Carbon dioxide cycle sources

Randerson, J. T., Thompson, M. V., Conway, T. J., Fung, I. Y. and Field, C. B. (1997). The contribution of terrestrial sources and sinks to trends in the seasonal cycle of atmospheric carbon dioxide. Global Biogeochem. Cycles 11,535-560. 

However, if the major source of hydrogen is reformed natural gas, the cost of generating electricity with a low-temperature fuel cell would be about 0.20 per kilowatt-hour. This is more than double the average price for electricity. It would also produce 50% more carbon dioxide emissions than the most efficient natural gas plants which are combined cycle natural gas turbines. Low-temperature fuel cells operating on natural gas are not as efficient at generating electricity. A stationary fuel cell system achieves high efficiency by cogeneration. 

The citric acid cycle is at the heart of aerobic cellular metabolism, or respiration. This is true of both prokaryotic and eukaryotic organisms, of plants and animals, of organisms large and small. Here is the main point. On the one hand, the small molecule products of catabolism of carbohydrates, lipids, and amino acids feed into the citric acid cycle. There they are converted to the ultimate end products of catabolism, carbon dioxide and water. On the other hand, the molecules of the citric acid cycle are intermediates for carbohydrate, lipid, and amino acid synthesis. Thus, the citric acid cycle is said to be amphibolic, involved in both catabolism and anabolism. It is a sink for the products of degradation of carbohydrates, lipids, and proteins and a source of building blocks for them as well. 

Carbon dioxide mainly exits the oceans at the interface with the atmosphere. Warm surface waters easily release carbon dioxide into the atmosphere. When warm waters rise to the surface, mainly near the equator, carbon dioxide is transferred from the water to the air. Because of this, the sea is a source of carbon for the carbon cycle as well as a carbon sink. 

In the previous section the present-day cycle of carbon was discussed in some detail. We can now turn our attention to the sources of carbon to the ocean, and of calcium and magnesium, the major elements (other than oxygen and hydrogen) with which carbon interacts. Because carbon dioxide is the major acid gas involved in both carbonate and silicate mineral weathering reactions at the Earth s surface, it is informative to consider the sources of other elements as well. 

Water and carbon dioxide provide the most attractive and abundant source materials for the fuel products of an artificial photosynthetic device, as seen in Fig. 11. These two compounds are the outcome of the heterotrophic cycle providing the energy sources of the living organism, and the result of consumption of fossil fuels by mankind. Not surprisingly, both of the materials exhibit high chemical... 

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