Nutrient Cycles

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Nutrient Cycles

The Carbon Cycle

  • All nutrient cycles have one simple sequence that they follow:
  • The nutrient (sunlight) is taken up by producers as simple, inorganic molecules.
  • The producer incorporates the nutrient into complex organic molecules.
  • When the producer is eaten, the nutrient passes onto the consumers.
  • It then passes along the food chain, when these animals are eaten by other consumers.
  • When producers and consumers die, their complex molecules are broken down by saprobiotic microorganisms (decomposers) that release the nutrient in its original form.
  • The cycle is then complete.
  • Carbon is a component of all major macromolecules in living organisms.
  • The mains source of carbon for terrestrial organisms is carbon dioxide from the atmosphere, which only makes up 0.04% of the overall gases in the atmosphere.
  • Photosynthetic organisms remove it from the air to build it up into macromolecules such as carbohydrates, fats and proteins.
  • All organisms return carbon dioxide back to the atmosphere through respiration. 
  • Variations in the rates of respiration and photosynthesis give a rise to short-term fluctuations in the proportions of oxygen and carbon dioxide in the atomosphere, i.e. lower during the day than at night.
  • Globally the level of carbon dioxide in the atmosphere has increased over the past few hundred years. 
  • This is due to two main human activities:
  • The combustion of fossil fuels, such as coal, oil and peat, has released carbon dioxide that was previously locked up within these fuels.
  • Deforestation, especially of the rain forests, has removed enormous amounts of photosynthesisng biomass and so less carbon dioxide is being removed from the atmosphere. Trees also contain a massive store of carbon within then and when they are burnt, this massive store is released into the atmosphere as carbon dioxide.
  • Carbon dioxide is a greenhouse gas and so contributes to global warming.
  • Oceans contain a massive reserve of carbon dioxide. This store is 50 times greater than that in the atmosphere.
  • This store helps keeps the atmospheric concentration more or less constant.
  • When atmospheric levels are too low, the reverse occurs.
  • Aquatice photosynthetic organisms use this dissolved carbon dioxide to form the marcromolecules that make up their bodies.
  • The carbon in photosynthetic organisms passes along food chains to animals.
  • Plant and animals are broke down by saprobiotic microorganisms, known as decomposers once they have died.
  • Saprobiotic microorganisms secrete enzymes on the dead organisms. These enzymes break down complex molecules into smaller, soluble molecules that they then absorbe by diffusion. The carbon in the dead organisms is then released as carbon dioxide during respiration by the decomposer.
  • If decay is prevented for any reason, organisms may become fossilised into coal, oil or peat.
  • Not all parts of organisms can be decomposed, for example the shells and bones of aquatic organisms sink to the bottom of the oceans and, over millions of years, form carbon containing sedimentary rocks such as chalk and limestone. This carbon eventually returns to the atmosphere as these rocks are weathered - this is a process known as carbon


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