Cambridge A Levels A2 Biology Chapter 18 Classification
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Cambridge A Levels A2 Biology
Institution
Chapter 18 Classification: 43 pages
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Chapter 18 Classification, Biodiversity and Conservation
18.1 Classification
18.2 Biodiversity
18.3 Conservation
18.1 Classification
The Species Concept
Biological species
A group of organisms with similar morphology and physiology, which can breed together to
produce fertile offspring and are reproductively isolated from other species.
Morphological species
A group of organisms that share many physical features that distinguish them from other
species.
Ecological species
A population of individuals of the same species living in the same area at the same time.
Population:
All of the organisms of the same species present in the same place and at the same time that
can interbreed with one another.
Hierarchical Classification
Taxonomy
the study and practice of naming and classifying species and groups of species within the
hierarchical classification scheme.
Biological classification
the organisation of living and extinct organisms into systematic groups based on similarities
and differences between species.
Hierarchical classification
The arrangement of organisms into groups of different rank. The lowest rank is the species;
similar species are grouped together into the next rank, which is the genus; this continues to
the highest rank, which is the domain where many species are grouped together.
,Taxonomic rank
One of the groups used in the hierarchical classification system for organisms, e.g. species,
genus, family, order, class, phylum, kingdom and domain.
Taxon (plural: taxa)
A taxonomic group of any rank, such as a particular species (e.g. Giraffa camelopardalis), a
family (e.g. Elephantidae), a class (e.g. Mammalia) or a kingdom (e.g. Plantae).
,The Three Domains
Bacteria:
The domain that contains all prokaryotic organisms except those classified as Archaea.
Archaea
The domain of prokaryotic organisms that resemble bacteria but share some features with
eukaryotes.
Eukarya
The domain that contains all eukaryotic organisms: protoctists, fungi, plants and animals.
▪ Biologists used to divide organisms into two large groupings based on their cell structure.
▪ Prokaryotes and eukaryotes have significantly different cellular structures.
▪ In the 1970s prokaryotes were discovered living in extreme environments, such as hot springs where
temperatures often exceed 100 °C.
▪ These organisms are called extremophiles and are not like typical bacteria.
▪ Studies revealed that the genes coding for the RNA that makes up their ribosomes were more like
those of eukaryotes.
▪ The extremophiles were found to share features with both typical bacteria and eukaryotes.
▪ At this time, studies of molecular biology assumed a much greater significance in taxonomy.
▪ This meant that a new taxon, the domain, had to be introduced to reflect the differences between
these extremophiles and typical bacteria.
▪ The domain is the taxon at the top of the hierarchy.
▪ The prokaryotes are divided between the domains Bacteria and Archaea and all the eukaryotes are
placed into the domain Eukarya.
▪ Many Archaea live in extreme environments, such as hot springs, around deep volcanic vents (black
smokers) in the oceans and in lakes where there is a very high concentration of salt.
, ▪ Some of them produce methane, cannot survive where there is oxygen and have many unusual
enzymes.
▪ Since they were discovered in extreme environments, they also have been found in many less extreme
environments; for example, they form an important part of the plankton in the oceans.
▪ In several ways, the Archaea appear to have more in common with the Eukarya than with Bacteria.
▪ It is thought that Bacteria and Archaea separated from each other very early in the evolution of life.
▪ The Archaea and Eukarya probably diverged later.
Domain Bacteria
Characteristic features of bacteria
▪ Bacteria are prokaryotic as their cells have no nucleus.
▪ They are all small organisms that vary in size between that of the largest virus and the
smallest single-celled eukaryote. (Up to 10µm)
▪ Cells with no nucleus.
▪ DNA is circular and does not have histone proteins associated with it.
▪ Smaller circular molecules of DNA called plasmids are often present.
▪ No membrane-bound organelles (such as mitochondria, endoplasmic reticulum, Golgi
body, chloroplasts) are present.
▪ Ribosomes (70S) are smaller than in eukaryotic cells.
▪ Cell walls are always present and contain peptidoglycans (not cellulose)
▪ Cells divide by binary fission, not by mitosis.
▪ Usually exist as single cells or small groups of cells.
▪ May have flagellum, therefore they show motility.
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