Site author Richard Steane
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Matter cannot be created or destroyed in a chemical reaction - or a biological reaction, for that matter. This means that elements are simply incorporated into different compounds, which are then transformed into other compounds, and, only if energy is available to drive the reactions, they may be changed to different compounds, probably repeating the process.
In other words, elements are recycled in Nature and re-used by living organisms. In many cases, living organisms use these elements so much that they cannot get enough of them elements - they are thus a limiting factor for growth - and so the rate at which they are recycled is critical. This is especially true for plants, but also for the whole food chain of organisms which depend on them. An ecosystem consists of an environment together with all the naturally occurring organisms in it.
The elements and compounds which are the most essential for the growth of living organisms are said to have their own (interlinked) cycles, but similar cycles exist for elements of "lesser" importance. These are transformation processes, in that chemical elements are moved from one compound to another.

The Carbon Cycle

click for an explanation including a photo of experimental results All living organisms contain the element carbon, in a combined form. If it forms the main structure of a molecule (by C-to-C and other bonds), it is said to be organic.
This term was originally defined to cover "chemicals" of biological origin, but it became modified to cover similar compounds of synthetic origin.
More recently, the term organic has been used in marketing and generally to mean a "natural" product, hopefully intrinsically pure, and not associated with chemicals and industry.

Name some general categories of organic compound which may be in a living organism.
> carbohydrate > fats and oils > protein

Name some actual examples of common inorganic carbon compound (with chemical formulae?):

> carbon dioxide > Calcium carbonate/carbonate ion/hydrogencarbonate ion

CO2 CaCO3 CO32- HCO3-

What sort of organisms carry out decay?

> fungi, bacteria (saprophytes)

[decomposers - term describing ecological role]

What sort of organisms carry out respiration?

> plants, animals, fungi, bacteria, i.e. ALL living organisms

What sort of organisms carry out photosynthesis?

> GREEN plants

What other element is being recycled during respiration and photosynthesis?

> Oxygen: ( CO2 = C + O2 !)

What is the normal proportion of carbon dioxide in the air?

> 0.03 % - equivalent to 300 parts per million

to 0.04 400

Below is a representation of the carbon cycle.
Fill it in, and add arrows.
You could use different colour codes to show the quickest routes round the cycle, and the slowest.


What do the capital letters in boxes represent?
> forms of carbon - C compounds

What do the lower case letters on arrows represent?
> processes - transformations

What (form of energy) powers the cycle of conversions above?
> SunLIGHT - NOT heat

According to the simple treatment of the carbon cycle in some books, the amount of carbon dioxide in the air always stays the same. In fact for years many people took this situation for granted, ignoring Biologists' concern over several linked worldwide environmental, agricultural, political and industrial issues.
Explain why it was thought that the amount of carbon dioxide would always stay the same.
> Plants would use up excess carbon dioxide from combustion, etc., by photosynthesis

Which environmentally associated issues do you think fall into the categories mentioned above?

- agricultural > cut & burn - forest clearance

- political > self-sufficiency /
to get foreign currency from cashcrops

- industrial > electrical power needs fuel (coal/oil)
to be burnt

What other carbon compounds contribute to the greenhouse effect?

> methane CH4, chlorofluorocarbons CFCs

(as well as water vapour, nitrous oxide etc.)

Briefly explain some of the consequences which are feared.

> rise in temperature - climatic effects

> rise in sea levels, etc

The Nitrogen Cycle

All living organisms also contain the element nitrogen, in a combined form. It is also said to exist in inorganic forms (which plants need) and organic forms (which animals need). Green plants are the main converters of inorganic nitrogen to organic forms, which is another reason why animals depend on plants.
Fungi and bacteria may also do this, and much biotechnological effort has been put into using them to try to solve world food problems, particularly shortage of protein.
What other chemical element or elements will be present along with nitrogen in organic compounds?
> C carbon H hydrogen O oxygen

Name some categories of organic nitrogen compound which may be in a living organism.

> protein > amino acids (& nucleic acids)

Name some (fairly specific) examples of common inorganic nitrogen compound.

> nitrates (NO3-) > nitrogen gas (N2)

Look at the diagram of the nitrogen cycle.
Note that most of the important conversions of nitrogen compounds take place in the soil. Despite being 78% nitrogen, air is only of minor importance as a nitrogen source to most living organisms!.

What is/are the main sort of organisms that use nitrates?

> GREEN plants

What do you understand by "the nitrifying process"?

> processes producing NITRATES

What sort of organisms carry out the nitrifying process?

> (nitrifying) BACTERIA

How are they connected with the carbon cycle? What name was given to the process there?

> the same micro-organisms (decomposers) > decay/ decomposition

What do you understand by "the denitrifying process"?

> processes losing NITRATES

What sort of organisms carry out the denitrifying process?

>(denitrifying) BACTERIA

Denitrifiers and nitrifiers grow in different places. Explain.

> Denitrifiers respire anaerobically i.e live in waterlogged soils - no O2

Nitrifiers are aerobic i.e found in ordinary soils

What do you understand by "nitrogen fixation"?

>conversion of gaseous nitrogen to soluble form (which stays "fixed" in soil)

What sort of organisms fix nitrogen?

> (nitrogen fixing) BACTERIA

There are two sorts of these. How do they differ?

> "free living" - in soil "symbiotic" - in root nodules

For what reason might plants benefit from a thunderstorm?

> oxides of nitrogen dissolve to make fertilisers
Thunderstorms are not a very important process in the cycle, but farmers, especially in the Western world, manipulate the nitrogen cycle by adding chemical "fertilisers" in order to improve agricultural productivity. Do not confuse this with fertilisation, which is part of the sexual reproduction process, which may also have a part to play in agricultural productivity!

There is a link between this unit and the topics of Plant mineral nutrition and Soil .

What are the main "chemicals" (as distinct from products of animal origin) by which farmers add nitrogen to the soil?

> nitrates e.g. Ca(NO3)2, NH4NO3

Where does the nitrogen come from, ultimately?

> air

There are several environmental issues connected with the use (or over-use) of fertilisers. For example, nitrates and other plant nutrients may leach or wash out of the soil or into deeper layers.

Briefly mention a few consequences:

> runoff and leaching going to rivers, causing algal blooms, pollution

> nitrates in river and other water enter drinking water system - suspected of link with cancer

Once again, fill in this version of the nitrogen cycle, adding arrows.

You could use different colour codes to show the major routes round the cycle, and the minor routes.


What is the significance of the following on the diagram above?

boxes > forms (compounds) of nitrogen

words on arrows > types of bacteria which transform

Add "leaching" to the cycle above.

This topic has connections with other BioTopics units on:-

Feeding relationships 
Food chains and webs
Examples of food chains
Measuring population sizes
Ecological pyramids
Conversion of biomass and energy
Predator prey interdependence
Natural Selection

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