Steps to Sustainability: Photosynthesis

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Tompkins Weekly 12-24-19

By Richard W. Franke

We meat-eaters often forget that when we eat animal flesh, we actually eat plants but in a most inefficient way. The animals we consume depend themselves on plants but are able to capture only about 10% of plant biomass for growth and/or tissue replacement. Put another way, it takes about 10 pounds of grain to produce one pound of beef. This is known in ecology as the “10% rule.”

Fortunately, the inefficiencies described by the 10% rule do not prevent earth’s plant and animal species from thriving. Our sun sends out daily 20 billion tons of photons as part of its solar wind. A tiny portion of this enormous energy blast is captured by two green food producers – phytoplankton in the oceans and green plant leaves on land. This process is called photosynthesis.

Image provided.

The secret of this amazing solar energy capture is the chlorophyll molecule. This molecule contains 137 atoms, one of them a magnesium atom surrounded by a ring of four nitrogen and five oxygen atoms and an extended tail of 55 carbon and 72 hydrogen atoms somewhat interwoven. The overall shape can be compared to a tadpole.

Exactly how the chlorophyll molecule snags photons – particles of light traveling at the speed of light – is not well understood. The chlorophyll molecule appeared possibly about 3.9 billion years ago, a little after the first living cells developed around 4 billion years back. Of course, 900 million years is a fair amount of time, even in evolutionary terms.

The overall process of photosynthesis is a little less mysterious but fairly complex. The light energy of the photons is channeled to split water into its chemical components: hydrogen and oxygen. The hydrogen is then applied to a second process where the plant uses CO2 to produce glucose, a sugar the plant can use to store energy. The oxygen (the O in H2O or water) is sent off as a byproduct.

Most or perhaps all biological and chemical processes on earth are influenced by temperature.

Scientists are only at the beginning of understanding the complex relationship between temperature and photosynthesis. Like many phenomena in nature, photosynthesis follows a common response curve in the shape of an upside-down “U.”

At least some current food crops are near the top of the U – a location labeled their “thermal optimum.” This implies that additional heat from global warming could tip them over the top of the U and actually lead to output declines. One study in India, published in 2001, found that a one-degree increase in average temperatures had no effect on yields, but a 2-degree Celsius (3.6 degrees Fahrenheit) temperature increase led to an 8.5% to 38% decline in irrigated wheat output. An Ohio study from 1999 found that photosynthesis in plants generally increases to 20 degrees C (68 degrees F) then plateaus as temperature rises to 35 degrees C (95 F). At 40 C or 95 F, photosynthesis ceases entirely. These findings suggest that global warming – while it may raise output in many areas (below or up to 95 F) and in many of the world’s warmest current climates with most of the world’s poorest people – it may decline.

Finally, besides its crucial role in manufacturing food for plants and thus eventually animals, photosynthesis plays a large role in maintaining (and possibly in creating) the balance of gases in the earth’s atmosphere – especially its oxygen content. As put by physicist Fritjof Capra: “By blending water and minerals from below with sunlight and CO2 from above, green plants link the earth and the sky.”

Richard W. Franke is professor emeritus of anthropology at Montclair State University, a resident of Ecovillage at Ithaca and a board member and treasurer of Sustainable Tompkins. To access all of Franke’s Steps to Sustainability Essays, go to

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