International A Level Biology / Classification
The three-domain system revision notes
Edexcel International A Level Biology (YBI11), chapter 11: Classification, specification point 4.14. Free lesson.
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The three-domain system uses molecular evidence to divide life into Bacteria, Archaea and Eukarya. It explains why the old kingdom Prokaryotae was split, and how scientific evidence is checked before a classification changes.
What the five kingdoms were based on
Before molecular evidence, organisms were placed into five kingdoms mainly by features that could be observed: their cell type, whether they were single-celled or multicellular, and how they obtained food. Prokaryotae contained organisms with no nucleus or membrane-bound organelles. Every prokaryote was placed here because they appeared similar under a microscope. Protoctista were eukaryotic organisms, mostly single-celled, such as Amoeba and Chlorella. Fungi were eukaryotic, had chitin cell walls and fed saprotrophically. They digested food outside their bodies and absorbed it.…
How molecular evidence separated Bacteria and Archaea
In the 1970s, Carl Woese compared the base sequences of ribosomal RNA (rRNA) in many prokaryotes. All cells have ribosomes, so rRNA allows organisms to be compared. The results showed that Archaea are as different from Bacteria as they are from eukaryotes, even though both groups look alike under a microscope. Bacteria and Archaea are both prokaryotic, so neither has a nucleus. Bacteria have unbranched fatty acids joined to glycerol by ester bonds. Archaea have branched chains joined by ether bonds. Bacterial cell walls contain peptidoglycan. Archaea have no peptidoglycan…
How new evidence is checked
A new result does not change classification straight away. The scientific community checks whether the evidence is reliable and whether other scientists can repeat it. Woese and George Fox published their rRNA work in 1977. In 1990, Woese and colleagues proposed the three-domain system.
How to identify the three domains
A domain is a classification level above a kingdom. Every living organism belongs to exactly one of the three domains. The old kingdom Prokaryotae was divided into Bacteria and Archaea, while the four eukaryotic kingdoms were placed in Eukarya. Bacteria are prokaryotic, have peptidoglycan cell walls and ester-linked lipids. Examples include E. coli and Lactobacillus. Archaea are prokaryotic, have no peptidoglycan and have ether-linked lipids. Examples include methanogens and halophiles. They can live in extreme environments, but also in places such as soil, oceans and animal guts.…
Choose the domain, not just the kingdom
A plant with a nucleus and chloroplasts belongs to the kingdom Plantae, but its domain is Eukarya. Chloroplasts can help identify the kingdom, while the nucleus identifies the domain. Do not give Plantae as the domain. It is a kingdom inside Eukarya. Do not give Archaea for a cell with a nucleus. Archaea are prokaryotic.