Cycling Through Nature

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Cycling Through Nature

Food webs and the size of populations has probably made you realise how interconnected things are in the environment.

This is not just true about the plants and animals but it is also the case for all the chemical molecules present on the Earth. The atoms that make them up cannot be destroyed, they just get moved around and around.

So if you look at a particular type of chemical you can follow it as it goes through a cycle in the environment. The best known are the carbon and nitrogen cycles.

First of all the chemicals need to get out of the animals or plants that they are in, this occurs through the process of decomposition.

All things die, sadly. But their molecules go marching on!

The cells and bodies of plants and animals decay by the action of soil bacteria and fungi. They are often called the decomposers. (There's a joke about Beethoven there somewhere).

The molecules released during decay are absorbed by the bacteria, fungi and also back into the soil from where plants can draw them up through their roots.

The decomposers work best at their optimum conditions since they use enzymes. The kind of conditions needed are shown nicely by the humble compost heap.


There are five necessary conditions or factors for good decomposition:

  1. Organic material: plant or animal material to decay
  2. Water: the right amount of moisture
  3. Oxygen: air must be able to get to the material
  4. Warmth: the temperature cannot be too hot or too cold
  5. Decomposers: bacteria or fungi are needed to do the job

You would not get decomposition if one of these was missing.

The molecules released by this decay process include the most important elements carbon and nitrogen which have their own environmental cycles.

Once carbon compounds from plants and animals are released by decomposition they are taken into the soil. Slowly they are broken down by bacteria and released back into the air in the atmosphere.

Carbon Cycle

There is only one route in which carbon compounds return and that is by photosynthesis carried out by plants. That is the only way that animals can get glucose, without plants there would be no animals on the Earth! As well as carbohydrates, plants also form fats and proteins.

The material from plants can decay or be burned and release the carbon back into the atmosphere as carbon dioxide. Otherwise this material can be transferred to animals as the plants are eaten.

The animals, or those that eat them, also form the molecules into carbohydrates, fats or proteins within their own bodies.

So the carbon cycle isn't as complicated as it appears at first!

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Nitrogen is not a glamorous element. It is not very reactive, and does not look particularly attractive!

The air contains 78% nitrogen but it is not altered during our breathing. It cannot be used directly by plants or animals.

Nevertheless nitrogen is important in Biology. It occurs in DNA, proteins, nitrates (NO3) and ammonium compounds (NH4). Plants need nitrates to grow well.

Nitrogen ends up in these molecules through 3 different routes:

  1. Lightning
  2. Nitrogen fixing bacteria
  3. Artificially through the synthetic, human reaction called the Haber process

The nitrogen cycle links these different compounds and routes in a continuous and inter-linked series of reactions.

nitrogen cycle links

You can see from this that key roles are played by the four different types of bacteria:

 four different types of bacteria

The nitrogen-fixing bacteria live in a mutualistic relationship in the root nodules of Leguminous plants (Legumes), such as clover. The bacteria provide the plant with nitrates and receive glucose in return, so both benefit.

Other bacteria live in the soil itself, they make nitrates and energy for themselves.

Lightning is the other natural way in which nitrogen can return to the soil. The energy in a lightning bolt is enough to zap nitrogen and oxygen in the air together to make the oxides of nitrogen. These dissolve in water in rain to form nitrates.

Now see if you can sort the descriptions out to match the names of the different bacteria.

Just work through the cycle stuff until you have got it. It's not too hard, honest!

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