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Leaf-macro

by Edmund Hazzard (Submitted: 09/08/2008)

[screen shot]

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WHAT IS IT?

This is a model of photosynthesis in a chloroplast which is inside a leaf. It was written for grades 5-6.

This model is used by the Concord Consortium in the UDL (Universal Design for Learning) project. To see the model in the context of an activity, go to http://udl.concord.org. Find the Intermediate Plants Unit (Grades 56) and go to the activity called "Photosynthesis: the big picture".

HOW IT WORKS

The following inputs can be adjusted with sliders:
-- light rays radiating from the sun (yellow arrowheads)
-- water supplied to the stem (blue circles)
-- carbon dioxide (CO2) molecules in the atmosphere (orange and black)

In the center of the leaf are several black circles representing chloroplasts. The rules for the model are as follows:
1. Water in the stem is drawn up into the leaf as it goes by, up to a maximum of 30 water molecules.
2. If a light ray and a water molecule are both inside a chloroplast, it goes into an excited state (NADPH). The circle gets larger and turns orange. One water is used up and one oxygen (gray circle) is created.
3. If a CO2 and a water are both inside a chloroplast when it is in an excited state,
it returns to an unexcited state. The circle gets smaller and turns black. One CO2 is used up and one sugar (white jagged shape) is created.
4. The sugars go down the leaf stem and disappear. The idea is that they are being used by the plant in some way to build leaves, stems, roots, and seeds for instance. The sugar production rate on the graph is the number of sugars on the screen at any moment.
5. The oxygens go out in all directions and disappear when they get to the edge of the screen. The oxygen production rate on the graph is the number of oxygens on the screen at any moment.

HOW TO USE IT

Run the model and watch for the reactions that are described above. Slow the model down if that is helpful.

Water, carbon dioxide in the air, and sunlight can be controlled with the sliders. Also, the day-night switch moves the sun across the sky and changes it into a moon on the next crossing, imitating a day-night cycle.

THINGS TO NOTICE

Notice that there are two reactions needed to make sugar. The first one is energy-dependent (#2 above); the second is energy-independent (#3 above). So a leave can continue to make sugar for a while after the sunlight goes away, but soon it runs out of excited-state chloroplasts and stops making sugar.

THINGS TO TRY

Change each of the sliders in turn and see what effect they have on the production of sugar.

EXTENDING THE MODEL

This model includes only the main inputs and final products of the photosynthesis reaction, which is very complex. One could try to expand the reactions that are included in the model.

Think of a better way to graph rates of production. Figure out how to make the graph smoother.

Have the leaf do something (grow?) in response to the production of sugar. Have it turn brown if water is turned off for too long.

The photosynthesis reaction is not linear; it also has an initial threshold. Figure out how to add this to the model.

NETLOGO FEATURES

Note that the image "aspenleaftrans.png" must be in the same folder as the model in order to be displayed. This image underlies but does not interact with the model.

RELATED MODELS

A related model, "leaf-micro.nlogo", looks at the same process but with a slightly different presentation. It is also posted on Community Models.

CREDITS AND REFERENCES

This model was written by Edmund Hazzard as part of the Universal Design for Learning Project (UDL) at the Concord Consortium (http://www.concord.org) in March of 2008.

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