Features of organisms

Biology · Classification. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Every organism is placed in one of five kingdoms: animals, plants, fungi, prokaryotes (bacteria) and protoctists. The questions to ask are: does the cell have a nucleus, does it have a cell wall, is the organism made of one cell or many, and how does it feed?

Inside the animal kingdom you must recognise the five groups of vertebrates (animals with a backbone) and the four groups of arthropods (jointed legs and a hard outer skeleton). In the plant kingdom you must tell ferns from flowering plants, and monocotyledons from dicotyledons.

Viruses are not in any kingdom. They are not cells and can only replicate inside a living cell.

Rules to remember

Animals: many cells, nucleus, no cell wall, feed on other organisms. Plants: many cells, nucleus, cellulose cell wall, chloroplasts, photosynthesise.
Fungi: nucleus, cell wall not made of cellulose, no chlorophyll, body of thread-like hyphae, feed by digesting food outside the cells. Prokaryotes: one cell, no nucleus, circular DNA, cell wall. Protoctists: mostly one cell, with a nucleus; some have chloroplasts.
Vertebrates: mammals (fur or hair, milk from mammary glands), birds (feathers, beak, hard-shelled eggs), reptiles (dry scaly skin, eggs with leathery shells laid on land), amphibians (moist skin without scales, eggs laid in water), fish (wet scales, gills, fins).
Arthropods: insects (3 pairs of legs, 3 body parts, 1 pair of antennae), arachnids (4 pairs of legs, 2 body parts, no antennae), crustaceans (more than 4 pairs of legs, 2 pairs of antennae), myriapods (many segments, each with legs, 1 pair of antennae).
Ferns: roots, stems and leaves (fronds), reproduce by spores, no flowers or seeds. Flowering plants: reproduce by seeds made in flowers.
Monocotyledons: one cotyledon, parallel leaf veins, flower parts in threes. Dicotyledons: two cotyledons, branching network of veins, flower parts in fours or fives.
Virus: a protein coat around genetic material; no cytoplasm, no cell membrane, no ribosomes.

Common mistake

Counting legs wrongly: the rule is in pairs. A spider has 4 pairs (8 legs) and is an arachnid, not an insect. Insects have 3 pairs (6 legs).

Worked example

A small animal has jointed legs, a body in three parts, three pairs of legs and one pair of antennae. To which group does it belong?

Jointed legs show that it is an arthropod.
Three pairs of legs rules out arachnids (four pairs), crustaceans (more than four pairs) and myriapods (many pairs).
Three body parts and one pair of antennae fit the insects.

Answer: It is an insect.

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O Level Biology Notes (Set 2)

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