Organisms are living beings that have organized structures, know their types and characteristics.

Prokaryotic cells are represented by mitochondria, plastids, endoplasmic reticulum, Golgi apparatus, lysosomes and endosomes. Due to the presence of a larger cytoplasmic structure, eukaryotic cells are larger than prokaryotic ones. What prokaryotic cells and eukaryotic cells have in common is the presence of genes that store genetic information. Ribosomes (cytoplasmic structures that function as sites for protein synthesis) are also present in both cell types. Examples of prokaryotes are bacteria and archaea.

2. Eukaryotes

Eukaryotic cells are the site of DNA replication (the process by which DNA segments are duplicated) and transcription (the process by which mRNA transcripts are produced). Instead, this process takes place in the cytoplasm of the prokaryotic cell. The presence of a nucleus organizes the genetic material and these processes. The nuclear envelope prevents molecules from easily penetrating and thus regulates the passage of molecules in and out of the nucleus. Examples of eukaryotes include protists, fungi, plants, and animals.

3. Unicellular organisms

Unicellular organisms are organisms that consist of only one cell. Examples of unicellular prokaryotes are bacteria and archaea, while unicellular eukaryotes are some protists and fungi. Multicellular organisms include plants and animals.

4. Multicellular organisms

Multicellular organisms are made up of many cells that act as a unit to perform a specific function. In multicellular organisms, a group of cells forms a network. Cells in tissue have a similar structure and function. Below is an example network:

1. Animal tissue is nervous tissue, muscle tissue, vascular tissue and connective tissue.
2. Plant tissue is meristematic tissue, permanent tissue and reproductive tissue.
3. A group of tissues organized into an anatomical unit is called a biological organ. Examples of animal organs: heart, lungs, brain, stomach, skin, pancreas, liver, intestines, kidneys, and reproductive organs. In plants, the organs are roots, stems, leaves, flowers, fruits, and seeds.

5. Taxonomic classification

Living beings are divided into three main areas proposed by Carl Woese. These are the domains of Archaea, Eubacteria (true bacteria), and Eucarya. Below the domain are the other major taxonomic levels: kingdom, phylum, class, order, family, genus, and species.

Archaea domain and eubacteria domain

Archaea and eubacteria are prokaryotes, and eukaryotes include eukaryotes. Thus, archaea and eubacteria do not have membrane-bound organelles. However, there are subtle differences between them that lead to their separation into different domains. Archaea have certain genes and metabolic pathways that are more closely related to eukaryotes than to eubacteria.

Eucharia domain

The living organisms included in the Eucarya domain are as follows:

1. Protists are living beings characterized by a relatively simple organization. Some of them are unicellular, others are multicellular.
2. Fungi are eukaryotes known for their heterotrophic mode of nutrition due to their lack of chlorophyll (an important pigment in photosynthesis).
3. Plants are multicellular, photosynthetic life forms. One of the main characteristics of plants is the presence of chloroplasts, which contain a chlorophyll system that collects light energy from light sources to be converted into chemical energy through photosynthesis.
4. Animals are multicellular eukaryotes. Cells in tissues are connected through cell junctions (eg, tight junctions, gap junctions, and desmosomes). Their lack of chloroplasts (and the green pigment chlorophyll) makes them incapable of photosynthesis. Thus, their survival depends on other organisms.

6. Viruses and viroids

Whether viruses are organisms or not is a matter of debate. Like living things, viruses have genetic material. However, they seem to only live inside the host. Whereas, viroids are another example of cell-free entities. They appear to be alive because they are pathogenic.