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
questions in the spaces below. (Kcal = kilocalories; m 2 = square meter)
Sunlight
1,000,000
Kcal /m2/100 days
Corn
5000 Kcal/m2/100 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.