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Biology 2nd Baccalaureate

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1.1. Levels of organization of living beings

Levels of organization of living beings

You will surely remember that last year we already saw the levels of organization of the human body. This course, in addition, we will see how living matter is organized at other levels higher than the organism, such as the population level and the ecosystem level.

Living matter is organized into different levels of organization that have increasing levels of complexity. A higher level gives matter properties that it did not have at lower levels.

The levels of organization of living matter can be classified into abiotic and biotic.

Niveles de organización de la materia viva

By Julien 212 (Template:Campbell Biologie 9ème édition) [CC BY-SA 3.0], via Wikimedia Commons

Abiotic organization levels

The abiotic levels include those levels found in inorganic matter and living beings, as the subatomic, atomic and molecular levels.

  • Subatomic level. Formed by the particles that make up the atom: protons, neutrons and electrons.

El átomo está formado por protones, neutrones y electrones

CNX OpenStax / CC BY

  • Atomic level. The atom is the smallest part of a chemical element that can take part in a chemical reaction and has the characteristics of that element. The atoms that are part of living matter are called bioelements, and the most important are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S) and phosphorus (P).
  • Molecular level. Molecules are formed by the union, through chemical bonds, of two or more atoms. The molecules that are part of living matter are called biomolecules, which can be:
    • Inorganic biomoleculeswater (H2O), calcium carbonate (CaCO3), gases such as oxygen (O2), etc.
    • Organic biomolecules: they are mainly made up of carbon atoms joined by covalent bond. The carbohydrates such as glucose (C6H12O6), the lipids, the protein and nucleic acids would organic biomolecules.

There are also various levels of complexity within molecules. For example, macromolecules are formed by the union of simpler molecules. A polymer is formed by the union of monomers. Thus, starch (polysaccharide) is a polymer formed by many glucose units (monomer), proteins are formed by the union of amino acids, and nucleic acids, by the union of nucleotides.

Cell organelles, made up of supramolecular complexes, should also be included at this level, although they have a certain entity of their own, they are not living beings, since they do not fulfill the three vital functions of nutrition, relationship and reproduction. Examples of organelles are mitochondriaribosomeschloroplasts, etc.

The virus would also be at this level, they are macromolecular complexes formed by proteins and nucleic acids nor fulfill vital functions.

Biotic levels of organization

The levels of biotic organization are present in living beings. They are as follows:
  • Cellular level. The cell is the basic unit of living beings. This level is much more complex than the previous ones, since it is the first that has life, since the cells can carry out the functions of nutrition, relationship and reproduction. The cell is the anatomical and functional unit of living beings. They are made up of a membrane that surrounds a cytoplasm. Two types of cells are distinguished:
    • Prokaryotic cells: they have DNA dispersed in the cytoplasm, without an envelope that surrounds it, condensed in a region called the nucleoid. They are single-celled beings. Example: bacteria.
    • Eukaryotic cells: they have DNA surrounded by a nuclear envelope that protects and isolates it, forming the nucleus. They can be unicellular beings (such as protozoa) or form part of multicellular beings, much more complex.
  • Multicellular level. Eukaryotic cells can be part of multicellular beings, made up of more than one cell. This level can be subdivided into several sublevels:
    • Organs: Structures made up of several different tissues that collaborate to perform a specific function.
    • Systems and devices: When several organs are coordinated to perform a certain function, they are distinguished:
  • Population level. A population is the set of individuals of the same species that live in a certain place and time that can be related to each other. Organisms are not considered as concrete individuals, but rather the relationships that are established between them.
  • Ecosystem level. An ecosystem encompasses both the set of different populations that live interrelated in a given place (biocenosis or community), as well as the place where they inhabit (the biotope), with its determined physicochemical conditions. The great ecosystems of the Earth are called biomes, and the set of biomes is the ecosphere, which can be considered as the most complex level of organization of living beings.


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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.