The Five Kingdoms of Life: A Visual, Step-by-Step Guide
Classifying Living ThingsClick thumbnail to view full-size
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 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.
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!
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)
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.
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.
- 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
Knowledge is Power
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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:
- 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
- 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.