As its most common meaning, the term refers to all living beings that inhabit our planet, regardless of their shape, size, as well as any other physical characteristic or primordial element. Thus, within the category of organisms are from microscopic bacteria to gigantic redwood trees over 100 meters tall.
There are numerous ways to classify organisms according to the most varied criteria, but following the most common theory, we can highlight three major divisions:
By the number of cells that compose them: multicellular or unicellular
As their name implies, multicellular organisms are composed of a large number of cells, while single-celled organisms are integrated by only one cell . Among the former are included human beings, animals, plants, among others; in the group of unicellular ones, bacteria stand out.
An interesting point is that all multicellular organisms in their origin were once unicellular, as this is a condition by which all organisms are forced to pass through. In fact, although it may seem the opposite, the group of unicellular organisms contains in its interior the vast majority of living organisms on the planet.
By their mode of feeding: autotrophs or heterotrophs
Autotrophic organisms are capable of making their own food, as is the case with plants. In contrast, heterotrophs must ingest organic substances manufactured by other organisms; this is the case of animals that can feed on both autotrophs and other heterotrophs.
A particular quality of human beings is that, thanks to their development, they can ingest foods made from raw materials that undergo an elaboration process in production plants .
On the other hand, the food system of autotrophic organisms consists of creating the substances needed to live from inorganic substances such as light. Therefore, it is important to emphasize that even without needing other organisms to feed themselves, other external factors are needed to correctly carry out their metabolic cycle.
By your metabolism: aerobic or anaerobic
Those that need oxygen to survive belong to the aerobic group, as shown by humans. In contrast, anaerobic organisms are those that do not need oxygen in their metabolism .
In the second group, depending on the substance that replaces oxygen, a new classification can be performed to differentiate these that carry out fermentation or anaerobic respiration.