What life is


Life can be studied as a hierarchical structure

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 structural 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


All species in an ecosystem

Only includes living things from bacteria, to fungi, to plant to animal


All individuals of a single species in a given area

Includes only individuals from a specific species such as a plant, an animal, a bacterial colony


One single individual

Serves as a representative of the species and describes overall form and function of an organism

Organ system

A specialized functional system of an 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


A single large or small molecule such as a protein, DNA, sugar or fatty acid

Molecules are the smallest part of biological systems; they can be studied for their chemical, physical properties, but are of particular interest for their role they play in biological systems.

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 © 1999-2011 Lukas K. Buehler