Biological compounds

The biosphere refers to all living organisms, both plants and animals, on Earth.

Compounds in living organisms

Living things are mainly made from compounds that contain the elements carbon, hydrogen, oxygen and nitrogen. There are smaller amounts of other elements such as phosphorus and sulfur. There are also traces of most of the other elements.

Amino acids and proteins

Proteins are polymers. They are built up by the joining together of monomers called amino acids. There are only 20 different amino acids, but each protein molecule has hundreds, or even thousands, of them joined together in a unique sequence. This gives each protein its own individual properties. The proteins in hair, skin and muscle each have properties suited to their function.

The diagrams show two of these amino acids:



Carbohydrates have the empirical formula CH2O. This means that each carbohydrate has twice as many hydrogen atoms as carbon or oxygen atoms.

The simplest carbohydrates are sugars such as glucose. These sugar molecules can polymerise to make giant carbohydrate molecules such as starch and cellulose.

Glucose has the molecular formula C6H12O6.

The structure of a glucose molecule is shown in the diagram below:

The structure of glucose


Fats are also compounds of carbon, hydrogen and oxygen. They are esters of fatty acids and glycerol.


DNA molecules are found in the nuclei of cells. They carry genetic information.

A DNA molecule consists of a backbone polymer of sugar molecules and phosphate groups. Attached to this are the four bases adenine, guanine, cytosine and thymine. The sequence of these bases comprises the genetic code.

Carbon and oxygen cycles


Almost all molecules in living things contain carbon. Carbon moves in a cyclical way, passing from one organism to another in the biosphere, and between the other spheres of the environment (lithosphere, hydrosphere and atmosphere). The carbon cycle is the key factor in maintaining the balance of carbon dioxide in the air.

It works like this:

  • Plants photosynthesise, taking carbon from the atmosphere in the form of carbon dioxide and making the carbohydrate glucose.

  • Plants also respire, giving carbon dioxide back to the atmosphere; but they take in much more carbon dioxide than they give out.

  • Animals get their carbon from eating either plants (carbohydrates) or other animals (proteins and fats), which they then digest. They respire, giving off carbon dioxide into the air.

  • Waste carbon-based material is excreted by animals, and then digested by decomposers - mainly microbes and fungi. The decomposers also respire, releasing carbon dioxide into the air.

  • When animals die, their remains may be either eaten by scavengers (for example, crows) or digested by decomposers. Both scavengers and decomposers respire, giving off more carbon dioxide into the air.

  • In certain conditions, both animal and plant remains may become fossilised, eventually forming carbon-based fossil fuels (coal, oil and gas). Both fossil fuels and plant material (wood) may later be burned, releasing still more carbon dioxide into the air.

Therefore, all of the carbon taken out of the air by plants is later returned to the air. This is the carbon cycle. This slideshow should help you to understand how the cycle works:

Step 1 - carbon in the atmosphere can come from the respiration of plants and animals, and combustion (burning of fuels)

Step 2 - plants perform photosynthesis which removes carbon from the air. Plants are eaten by animals.

Step 3 - animals die and decay

Step 4 - decay of animals goes back into the earth, fossilisation occurs under suitable conditions


Oxygen is cycled in a similar way:

  • Plants produce oxygen during the process of photosynthesis. This is released into the air. Plants also use some oxygen for respiration, but they produce more than they use.

  • Animals use oxygen from the air for the process of respiration.

  • There is a balance between the excess oxygen given out by plants and the oxygen used by animals, so that the percentage of oxygen in dry air is always the same.

The nitrogen cycle

Seventy-nine per cent of the air around us is nitrogen. Living things need nitrogen to make proteins, but they cannot get it directly from the air because nitrogen gas is too unreactive to be used to make new compounds within an organism.

Plants can take up and use nitrogen when it is in a more reactive form - for example, in nitrates or ammonium salts. Changing nitrogen into a more reactive substance is called nitrogen fixation.

Nitrogen fixation

Nitrogen fixation happens in three different ways:

  • The energy in a lightning bolt can split nitrogen molecules in the air, allowing each nitrogen atom to react with oxygen to form nitrogen oxides. The rain washes these oxides to the ground, where they form nitrates.

  • The Haber Process is used by industry to produce ammonia from nitrogen. Ammonia is then used to make the fertiliser that farmers spread on the soil to feed their crops.

  • Nitrogen-fixing bacteria found in both the soil and root nodules of leguminous plants fix nitrogen into a form that can be used by plants.

When plants are eaten by animals, the nitrogen compounds are passed on.

Nitrogen compounds are returned to the soil by excretion and egestion from animals, or when plants and animals die and decay.

The nitrogen compounds returned in this way are changed back to nitrogen gas by denitrifying bacteria which live in the soil. This completes the cycle, so that the percentage of nitrogen in the air remains constant.

This slideshow illustrates the processes involved.

Diagram showing the 3 different ways of nitrogen fixation

how nitrogen compounds are passed on by animals eating plants

Diagram showing how nitrogen compounds are returned to the soil by excretion and egestion from animals

Diagram showing how nitrogen compounds are returned to the soil when plants and animals die and decay.

Step 5 - conversion of nitrates to nitrogen by dentrifying bacteria

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