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Biochemical Cycles

The Earth has a fertile surface. The biosphere is an extremely thin layer, with interfaces between water, land and air, where all life exchanges the necessities of life.

Every portion of this biosphere is in constant motion, and life is a process of continuous energy and matter exchange. None of these are closed systems, but it is useful to consider the paths of substances vital to life through the biosphere, their removal from it, and eventual return, in terms of global biogeochemical cycles.

Some major element cycles are: carbon, oxygen, nitrogen, sulphur, phosphorus.

Other cycles include: hydrological cycle, biomass-fossil fuel cycle, nutrient cycles, erosion and pollutants.

Element Cycles

Carbon: king of life

Joseph Priestley and Antoine Lavoisier are credited as being the discoverers of the carbon cycle.

SourceStorageImmissionSinkNet carbon balance
Atmospheric pool720; 1.6%
Biosphere2,000; 4.4% 10.390
Fossil fuels4,130; 9.1% 0
Oceans38,400; 84.7%8.3 -1.6-2.4

The figures in the table above refer to 1015 molecules of CO2 (carbon dioxide).


Oxygen is one of the most abundant elements on and in Earth. It is a primary constituent of silicon dioxide (SiO2), or quartz, and as O2 is just under 21% of the atmosphere by volume. By mass, it is 89% of water (H2O). In the Stratosphere, oxygen takes the form ozone (O3), creating an essential shield against UV radiation from the Sun.

Oxygen O2 takes the central role in cellular respiration, being breathed in by animals, and exhaled as CO2. The counterpart to animal respiration is plant transpiration, whereby during photosynthesis chloroplasts in leaves 'breathe in' CO2 and 'exhale' O2.


Nitrogen (N) is involved in both biotic and abiotic processes: fixation, ammonification, nitrification and denitrification.

78% of the atmosphere is molecular nitrogen (N2). However, different to oxygen, nitrogen cannot be used by organisms directly. Plants are dependent on nitrobacteria for their nitrogen. This strain of bacteria 'fix' nitrogen from the air, and provide the nitrogen in nodules at the roots of plants.

Nitrates (NO3-) are manufactured from the air by the Haber-Bosch Process, and the release of large quantities into agriculture can cause accumulation in waterbodies, resulting in eutrophication.

Sulphur (S)

Sulphur (or Am. sulfur) is an essential element for life, and moves to and from the soil minerals through biological and physical processes.

The forms of sulphur are H2S (hydrogen sulphide, or 'rotten egg gas'), sulphide minerals (e.g. pyrite (fool's gold) FeS2, and elemental sulphur. Sulphur is expelled during volcanic activity.

As an impurity in coal and oil, sulphur dioxide SO2 is a major pollutant during fossil fuel combustion, and sulphur oxides (SOx = SO, S2O, SO2) are responsible for the formation of acidic rain.

Phosphorus (P)

Phosphate (PO4-3) is an artificially produced and natural fertiliser. The over abundance of phosphate in a water catchment area can lead to eutrophication of lakes and rivers. Organophosphorus compounds are a commercially important group of phosphorous compounds. Living cells also use phosphate to transport cellular energy in the form of adenosine triphosphate (ATP).

The oceans hold moles of phosphorus. Approximately 1010 moles of phosphorous are deposited by sediments in the oceans each year. Sediment carrying P are of three types:

  1. Phosphorous associated with calcium carbonate
  2. Organic phosphorous (associated with organic carbon)
  3. Apatite

Structure of Earth's Atmosphere


Ground to 12 km. Half of the mass of the entire atmosphere is found in the first 5.6 km of the troposphere. This part of the atmosphere is dense and turbulent, and is where nearly all clouds and meteorological phenomena occur.


12 to 50 km. The tropopause separates the troposphere from the stratosphere. Air pressure at the top of the stratosphere is about 1000 pascals, or one-thousandth the air pressure at sea level. It is where the vital ozone layer is located, a protective shield of $0_3$, preventing dangerous solar UV light reaching the Earth's surface. The temperature at the tropopause may be as low as -60°C, but rises to around 0°C at the top of the stratosphere.


50 to 80 km. Temperature decreases with increasing altitude in the mesosphere. The mesosphere is too high to be accessed by aircraft or balloon, and too low for satellites.


80 to 700 km. The exact height of the thermosphere varies with solar activity. The ionosphere (the part of the atmosphere ionised by solar radiation) is located in the lower part of the thermosphere. The atmosphere is very low density, and the molecules are very energetic. Temperature increases with altitude in the thermosphere. There is no water vapour, so there are no clouds. The ISS (International Space Station) orbits in the thermosphere, at 320 - 380 km.


700 to 10,000 km. The exosphere has practically no atmosphere, and is the zone where most satellites are located.

Composition of the Atmosphere

$N_2$ = $78.084$ % by volume

$O_2$ = $20.946$ % by volume

$Ar$ = $0.9340$ % by volume

$CO_2$ = $0.043$ % by volume

$Ne$ = $0.00182$ % by volume

$He$ = $0.000524$ % by volume

$CH_4$ = $0.00018$ % by volume

$Kr$ = $0.000114$ % by volume

$H_2$ = $0.000055$ % by volume

There is also a variable amount of water vapour (0.001 - 5%), or an average of about 0.25%

All of the gases in the atmosphere have a total mass of $5.1480×10^{18}$ kg (5 million billion tonnes).


Life is a chemical process. Beautiful, mysterious, enigmatic, and wonderfully diverse. But a chemical process, nonetheless.

Chemistry is summarised quite neatly by the Periodic Table. And the periodic Table shows how elements on the right are non-metallic in properties, and metallic on the left, with a few fence-sitters along a jagged line down the middle, who swing between metallic and non-metallic properties.

However useful this distinction is, in biology it makes even more sense to talk of elements which are generally involved in life processes (biotic), and those that generally are not (abiotic).

Essential Minerals

These are the biotic elements:

  • Primary Elements: CHNOPS
  • Carbon C, Hydrogen H, Nitrogen N, Oxygen O, Phosphorus P, Sulphur S

  • Other Macrominerals:
  • Chloride Cl, Magnesium Mg, Potassium K, Sodium Na, Calcium Ca

  • Trace minerals:
  • Boron B, Cobalt Co, Chromium Cr, Copper Cu, Fluorine F, Iodine I, Iron Fe, Manganese Mn, Molybdenum Mo, Selenium Se, Zinc Zn

The primary elements compose the major energy sources for life: carbohydrates, proteins, and fats. All life also requires the compound water, used in reactions such as respiration. involves atmospheric oxygen ($O_2$), and photosynthesis carbon dioxide ($CO_2$).

Carbon, hydrogen and oxygen make up about 95% of dry weight of the world's living biomass. The other macrominerals are essential for all life. There are some elements which are essential for some organisms, but not others: e.g. silicon, boron and tin.

Although the trace minerals are present only in tiny amounts (0.001% of the total mass), their importance to life processes are paramont. Although essential in small quantities, some elements, such as copper and zinc, are toxic at higher concentrations.


The name carbohydrate means carbon and water (Greek Hydra = water). Water is a compound, which means it consists of more than one element, two hydrogen atoms and one oxygen, bonded together in a molecule. In carbohydrates, there is typically a chain of carbon atoms, and branches of hydrogen and oxygen atoms in the ratio of water: 2 hydrogens to 1 oxygen.


Content © All rights reserved. Created : May 6, 2015 Last updated :March 20, 2016

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