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Carbon is for all of us

By David Smith - posted Monday, 26 November 2007

It should never be forgotten that a carbon economy underpins life on earth. Next to the spark of life, the greatest marvel is the plant. We must remember our childhood awe at seeing the shoot from the seed push through the soil, raise itself into the air and turn green by nightfall.

This turning green indicates two things, interdependent, but vital for life: first, that the plant is capturing solar energy from that virtually inexhaustible supplier, the Sun, and second, in this process the plant is taking carbon dioxide from the atmosphere and water from the soil and forming carbohydrates. In these, carbon is bonded to the other elements, hydrogen and oxygen and these bonds are the energy store. The process of green plants capturing this energy is photosynthesis.

These carbohydrates are the real stuff of life. Some may be molecules made up of a short chain with perhaps six carbon atoms or many more: there are various forms. Organisms can use these carbon compounds, breaking them down to access the energy. Plants make their own. Other living things, such as animals or most insects, use plants as their source.


This process of breaking down carbohydrates and releasing energy is called respiration, in a sense the reverse of photosynthesis, freeing carbon dioxide and hydrogen again and releasing energy. For animals we associate this with breathing - oxygen in, carbon dioxide out.

Some carbohydrates are used in body building, especially with nitrogen added to the carbohydrates to make amino acids which in chains form proteins.

Proteins lead us to a third marvel - the capture of nitrogen from the atmosphere by the legume plant family by providing a "home" for certain bacteria on their roots and exchanging carbohydrates for nitrogen compounds. Uniquely, legumes can access atmospheric nitrogen through these bacteria, a huge saving on using energy to make artificial fertilisers.

Some carbohydrates have only a short life - they are used up soon, for example as the energy source for the plant that made them. Some last a bit longer as the "body" of the organism, perhaps eaten by an animal, perhaps only broken down after a plant's death.

Wood is an interesting example: complex, long life carbohydrates largely resistant to rotting - such as breakdown by some other organisms - and so lasting perhaps hundreds of years. This is why trees are so important in locking up - or sequestering - carbon.

Some carbohydrates or their derivatives have a long life. The "bodies" of the organisms have become trapped when earth-forming processes occur and deeply buried in the earth's crust. Because these are well away from oxygen, when changes occur they may form what we call hydrocarbons.


Again, some are short chains with a low number of carbon atoms, and these tend to be gases such as methane. Longer chains are liquids - such as high-octane petrol and kerosene. Really longer ones are solids - greases and coal.

Then soils, though mineral in origin, "work" best for us as a place for plants to grow if there is plenty of carbon material - organic matter - in them, some in the many organisms and some as more inert soil organic matter. With lower carbon levels, soils become hard and inhospitable and unproductive and often erode. Furthermore, highly productive crops and pastures do a lot of photosynthesis - taking up a lot of carbon dioxide from the atmosphere.

While it sounds complicated, it is really orderly and systematic. The most important thing to recognise is the unity of it all, the centrality of carbon and how it begins with that shoot "plugging in" to the sun. Therefore we have short-term solar energy, captured and used the same day; longer term in plant storage organisms such as grains and tubers, and, as we eat them, in our bodies; much longer-term solar energy storage as in trees; and long-term storage in things such as coal, oil and gas.

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First published in The Age on November 19, 2007.

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About the Author

Dr David Smith is a senior associate in the school of agriculture and food systems at the University of Melbourne.

Creative Commons LicenseThis work is licensed under a Creative Commons License.

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