VIEWS: 15 PAGES: 5 POSTED ON: 7/7/2011
1. Understanding box-and-arrow diagrams Notes to faculty Outcomes: Students better understand energy transformation in the process of photosynthesis - that energy is “lost and carbon changes from one form to another (but is conserved). Students will understand the basics of ecosystem box-and-arrow diagrams including terms such as “pool” and “process” (e.g. what the boxes and arrows indicate) and will be able to construct simple diagrams of their own. What students do: work with a simple two-step box and arrow model in groups to discuss and ask questions about box-and-arrow representations, photosynthesis as an ecosystem process, and energy transfer/energetics (sunlight to plant). To demonstrate their understanding, students use values in a table to construct a box-arrow diagram of energy flow in a forest. What to pay particular attention to: A main purpose of the activity if for students to really think about the transformations that take place during photosynthesis – both in terms of carbon and energy. In discussion you should push the students to “really” explain what they mean by phrases such as “ light is used to …”. When you think about it, these concepts are not easy to grasp! Look at the associated DQC pages to see what students say and common misunderstandings. Logistics: There are 3 steps here which can take a fair amount of time, depending on your students understanding. You can use some of the parts and not others. Part “a”: Students work in groups on a simple two-part box-arrow diagram about energy assimilation by corn. Lead a discussion based on their questions and comments. Issues to emphasize: What boxes represent (you can introduce the term “pool”) and that arrows are processes. You can use the representation of energy flow in the textbook to discuss how/whether this is confusing to students generally. “Energy” is very difficult to explain, in part because it takes such different forms. The unit Kcal is described mechanistically which can help with this confusion. Part “b”: Students return to the groups to address question “b” which asks them to make the connection between the process of photosynthesis and the energy flow diagram. They also calculate the energetics of this simple system. Differences in students understanding may be a problem here; you can take advantage of this by asking the more advanced students to describe photosynthesis relationship and/or the calculation in their own words. Part “c”: This is the evaluation component. Students are given a table and asked to make a box-and-arrow diagram of energy flow in a temperate forest ecosystem based on these values. This could be given as a quiz or homework. Values are percents, which is different from the example above and may be confusing. You can develop a rubric for evaluating this step. In smaller classes, student could work in pairs with the rubric to discuss their own individual efforts. If you give students this question, be sure to explain what you looking for (perhaps with an example). Units and Quantitative Skills: Energy concentration (Kcal), in a defined area (Kcal/m2), and rate (Kcal/m2/day); energy as a percent; efficiency of trophic transfer. Hidden Curriculum addressed • Principles: Conservation of Energy (Energy flow within a designated ecosystem - where the energy comes from and goes); energy and matter are linked but not interchangeable. • Processes Generation (photosynthesis); Transformation (building of biomolecules within an organism) • Scale & Time: Atomic/molecular/cellular/organismal scale • Forms & Representations: Energy (sunlight, chemical potential energy within an organisms’ molecules); Ecosystem Energy Flow (box and arrow diagrams) Student Directions 1. Below is a box-and-arrow diagram illustrating sunlight energy reaching a square meter of corn over a growing season of 100 days in the Midwest of the U.S. and how much of that light energy is converted to corn energy during this time. Discuss the questions below with other students in preparation for an all-class discussion. You can write answers and 2 questions in the spaces below. (Kcal = kilocalories; m = square meter) Sunlight Corn 1,000,000 2 2 5000 Kcal/m /100 days Kcal /m /100 days a. What do the boxes and arrows represent – that is, what are they meant to show? If you were going to write a phrase or sentence about each box and about the arrow, what would you write? What questions do you have about this box-and-arrow representation of energy flow in a corn ecosystem? If this picture were in a biology textbook, what do you think students might be confused about? b. Lets try to see what is going on here. A million kilocalories of sunlight energy strikes one square meter (picture that – a meter is about a yard) of ground in Iowa (for example) over the 100 day time period (roughly 3 summer months). Then, after the 100 days the corn plants in the square meter contain 500 kilocalories of energy. Can you picture this? Sketch this below. Any questions? Definition: A kilocalorie is the amount of heat needed to warm 1 kg of water 1 degree Celsius (°C). So if you burned the corn plants, you could heat 1 kg (about 2 lbs) of water 5000 degrees Celsius. This is what the phrase “contains” kcal means operationally (e.g. what you can measure). o What does the phrase “sunlight energy is converted to corn energy really mean biologically? What is the main biological process? Hint: there is a “famous” organic molecule at the end of this process. Questions? What might other students be confused about here? o Compare 1,000,000 Kcal of sunlight energy to 5000 Kcal of corn energy. What strikes you about the difference between these two numbers? How efficient is the corn in converting sunlight energy into corn energy? How would you calculate this? Since energy is never lost within an ecosystem, where do you think the energy not converted to corn energy “goes”? Draw a picture showing this as best you can. Write down any questions that come up. c. Make you own box-and-arrow diagram with the data below. This is an energy budget for a forest ecosystem. The units are percent of solar radiation (sunlight) in a year. The system is a temperate deciduous forest Percent of solar radiation Parts of forest ecosystem 100 Sunlight 15 Reflected sunlight 50 Heat 2 Gross plant production 1 Net plant production 0.01 Plant ingestion by herbivorous insects 0.005 Respiration by herbivorous insects Values are from Gosz et al. 1978. The flow of energy in a forest ecosystem. Scientific American 238: 92-102.
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