Characteristics and classification of living organisms

Cambridge IGCSE Biology 0610 · Core and Extended

Syllabus points

• List and describe the seven characteristics of living organisms.
• Explain that organisms are classified into groups by shared features.
• Describe the binomial system of naming species.
• Explain that classification can be based on sequence data as well as on visible features.
• Describe the main features of the five kingdoms, the main groups of arthropods and vertebrates, and the main groups of plants.
• Use and construct simple dichotomous keys.

The seven characteristics

Every living organism does all seven of these. That word all is what questions test: a car moves and uses fuel, but it does not reproduce or grow, so it is not alive.

• Movement — an action that changes position or place.
• Respiration — the chemical reactions that break down nutrient molecules in living cells to release energy.
• Sensitivity — detecting stimuli and responding to them.
• Growth — a permanent increase in size and dry mass.
• Reproduction — making more of the same kind of organism.
• Excretion — removing the waste products of metabolism and substances in excess.
• Nutrition — taking in materials for energy, growth and development.

Two of these are regularly confused in exams.

Excretion is not egestion. Excretion removes waste your own cells MADE, such as carbon dioxide from respiration or urea from breaking down excess amino acids. Egestion removes food that was never absorbed, so it never took part in metabolism at all. Faeces is egested, not excreted.

Growth means dry mass, not just size. A plant that takes up water swells but has not grown. Dry mass excludes water, so it measures material actually built into the organism.

Why we classify

Classification puts organisms into groups based on shared features. The purpose is not tidiness: a good classification reflects how closely related organisms are, so it also tells you something about their evolutionary history.

Organisms in the same group share more features with each other than with organisms outside it, and the groups sit inside one another: kingdom, phylum, class, order, family, genus, species.

A species is a group of organisms that can reproduce to produce fertile offspring. The word fertile is the whole test. A horse and a donkey produce a mule, but a mule cannot breed, so horses and donkeys stay separate species.

The binomial system

Every species has a two-word Latin name: the genus first with a capital letter, then the species in lower case. Both are italicised, or underlined in handwriting.

Homo sapiens, Panthera leo, Escherichia coli.

The system is international. A biologist anywhere reads the same name for the same organism, which common names do not allow: "robin" means one bird in Britain and a different one in North America.

Classifying by sequence data

Visible features can mislead. Two unrelated organisms living in the same conditions can end up looking similar, because the same environment favours the same solutions.

Comparing DNA base sequences, or the amino acid sequences of proteins, avoids that. Organisms that are closely related share more of their sequence, because less time has passed for mutations to accumulate since they diverged. Where sequence evidence and appearance disagree, sequence evidence is taken as the better guide, and several groups have been reorganised because of it.

The five kingdoms

• Animals — many cells, no cell wall, no chlorophyll, feed on other organisms.
• Plants — many cells, cellulose cell wall, chlorophyll, make their own food by photosynthesis.
• Fungi — usually many cells arranged as hyphae forming a mycelium, cell wall not made of cellulose, no chlorophyll, feed by secreting enzymes onto food and absorbing the products.
• Prokaryotes — one cell, no nucleus, DNA loose in the cytoplasm, often a plasmid.
• Protoctists — mostly one cell, with a nucleus; the group that holds everything not fitting the other four.

Viruses sit in none of them. They have no cytoplasm, no organelles and carry out no metabolism of their own, so most biologists do not treat them as living.

Arthropods

All arthropods have an exoskeleton and jointed legs. The four groups are told apart by counting.

Counting legs settles almost every arthropod question, so count before reading the options.

Vertebrates

All have a backbone. The distinguishing feature is usually the body covering or how the young develop.

• Fish — wet scales, gills, external fertilisation, eggs without shells.
• Amphibians — moist smooth skin, larvae with gills, eggs laid in water.
• Reptiles — dry scaly skin, eggs with leathery shells laid on land.
• Birds — feathers, beak, hard-shelled eggs, constant body temperature.
• Mammals — fur or hair, mammary glands, young born live, constant body temperature.

Feathers alone identify a bird, and fur alone identifies a mammal. Wings do not: bats are mammals and many insects fly.

Plants

• Ferns — leaves called fronds, reproduce by spores, no flowers.
• Flowering plants — reproduce by seeds formed inside flowers. Split into monocotyledons, with one seed leaf, narrow leaves and parallel veins, and dicotyledons, with two seed leaves, broader leaves and branching veins.

Dichotomous keys

A dichotomous key is a series of paired statements. Each pair splits the remaining organisms in two, and following the branches leads to a name.

Two rules make keys work in an exam:

• Use only visible features. "Lives in water" is not usable from a picture; "has four legs" is.
• Each pair must be a genuine either/or, so every organism follows exactly one branch.

Work through a key one numbered step at a time and do not skip ahead, because a key gives a wrong answer confidently if a step is missed.