History of the Classification of organisms into kingdoms
Until relatively recently it was generally agreed that all living organisms should be placed into two kingdoms, the animal and plant kingdoms. The basic different between plants and animals was that animals fed on organic material (are heterotrophic) whereas plants synthesized their own organic requirements from inorganic compounds (are autotrophic).
More precisely, a heterotrophic organism is one who has an organic source of carbon, and an autotrophic organism is one which has an inorganic source of carbon, namely carbon dioxide.
Animals typically search for their food and so show locomotion. For this, they require a nervous system for coordination in the more complex animals, whereas plants are stationary and do not show locomotion or possess a nervous system.
However, this classification ignored the fact that all cellular organisms fall into two natural groups, now known as prokaryotes and eukaryotes. These two groups are fundamentally different.
The term prokaryote and eukaryote refers to differences in the location of DNA (the genetic material).
In prokaryotes the DNA is not enclosed by nuclear membranes and lies free in the cytoplasm. The cells therefore lack true nuclei (pro, before; karyon, nucleus).
The cells of eukaryotes, however do contain true nuclei (eu, true). Eukaryotes evolved from prokaryotes.
Classifying all organisms as animals or plants presented other difficulties. For example, fungi are heterotrophic but non-motile, so should they be classified as animals or plants? Such problems have been by accepting that there should be more than two kingdoms.
In 1982, Margulis and Schartz proposed a system which used five kingdoms, the prokaryotae and four eukaryote kingdoms. This has been widely accepted and is currently recommended by the Institute of Biology.
The eukaryotes can be regarded as belonging to a superkingdom, the eukaryotae. The most controversial group is the protoctista because it is probably an unnatural group.
The five kindoms
- Prokaryotae: they are called prokaryotes, consists bacteria and cyanobacteria (blue green bacteria), autotrophic or heterotrophic, motile or non-motile.
- Protoctista: members are called protoctists, organisms resembles the ancestors of plants, animals and fungi; include algae, protozoa, slime moulds, oomycotes (early fungi)
- Fungi: heterotrophic, non-motile, food is digested outside the body and products of digestion absorbed.
- Plantae: members are called plants, they are autotrophic and non-motile.
- Animalia: members are called animals, they are heterotrophic and motile, food is ingested (taken into the body) before digestion.
The kingdoms prokaryotae and protoctista are predominantly unicellular and are the evolutionary ancestors of the more advanced kingdoms; fungi, plantae and Animalia (which are predominantly multicellular).
One group of organisms that does not fit in neatly into any classification scheme is the viruses. Viruses are extremely small particles consisting only of a piece of genetic material (DNA or RNA) in a protective coat of protein. They do not have a cellular structure, unlike all other organisms and can only reproduce by invading living cells.
Although it is not a natural grouping, the smallest organisms are often collectively known as microorganisms or microbes. These include the bacteria (prokaryotae), viruses, fungi and protoctists.
The grouping is a useful one for practical reasons because the techniques used in their study are often similar. For example, a microscope is needed to see them and sterile (aseptic) techniques are needed for culturing them. The study of microorganisms is a branch of biology known as microbiology.
Microorganisms have become increasingly important in the areas of biochemistry, genetics, agriculture and medicine and are the basis of important section of industry known as biotechnology.
Microorganisms such as bacteria and fungi are also important ecologically as decomposers.
Major differences between prokaryotes and eukaryotes.
|Organisms||Bacteria||Protoctists, fungi, plants and animals|
|Cell size||Average diameter of 0.5-1µm||10-100µm diameter common, commonly 1000-10000 times volume of prokaryotic cells.|
|Form||Mainly unicellular||Mainly multicellular (except protoctista, many of which are unicellular)|
|Evolutionary origin||3.5 billion years ago||1.2 billion years ago, evolved from prokaryotes|
|Cell division||Mostly binary fission, no spindle||Mitosis, meiosis, or both; spindle formed|
|Genetic material||DNA is circular and lies free in the cytoplasm (no true nucleus) DNA is naked (not associated with proteins or RNA to form chromosomes)||DNA is linear and contained in a nucleus. DNA is associated with proteins and RNA to form chromosomes.|
|Protein synthesis||70S ribosomes (smaller) No endoplasmic reticulum present (many other details of protein synthesis differ, including susceptibility to antibiotics, e.g prokaryotes inhibited by streptomycin)||80S ribosomes (larger) Ribosomes may be attached to endoplasmic reticulum|
|Organelles||Few organelles None are surrounded by an envelope (two membranes) Internal membrane scarce; if present usually associated with respiration of photosynthesis.||Many organelles Envelope-bound organelles present e.g nucleus, mitochondria, chloroplasts. Great diversity of organelles bounded by single membranes e.g Golgi apparatus, lysosomes, vacuoles, microbodies, endoplasmic reticulum.|
|Cell walls||Rigid and contain polysaccharides with amino acids; murein is main strengthening compound.||Cell walls of green plants and fungi rigid and contain polysaccharides; cellulose is main strengthening compound of plant walls, chitin of fungal walls (non in animal cells).|
|Flagella||Simple, lacking microtubules; extracellular (not enclosed by cell surface membrane) 20nm diameter||Complex with ‘9+2’ arrangement of microtubules; intracellular (surrounded by cell surface membrane) 200nm diameter|
|Respiration||Mesosomes in bacteria, except cytoplasmic membranes in blue-green bacteria||Mitochondria for aerobic respiration|
|Photosynthesis||No chloroplasts; takes place on membrane which show no stacking||Chloroplasts containing membranes which are usually stacked into lamellae or grana|
|Nitrogen fixation||Some have the ability||None have the ability.|
SOURCE: Biological science 1 & 2
With this, I think we have discussed successfully the classification of living organism. Do ensure to search for answers with our search box should you have any question regarding any of the kingdoms. +��-