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The Five Kingdoms of Life: A Visual, Step-by-Step Guide

Updated on June 3, 2013

Classifying Living Things

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The current system of classification uses 8 taxa, with species as the basic unit This means that whilst all members of a species show variation, they are all essentially the same. As you go up, variation increases & relatedness decreases.Phylogeny is the study of how closely different species are related - it reflects the evolutionary relationships of the species. The more closely two species are related, the closer they appear on the evolutionary tree. Carl Linnaeus devised the system of classification still in use today - albeit in a modified and expanded form. His original system only contained 5 taxa, compared to the 8 in use today.A more visual representation of how relatedness decreases as you ascend the modern classification system taxa
The current system of classification uses 8 taxa, with species as the basic unit This means that whilst all members of a species show variation, they are all essentially the same. As you go up, variation increases & relatedness decreases.
The current system of classification uses 8 taxa, with species as the basic unit This means that whilst all members of a species show variation, they are all essentially the same. As you go up, variation increases & relatedness decreases. | Source
Phylogeny is the study of how closely different species are related - it reflects the evolutionary relationships of the species. The more closely two species are related, the closer they appear on the evolutionary tree.
Phylogeny is the study of how closely different species are related - it reflects the evolutionary relationships of the species. The more closely two species are related, the closer they appear on the evolutionary tree. | Source
Carl Linnaeus devised the system of classification still in use today - albeit in a modified and expanded form. His original system only contained 5 taxa, compared to the 8 in use today.
Carl Linnaeus devised the system of classification still in use today - albeit in a modified and expanded form. His original system only contained 5 taxa, compared to the 8 in use today. | Source
A more visual representation of how relatedness decreases as you ascend the modern classification system taxa
A more visual representation of how relatedness decreases as you ascend the modern classification system taxa | Source

Why do we Classify Living Things

This hub is all about the classification of living things (Taxonomy). The list to the right shows the current system of classification with species at the base and domain at the top. The first question we must answer is "What is the point?". We classify:

  • for convenience
  • to make the study of living things more manageable
  • for ease of identification
  • to analyse inter-species relationships (Phylogenetics)

More than 250 years ago, a Naturalist named Carl Linnaeus classified more than 70,000 organisms according to methodical observation of their physical features. Each organism was placed into a series of ranked categories named taxa (singular: taxon).

The modern classification system is an expanded version of that used by Linnaeus - as you rise through the taxa, the individuals grouped together show less and less similarity. Individuals of a Species (at the bottom) are all essentially the same; individuals of a kingdom (near the top) show huge variation and are much less related.

Prokaryotes (Bacteria) are the only division of life that do not have a nucleus. Estimates suggest that there are 10 times more bacterial cells in your body than human cells!
Prokaryotes (Bacteria) are the only division of life that do not have a nucleus. Estimates suggest that there are 10 times more bacterial cells in your body than human cells! | Source

1. Prokaryotae

Prokaryotes get their name from the Greek meaning 'before nucleus,' they evolved before the nucleus became a storage organelle for DNA. As such, all members of the prokaryotae taxon lack a nucleus. In addition to this, prokaryotes:

  • do not arrange their DNA in linear chromosomes, but instead as a loop of naked DNA (not associated with histone proteins)
  • have no membrane-bound organelles
  • are smaller than eukaryotes
  • have smaller ribosomes than eukaryotes

Whilst we may consider humans as the pinnacle of evolution (a fallacy in itself), by sheer weight of numbers, or even sheer weight, this planet is ruled by the prokaryotes.

Paramecium - an example of a protoctist. This kingdom shows huge variety, as is more like the 'junk-drawer' of the classification system
Paramecium - an example of a protoctist. This kingdom shows huge variety, as is more like the 'junk-drawer' of the classification system | Source

2. Protoctista

The protoctists are a hodge-podge of different organisms. Some are single-celled, others are multicellular; some are autotrophic, others are heterotrophic. Pretty much the only thing they have in common is that they are eukaryotes (have a nucleus and membrane bound organelles) and don't fit in the other kingdoms. The protoctists:

  • show huge variety
  • are eukaryotic

Just because humans want everything to fit into neat little boxes, doesn't mean that nature follows suit!

Perhaps the most important Kingdom of them all. If animal life disappeared tomorrow, Earth would carry on much the same. If fungi disappeared, most of the planet's ecosystems would collapse.
Perhaps the most important Kingdom of them all. If animal life disappeared tomorrow, Earth would carry on much the same. If fungi disappeared, most of the planet's ecosystems would collapse. | Source

3. Fungi

As my old university tutor would say, fungi are the most important organisms on the planet. They are saprophytic (break down dead organic matter), form symbiotic relationships with plants to help them collect water and are edible. The mushrooms we associate with a fungus is only the 'fruiting-body' - the equivalent of flowers on a plant - and make up a tiny percentage of the organism. Fungi are also the largest organisms on the planet - one individual in Oregon's Blue Mountains occupies 2,384 acres of land, and could be up to 8,650 years old!

Together, the fungi:

  • are eukaryotes (have a nucleus and membrane bound organelles)
  • are made up of a network of strands (like you see in mouldy bread) called hyphae, collectively called a mycelium
  • have cell walls made of chitin
  • have cytoplasm with several nuclei (multinucleate)

Plants - the basis of most ecosystems. Characterised by photosynthetic organelles and a cellulose cell wall
Plants - the basis of most ecosystems. Characterised by photosynthetic organelles and a cellulose cell wall | Source

4. Plantae

We live on a blue and green planet - blue from liquid water, green from the vast abundance of plant life. Plants use energy from the sun to combine carbon dioxide and water to make sugars. A happy by-product of this process - known as photosynthesis - is the oxygen we and all other animals breathe. The ability to photosynthesise is a key feature used to classify the plants. The plants:

  • are eukaryotes
  • are multicellular
  • have a cellulose cell wall
  • are autotrophic (make their own food); use photosynthesis to make sugars
  • produce multicellular embryos from fertilised eggs.

All life on Earth is interconnected, so all kingdoms are equally important. The animals, however, are most charismatic and secure funding for most environmental causes.
All life on Earth is interconnected, so all kingdoms are equally important. The animals, however, are most charismatic and secure funding for most environmental causes. | Source

5. Animalia

The animals are the most recognisable and charismatic organisms on our planet. Just like every other kingdom, they show huge variety and have evolved wonderful adaptations to their environments. At face value, it may be difficult to figure out what makes animals unique and related, but all animals:

  • are eukaryotic
  • are multicellular
  • are heterotrophic (consume other organisms for food)
  • have fertilised eggs that develop into a blastula (ball of cells)
  • are usually motile.

Key Features

  • Prokaryotes - have no nucleus
  • Protoctists - the 'miscellaneous' taxon; contains organisms that don't fit into the other kingdoms
  • Fungi - cell wall made of chitin, decay organic matter
  • Plantae - multicellular photosynthesisers
  • Animalia - heterotrophic, multicellular eukaryotes


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Modern Classification

The protoctists are an excellent example of how difficult classification is. In the 250 years since Linnaeus, many animals have been reclassified - the reason for this is simple: originally, classification was based on morphology alone. Now, with proteomics, genomics and other phylogenetic tools, it is clear some of the animals once classified as close cousins, aren't as closely related as we thought. Modern phylogeny uses biochemistry to supplement morphological analysis.

Certain large biochemical molecules are ubiquitous in the living world. However, they may have subtle differences; these differences reflect evolutionary relationships:

  1. Cytochrome c - A respiratory protein found in almost all living things. By comparing the amino acid sequences of cytochrome c in to species we can see how closely related the species are. The more different the sequence, the more distantly related
  2. DNA - Comparison of DNA provides a way to classify sequences, much like proteomics (the analysis of proteins such as cytochrome c) This is the most accurate way to demonstrate the relatedness of different species.

Do YOU think there is any point to classifying organisms in this way?

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    • profile image

      Akpan, Samuel S. 18 months ago

      Living organisms from 1 to 1000 or more than that be listed out for easiest use.

      Having said that, I also recommend this book to all science students, teachers, and lecturers.

    • profile image

      Areeb Irfan 5 years ago

      very useful it is very helping and in easy language hope the notes 4rm it will help me in GCE

    • scottcgruber profile image

      scottcgruber 5 years ago from USA

      Very interesting stuff! The Linnaean system was definitely ahead of its time, though it's becoming a bit obsolete. Life has a funny way of defying our attempts to categorize, forcing biologists to invent superfamilies and suborders and superphyla to accurately classify living things.

      I've been trying to learn more about cladistics lately - it seems a more useful method of classification, reflecting how life organizes itself rather than how humans want to organize it.

    • TFScientist profile image
      Author

      Rhys Baker 5 years ago from Peterborough, UK

      @thebiologyofleah: thanks for the positive feedback! I'm glad you enjoyed it :)

    • thebiologyofleah profile image

      Leah Kennedy-Jangraw 5 years ago from Massachusetts

      great article, nice review for me- I haven't looked at classification in a while.