The Carbon Cycle

Yesterday, we got a quick look at organic chemistry, and identified several different substances that contain carbon. These include carbon by itself – as an element – carbon dioxide (CO2), and biomolecules (carbohydrates, fats, and proteins.)

How did carbon join with other elements to form biomolecules? This miracle is accomplished primarily by photosynthesis – the most common and arguably the most important chemical reaction on Earth. In photosynthesis, green plants – including microscopic and aquatic forms – convert carbon dioxide into carbohydrates. In this process, solar energy is converted into chemical energy that plants use to grow and multiply. As part of their growth and reproduction, plants convert the carbohydrate molecules into all the various biomolecules needed to make the plant.
Eventually, the plants die. They may rot in place and thus provide a food supply for microorganisms. Or they may be consumed by animals in search of energy and biomolecules. The animals eventually die, too, and meet similar fates. In this “Circle of Life,” carbon atoms change places and molecular partners, but the law of conservation of mass dictates that the overall number of carbon atoms remains constant.
Where does the carbon dioxide come from? To read today’s news, you would be led to believe it all comes from SUVs or such trappings of modern life. These do produce carbon dioxide, as a product in the combustion of the fuel. “Fossil fuels” – wood, coal, natural gas, and petroleum – are derived from long-dead plants and animals. But all living things engage in some form of combustion – that is how they derive the energy to grow and reproduce. And in the process of biological combustion – called respiration – all living things produce carbon dioxide. So it’s the biosphere – life on Earth, predominantly microscopic, as noted in January’s “Death to the Respirators” blog – that is the main source of carbon dioxide.
Fortunately, this same biosphere is the main consumer of carbon dioxide, via photosynthesis. The principal tenet of the science of ecology is this: if a food supply exists, a biological population will arise to consume it. For green plants, their food supply is carbon dioxide. More carbon dioxide means more photosynthesis will occur. This truly keeps carbon and “Earth in the Balance.”
Carbon is present on Earth in many forms, and life on Earth is constantly changing carbon from form to form. Photosynthesis converts carbon dioxide to biomolecules, and respiration converts biomolecules back into carbon dioxide. Just as with the hydrologic cycle – where humans “borrow” water from the Earth, but return every single molecule – humans “borrow” carbon in various forms from the Earth, but we return every single atom. No matter how evil one might be, one cannot violate the laws of Nature, including the law of conservation of mass.

2 Responses to The Carbon Cycle

  1. avatar Tom says:

    I'm with you on this whole "Earth in the Balance" thing, but for the sake of discussion, what happens to the "principal tenent of the science of ecology" when the rainforests disappear? Do the (disappearing) trees in these forests play a big enough role photosynthesis wise to throw the world out of balance, or do the world's oceans pick up the slack?

  2. avatar Steve McLean says:

    Ecology tells us that some population will capitalize on the available food supply. Destroying rain forests definitely puts a dent in a significant population that uses carbon dioxide. That "stress" on the rain forests would provide a competitive advantage to other life forms vying for the same carbon dioxide. So, yes, the oceans that dominate our planet's surface are the most likely location for additional photosynthetic growth. The science of ecology would predict larger marine algae populations as a response to rain forest declines.

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