The seven characteristics of life

All living organisms share seven characteristics. Remember them with MRS GREN:

• Movement — an action causing a change of position or place.
• Respiration — chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
• Sensitivity — the ability to detect and respond to changes in the internal or external environment.
• Growth — a permanent increase in size and dry mass.
• Reproduction — the processes that make more of the same kind of organism.
• Excretion — removal of the waste products of metabolism and substances in excess of requirements.
• Nutrition — the taking in of materials for energy, growth and development.

Watch out: growth is measured as increase in dry mass (not just size — a wilting plant can lose water and shrink yet still have grown). Excretion is getting rid of metabolic waste; it is not the same as egestion (removing undigested food).

Which characteristic?

Species & the binomial system

Organisms are classified into groups by the features they share. The smallest group is the species:

• A species is a group of organisms that can reproduce to produce fertile offspring.
• The binomial system is an internationally agreed way of naming species. Each scientific name has two parts: the genus then the species.

Supplement — why classify this way? Classification systems aim to reflect evolutionary relationships. The sequences of bases in DNA are used as a means of classification: organisms sharing a more recent ancestor have more similar DNA base sequences than those sharing only a distant ancestor.

Read the name

Dichotomous keys

A dichotomous key identifies an organism using a series of paired questions. Each step has two options ("di" = two) and each answer either names the organism or sends you to the next question.

Exam tip: good keys use observable, contrasting features (has/has not; more than/fewer than) — never vague ones like "big" or "pretty".

The five kingdoms

All organisms can be placed into one of five kingdoms, using their main features:

• Animal — multicellular; cells have no cell wall; feed on other organisms (nutrition by ingestion).
• Plant — multicellular; cells have cellulose cell walls and often chloroplasts; make food by photosynthesis.
• Fungus — often multicellular threads (hyphae); cell walls not made of cellulose; feed by saprotrophic nutrition (e.g. moulds, yeasts).
• Prokaryote — single-celled; no nucleus; DNA is a circular loop (e.g. bacteria).
• Protoctist — mostly single-celled with a nucleus; a "leftover" group (e.g. Amoeba, Chlorella).

Supplement — viruses: viruses are not placed in a kingdom. Their only features are a protein coat and genetic material inside; they are not cells.

Which kingdom?

Tap a feature, then tap the kingdom it best describes.

Vertebrates & arthropods

Within the animal kingdom, IGCSE focuses on two big groups.

Vertebrates (animals with a backbone) fall into five classes:

• Mammals — fur/hair, produce milk, external ears; e.g. dog.
• Birds — feathers, beak, lay hard-shelled eggs.
• Reptiles — dry scaly skin, lay rubbery-shelled eggs; e.g. lizard.
• Amphibians — moist permeable skin, lay jelly-covered eggs in water; e.g. frog.
• Fish — wet scales, gills, fins.

Arthropods (jointed legs, exoskeleton) fall into four groups:

• Insects — 6 legs, 3 body parts, usually wings (e.g. bee).
• Arachnids — 8 legs, 2 body parts (e.g. spider).
• Crustaceans — many legs, chalky exoskeleton, mostly aquatic (e.g. crab).
• Myriapods — many body segments, a pair (or two) of legs per segment (e.g. centipede).

Supplement — plant groups: within the plant kingdom, distinguish ferns from flowering plants (which split into monocotyledons and dicotyledons).

Your turn — identify by legs

Reading relatedness from DNA

Match the feature to its group

Tap a feature on the left, then its group on the right.

What makes a species?

The big ideas to know

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