Teacher Notes for Where does a plant's mass come from? Learning Goals

Transcription

1 Teacher Notes for Where does a plant's mass come from? 1 Students analyze evidence to evaluate four hypotheses about where a plant s mass comes from. For example, students analyze Helmont s classic experiment, and evaluate whether his interpretation was supported by his evidence. Thus, students engage in scientific practices as they learn that plants consist mainly of water and organic molecules and most of the mass of organic molecules consists of carbon and oxygen atoms originally contained in carbon dioxide molecules from the air. Learning Goals In accord with the Next Generation Science Standards 2 : Students learn the Disciplinary Core Idea LS1.C: "The process of photosynthesis converts light energy to stored chemical energy by converting carbon dioxide plus water into sugars plus released oxygen. The sugar molecules thus formed contain carbon, hydrogen, and oxygen; their hydrocarbon backbones are used to make amino acids and other carbon-based molecules that can be assembled into larger molecules (such as proteins or DNA), used for example to form new cells." Students engage in Scientific Practices, including: o "Analyzing and Interpreting Data Evaluate the impact of new data on a working explanation and/or model of the proposed process or system." o Constructing Explanations [and Designing Solutions] Apply scientific ideas, principles, and/or evidence to provide an explanation of phenomena [and solve design problems, taking into account possible unanticipated effects]. o "Engaging in Argument from Evidence Compare and evaluate competing arguments [or design solution] in light of currently accepted explanations, new evidence, [limitations (e.g. trade-offs), constraints and ethical issues]." This activity reinforces student understanding of the Crosscutting Concept, Energy and matter: Flows, cycles and conservation, including matter is conserved because atoms are conserved in physical and chemical processes and without inputs of energy (sunlight) and matter (carbon dioxide and water), a plant cannot grow. This activity helps students to understand the Nature of Science: o Most scientific knowledge is quite durable but is, in principle, subject to change based on new evidence and or reinterpretation of existing evidence. o Science knowledge has a history that includes the refinement of, and changes to, theories, ideas, and beliefs over time. This activity helps students to prepare for Performance Expectations: o HS-LS1-6, Construct and revise an explanation based on evidence for how carbon, hydrogen, and oxygen from sugar molecules may combine with other elements to form amino acids and/or other large carbon-based molecules. o HS-LS2-5, "Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere " 1 By Dr. Ingrid Waldron, Department of Biology, University of Pennsylvania, These Teacher Notes and the related Student Handout are available at 2 Quotations are from Next Generation Science Standards ( and %20The%20Nature%20of%20Science%20in%20the%20Next%20Generation%20Science%20Standards% pdf).

2 Instructional Suggestions and Background Information Before students begin this activity they should understand basic biological chemistry, including how to interpret chemical formulae and equations. They should also understand that matter has mass, energy does not have mass, and energy cannot be converted to matter. The following analysis and discussion activities can provide additional useful preparation for this activity: "Using Models to Understand Photosynthesis ( Photosynthesis and Cellular Respiration Understanding the Basics of Bioenergetics and Biosynthesis ( at least the first three pages). To maximize student participation and learning, I suggest that you have your students work in pairs (or individually) to complete groups of related questions and then have a class discussion after each group of related questions. In each discussion, you can probe student thinking and help them develop a sound understanding of the concepts and information covered before moving on to the next group of related questions. If you use the Word document to make changes in the Student Handout, please consult the PDF file to see the correct format for the Student Handout. A key is available upon request to Ingrid Waldron (iwaldron@sas.upenn.edu). Additional background information and instructional suggestions are included in the paragraphs below. The hypotheses in the cartoon in question 1 in the Student Handout provide an introduction to the issues that are analyzed in the remainder of the activity. The third hypothesis mentions holes in the plant s leaves ; to help your students understand this, you may want to show your students the stoma (singular of stomata) in this diagram of a cross-section of a leaf. ( ) 2

3 The results shown in question 2 are representative of findings for smaller plants and generally similar results are observed for larger plants. Actively growing tissues such as leaves and root tips are ~75-90% water, and woody parts such as a tree trunk are ~45-60% water. Question 2 introduces the concept of biomass, defined as the dry mass of a plant or the mass of the organic molecules in an organism. For plants, ~96% of the dry mass consists of organic molecules, so this activity does not distinguish between dry mass and the mass of organic molecules. 3 The bottom half of page 1 in the Student Handout reviews photosynthesis and one example of how plants use the sugar molecules produced by photosynthesis to make other organic molecules. Question 3a provides the opportunity to reinforce student understanding that energy is not converted to matter or vice versa. During discussion of questions 3-5, it is also important to reinforce student understanding of the Crosscutting Concept, without inputs of energy (sunlight) and matter (carbon dioxide and water), a plant cannot grow. The three-part figure on the top of page 2 of the Student Handout shows how a plant is made up of cells which contain chloroplasts which make sugars which are converted to other organic molecules in plants cells. You may want to ask your students where cellulose, starch, and proteins would be located in the cell (cell wall, starch grain, and throughout the cytoplasm, nucleus and cell membrane, respectively). The Student Handout figure includes an edited version of the figure which is shown below in a large enough size for you to be able to see the detail. Most of this detail is not relevant for this activity and is not visible in the smaller version of this diagram in the figure in the Student Handout. ( This paragraph summarizes evidence that atoms from CO2 are the primary source of the mass of the glucose molecules produced by photosynthesis. Based on the chemical equation for 3 An organic molecule (also called an organic compound) is a complex carbon-containing molecule. Organic molecules are normally produced by and found in living organisms. Unfortunately, different scientists use different definitions of mass and biomass. Some scientists use mass to refer to the weight of the organic molecules in a plant, and other scientists use biomass to refer to the total weight of an organism; neither of these definitions is used in this activity. For the purposes of this activity, weight is a good estimate of mass. 3

4 photosynthesis, it is obvious that the carbon atoms in glucose must come from CO2, rather than H2O. Experiments using isotopes of oxygen have shown that the oxygen atoms in the O2 produced by photosynthesis come from H2O, while the oxygen atoms in the sugar molecules produced by photosynthesis come from CO2 ( If your students are familiar with atomic weights, you may want to remind them that carbon and oxygen have much higher atomic weights than hydrogen, so most of the mass of glucose is due to the carbon and oxygen atoms. Atom Atomic weight Percent of molecular weight of glucose C % O % H 1.0 7% After question 5, you may want to ask your students to identify points learned thus far that are important for evaluating the four hypotheses about where a plant s mass comes from. Much of a plant s mass is water. Most of the dry mass of plants consists of organic molecules. Plant cells synthesize organic molecules in large part from sugars produced by photosynthesis. Most of the mass of the sugar molecules produced by photosynthesis comes from carbon and oxygen atoms originally contained in carbon dioxide molecules from the air. To help students understand that the gas, CO2, actually has mass, you can use one or both of the following demonstrations. Have the students hold some dry ice (wearing suitable protective glove) and notice its weight and how it gives off CO2 gas; discuss how the same molecules/atoms are present in both the solid and gas, but are more spread out in the gas. Have the students measure the weight of a bottle or cup of carbonated soda immediately after removing the cap, and then three other times over a class period as more and more of the CO2 bubbles off. For question 6a some students may need a reminder that there are 16 ounces in a pound. In answering question 6b, students should recognize that: This experiment established that most of the increase in weight of the tree did not come from the dried soil, but the experiment did not test whether the increase in weight came from water or something in the air. This experiment with one tree requires replication before drawing conclusions about plants in general. Although Helmont's experiment is widely cited in biology textbooks, his experiment and interpretation are flawed even by the standards of the seventeenth century (see In interpreting his results, Helmont fell prey to the relatively common error of failing to consider alternative interpretations of his results. His results do not eliminate other possible sources of weight (e.g. carbon dioxide which was called sylvestre by Helmont who knew it was produced by burning dried plant matter). Also, Helmont did not replicate his experiment, and he gave an improbably precise measurement of the weight of the soil. Boyle did a similar experiment with replication in the 1640s and found a decrease in soil weight of 0 pound in one case and 1.5 pounds in the other. 4

5 In 1627, Bacon had concluded from his experiments growing terrestrial plants in water instead of soil that "It seemeth by these instances of water, that for nourishment the water is almost all in all, and the earth doth but keep the plant upright, and save it from overheat and over-cold". As discussed on the top of page 3 in the Student Handout, we now know that, although only a small part of a plant's mass consists of minerals from the soil, plant health requires minerals such as nitrogen and phosphorus in order to make protein and DNA molecules. You may want to mention hydroponic agriculture as a practical implication of these insights. This example illustrates several general principles about the nature of science. As is often the case, multiple researchers were addressing the same question at about the same time and, taken together, their results provided strong evidence that most of the weight of plants does not come from the dry soil. The subsequent change in the interpretation of their experimental results illustrates how conclusions are subject to change based on new evidence and or reinterpretation of existing evidence. This example illustrates that science knowledge has a history that includes the refinement of, and changes to, theories, ideas, and beliefs over time. The distinction between mass and biomass in question 8 allows a more sophisticated evaluation of the original hypotheses presented in the cartoon on page 1 of the Student Handout. I recommend that you discuss with your students how this type of more sophisticated reformulation of questions and hypotheses is characteristic of scientific progress. Discussion during this activity may lead to comments or questions about the role of growing forests in reducing CO2 concentration in the atmosphere and thus reducing global warming. It should be noted that trees have other effects on temperature, in addition to effects on CO2 concentration. Plants result in cooling through evapotranspiration and plants can result in warming by absorbing more sunlight than snow or pale sand or rock; at high latitudes the latter effect is sufficiently strong that it may outweigh the benefits of the heat-lowering effects ( For more information about global warming, see Resources for Teaching about Climate Change ( Additional Resources This activity is part of an integrated sequence of learning activities described in Cellular Respiration and Photosynthesis Important Concepts, Common Misconceptions and Learning Activities ( "Lessons From Thin Air" explores how and why even intelligent students who have received thoughtful teaching about photosynthesis often do not understand that much of a plant s mass comes from CO2 (available at Sources for Figures in the Student Handout Cartoon on the top of page 1 from "Hard-to-Teach Biology Concepts" by Susan Koba with Anne Tweed, NSTA Press Cellulose figure on the bottom of page 1 modified from 5

6 Figure on the top of page 2 constructed by author using edited images from and 6