Plants Pre-Lesson 1 Background Information

The plants that are set up in the Gel Protocol (2-turtle) Pre-Lesson need about four weeks to grow to a point where they have gained enough mass to produce significant results in Lesson 3. The Paper Towel Protocol (1-turtle) may take as few as 2 weeks to achieve significant mass gain results. Thus, the Pre-Lesson(s) should be conducted one to three weeks before you plan to begin Lesson 1 of the Plants Unit, depending on which pathway you choose.

Students may begin this Unit thinking that plants build the majority of their mass from minerals and nutrients in the soil. Although atoms from soil minerals and water contribute trace amounts of materials during biosynthesis (e.g., nitrogen, potassium, and phosphorous from the soil), the majority of biomass is built from carbon from CO₂ in the air. By growing plants from seeds and taking measurements of plant dry mass and the dry mass of the rest of the system, this investigation gives students evidence that plants do not build their mass from the growing medium or water. Lesson 4 (Photosynthesis & Cellular Respiration) and especially Lesson 5 (Biosynthesis) give students further evidence that plants build their mass from materials they get from the air.

This Pre-Lesson, then, sets up for students not only the actual growing of the plants, but it also introduces to students some foundational knowledge about the importance of measuring a plant's dry mass. Students will take some initial measurements of the plant systems they are setting up as a reference for use in Lesson 3. In preparation for the investigations in Lesson 3, keep in mind that inevitably things go wrong when growing plants, so make sure to plant some extra radishes (we recommend one extra per group of students). Additionally, you may want to have some extra growing plants in the classroom or at home that are available to potentially do the Plants in the Light and Dark investigation (Activity 3.3). If you decide to have students harvest their radish plants first (in Activity 3.2), they will not have their own radish plants available for Activity 3.3. Therefore, they will either need to rely on the extras that you plant in this Pre-Lesson, or on other growing plants that are available in the classroom or that are brought in from home.

There are two pathways from which to choose when implementing this Pre-Lesson, which will then extend into Lesson 5. The storyline supporting each pathway is the same: students will discover that their radish plants end up with more dry mass in them than they introduced into the system. Where did that mass come from? In Lesson 5, the task for students will be to explain where this mass came from. In this Pre-Lesson, each pathway will help students establish a foundation on which to understand that the mass didn't come from the materials they put into the system. Each pathway will also utilize a text and/or a video to help establish this problem for students. However, each pathway will engage students in different levels of measurements and calculations in order to meet this same goal.

You may choose a pathway based on the amount of instructional time you have to teach the unit, the complexity of measurements and calculations your students can handle while still understanding the underlying concept, or the learning goals you have for your students in this unit. Please refer to the following table for a brief outline of each pathway in the Pre-Lesson.

The following table is excerpted from the Plants Unit Front Matter document and is also in the Student Challenges and Teacher Choices in the Plants Unit document. It illustrates the options in the Pre-Lesson and how they will impact a sequence of Pre-Lesson and Activities 3.1, 3.2, and 3.4. Please refer to the Plants Unit Front Matter document and the Student Challenges and Teacher Choices in the Plants Unit document for more information about how you might make this choice.

A note on mass and weight: Grams and kilograms in the SI (metric) system are units of mass—the amount of matter in a system. On the other hand, pounds and ounces in the English system are units of weight—the force of gravity on a particular mass. As long as gravity doesn’t change, these units are interconvertible: The force of gravity on a 1 kg mass is about 2.205 pounds. Since most American students are more familiar with the English units of weight, we sometimes use “weigh” and “weight,” especially when encouraging students to express their own ideas. When referring to measurements in grams, we use “mass” as both a verb and a noun.

Key Carbon-Transforming Processes: Photosynthesis