The good Earth

With the coming of spring and the return of green to the Northern Hemisphere, what better way than the Web to explore the extraordinary process of photosynthesis? If you're lucky enough to cultivate your own garden, you know you must seed on a tight schedule, often dictated by the arrival of the final frost of winter. To find maps showing average annual minimum temperatures in the United States, visit the monthly garden zine (www.thegarden helper.com). Under the title, click on "Garden Encyclopedia," then scroll down until you find the list of links for "USDA Hardiness Zones." On the same page, check out the schedule of gardening tasks for April.

Not surprisingly, perhaps, the process that greens the planet also protects it. Photosynthesis helps cool the atmosphere by absorbing carbon dioxide, a greenhouse gas, and it is a critical factor to consider in forecasting climate change and global warming [see "Heat Exchange," by Robert Ehrlich, page 58]. I found a remarkable, fresh perspective on photosynthesis at NASA's "Earth Observatory" site, in an animation that accompanies an article on Earth's carbon "metabolism" (earthobserva tory.nasa.gov/Newsroom/NPP/npp.html).

Every eight days for the years 2001 and 2002, NASA combined land-based data with data from Terra and Aqua, two Earth-orbiting satellites, to generate composite maps of the world's "net primary production" of carbon dioxide--the amount absorbed during photosynthesis, minus the amount given off during respiration. Scroll down the "Earth Observatory" site to "animations" on the right, where you'll find the composite maps compressed into a short film (download the larger, twenty-megabyte version if you have a fast Internet connection). In the film, tides of photosynthesis ebb and flow with the seasons across the oceans and continents of our planet [two images made from the maps are shown below].

NASA also keeps track of the abundance of aquatic life by measuring the chlorophyll in the microscopic marine plants known as phytoplankton. At nasa.gov/Observatory/ Datasets/chlor.czcs.html you can use data sets obtained by satellites to "build" your own animated illustrations of the shifting concentrations of chlorophyll.

If you want to start your acquaintance with photosynthesis with some history and a simple explanation, go to www.chm.bris.ac.uk/motm/ chlorophyll/chlorophyll_h.htm, a site run by Paul May, a chemist at the University of Bristol in England. Another approach--one my children prefer--is through well-illustrated pages, such as the ones at David Watson's "Flying Turtle" site (www.ftexploring.com/photosyn/photo synth.html).

Devens Gust, a biochemist at Arizona State University in Tempe, offers a good introduction to the importance of photosynthesis for life on Earth (go to photoscience.la.asu. edu/photosyn, scroll down to "Educational Resources," and click on "Why Study Photosynthesis?"). Gust works at the university's Center for the Study of Early Events in Photosynthesis, which has become a Web clearing house for information on the subject. In the same resources list, you'll also find a long, well-organized directory of related sites (click on "Photosynthesis and the Web").

As the name implies, photosynthesis is powered by electromagnetic radiation. Even a cursory glance at a diagram of the chain of events needed to convert photons into food hints at what a complex process it is. (See, for instance, the diagram at www.uqtr.ca/labcarpentier/ eng/home_frames.htm of "Electron transport in the photosynthetic membrane," by Robert Carpentier, a biochemist at the University of Quebec at Trois-Rivieres.) For more information about how the photosynthetic process got under way on Earth, you can try www.ucmp.berkeley.edu/bacteria/cyan ointro.html, a site run by UCLA's Museum of Paleontology.

Perhaps the most intriguing aspect of the study of photosynthesis, however, is the promise it holds for a sustainable future. The world's hunger for fossil fuels is rapidly depleting the photosynthetic bonanza that took hundreds of millions of years to store as coal and oil. But biochemists may soon figure out how to duplicate nature's most important chemical reaction, and harness it more directly to make our fuel from scratch.

Robert Anderson is a freelance science writer living in Los Angeles.

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