Biological Hierarchy: Levels of Organization

Biological Levels of Organization

All living things are composed of cells that can be grouped, giving rise to complex structures such as tissues and organs. However, cells are made up of smaller components. These, in turn, consist of even smaller units. Levels of organization refer to the different degrees of complexity in which matter is organized.

In this organization, the elements of each level are grouped to form other more complex levels, with new features and properties that go beyond the mere grouping of elements from the previous level.

Subatomic Level

This level corresponds to the particles of atoms: protons, neutrons, and electrons.

Atomic Level

Formed by atoms, which are the smallest constituents of matter that maintain their properties. For example, the oxygen or carbon atom.

Molecular Level

Molecules result from the bonding of different atoms. Some are relatively simple, such as water, while others are much more complex, such as proteins. The molecules that make up living organisms are known as immediate principles and can include:

• Inorganic: Present in both living and inert matter. These include water and mineral salts.
• Organic: Present exclusively in living matter. They are usually polymers, i.e., chains formed by the union of one type of molecule called a monomer. The organic immediate principles are: carbohydrates, lipids, proteins, and nucleic acids.

Cellular Level

This is the first biotic level (i.e., alive). To this level belong, for example, epithelial, muscle, and bone cells.

Tissue Level

These are sets of specialized cells with the same function and origin. Examples include muscle, epithelial, and bone tissues.

Organ Level

Organs are formed by different tissues grouped together to perform a specific function. The stomach, heart, and bones are organs.

System Level

A system is a set of organs that perform a similar function and are formed by the same tissue type. For example, the skeletal, muscular, or nervous systems.

Apparatus Level

This corresponds to a set of different organs, each with a specific role, participating in one or more higher functions. Examples include the musculoskeletal or digestive systems.

Organism Level

This level represents the living being as a whole. A human being belongs to this level. It includes both multicellular organisms (consisting of many cells) and unicellular organisms (consisting of a single cell). In the latter case, the cellular level and the organism level coincide.

There are other levels formed by the grouping of different organisms, such as populations, communities, and ecosystems.