The Five Kingdoms And Classification Of Living Things
The Five Kingdoms
In the past there used to be 2 categories (referred to as kingdoms) that any living thing would be put into, these were 'plants' and 'animals'. As time progressed and people understood more and more about living things, they realised that they couldn't necessarily be put into either one of these 2 categories and so more kingdoms were made. Now every living organism in the world is in one of the following 5 kingdoms:
In this hub I will go over the 5 kingdoms in more detail including what characteristics the organisms share with one and other.
Prokaryotes belong to to the kingdom Prokaryota.
Organisms that belong to this kingdom include bacteria and the cyanobacteria. Prokaryote means 'before nucleus'.
The main features of Prokaryotes include:
- No nucleus.
- 'Naked DNA' that is not arranged in linear chromosomes.
- No membrane-bound organelles.
- Carry out respiration on mesosomes and not in mitochondria.
- Smaller ribosomes.
- Some cause diseases.
The fungi are a group of organisms the body of which consists of lots and lots of strands of hyphae which make up something called mycelium. The cytoplasm of fungi are made up of a wall of a thin polysaccharide, 'chitin'.
Main features of fungi are:
- Many nuclei.
- They are eukaryotes (have a real nucleus)
- They are mostly free-living.
- They are saprophytic which means that they cause the decay of organic matter.
These organisms belong to the kindom Animalia. They are all multicellular organisms that digesting organic matter and absorb it in order to gain the nutrition that they need (heterotrophic).
Main features of animals include:
- All eukaryotic.
- Have fertilised eggs that form blastula.
- Are (usually) able to move around.
These organisms belong to the kingdom Plantae. They get their nutrition via photosynthesis and this makes them 'autotrophs'.
Main features include:
- All eukaryotes.
- All multicellular.
- Their cells are surrounded by a cell wall made of cellulose.
- They all produce multi-cellular embryos from fertilized eggs.
These organisms belonged tot he group Protoctista and consist of the organisms that don't really fit into any other kingdom.
Some of the main features include:
- Are all eukaryotes.
- Some multi-cellular and some single celled.
- Some plant like and some animal like features.
- Mostly free living.
- Can be heterotrophs, autotrophs or even digest food with extracellular enzymes.
The system of classification
Above I described the 5 kingdoms that organisms are categorised into, however that is not as far as it goes.
The current system of classification uses eight taxa (the ranks of classification) and these are:
Species is the most basic unit of classification. Organisms in this category are all essentially the same but with some variation. As you rise through the taxa then the organisms will show more and more diversity and the similarities between the individuals will be smaller.
The 3 domains are Archaebacteria, Eukaryote and Eubacteria and all living things are classified into these.
But why do we need to classify organisms at all let alone in such a complex way? Well it is currently estimated that there are around two million different species of living organisms in the world and we order these to make the study of living things more manageable and help us determine the relationships between species. Understanding the relationship between different species helps us understand our own evolution.
To help you understand this a bit more below is how humans are classified in the 8 different taxa:
- Domain - Eukaryota
- Kingdom - Animalia
- Phylum - Chordata
- Class - Mammalia
- Order - Primate
- Family - Hominidae
- Genus - Homo
- Species - Sapiens
- Common name - Human