Carbon Cycle, Nitrogen Cycle, Water Cycle and Decomposition

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 This page will help you answer questions such as ...

 Be able to describe and understand the steps in the carbon cycle?

 Be able to describe and understand the steps in the nitrogen cycle?

 Be able to describe the water cycle?

What factors affect the rate of decomposition of organic material?

Introduction to cycles

An ecosystem is all the living organisms in a particular area including non-living conditions such as temperature or soil quality, water sources.

Materials are recycled in many complex ways involving biotic (living) and abiotic (non-living) components in ecosystems.

For more on this see Ecosystems and interactions between organisms   gcse biology revision notes

All living organisms are composed of the chemical elements they obtain from the environment in some way.

Plants will take in compounds of carbon (e.g. carbon dioxide), hydrogen (e.g. water) and nitrogen (e.g. nitrates) and the element oxygen in air. Some plants can metabolise nitrogen from the air directly (e.g. legumes).

These elements and simple compounds are converted into all the complex biological molecules you find in plants such as carbohydrates, fats and proteins.

These are then consumed by plant eating animals.

All of these elements and compounds must be recycled by some means back into the environment.

The material might be recycled back into the soil or into the air.

This recycled material may come from animal waste products e.g. faeces or emitted gases!

When plants or animas die, their remains will slowly decay.

The decayed material in the soil is recycled by new plant growth and so re-enters the many complex food chains in an ecosystem.

All dead organisms decay because they are broken down by decomposers (often microorganisms) which replaces lost nutrients in the soil.



The Carbon Cycle

  • Know and understand that the constant cycling of carbon is called the carbon cycle.

    • Carbon is an important element in many of the compounds of living organisms and because there is only a fixed amount (ignoring fossil fuel burning) in the 'biosphere' it must be constantly recycled.

    • Know and understand that in the carbon cycle:

      • a) Carbon dioxide is removed from the environment by green plants and algae for photosynthesis.

        • Most carbon is 'captured' by photosynthesis in the leaves/stems of plants on land on land, but carbon dioxide dissolves in water so algae in lakes, seas and oceans make a major contribution to capturing and recycling carbon too.

        • Also note that photosynthesis is the only way carbon is absorbed from the atmosphere (only one arrow points down from the atmosphere).

      • b) The carbon from the carbon dioxide is used to make carbohydrates, fats and proteins, which make up the body of plants and algae and ultimately the food of animals.

        • Photosynthesis uses sunlight energy to convert water and carbon dioxide into sugars like glucose, the 'waste product' being oxygen - though plants need oxygen for their respiration at night!

        • the simplest equation to illustrate photosynthesis is

        • water + carbon dioxide (+ sunlight) == chlorophyll ==> glucose + oxygen

      • c) When green plants and algae respire, some of this carbon becomes carbon dioxide and is released into the atmosphere.

        • the overall simplest equation for respiration is the opposite of photosynthesis

        • glucose + oxygen ==> water + carbon dioxide (+ energy)

      • d) When green plants and algae are eaten by animals and these animals are eaten by other animals, some of the carbon becomes part of the fats and proteins that make up their bodies, so carbon based materials and energy are moved up the food chain.

      • e) When animals/plants respire some of this carbon becomes carbon dioxide and is released into the atmosphere.

      • f) When plants, algae and animals die, some animals and microorganisms feed on their bodies.

      • g) Carbon is released into the atmosphere as carbon dioxide when these microorganisms respire.

      • h) By the time the microorganisms and detritus feeders have broken down the waste products and dead bodies of organisms in ecosystems and recycled the materials as plant nutrients, all the energy originally absorbed by green plants and algae has been transferred.

      • i) Combustion of wood and fossil fuels releases carbon dioxide into the atmosphere.

        • It takes fossil fuels millions of years to form, but using biofuels is MUCH faster!

        • Ignoring fossil fuels, through the carbon cycle, carbon is recycled over and over again, through the atmosphere and food chains over a relatively short period of time.

        • The balance has been shifted in favour of atmospheric carbon dioxide (slowly rising) because of the rapidity with which we are burning fossil fuels which take so long to form.

      • j) The decomposition of these carbon based materials ensures that habitats can be maintained to support the organisms that live in that particular ecosystem. If this decomposition did not take place via microorganisms, all the waste would stack up and the nutrients needed to sustain new life would not be recycled.



The Nitrogen Cycle

  • Nitrogen is an extremely important element for all plant or animal life! It is found in important molecules such as amino acids, which are combined to form proteins.
    • Protein is used everywhere in living organisms from muscle structure in animals to enzymes in plants/animals.
  • Nitrogen from the atmosphere:
    • Action of nitrifying bacteria, (nitrogen-fixing bacteria) e.g. they function in the root nodules of certain plants like peas/beans (the legumes), can directly convert atmospheric nitrogen into nitrogen compounds in plants e.g. nitrogen => ammonia => nitrates which plants can absorb.
      • However, most plants can't do this conversion from nitrogen => ammonia, though they can all absorb nitrates, so the 'conversion' or 'fixing' ability might be introduced into other plant species by genetic engineering.
    • The nitrogen from air is converted into ammonia in the chemical industry, and from this artificial fertilisers are manufactured to add to nutrient deficient soils. However, some of the fertiliser is washed out of the soil and can cause pollution.
    • The energy of lightning causes nitrogen and oxygen to combine and form nitrogen oxides which dissolve in rain that falls on the soil adding to its nitrogen content. This is described as a nitrogen fixation process - atmospheric elemental nitrogen converted to a compound that enters the soil for plants to use.
      1. N2(g) + O2(g) ==> 2NO(g)
      2. then 2NO(g) + O2(g) ==> 2NO2(g) 
      3. NO2(g) + water ==> nitrates(aq) in rain/soil
      4. Incidentally, reactions 1. and 2. can also happen in a car engine, and NO2 is acidic and adds to the polluting acidity of rain as well as providing nutrients for plants! See air pollution and acid rain
      5. Note that the Haber synthesis of ammonia is a synthetic method of nitrogen fixation.
  • Nitrogen recycling apart from the atmosphere:
    • Nitrogen compounds, e.g. protein formed in plants or animals, are consumed by animals higher up the food chain and then bacterial and fungal decomposers break down animal waste and dead plants/animals to release nitrogen nutrient compounds into the soil (e.g. in manure/compost) which can then be re-taken up by plants. 
  • Nitrogen returned to the atmosphere:
    • However, the action denitrifying bacteria will break down proteins completely and release nitrogen gas into the atmosphere.
  • -

  • More 'biological detail' of the NITROGEN CYCLE with reference to the above diagram so you can show an understanding of how nitrogen is recycled.
    • Nitrogen gas in the air (78%, ~4/5th) cannot be used directly by most plants and all animals.
      • No animals and only a few specialised plants can directly use the very unreactive nitrogen from air, but all plants nitrogen in some form to synthesise amino acids and proteins for growth and maintenance and for DNA in cell reproduction.
      • However, nitrogen can be changed into nitrogen compounds like nitrates which the plants can use.
      • Animals rely on plants or other animals in the food chain for their source of nitrogen compounds e.g. protein in grass, crops or other animals.
    • The action of lightning can convert nitrogen gas into nitrates.
      • The very high electrical energy discharges from lightning activates nitrogen and oxygen molecules to react and form nitrogen oxides. These dissolve in rain to form nitrates which end up in the soil when rainwater trickles into the soil.
    • There are four types of bacteria involved in the nitrogen cycle
      • (1) Action of Nitrogen-fixing bacteria living in root nodules of plants or in the soil, their function is to fix nitrogen gas from the atmosphere into a chemical form the plant can metabolise.
        • Leguminous plants like peas, lentils, clover and beans can absorb nitrogen from the air via their root nodules (swellings on the root surface) which contain enzymes capable of converting ('fixing') atmospheric nitrogen into soluble nitrate - a nutrient essential for amino acids, proteins and therefore plant growth.
          • Legumes and their root nodule bacteria are an example of mutualism, because the plant root supplies the bacteria with carbohydrate food (sugars) and minerals and the bacteria supplies the plant with nitrogen in the form of the nitrate ion.
          • This mutual relationship benefits both the plant and its associated bacteria.
          • The process of converting nitrogen in air into nitrogen compounds is sometimes called 'nitrogen fixation'.
      • (2) Action of decomposers - soil bacteria: Their function is to break down dead animals and plants
        • Their function is to convert proteins and urea into ammonia or ammonium ions.
        • Decomposer bacteria in the soil break down proteins from dead plants/animals and urea in animal urine/droppings into ammonia/ammonium ion compounds.
        • Decomposers include various organisms like bacteria, fungi or worms can break down dead animals or plants.
        • The decomposers break down proteins in rotting plants and animals and urea in animal waste into ammonia which forms ammonium ions, this returning the nitrogen to the soil. The ammonium ions are oxidised to nitrate ions, which plants can absorb through their roots.
          • This process is sometimes called putrefaction by putrefying bacteria.
      • (3) Action of nitrifying bacteria: Their function is to convert this ammonia to nitrates - the process of nitrification
        • Nitrifying bacteria oxidise ammonia/ammonium ions from the decayed material to form nitrates, the nitrate ion can be absorbed by plants through their root systems.
      • (4) Action of denitrifying bacteria:
        • Denitrifying bacteria convert nitrates back into nitrogen gas which is returned to the atmosphere.
        • These particular bacterial organisms can remove the oxygen from nitrate compounds to form the element nitrogen gas.
        • The action of denitrifying bacteria is of no use to any living organism!
        • The function of denitrifying bacteria is the opposite of the nitrogen-fixing bacteria (1).
        • Denitrifying bacteria are most often found in waterlogged soils, conditions unsuitable for most plant or animal life.
        • These denitrifying bacteria live in anaerobic conditions like waterlogged soils and use the nitrate ion to respire - use the nitrate ion (NO3-) as a source of oxidation instead of oxygen gas.
    • Plants absorb nitrates from the soil.
      • Plants get their nitrogen from soil.
      • Plants absorb nitrates (soluble in water) in the moisture that the roots absorb from the surrounding soil.
      • Plants can use the nitrate ion in forming amino acids from which the plant can make its proteins.
      • Nitrogen compounds are then passed along the food chains e.g. animals eat plants (herbivores) and animals eat animals (carnivores).
    • Nitrates are needed by plants to make proteins for growth.
      • Nitrates are an essential nutrient for plants to synthesis amino acids and hence proteins.
    • Nitrogen compounds pass along a food chain or web of food chains.
      • All food chains involve the passing of carbon compounds e.g. sugars, carbohydrates, fats and proteins up to the next trophic level i.e. the consecutive eating along a food chain (and waste produced on the way).
        • e.g. grass ==> cow ==> human
        • Plants make their own protein from nitrates, but animals must obtain it from plants or other animals. In fact the protein is broken down in digestion to amino acids and each animal makes its own proteins from these amino acid residues.


The Water Cycle

1. What happens to water on the Earth's Surface?

The water on the Earth's surface is continually being re-cycled.

As it falls, rain water contains only dissolved gases but once it reaches the ground water becomes contaminated in various ways.

  • Water is the most abundant substance on the surface of our planet and is essential for all life.
    • Water in rivers, lakes and the oceans is evaporated by the heat of the Sun (endothermic process).
    • The water vapour formed rises in warm air convection currents into the atmosphere, cools and forms clouds of condensation (exothermic process).
    • Eventually this falls as rain and snow 'precipitation' which returns the water to land filling rivers and direct precipitation to seas and oceans.
    • This is known as the water cycle.
    • This flow of fresh water via the water cycle transports nutrients from location to another i.e. from one ecosystem to another.
  • Water is an important raw material and has many uses.

Decomposition - decay processes

As already described, waste materials from plants and animals must be recycled to maintain healthy ecosystems and avoid a build-up of too much waste, including dead plants and animals.

The decay processes ensure the constant recycling of the 'elements of life' for new growth in plants and ultimately providing food for animals.

The waste products and dead animals or plants are broken down by decomposers.

The two main types of decomposers are bacteria and fungi microorganisms.

Just as in GCSE chemistry, where you study the factors that control the rates of chemical reactions, we can look at what factors control the rate of decomposition of this organic material.

The rate of decomposition of organic material from dead organisms or their waste if living, is affected by various environmental factors.


(1) The availability of oxygen

Many decomposers require oxygen for aerobic respiration.

Therefore the rate of decomposition increases with increase in the ambient oxygen concentration.

If the oxygen levels are low the rate of decomposition is reduced - you get this in water-logged soils.

However, some decomposers can respire anaerobically (not needing oxygen) but this is transfers less energy (less exothermic) and so such decomposers work more slowly.

See Respiration - aerobic and anaerobic in plants and animals.  gcse biology revision notes


(2) The temperature conditions

Most decomposers work most efficiently in warm conditions, but not at to high a temperature.

(Remember that enzymes are denatured at high temperatures and these enzymes control the digestion breakdown of organic material at the molecular level.)

(c) doc bAll chemical reactions speed up with increase in temperature, but enzymes work best (fastest) within an optimum temperature range, often around 50oC. The diagram on the right is for an enzyme with an optimum temperature around 35oC.

At higher temperatures, reactant molecules have a greater average kinetic energy to overcome the activation energy barrier and so reactions can proceed faster.

The enzymes in the decomposers that digest the dead or waste material, can therefore work faster, more efficiently, as the temperature is increased.

However, at temperatures above ~50oC, the protein structure of the enzymes begins to breakdown and the decomposition rapidly decreases with further increase in temperature.

The resulting graph shows the result of the two competing effects - showing the optimum temperature and at high temperatures the enzyme controlled digestion reactions stop.


(3) The water content of the soil

All decomposers (like all of life) require water to survive, no matter where their location.

Decomposition increases in moist conditions compared to dry conditions.

However, there are situations where there is little oxygen and too much water!

A good example is water-logged soil, where it is difficult for air to permeate into the mud.

As a consequence, the water contains little dissolved oxygen and decomposers do need it to respire.

Consequently, in water-logged soils, the rate of decomposition is considerably slowed down.



Some revision points based on the above notes

  • Know that many trees shed their leaves each year and most animals produce droppings at least once a day.

  • All plants and animals eventually die and know that microorganisms play an important part in decomposing this material so that it can be used again by plants.

  • Appreciate that the same material is recycled over and over again and can lead to stable communities.

    • You are expected to use your skills, knowledge and understanding to evaluate the necessity and effectiveness of schemes for recycling organic kitchen or garden waste.

    • Like using a compost bin!, to which you can add garden waste and kitchen waste. Its best if the compost material is shredded and the compost bin sides have holes/mesh to allow air in and circulate. You can add compost makers (decay accelerators) to speed up the process, but hopefully the bulk of the composting material warms up by heat released by the decomposition reactions to further speed up the decay.

  • a) Appreciate that living things remove materials from the environment for growth and other processes.

    • Plants need carbon as carbon dioxide from air, hydrogen as water, oxygen from air/water and nitrogen from air/soil or as nitrates from the soil, plus other minerals via water through the roots.

    • From these elements and compounds, plants can make carbohydrates, fats and proteins as well as a source of absorbed minerals.

    • Therefore, when animals eat plants they digest the carbohydrates, fats, proteins and minerals, and then convert these materials into their own fats and proteins.

    • Know that these materials are returned to the environment either in waste materials or when living things die and decay.

      • These means all the essential elements of life are recycled through the soil and can re-enter the food chain again.

  • b) Know and understand materials decay because they are broken down (digested) by microorganisms.

    • Know that microorganisms are more active and digest materials faster in (i) warm, (ii) moist and (iii) aerobic conditions.

      • (i) Warm conditions help speed up the chemical reactions of decay.

      • (ii) Water is an important medium for the reactions of living organisms.

      • (iii) Aerobic conditions require well oxygenated soil ie air able to circulate into the soil and litter.

  • c) The decay process releases substances that plants need to grow.

  • d) Know and understand that in a stable community, the processes that remove materials (plant/animal growth) are balanced by processes that return materials (microorganism decay) and the materials are constantly cycled (in a way 'recycled').


  • Revise and practical work-investigations that helped develop skills and understanding which may have included the following (which should also be revised, helps in understanding 'how science works' and context examination questions):

    • designing and carrying out an investigation to measure the rate of decay of bread by, for example, exposing cubes of bread to air before placing them in sealed Petri dishes at different temperatures and/or different moisture levels,

    • investigating the rates of decay using containers (eg thermos flasks) full of grass clippings, one with disinfectant, one with dry grass, one with wet grass and one with a composting agent.

      • If the container is sealed, a thermometer or temperature probe can be placed through a cotton wool plug to monitor the temperature

    • potato decay competition, using fresh potatoes - you decide on the environmental conditions and the rate of decay is measured over a 2 week period,

    • using a sensor and data logger to investigate carbon dioxide levels during the decay process.


See also

Ecosystems - biotic & abiotic factors - interactions between organisms - interdependency  gcse biology notes

Food chains, food webs and biomass   gcse biology revision notes


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