Cycles in Nature

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Cycles in Nature

• HS Biology

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Earth Cycles

• Earths biosphere contains a fixed amount of
  water, carbon, nitrogen, oxygen, and other
  materials that cycle through the environment and
  are reused by different organisms.

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Water Cycle

• How water moves from the Earths surface to the
  atmosphere and back to the surface again

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Components of the Water Cycle

• Evaporation when liquid water changes into water
  vapor and enters the atmosphere
• Condensation the process of changing water from
  a gas to a liquid
• Precipitation when water drops become large
  enough and fall to the ground as rain, snow,
  sleet, etc.
• Transpiration Transpiration is the process by
  which plants lose water out of their leaves. 
  Transpiration gives evaporation a bit of a hand
  in getting the water vapor back up into the air.

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Nitrogen cycle

• The transfer of nitrogen from the atmosphere to
  the soil, to living organisms, and back to the
  atmosphere
• The growth of all organisms depends on the
  availability of mineral nutrients, and none is
  more important than nitrogen, which is required
  in large amounts as an essential component of
  proteins, nucleic acids and other cellular
  constituents.

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Nitrogen Cycle (cont.)

• There is an abundant supply of nitrogen in the
  earth's atmosphere - nearly 79 in the form of N2
  gas. However, N2 is unavailable for use by most
  organisms because there is a triple bond between
  the two nitrogen atoms, making the molecule
  almost inert.

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Nitrogen Fixation

• In order for nitrogen to be used for growth it
  must be "fixed" (combined) in the form of
  ammonium (NH3) or nitrate (NO3) ions.
• some bacteria can convert N2 into ammonia by the
  process termed nitrogen fixation these bacteria
  are either free-living or form symbiotic
  associations with plants or other organisms (e.g.
  termites, protozoa)
• Nitrogen-fixing bacteria can be found in the soil
  as well as on submerged objects in lakes and
  ponds. These are called cyanophytes (blue-green
  algae).

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Legume symbioses

• The most familiar examples of nitrogen-fixing
  symbioses are the root nodules of legumes (peas,
  beans, clover, etc.).

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More ammonia

• A small amount of nitrogen fixation also results
  from the passage of ultraviolet light and
  lightning through the air, causing nitrogen to
  react with oxygen to form nitrate ions.
• Additional amounts of nitrate and ammonia are put
  into the atmosphere by volcanoes, by combustion
  of fossil fuels, and by forest fires.

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Other nitrogen

• Farmers replace nitrogen in the soil by growing
  nitrogen-fixing crops or using fertilizers that
  contain nitrogen compounds that plants need for
  growth.

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Ammonification

• At any one time a large proportion of the total
  fixed nitrogen will be locked up in the biomass
  or in the dead remains of organisms.
• The excretions of animals and dead bodies are
  broken down in the soil by decomposers
• This produces ammonia.

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Nitrification

• Some bacteria called nitrifying bacteria take the
  ammonia and change it to a nitrite ion, and then
  a different group of nitrifying bacteria change
  that nitrite ion to a nitrate ion.
• This then can be taken up into the roots of the
  plant and be used.

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Problems of nitrification

• The ammonium ion is readily adsorbed onto the
  clay soil and soil organic matter, preventing it
  from being washed out of the soil by rainfall.
• In contrast, the nitrate ion is not held on soil
  particles and so can be washed down the soil -
  the process termed leaching
• In this way, valuable nitrogen can be lost from
  the soil, reducing the soil fertility.

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The problems with too many nitrates

• The nitrates can then accumulate in groundwater,
  and ultimately in drinking water.
• There are strict regulations governing the amount
  of nitrate that can be present in drinking water,
  because nitrates can be changed to nitrites by
  microorganisms in the gut.
• Nitrites are absorbed from the gut and bind to
  haemoglobin in your blood, reducing its
  oxygen-carrying capacity.

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Denitrification

• If the soil becomes too compacted or gets too
  wet, air cannot penetrate.
• This allows bacteria called denitrifying bacteria
  to convert nitrate to nitrous oxide or nitrogen
  gas which is then lost to the atmosphere.
• That is why it is important for farmland to be
  kept well drained and plowed.

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Carbon Cycle

• How carbon molecules move between the living and
  nonliving world
• The same carbon atoms in your body today have
  been used in countless other molecules since time
  began.
• Plants absorb carbon dioxide from the atmosphere
  and use it, combined with water they get from the
  soil, to make the substances they need for
  growth.

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Carbon Cycle (cont.)

• Animals, like a rabbit, eat the plants and use
  the carbon to build their own tissues.
• Other animals, such as the fox, eat the rabbit
  and then use the carbon for their own needs.
• These animals return carbon dioxide into the air
  when they breathe, and when they die, since the
  carbon is returned to the soil during
  decomposition.

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Carbon cycle (cont.)

• The carbon atoms in soil may then be used in a
  new plant or small microorganisms.
• Ultimately, the same carbon atom can move through
  many organisms and even end in the same place
  where it began.

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