CHEMICAL COMPOSITION OF LIVING ORGANISMS

Synopsis:-

• Introduction
• Composition of elements in living system
• Biogenic elements
• Water and Mineral substances
• Organic Matters
• Conclusion
• Reference 

        1.      Introduction to the topic:-                                                                                                                               The life on Earth can be understood as a form of existence of matter. All the living matter, i.e. living organisms, is composed of the same particles (atoms, ions, molecules) as the non-living organisms and chemical laws and laws of physics are applicable to both of them. There is a close connection between living and non-living nature, however they differ in chemical composition, structure, complexity and organization. While the chemical composition of the non-living nature is varied, the existence of living organisms is based on the presence of a few chemical elements, especially carbon, oxygen, nitrogen and hydrogen.

       All the chemical compounds in living organisms are composed of chemical elements. In these days almost 120 chemical elements are known. Out of this number 92 elements are naturally present in the nature (the rest were made in laboratories). Out of the 92 elements only 30 elements create the living matter and they are called biogenic elements.

There are 92 elements in the Earth´s crust. Oxygen and silicon represent the highest percentage - 75% of all elements. Both these elements of the Earth´s crust, as well as the other elements, are bonded especially in minerals (e.g. oxides, silicates) and rocks. The remaining 90 elements represent about 25 % of all elements.

Fig.: Representation of chemical elements in the Earth´s crust

2.     Composition of elements in living systems:-

                                     In all living systems we can always find 4 basic elements: carbon, oxygen, nitrogen and hydrogen. Carbon is the basic building unit contained in living matter. The percentage of carbon in the mass of living matter is 19.4 %. Oxygen and hydrogen are present in almost all organic compounds which create living organisms. The percentage of oxygen in the mass of living systems is 62.8 %, the percentage of hydrogen is 9.3 %. The source of hydrogen for organisms is water, the source of oxygen is water and the atmosphere. Nitrogen is bonded mainly in amino acids, proteins and nucleic acids. Its percentage is 5.1 %.

Chemical element	Average representation in living matter (%)	Average representation in non-living matter (%)
Carbon	19.37 %	0.18 %
Oxygen	62.80 %	49.40 %
Hydrogen	9.31 %	0.95 %
Nitrogen	5.14 %	0.63 %

2.1  Biogenic elements

 

       All elements contained in living matter are called biogenic elements. They are present in compounds, in the form of ions and in some special cases they are unbound (e.g. oxygen). According to their representation in organisms, the biogenic elements are divided into 3 groups: macrobiogenic, microbiogenic and trace elements. Trace and microbiogenic elements are sometimes also called oligobiogenic elements.

 

       I. Macrobiogenic elements – C, O, H, N, S, P, Na, K, Ca, Mg, Cl, Fe.  Four of these elements – O, C, H, N – represent up to 95 % of living matter. The rest of the elements mentioned above represent up to 4.9 %. Macrobiogenic elements have a building function.

      Carbon is the basis for all living matter. The typical feature of carbon atoms is the ability to bond to each other or to atoms of other elements. That is why there are many organic compounds of carbon. Carbon is also present in carbon dioxide and carbonates.

      Oxygen and hydrogen in organisms they are present both in the form of organic and inorganic compounds and they are a part of the basic micromolecule – water. Oxygen is produced by autotroph organisms (especially by plants and cyanobacteria) during the process of photosynthesis.

Nitrogen is a component of proteins and nucleic acids. It is also a part of nitrates and ammonium carbonate, which are necessary for the mineral nutrition of plants and also the synthesis of plant proteins.

      II. Microbiogenic elements – Cu, I, Mo, Mn, Zn, Co. The average content of these elements in living organisms is less than 0.1%. Microbiogenic elements have catalytic function, i.e. they are part of enzymes.

      III. Trace elements – e.g. Al, As, B, Br, F, Li, Ni, Se, Si, Ti, V. Their content in organisms is less than 0.001 %. As well as microbiogenic elements, trace elements are parts of enzymes and their function is catalytic.

 

3.  Chemical composition of living systems

 Living organisms are composed of several types of substances called biomolecules. According to their molecular weight, substances in living organisms are divided into two groups:

 1. Low molecular substances (M_r < 10 000)

•         water

•         inorganic (mineral) substances

•         intermediates of metabolic pathways (carboxylic acids etc.)

• final products of metabolic pathways (amino acids, monosaccharides, lipids, nucleotides)

 2. High molecular substances (M_r > 10 000)

•         proteins

•         polysaccharides

•         nucleic acids

      High molecular substances, which are present in living organisms, are also named as biological macromolecules or biopolymers. The building units of proteins are amino acids, the building units of polysaccharides are monosaccharides, and the building units of nucleic acids are nucleotides.

      According to their origin, the substances included in the living organisms are divided into inorganic substances (water, carbon dioxide, mineral substances) and organic substances (the most important are nucleic acids, proteins, saccharides, lipids).

  

Fig.: Average representation of the main groups of substances in organisms

3.1.  Water and mineral substances

The most frequent and the simplest biomolecule in living systems is water. Water  is basic and the most spread inorganic compound contained in living organisms. The average content of water in organisms is 60-70 %. The amount of water depends on the surroundings  in which the organism lives, on a kind of organism, on its age. The amount of water also depends on specific parts of body, e.g. the highest percentage of water in human body is in body fluids and the lowest in fatty, dental or bone tissue. Water in organisms helps to create their inner environment and keep their stability. Water is a dissolving agent, transporting medium and a thermoregulator. Bochemical reactions in living systems happen in water environment.

      

Inorganic salts can be either water-soluble, i.e. dissociated into ions, or insoluble. Insoluble salts are present in hard connective tissue such as teeth, bones or shells. Examples: Ca₃(PO₄)₂ (bones, teeth), CaCO₃ (bones, shells of invertebrates), CaF₂ (teeth). Soluble salts in the form of ions are mainly in body fluids. The main extracellular ions are cation Na⁺ and anion Cl^-.  The main intracellular ions are cation K⁺ and cation  Mg²⁺ .

     Very important compound is carbon dioxide, which is necessary for photosynthesis. It is produced in metabolic (catabolic) processes, e.g. when breathing.

 

3.2.  Organic matters                  

 

The most important organic matters necessary for organism structure and function are:

ü  proteins

ü  nucleic acids

ü  saccharides

ü  lipids

       Organic matters represent more than 30% of organism mass. The rest is represented by water and mineral matters.