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Organism: Definition & Explanation

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  • 0:00 Definition of an Organism
  • 1:25 Taxonomy and Carolus Linnaeus
  • 3:55 Example of Classification
  • 5:05 Naming Organisms
  • 6:30 Lesson Summary
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Lesson Transcript
Instructor: Honey Whitney
An organism is any living thing. There are millions of organisms, so how do scientists organize them all? Learn more about the diversity of life on Earth and how scientists classify them, then take a quiz to test your knowledge!

Definition of an Organism

Take a look around you. There may be a computer, a plant, some writing utensils, or another person sitting right next to you. Which of the things around you would you characterize as living? If you said the plant and the person, brownie points to you! While it may sound simplistic, it is important to distinguish the difference between living and non-living things for purposes of science.

Examples of life on Earth
Life on Earth Examples

In biology, specifically, scientists study organisms and their living environment. An organism is simply defined as any living thing, ranging from microscopic bacteria to the large African bush elephant and everything in between. So how many different organisms currently exist? Scientists estimate about 8.7 million different species of organism, not including microorganisms like bacteria, are on planet Earth right now. Of those possible millions of species of organisms, only about 1.2 million have been identified; that means there are still more than 85 percent left that we have not even discovered!

With so many differences between all the forms of life, classifying organisms can be pretty difficult. There is an entire science dedicated to organizing and describing all of the organisms that previously existed, currently exist, or have yet to be discovered; that science is called taxonomy.

Taxonomy and Carolus Linnaeus

Carolus Linnaeus (circa 1739)
Karl von Linne

Most early biologists and naturalists, including the likes of Aristotle, spent a lot of time discovering and naming new organisms and determining how they were related to each other. The idea of classifying organisms goes back to classical times. The system used today was not developed until the 1758. Karl von Linne, a Swedish biologist more commonly known as Carolus Linnaeus, came up with a logical system that separates groups based on similarities and differences in body structure, and allows scientists everywhere to refer to a particular organism in the same way.

Levels of classification
Linnaean Hierarchy

Linnaeus' original system had seven levels of taxonomic categories. The highest category, Kingdom, separates organisms based on their major characteristics. Originally, all life was divided into the Kingdoms -- Animalia (multi-celled organisms that eat organic matter) and Plantae (multi-celled, usually non-mobile organisms that make their own food). With the invention of the microscope, life was further divided into three more kingdoms -- Protista (most single-celled organisms), Fungi, and Monera (blue-green algae and bacteria). Following Kingdom, organisms are further divided into Phylum, Class, Order, Family, Genus, and Species, getting more specific and closely related going down the list. A species represents a single type of organism, such as a great white shark or a red maple.

An eighth category above Kingdom was introduced by microbiologist Carl Woese in 1990, using newer genetic techniques, separating all organisms into the Domains of Archaea, Bacteria, and Eukarya. Archaea and Bacteria are both single-celled organisms, but each use different chemicals in their own biological processes and also have different evolutionary origins. Eukarya encompass all of the multi-celled organisms.

With all of these different categories, it gets a little confusing to remember which order they go in. Luckily, there are some clever sentences to remember the proper order. A few are:

  • Do Koalas Prefer Chocolate Or Fruit Generally Speaking?
  • Did King Philip Come Over For Good Spaghetti?
  • Databases Keep Precious Creatures Organized For Grumpy Scientists

See if you can come up with some of your own to help you remember!

Example of Classification

Going to a zoo gives you a great chance to learn about some really cool organisms found in different parts of the world. One of the most fascinating creatures that you may find is the common ostrich, a large bird from Africa. Ostriches do not fly but instead use their large, powerful legs to run fast. You may notice that these birds, like humans, run on two legs instead of four. Both ostriches and humans also engage in dancing as a courtship ritual. What other things do humans and ostriches have in common? Look at the table to see how the two compare when classified according to the Linnaean system.

Classification of a Human versus Ostrich (according to Linnaeus)
Human vs Ostrich

Despite their dance moves in common, as well as being multi-celled animals with a spinal column, humans and ostriches are categorized very differently. The Linnaean system was designed to describe organisms via structural differences alone, a major feat since the field of genetics had not been discovered yet, so evolutionary relationships were not known. Both humans and ostriches were classified and described by Linnaeus himself; it is amazing to think that many of the organisms on the planet are still classified according to Linnaeus' original descriptions!

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