What Is an Organism?

An organism is any living entity. To better understand this, it may be helpful to learn what three domains (types) of organisms exist. The three domains of organisms are Bacteria, Archaea, and Eukarya.

Bacteria

*Salmonella* Photo credit: https://www.eehealth.org/blog/2018/07/salmonella-infection/

Bacteria are the least complex type of organisms. They are prokaryotes, meaning they are unicellular (single-celled) organisms.

A bacterium cell’s DNA (genetic material) is located in the nucleoid. The DNA is not separated from other parts of the cell by a nucleus, like eukaryotic cells.

Ribosomes lie within the cytoplasm, which is surrounded a cell membrane, cell wall, and capsule. Pili are found on the outside of the cell, and are used to attach to various things. A flagellum is located at the end of the cell, and is used to propel the cell.

Photo credit: https://www.vectorstock.com/royalty-free-vector/bacteria-cell-structure-vector-1858138

Why are bacteria important? Bacteria have many purposes vital for sustaining life.

For one, they provide the majority of the oxygen on our planet. However, some bacteria are harmful, such as salmonella.

Bacteria are masters of decomposition, decomposing dead matter to be reused by other organisms as nutrients. They also boost our immune system, to help combat antigens (harmful substances). There are approximately 10 times more bacteria cells in the human body than human cells.

Archaea

Originally thought to be bacteria when discovered in Yellowstone National Park, archaea are the oldest of organisms. From hydrothermal vents on the ocean floor to the saltiest bodies of water, archaea live in the most extreme environments on earth.

Like bacteria, archaea have prokaryotic cells (unicellular). The chromosomal DNA is coiled in a ring. Unlike bacteria, the DNA is not in a nucleoid.

Plasmid DNA may be present if picked up from other cells or the environment. Fimbriae help the cell attach to its surroundings. The remaining parts of the cell are similar to bacterium cells, though may differ in size, thickness, etc.

Photo credit: https://m.wisegeek.com/what-are-archaea.htm

Archaea help support other life forms. Methanobrevibacter smithii is found in our digestive system, breaking down complex sugars.

Eukarya

*African bush elephant (Loxodonta africana)*

Eukarya are eukaryotes (multicellular organisms) made up of eukaryotic cells. These cells have a nucleus housing the cell’s DNA, separating it from the rest of the cell. Membrane bound organelles inside the cell carry out various functions for the cell.

Some eukaryotes, like ourselves, have tissues and organs made up of a large numbers of cells. Not all eukaryotic cells are the same. For instance, plant cells have a cell wall, whereas animal cells do not.

For more information on the structure of a eukaryotic cell, an animal cell in particular, read: https://the-worlds-deadliest.com/2020/04/14/what-are-animal-cells-a-brief-overview-of-the-structure-and-functions/. Examples of eukarya include animals, plants, and fungi.

Eukarya are important because us humans are this type of organisms; without them we would not exist. A symbiotic relationship is an interaction between two organisms. For example, plants produce oxygen (which humans need to survive) through photosynthesis and animals produce carbon dioxide (which plants need to survive) through respiration.