Reviewed by: BD Editors

Organism Definition

An organism is a single individual, or being. While it may have many separate parts, the organism cannot survive without the parts, as the parts cannot survive without the organism. Some organisms are simple and only contain an information molecule describing how to obtain energy and reproduce the molecule. Other more complex multi-cellular organisms go through complex mating rituals to introduce two haploid cells together which will fuse and become a new organism. As the variety of life on Earth is huge, the definition of organism is still in flux, and new definitions for what is considered an organism are presented all the time.

Types of Organisms

Scientists classify organisms into 3 domains and 6 kingdoms, although this has changed throughout history. There are 3 recognized domains, or broadest classification of organism. These are Bacteria, Archaea, and Eukarya.


In the simplest case, an organism can be a bacteria, a DNA molecule containing genetic information wrapped in a protective plasma membrane. Organisms tend to separate their information molecules from the outside environment, where pH changes and unknown chemicals could do damage to the molecule. Bacteria contain their DNA in a simple ring, and replicate it through a process known as binary fission. The DNA is replicated so that two rings exists, and the cell divides its contents in half, each getting one ring of DNA.

Although bacteria are some of the smallest organisms on Earth, the can produce a huge effect. It is believed that soil bacteria can speed the effects of climate change, and that the bacteria in cow’s guts may be responsible for a large portion of the greenhouse gasses in the atmosphere. Other bacteria help us digest food, and some can make us sick.


The domain Archaea contains bacteria-like organisms that are unrelated to bacteria, and can perform a wide variety of functions. For instance, many archaea live in the most extreme environments on the planet, from hydrothermal vents to lakes so salty that no other life can become established. However, the archaea also exist in most “normal” habitats. It is thought that organisms in the Archaea, Bacteria, and Eukarya branched off from each other in the early history of life on Earth. The Archaea show a high level of antibiotic resistance, and it is thought that they may have evolved in response to a simple antibiotic produced by organisms at the time of divergence.

One of the newest genetic techniques, Polymerase Chain Reactions (PCR), is based on an enzyme produced by the Archaean organism, Thermus aquaticus. The polymerase enzyme produced by this organism is highly effective in replicating DNA, and can work in high temperatures. This is good because PCR requires cycling the DNA into high and lower heats to replicate it faster. The Taq polymerase enzyme, as it is nicknamed, allows us to produce an enormous amount DNA in a short time. This this magnified amount of DNA, it becomes easy to study the genetic code of organisms.


In a eukaryote, or an organism that has a membrane bound nucleus and organelles, the DNA is contained in the nucleus, and the highly specialized organelles complete the various functions of the cell. Some eukaryotes become highly complex, multi-celled organisms. The individual cells then group into tissues, which form organs. These organs allow large animals like ourselves to move, eat and reproduce. Most organisms that you can think of are eukaryotes.

All eukaryotic life starts as a single cell. The cell divides through the process of mitosis, and becomes many cells. As the cells begin to specialize, they are sent different signals either chemically or electrically, and grow or change as needed. In this way, large organisms can manage the processes of their bodies through the release of chemicals or through the nervous system.

Organisms in the Eukarya include 5 kingdoms: Animalia, Plantae, Fungi, Protozoa and Chromista. The protozoans and chromistans are single-celled organisms that have membrane-bound organelles and nuclei. Fungi includes mushrooms, molds, and yeasts. Plantae is a large and diverse group that contains everything from single-celled algae to the largest organisms on the planet: trees. The Animalia contains most of the typical multi-celled organism that we would see in a zoo.


Some scientists even consider viruses to be organisms, as they are self-replicating information molecules usually protected by a protein shell. The virus then uses the mechanisms of a cell it infects to replicate itself. Proponents of classifying the virus as an organism point this out, while other scientist note that unlike a living organism, the virus does not create or store energy or the mechanisms to do so. While the debate rages on, it is important to note that the definition of life is not static. New evidence is found, methods of observation are created, and breakthroughs are made every day. It may not be long before life is found on another planet that operates in a completely different way than life on Earth.

Examples of Organism


Bees are an example of organisms that live socially. Many bees work to collect sugary nectar from flowers, which they store in their hive. They protect the hive and work cooperatively to build and repair it. The hive is usually attached to another organism, a tree. This is an example of a mutualistic relationship between organisms. The bees are provided a place off the ground, away from bears and other animals that want to eat their honey. The tree is provided with a source of pollination for reproduction. Bees are a major pollinator of agricultural crops as well. In fact, it has been estimated that without the bees, billions of dollars of crops would not be able to pollenate. That is a scary fact, considering bees have been in decline globally for decades.


Tapeworms are an example of a parasitic organism, or an organism that feeds off of other organisms to survive. The tapeworm lives in the intestines of mammals, and feeds off the dissolved nutrients that the mammal has worked so hard to gather. Tapeworms reproduce in the gut, lay eggs in the feces, and new animals are exposed when they come into contact with the eggs, which can lay dormant in the soil for years. Parasitism is a type of relationship between organisms in which one organism benefits and one organism suffers. Single parasites do not often kill their host, because in doing so they would lose a home. However, large infestations of parasites can lead to malnourishment and even death if not treated.

Great White Shark

Considered the top of the food chain in the ocean, the great white shark is the ultimate predatory organism. The shark’s keen sense of smell allows it to track the scent of blood for miles underwater, leading it to wounded animals and corpses it can devour. The great white is one of only a few sharks ever documented leaping from the water in a strike on prey. Great whites often feed on seals, which are very agile and can outturn the shark. However, the sharks usually strike from below, honing in on the seal and hitting it at great speeds. Cells around the shark’s mouth are sensitive to small electrical impulses given off by prey, and the shark can literally feel its prey before it touches it. This makes the great white one an ultimate predatory organism.

  • Organelle – A membrane-bound sac that specializes in a particular function for a cell.
  • Organ – A collection of tissues, or group of cells, in the body that specializes in a particular function for the body.
  • Eukaryotic – A cell with membrane-bound organelles and a nucleus.
  • Prokaryotic – A cell without membrane-bound organelles or a nucleus.


1. A single blue whale weighs almost 40,000 pounds. A colony of aspen trees that share a root system and were derived from the same seed weighs almost 13,000,000 pounds. The combined weight of all bacteria on Earth is somewhere around 1.1 x 1014, or 110,000,000,000,000 pounds. What is the largest organism on Earth?
A. Blue Whale
B. Aspen Tree
C. Bacteria

Answer to Question #1
B is correct. While the blue whale is massive, it is no match for a clonal grove of aspen trees. Although the trees look like individuals to us, they are all connected to a massive underground root system. The oldest tree known, “Pando”, is thought to be 80,000 years old. The bacteria, while massive together, represent trillions of individual organisms, each independent of each other. Take out one bacteria, the rest will be fine. Ruin Pando’s roots and millions of trees could be lost. That is what makes the Aspen tree the largest living organism.

2. Many plants produce offspring in the form of seeds. To create seeds, the male gamete has to meet the female gamete and fertilization must occur. This can occur to many eggs at once and many plant prepare a huge number of seeds at the same time. You see a dandelion plant in your yard. The yellow petals of the three different flowers have been replaced by a white puffball connected to hundreds of seeds. How many organisms are present?
A. 1
B. <300
C. >300

Answer to Question #2
C is correct. The plant itself represents 1 organism. Each seed also represents an individual organism, because they were created after two haploid cells created a zygote. That zygote can become an entire plant. Therefore, with 3 flowers and over a hundred seeds each, there are well over 300 organisms present in a single dandelion plant. Cool!

3. Aliens visit our planet. They add us to their categorization of life as the domain “Eartharia”. Would they be wrong?
A. Yes
B. No
C. Hmm…

Answer to Question #3
B is correct. Classification systems are simply a representation of the different relationships between animals. Because all life on Earth is thought to share the same root, scientists are focused on the relationships between life on Earth. From an alien’s perspective, it might be perfectly reasonable to classify all Earthly organisms together.

Cite This Article

MLAAPAChicago Editors. "Organism." Biology Dictionary,, 06 Dec. 2016, Editors. (2016, December 06). Organism. Retrieved from Editors. "Organism." Biology Dictionary., December 06, 2016.

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