- 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; m2 = square meter)
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
Gross plant production
Net plant production
Plant ingestion by herbivorous insects
Respiration by herbivorous insects
Values are from Gosz et al. 1978. The flow of energy in a forest ecosystem. Scientific American 238: 92-102.