IGCSE NOTES : Biology - Characteristics and classification of living organisms

All living organisms, whether they are single celled or multicellular, plants or animals, show the characteristics included in the definitions above: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition.

Species

The smallest natural group of organisms is the species. A species can be defined as a group of organisms that can reproduce to produce fertile offspring. Members of a species also often resemble each other very closely in appearance, unless humans have taken a hand in the breeding programmes. All cats belong to the same species but there are wide variations in the appearance of different breeds  An American Longhair and a Siamese may look very different but they have few problems in breeding together. Robins, blackbirds and sparrows are three different species of bird. Apart from small variations, members of a species are almost identical in their anatomy, physiology and behaviour. Closely related species are grouped into a genus (plural: genera). For example, stoats, weasels and polecats are grouped into the genus Mustela.Binomial nomenclatureSpecies must be named in such a way that the name is recognised all over the world. ‘Cuckoo flower’ and ‘Lady’s smock’ are two common names for the same wild plant. If you are not aware that these are alternative names this could lead to confusion. If the botanical name, Cardamine pratensis, is used, however, there is no chance of error. The Latin form of the name allows it to be used in all the countries of the world irrespective of language barriers. People living in Britain are familiar with the appearance of a blackbird – a very common garden visitor. The male has jet black plumage, while the female is brown. Its scientific name is Turdus merula and the adult is about 24 cm long. However, someone living in North America would describe a blackbird very differently. For example, the male of one species, Agelaius phoeniceus, has black plumage with red shoulder patches and yellow flashes, while the female is speckled brown. It is about the size of a sparrow – only about 20 cm long. A British scientist could get very confused talking to an American scientist about a blackbird! Again, the use of the scientific name avoids any confusion. The binomial system of naming species is an internationally agreed system in which the scientific name of an organism is made up of two parts showing the genus and the species. Binomial means ‘two names’; the first name gives the genus and the second gives the species. For example, the stoat and weasel are both in the genus Mustela but they are different species; the stoat is Mustela erminea and the weasel is Mustela nivalis.
The name of the genus (the generic name) is always given a capital letter and the name of the species (the specific name) always starts with a small letter. Frequently, the specific name is descriptive, for example edulis means ‘edible’, aquatilis means ‘living in water’, bulbosus means ‘having a bulb’, serratus means ‘having a jagged (serrated) edge’.

Features of organisms

All living organisms have certain features in common, including the presence of cytoplasm and cell membranes, and DNA as genetic material. All living organisms also contain ribosomes in the cytoplasm, floating freely or attached to membranes called rough endoplasmic reticulum (ER).

The animal kingdom

Animals are multicellular organisms whose cells have no cell walls or chloroplasts. Most animals ingest solid food and digest it internally.