Where does a plant’s mass come from



Teacher Notes for Where does a plant's mass come from?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.I recommend that your students have a basic understanding of photosynthesis before they begin this activity. For this purpose, you may want to use "Using Models to Understand Photosynthesis” ().A hands-on activity that addresses the same basic question is “Photosynthesis, Cellular Respiration and Plant Growth” (). This hands-on, minds-on activity begins with the question of how a tiny seed grows into a giant Sequoia tree. Students analyze data from research studies on plant mass and biomass, and they conduct a hands-on experiment to evaluate changes in CO2 concentration in the air around plants in the light vs. dark. Students interpret the data to understand how photosynthesis makes an essential contribution to increases in plant biomass, and cellular respiration can result in decreases in biomass. This activity counteracts several common misconceptions about plant growth, photosynthesis, and cellular respiration.Learning GoalsIn accord with the Next Generation Science Standards: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:"Analyzing and Interpreting Data – Evaluate the impact of new data on a working explanation and/or model of the proposed process or system." “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].”"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:“Most scientific knowledge is quite durable but is, in principle, subject to change based on new evidence and or reinterpretation of existing evidence.”“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: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.”HS-LS2-5, "Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere…"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.To maximize student learning, I recommend that you have your students work in pairs to complete groups of related questions. Student learning is increased when students discuss scientific concepts to develop answers to challenging questions; students who actively contribute to the development of conceptual understanding and question answers gain the most (). After students have worked together to answer a group of related questions, I recommend having a class discussion that probes student thinking and helps students to develop a sound understanding of the concepts and information covered. To maximize student participation and learning, you can alternate between having student pairs work together to answer each group of related questions and class discussions of their answers and any related information you want to introduce. 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.( )Evidence – Part 1The results shown are representative of findings for smaller plants. 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. Almost all of the rest of a plant’s mass consists of organic molecules.How Plants Make Their Organic MoleculesThe three-part figure in the middle 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 a plant’s cells. The Student Handout figure includes an edited version of the figure shown below in a larger size. The detail shown in the figure below is not discussed in this activity. If you want your students to learn more about the multiple reactions involved in photosynthesis, you can use the discussion and analysis activity “Using Models to Understand Photosynthesis” (). Similarly, if you want your students to learn more about the multiple reactions involved in cellular respiration, you can use the discussion and analysis activity “Using Models to Understand Cellular Respiration” ().()Question 5c asserts that “Most of the mass of the sugar molecules produced by photosynthesis comes from CO2.” The following evidence supports the conclusion that atoms from CO2 are the primary source of the mass of the glucose molecules produced by photosynthesis. Since CO2 and H2O are the inputs for 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 sugar molecules produced by photosynthesis come from CO2, while the oxygen atoms in the O2 produced by photosynthesis come from H2O (). Note also 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. (See table below.)AtomAtomic weightPercent of molecular weight of glucoseC12.040%O16.053%H1.07%To help students understand that the gas, CO2, actually has mass, you can use either or both of the following demonstrations. Have a student who is wearing a suitable protective glove hold some dry ice. He or she should notice the weight of the dry ice and also 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 several other times over a class period as more and more of the CO2 bubbles off. The figure below provides additional information about how glucose molecules are used to synthesize other types of organic molecules.( )Question 6 provides the opportunity to reinforce student understanding that energy is not converted to matter or vice versa. After your discussion of question 7, I recommend that you reinforce student understanding of the Crosscutting Concept, “without inputs of energy (sunlight) and matter (carbon dioxide and water), a plant cannot grow”. You may also 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. Evidence – Part 2For question 8a some students may need a reminder that there are 16 ounces in a pound. In answering question 8b, 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. (The importance of considering alternative interpretations is reinforced in question 11.) 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. 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". 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”.ConclusionsQuestion 10 introduces the concept of biomass, defined as the mass of the organic molecules in an organism. For plants, ~96% of the dry mass consists of organic molecules, so biomass is often assessed as the dry mass. As discussed in question 11, the distinction between mass and biomass 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 an important part 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. However, you should be aware that these benefits are counteracted to varying degrees by other effects of trees (e.g. trees’ secretion of volatile organic compounds and the greater absorption of sunlight by leaves compared to more sunlight reflected by snow or light sand) (). For learning activities and more information about global warming, see “Food, the Carbon Cycle and Global Warming – How can we feed a growing world population without increasing global warming?” () and “Resources for Teaching about Climate Change” ().Additional ResourcesThis 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 top of page 2 – modified from – Composite figure on page 2 – constructed by author using edited images from and ................
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