The Biology of Life


Life can be studied as a hierarchical systems

The structure of living organisms including those of entire populations and ecosystems is organized in a hierarchical fashion that allows a systematic exploration of the question ‘What is life?’. In the biological sciences, the following hierarchical levels are recognized:



Special consideration


Rain forest, desert, fresh water lake, digestive tract of animal for bacteria

Includes all living organisms and non living matter such as air, water and minerals as well as heat (temperature) and pressure.


All species in an ecosystem

Only includes living things from bacteria, to fungi, to plant to animal and accompanying viruses.


All individuals of a single species in a given area

Includes only individuals from a specific species such as a plant, an animal, or microbes


One single individual

Serves as a representative of the species and describes overall form and function of an organism (can be multicellular or unicellular).

Organ system

A specialized functional system of a multicellular organism

The nervous system or immune system of an animal


A specialized structural system of an organism

The brain or the thymus of an animal


A specialized substructure of an organ

The nervous tissue and epithelial tissue are both part of the brain


A single cell

A neuron, a skin cell, a root cell, bacteria, yeast, paramecium

Macromolecule A polymer such as a protein, DNA, polysaccharide or fat Macromolecules are the functional and structural building blocks of cells and their organelles.


A single small molecule (often building block of a macromolecule) such as an amino acid, nucleotide, monosaccharide or fatty acid.

Molecules are the smallest unit of biological systems; they can be studied for their chemical and physical properties and are of particular interest for their role they play in metabolism and signaling.

In this hierarchical organization, each higher level exists only with all lower levels intact (single celled organism don’t include tissue and organ levels). Importantly, each higher level provides novel or emerging properties not found at any lower level, an important feature of complex systems. This phenomenon is known as ‘the whole is more than the sum of its parts’.

Lower level disturbances or changes affect higher level properties. This hierarchical model is well suited to explain the cause of diseases and the mechanism of evolution. The latter depends on random mutations occurring at the DNA level affecting higher order properties at the cellular and organism level.

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 Copyright © 1999-2016 Lukas K. Buehler