Classification in kingdoms has had a fairly long history of going backwards and forwards in terms of which kingdoms to include, and how many there are in total. For example, one of the latest classifications puts forward 7 kingdoms.
The five-kingdom system of Prokaryotae, Protoctista, Plantae, Fungi and Animalia is already half a century old, has been up for debate as well, and will probably continue to evolve as we learn more about new species and the relationships between different kinds of organism. So, without further ado, let’s learn the basic principles of these kingdoms and what kind of organisms they represent.
Prokaryotae (or Monera)
This kingdom was used to refer to single-celled prokaryotes such as bacteria and archaea, and it rose to prominence in the 1920s because the important distinction between eukaryotes and prokaryotes was just beginning to take hold.
Pictured Lactobacillus is a bacterium added to so-called “probiotic” foods. Archaea look similar to bacteria, so weren’t distinguished for a while, before molecular biology revealed that their genes and protein synthesis machinery are very different and more closely related to those of eukaryotes. Many famous archaea are what’s called extremophiles because they were found in extreme environments such as boiling water and extreme acidity.
Reproduction is by division i.e. binary fission.
Cell division in prokaryotic organisms such as bacteria is very simple. Termed binary fission (splitting in two), it involves duplication of the cell’s DNA and the even splitting of the copied genetic material into its two offspring cells, which in this unicellular organism effectively becomes two new individuals.
The cytoplasm is therefore also evenly divided alongside the respective genetic material. However, you might remember that these organisms have extra genetic information in plasmids alongside their main DNA. The number of copies of plasmid that each new cell receives from the parent cell during binary fission is variable.
In the diagram only the main DNA is pictured (plasmids are much smaller) as well as the cell wall which can be seen pinching from the sides of the new emerging offspring cells, and gradually tearing a path towards the middle to separate the two cells. Very much the same idea as cytokinesis in mitosis, although bear in mind these cells do also have a cell wall to worry about in addition to the membrane!
Protoctista (or Protista)
This is not the best kingdom to start with, because it’s defined mostly by organisms which do not make it into the other kingdoms more than anything else! It refers to unicellular or multicellular eukaryotes which do not fit under the kingdoms Plantae, Fungi or Animalia. Protoctista is more generic than Protista and covers the multicellular members of the kingdom such as kelps and red algae, while Protista refers to the microscopic organisms such as paramecium.
Some are heterotrophs e.g. paramecium, while others are autotrophs e.g. the giant kelp.
Plants – basically. More specifically, green plants. They are defined in this kingdom by cells with cellulose cell walls, the function of photosynthesis which underpins their autotroph feeding (meaning self-feeding by making their own nutrients rather than having to ingest other animals), and sexual reproduction with alternating generations.
Green plants evolved their ability to photosynthesise by endosymbiosis with cyanoabcteria. This means plant cells joined up with photosynthesising bacterial cells. These became today’s chloroplasts which contain the photosynthesising chemicals in green plants.
Although some green plants do reproduce asexually, sexual reproduction is a key feature of them. Alternating generations refers to some groups of this kingdom producing haploid specimens which give rise to diploid specimens, which give rise to haploid specimens, and so on and so forth alternating generations.
For example, a diploid fern (first generation) will produce spores which will result in a different variation of the same fern (second generation). This variation is haploid, and produces gametes instead of spores. These are haploid so will need to sexually reproduce by joining together to form a zygote. The zygote will grow into a new diploid fern (third generation).
Kingdom Fungi covers yeasts, moulds and mushrooms – all eukaryotic. Of course, all kingdoms are eukaryotic apart from Prokaryotae, there’s a hint. Fungi can be both microscopic and large.
Fungi set themselves apart into the kingdom by the prevalence of chitin in their cell walls (as opposed to cellulose in plant cell walls), no ability to photosynthesise, no ability to move except by growing in a particular direction, and their heterotrophic means of feeding.
They can excrete enzymes into their environment to break down nutrients for them to absorb for energy (e.g. decomposer lysotrophs). Fungi were misunderstood for a while and thought to be a subset of plants, only for it to later become apparent that they are their own thing, and if anything are more closely related to animals.
Their cells form web-like or sponge-like structures (hyphae that together form a mycelium) that can be underground, so they only make themselves obvious when they fruit, for example as mould on bread or mushrooms in the woods.
Finally, we’ve got to me and you! Les animaux… Animalia originates from the Latin animalis meaning having a soul. It also shares its meaning with words like animation and animated.
Unsurprisingly then, one of the distinguishing features for members of this eukaryotic kingdom is the ability to move at some point in their lives – motility. Other features include heterotrophic feeding, like fungi, as well as a fixed body plan, meaning that individuals develop into a specific rough body model (such as a vertebrate) which then stays the same.
Organisms in this kingdom include vertebrates (less than 5% of species, e.g. fish, reptiles, mammals) and invertebrates (lacking a spine). The latter include worms, leeches, corals, crabs, sponges and insects.
Three domains or five kingdoms?
An example of the role of the scientific community in validating evidence is the classification of life into 3 domains versus 5 kingdoms. Archaea, Bacteria and Eukarya or perhaps Monera, Protista, Plantae, Fungi and Animalia?
Domains are higher up than kingdoms, so refer to the very earliest differences between living things. Originally, the kingdoms were established based on superficial analysis of different organisms, and an even earlier classification called Linnean classification simply split everything into animals and plants, based on whether they moved!
Monera (or Prokaryotae) kingdom was split into bacteria and archaea because very fundamental differences between these organisms were found as a result of better comparisons such as molecular biology techniques such as those outlined above. The remaining 4 kingdoms could then be grouped under Eukarya.
The response of scientists to this evidence follows that older evidence was treated differently in light of newer, more insightful evidence. Evidence can always build up, break down or change in other ways to change our understanding of life. At first, archaea and bacteria looked similar, so they were treated as closely related. As their inner workings were investigated with the use of more sophisticated tools, it became apparent that their characteristics beyond appearance were very different.