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The Carbon Cycle

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Carbon is one of the elements most important to life. All living things have carbon in them, not as the element itself, but in compounds.

Trees, plants, algae and lichen all convert carbon dioxide into oxygen, by photosynthesis. (Carbon dioxide is the compound in which most carbon is present in the air.)

All living things produce carbon dioxide, but trees, plants, algae and lichen produce more oxygen then carbon dioxide. Carbon dioxide is produced as a product of respiration. Respiration can be summarised as: 

oxygen + food --> carbon dioxide + energy
Considering this from the viewpoint of humans, this basically means that oxygen (from air that you breathe in) and food that you eat get turned into carbon dioxide (which is breathed out) ans energy for doing everything.

Photosynthesis could be regarded as the opposite of respiration: 


Carbon dioxide + energy --> oxygen + complex carbon compounds 
                                      known as carbohydrates.
Photosynthesis takes place in chloroplasts, (which contain the green pigment chlorophyll, which is what makes trees' leaves green).

Carbon dioxide is also made when organic material decomposes. Usually, decomposition happens when an oraganism or part of an organism dies. When an organism dies, fungi and bacteria that find their way to the material excrete digestive juices (which living material has defence against.) The fungi and bacteria then digest the material.

Fires, too produce carbon dioxide. If the fuel that they burn contains a carbon-containing compound then carbon dioxide is produced. 

Carbon + oxygen (+heat) --> Carbon Dioxide + heat
                                   (+ light)

A description of the Carbon cycle

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All animals take in carbon compounds when they eat food: sugars, starches, fats, and proteins.

During respiration, food is effectively turned into energy, water and carbon dioxide. The water vapour and carbon dioxide are breathed out.

Carbon dioxide in the atmosphere is used by plants for photosynthesis. As mentioned above, plants also respire, but the overall effect is a production of oxygen, because they photosynthesise more than they respire.

Note: Only plants that have leaves can photosynthesise, but trees without leaves still respire. This means that if a tree is deciduous, it produces oxygen in the summer (while it has leaves), because it photosynthesising more than it respiring, and in the winter when it doesn't have any leaves, the tree produces carbon dioxide because it isn't photosynthesising, but it is respiring.

Plants store up the carbon that they take in from the atmosphere in the form of carbohydrates. Carbohydrates are used as structural material in plant cell walls and are a major constituent of plants that are used as food. Edible carbohydrates include starches in potatoes and grains, and sugars in fruits.

Animals store some food as the carbohydrate Glycogen, but convert more to fats, to use as reserves. Animals need to eat plant-derived complex carbon compounds including carbohydrates to obtain energy necessary to maintain life.

Animals and plants that live in water have to use oxygen and carbon dioxide that are dissolved in the water. Plankton in the oceans in fact is the major contributor to carbon fixation; i.e. the trapping of inorganic carbon by photosynthesis.

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Questions

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  1. Questions on the above lesson

    1. Name the process that produces oxygen from carbon dioxide, water, and light.
    2. Write in your own words a description of how carbon cycles in the biosphere.
    3. Below is a list of types of organisms. Choose from the list those organisms that produce carbon dioxide.
      • animals
      • plants
      • bacteria
      • viruses
      • algae
      • lichen
    4. What is carbon fixation? What type of organism does it?
    5. When plants trap Carbon Dioxide from the air, it is stored in plant cells as which of the following?
      • Carbohydrates
      • Glucose
      • Glycogen
      • Sugars
      • Other (please specify)
    6. Complete the following sentence: When an organism dies, and/or fungii the organic materials from which the organism was made, as a result of which carbon is produced.
    7. Above, in the diagram of the carbon cycle in water, there are two unlabelled arrows pointing to the air, a red arrow and a blue arrow. What should these be labelled?

  2. Questions related to the above lesson and to extend your knowledge.

    1. Name five carbon compounds found in living things not mentioned in the above lesson text.
    2. Write a paragraph or two to describe what you would think would happen if plankton was to stop fixing carbon.
    3. How do animals that live in water `extract' the dissolved oxygen from the water?
    4. Are plants and animals are dependant on each other? If so, why? Write a paragraph.


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Glossary

A compound is a substance made of two or more elements joined together by a chemical bond.

A chloroplast is a cell organelle found in plants, lichen, and algae.

A deciduous plant is a plant that grows leaves in the spring for the summer, and loses them in autumn for winter.

This page and material in it is copyright © John & Rosalind Stinchcombe Associates Ltd, Waitakere, Auckland, New Zealand. All rights reserved.