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Biology 2nd Baccalaureate

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1.2.1. Primary bioelements

Primary bioelements

If the chemical composition of living matter (biosphere) is compared with that of the atmospherehydrosphere and lithosphere, which are the three layers that living beings occupy, the following conclusions can be drawn:

  • In the biosphere we will find a high amount of H and O because living matter is made up of water in a percentage that varies between 65% (terrestrial organisms) to 90% (aquatic organisms). All chemical reactions that take place in living beings develop in water, so the existence of living matter without water is not possible. All of this indicates that life originated in water.
  • The rest of the primary bioelements (C, N, S and P) of the biosphere are not so abundant in the atmosphere, hydrosphere or lithosphere, so it can be deduced that living matter has not been formed from the most common elements. abundant, but from those (C, H, O, N, P and S) that thanks to their properties are capable of constituting it. These properties are:
    • Their atomic mass is relatively small, which favors the establishment of stable covalent bonds when combinedThe smaller an atom, the greater the tendency of the positive nucleus to complete its last orbital with the electrons that form the bonds, and, therefore, the more stable these bonds are.
    • Oxygen and nitrogen are highly electronegative elements, so when they join covalently with other atoms, they often give rise to dipole molecules. As water is also dipolar, these compounds dissolve well in it and can react with each other, making possible the biochemical processes essential for life.

The rest of the properties are not common, so they are discussed separately below.


It is the basic skeleton of all organic biomolecules and makes the difference between organic and inorganic matter.

It has four electrons in its outermost shell and can form covalent bonds with other carbons, which allow it to form long chains of atoms (macromolecules). These bonds can be single (C-C), double (C=C) or triple (C≡C). It can also bind to the different radicals formed by the other elements (-H, = O, -OH, -NH2, -SH, -H2PO4, etc.), thus allowing a large number of different molecules, that will intervene in a multitude of chemical reactions, and, thus, be able to take advantage of the great diversity of elements that exist in the environment.

Carbono tetraédrico

By Andreza Memelli (Own work) [Public domain], via Wikimedia Commons

The four covalent bonds form, in space, the vertices of an imaginary tetrahedron, allowing the formation of three-dimensional structures, such as the plasma membrane or other organelles.


The carbon also allows other macromolecule, the DNA, can contain all the information necessary to produce other molecules of the body and replicate to transmit that information to their descendants, being essential for life.

Biological song: The carbon cycle (The frustrating journey of a carbon atom). Julinky.


Together with oxygen, it is essential to form organic matter, which is defined as the matter basically made up of carbon and hydrogen.

For example, some lipids are only made up of carbon and hydrogen atoms. The same as oil and its derivatives (butane, gasoline, diesel, etc.), consisting only of carbon and hydrogen, which is why they are called hydrocarbons.

The electron in the hydrogen atom allows it to bind to any of the other primary bioelements. The covalent bonds that forms between hydrogen and carbon is strong enough to be stable, but not strong enough to prevent it from breaking, thus allowing the synthesis of other molecules. Molecules made up of only carbon and hydrogen are nonpolar covalents (insoluble in water).


It is the most electronegative primary bioelement, so when it binds with hydrogen, it attracts its only electron, originating electrical poles. Therefore, the radicals -OH, -CHO and -COOH are polar radicals. When these radicals replace some hydrogens of a chain of carbons and hydrogens, such as glucose (C6H12O6), they originate soluble molecules in polar liquids such as water.

Due to its electronegativity, oxygen has a great capacity to attract electrons from other atoms, leaving them oxidized. As this process involves the breaking of bonds and the release of a large amount of energy, the reaction of carbon compounds with oxygen, called aerobic respiration, is the most common way to obtain energy. The other route, fermentation, has been declining since algae and plants, through photosynthesis, began to enrich the primitive atmosphere with oxygen.

The oxidation of biological compounds is basically carried out by subtracting hydrogens from carbon atoms. Oxygen (more electronegative) attracts the electron from hydrogen more strongly than carbon, so it is able to remove it. Thus water is formed (oxygen plus hydrogen) and a large amount of energy is released, which living beings take advantage of. As the carbon atom goes from sharing an electron with hydrogen, to sharing fewer electrons with oxygen, it experiences a "loss" of electrons, that is, it oxidizes:

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy


The nitrogen has a great facility for forming compounds with both hydrogen (NH3) as with oxygen (NO-), allowing, to the switch from one form to another, the release of energy.

It is found in amino acids (molecules that make up proteins) forming amino groups (-NH2) and in the nitrogenous bases of nucleic acids. Although nitrogen is the most abundant gas in the atmosphere, very few organisms are able to take advantage of it. Virtually all nitrogen is incorporated into living matter by algae and plants, which absorb it dissolved in the form of nitrate ion (NH3-).


It is found in the form of the sulfhydryl radical (-SH) in certain amino acids such as cysteine and methionine. These radicals make it possible to establish, between two nearby amino acids, strong covalent bonds called disulfide bridges (-SS-), which maintain the protein structure.


It is very important because it forms energy-rich bonds. By breaking the bond that joins two phosphate groups -PO3- ~ PO3- ~ PO32-, generally of a molecule called ATP (adenosine triphosphate), the energy contained in said bond is released. The energy released in other reactions, such as oxidations of respiration, is stored in these bonds. 

In addition, phosphorus is very important because it is part of the nucleic acids (DNA and RNA), of the phospholipids of the plasma membrane and of the bones of vertebrates, and because it helps to keep the acidity of the internal environment of the body constant.


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