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On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Previous Week's Question Published: 2 August, 2006 Next Week's Question
Photosynthetic efficiency cannot be measured precisely
Question
How efficient is photosynthesis compared to the amount of light energy delivered to the earth?

Question
This is a very interesting question. If we consider only a single leaf and the light that hits it, only about 5% of the incident light energy shows up as the products of photosynthesis. Most of the energy that hits the leaf is either not absorbed by the plant or is lost as heat.

It is a more complex problem to figure out this efficiency on a global scale. The calculation involves a lot of big numbers which are best expressed in what is called scientific notation (300 = 3 x 102).

First let's consider the total input of solar energy to the earth because this number has been measured by scientists with reasonable accuracy. On average, the earth receives about 1 x 1024 calories of heat energy from the sun per year (that's a 1 with 24 zeros after it). The calorie unit we are using here is a measure of energy content and is 1/1000th of the dietary calories we are more accustomed to.

For global photosynthesis, it is really not possible to directly measure the total amount of sunlight used, but we can calculate it indirectly from estimates of the total amount of photosynthesis. The current estimate of the earth's annual photosynthesis (by terrestrial plants and algae in the ocean) is about 200 billion tons of carbon per year. This is carbon from atmospheric carbon dioxide that is converted into carbon in sugars by photosynthesis. The total energy stored (worldwide) in the sugar produced by photosynthesis is about 2 x 1021 calories per year.

Now all that remains is to take the ratio of energy stored in sugars by photosynthesis to total energy input from the sun. This value comes out to be about 0.2%. The reason that this is smaller than the 5% energy efficiency in a single leaf is that leaves (or other photosynthetic organisms) actually cover only a small fraction (less than 10%) of the earth's surface.

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