill or drying oven (domestic ovens work at low settings)
Markers to label plants by student or group (1 per group)

Resources Provided

Setup

Each student or group of 4 students should have 1-2 tubes. For each group, provide a container to collect the gel that the students will extract from the plant roots. Unless you choose to dry out the gel, it will be discarded after the investigation.
Provide a paper towel, small paper bags, or envelopes (1 per plant), on which to place the plants to dry.
After the activity, students can assist with cleanup by rinsing out the plant test tubes.
Prepare your computer and projector for the 3.2GL Observing Plants’ Mass Changes, Part 1 PPT.
Print one copy per student of the 3.2GL Observing Plants’ Mass Changes, Part 1 Worksheet.

Directions

1. Use the instructional model to show students where they are in the course of the unit.

Show slide 2 of the 3.2GL Observing Plants’ Mass Changes, Part 1 PPT.

2. Have students harvest the plants.

Follow the instructions in Part A of the 3.2GL Observing Plants’ Mass Changes, Part 1 Worksheet.

Show slide 3 and 4 of the PPT, which has the instructions and pictures of the process.
You may want to demonstrate how to remove the gel from the roots to preserve as much of the plant as possible. Emphasize to students that they do not want to pull on the roots or they will break. Squeezing will generally not break roots.
The Worksheet prompts students to mass their hydrated gel after separating it from their plants and estimate the dry mass by using the percentage dry mass provided = 1.4%. Alternatively, you could dry the gel in an oven and use the scales to measure this directly.
The worksheet also prompts students to mass the plant after harvesting. An estimated dry mass percentage of a radish seedling (7%) is provided. Students can use this value to estimate the dry mass present in their seedling. Make sure students save their initial measurements in a safe place as they will return to them in Activity 3.4.

3. Have students record their observations and estimate dry mass of gel and plants.

Record observations of the plants in Part B of 3.2GL Observing Plants’ Mass Changes, Part 1 Worksheet.

Show slide 5 of the PPT, which lists a few observation idea starters.

4. Dry the plants.

Students will be able to estimate the dry mass of their plants based on wet mass and compare this value to an actual dry mass they record.

Plants individually labeled can be placed in a small paper bag or envelope on a windowsill to dessicate or can be dried overnight at the lowest temperature (<170 degrees F) in a home oven. IF YOU USE AN OVEN—make sure that the material you place the plants on will not burn and start a fire! Windowsill plants will take a few days to dry if small, but may take longer if they are large.
Note: Radish plants contain only about 7% dry mass, so in order for most classroom scales to even record the dry mass of a single dried plant, the starting masses of the plants should be at least 0.75g. If your plants are smaller, either mass multiple plants together or use the 7% dry mass calculation to estimate dry mass from wet.

5. (Optional) Dry the gel.

Students may also want to desiccate their gel to see if the mass of the gel in their tubes changed over time and to see if they come up with a similar dry mass percentage (~1.4%).

The gel desiccates best in an oven at low heat (~200 degrees or less) for at least 3 hours. Be aware that the desiccated gel will stick to any surface it is dried on, so students will have to mass the drying surface (non-flammable!) prior to placing the gel on it. Aluminum foil or parchment paper (labeled with the student name or a unique number) works well.

6. After drying the plants/gel, continue with Activity 3.4.

Since it may take several days for the plants to dry, there are options for what to do with your students while waiting. You may move on to Activity 3.3 and conduct the Light and Dark Investigation. Or, if you started with Activity 3.3 first, you may move on to Lesson 4 which focuses on following up results from the Light and Dark Investigation (Activity 3.3) with explanations of photosynthesis and cellular respiration. Later, students will complete Evidence-Based Arguments Tool for this investigation (Activity 3.5) and explanations of biosynthesis in plants (Activities 5.1, 5.2, and 5.3). See the Plants Unit Front Matter for more information about this decision.

Assessment

Since this activity is primarily an opportunity for students to harvest their plants for Activity 3.4, there is little content in this activity to assess. You can use the 3.2GL Grading Observing Plants’ Mass Changes, Part 1 Worksheet to grade your students’ work in collecting observations on 3.2GL Observing Plants’ Mass Changes, Part 1 Worksheet.