All life on earth is made possible by sunlight. We know that life needs a low-entropy resource – photosynthesis – to survive and reproduce. Utilized by plants, algae, and some bacteria, photosynthesis involves a photochemical reaction that leads eventually to a process called carbon dioxide (CO2) fixation. But before we discuss this elegant process, let’s first look at the food chain here on earth, as depicted below.
Simplified food chain. For each movement upward, the efficiency of energy utilization is only 10%. This means, for example, that humans use 10% of the available energy from the food we eat.
The principle is well known: each step in the food chain serves as nutrition for the creatures on the next step, and each animal produces carbon dioxide while consuming oxygen. What is less well known is that there is a large amount of inefficiency in this food chain. (Efficiency is defined here as the proportion of energy actually used by an organism, compared to the total energy present in its food.) This can be illustrated with several examples [Glencoe, 2004]: 100 kilograms of grain are needed to produce 10 kilograms of beef, which create only 1 kilogram of human tissue. Similarly, 3000 blades of grass are needed to produce 250 grasshoppers, which will feed 25 birds that will be eaten by just one fox. In general, each higher level in the food chain transforms only 10% of the energy from the next level beneath it. From that point of view, it is rather inefficient to feed cattle with grain, and then eat the cattle; ecologically, we would be much better off if we just ate the grain ourselves. Also, energy efficiency isn’t very high in the photosynthetic process either (as we will see shortly), but the difference here is that solar energy is so abundant, photosynthetic efficiency is not a concern!
Overall, photosynthesis can be written as a chemical reaction:
6H2O + 6CO2 + hν → C6H12O6 + 6O2
In ordinary language, this says that six molecules of water and six molecules of carbon dioxide are transformed into one molecule of sugar and six molecules of oxygen. (The term hν stands for the light quanta that are needed to drive the reaction.) Massive research has shown that the fundamental chemical reactions involved in producing sugar and oxygen are the same in all photosynthetic organisms. The structure of a common sugar, β-D-glucose, is as follows:
Although the overall photosynthesis reaction suggests a rather simple mechanism, the reality is that photosynthesis is extremely complex, and even today is not completely understood. The first step in unraveling the process was Joseph Priestly’s discovery in 1770 that leafy plants produce a gas (oxygen) that supports combustion. In 1845, Julius Robert von Mayer conjectured that plants convert light energy into chemical energy.
Just a few numbers will give you a feeling for this essential life-enabling process [Whitmarsh, 1995]:
- Producing 1 oxygen molecule requires about 8 (red ) light quanta. To make 180 grams of glucose, you need about 3000 kJ of energy.
- Each year, 1014 kilograms of carbon are removed from the atmosphere by photosynthesis. The energy needed to do this represents only 0.1% of all solar energy received by the earth.
- Every year, more than 10% of the total atmospheric carbon dioxide is converted into carbohydrates (or glucose, a 6-carbon sugar).
Pretty neat stuff , isn’t it? In Part 2, I will describe the actual photosynthetic process in a bit more detail, along with its entropy aspects. Stay tuned!
(Taken from The Second Law of Life with permission of William Andrew Publishers)
General further reading:
– Glencoe, Biology the Dynamics of Life, McGraw-Hill (2004)
– Whitmarsh J. and Govindjee; Encyclopedia of Applied Physics, Vol 13 (1995)
– Manning, Richard, “The oil we eat: following the food chain back to Iraq”, Harper’s Magazine, February (2004)
 A few examples to illustrate the inefficiency of the “artificial food chain” [from Manning, 2004]: The agriculture industry needs about 35 J of fossil fuel to produce one J of beef and about 68 J to produce one J of pork. Processed food requires about 10 J to produce one J of food energy.
 The color of the light quanta is significant, since quanta energy depends on their frequency, each of which has a characteristic color.