Learning objectives
State and define the seven characteristics of living organisms
Define a species and name organisms using the binomial system of genus and species
Explain why classification aims to reflect evolutionary relationships and how comparing DNA base sequences improves it (Extended)
Describe the features shared by all cells, the features used to place animals and plants in their kingdoms (all five kingdoms at Extended), the vertebrate classes and the four arthropod groups
Distinguish ferns from flowering plants and monocotyledons from dicotyledons, and describe the structure of a virus (Extended)
Construct and use a dichotomous key based on observable features
Key definitions
Species
A group of organisms that can reproduce to produce fertile offspring.
Binomial system
An internationally agreed system in which the scientific name of an organism is made up of two parts showing the genus and the species.
Dichotomous key
A series of paired contrasting statements about observable features, each pair leading either to another pair or to the name of an organism.
Virus
A particle consisting only of genetic material inside a protein coat; it is not made of cells and can reproduce only inside a host cell (Extended).
Explanation
All living organisms share seven characteristics, remembered as MRS GREN. Movement is an action by an organism or part of an organism causing a change of position or place. Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism. Sensitivity is the ability to detect and respond to changes in the internal or external environment. Growth is a permanent increase in size and dry mass. Reproduction is the processes that make more of the same kind of organism. Excretion is the removal of the waste products of metabolism and substances in excess of requirements. Nutrition is the taking in of materials for energy, growth and development. Examiners reward the precise wording: growth must be a permanent increase in dry mass, so a plant swelling with water has not grown, and excretion concerns metabolic waste such as carbon dioxide and urea, not undigested food, which leaves the body by egestion.
A species is a group of organisms that can reproduce to produce fertile offspring. A horse and a donkey can breed, but the mule they produce is infertile, so they remain separate species. Each species has a two-part scientific name under the binomial system: the genus first with a capital letter, then the species in lower case, both in italics or underlined when handwritten, as in Panthera leo (the lion) and Panthera tigris (the tiger), which share a genus because they are closely related. Classification groups organisms by shared features, and modern systems aim to reflect evolutionary relationships. At Extended you explain that comparing the base sequences of DNA gives a more accurate classification than comparing visible features: species with more similar sequences separated from a common ancestor more recently, because fewer mutations have built up since. Similar-looking organisms such as birds and bats may be only distantly related, so appearance alone can mislead.
The cells of every living organism contain cytoplasm, a cell membrane, DNA as the genetic material and ribosomes for making proteins. Extra features separate the five kingdoms (Core needs the animal and plant kingdoms; the fungus, prokaryote and protoctist kingdoms and viruses are Extended). Animals are multicellular, have no cell walls or chloroplasts and usually show nervous coordination. Plants are multicellular with cellulose cell walls and chloroplasts, so they photosynthesise. Fungi have cell walls made of chitin rather than cellulose, contain no chloroplasts, are usually built of thread-like hyphae and feed by secreting enzymes onto dead organic matter and absorbing the products (saprophytic nutrition). Prokaryotes, such as bacteria, are single cells with no nucleus; their DNA is a circular loop, often with extra plasmids. Protoctists are mostly single-celled organisms with a nucleus; some, such as Chlorella, have chloroplasts, while others, such as Amoeba, feed like animals. A virus belongs to none of these kingdoms because it is not a cell: it is only genetic material inside a protein coat and can reproduce only inside a host cell.
Within the animal kingdom, vertebrates are identified by external features: fish have scales, fins and gills; amphibians have moist skin and lay jelly-coated eggs in water; reptiles have dry scaly skin and lay leathery-shelled eggs on land; birds have feathers, a beak and hard-shelled eggs; mammals have hair or fur and feed their young on milk. Arthropods have jointed legs and an exoskeleton: insects have three body parts, six legs and one pair of antennae; arachnids have two body parts, eight legs and no antennae; crustaceans have more than four pairs of legs and two pairs of antennae; myriapods have many similar segments, each carrying legs. Among plants (Extended), ferns have fronds and reproduce by spores, whereas flowering plants reproduce by seeds made in flowers; monocotyledons have one seed leaf, parallel leaf veins and flower parts in threes, while dicotyledons have two seed leaves, branching veins and flower parts in fours or fives. A dichotomous key identifies an unknown organism through a series of paired contrasting statements about visible features.
Worked examples
q: Four insects are collected: A has wings and a striped body; B has wings and a plain body; C has no wings and long antennae; D has no wings and short antennae. Construct a dichotomous key to identify them.
steps: Choose one observable feature that splits the group into two: 1a Has wings → go to 2; 1b Has no wings → go to 3.
Split each pair with a second contrasting feature: 2a Body striped → A; 2b Body plain → B.
3a Antennae long → C; 3b Antennae short → D. Every organism now ends at a single name, and each statement uses a feature that can be seen rather than size or behaviour, which vary between individuals.
In context
Customs officers and wildlife surveys identify species from tiny samples such as a fin or a feather by comparing DNA base sequences with a database.
DNA comparison showed that whales are more closely related to hippos than to fish despite their fish-like shape, confirming their place among the mammals.
Common misconceptions
Confusing excretion with egestion: faeces are undigested food that was never part of metabolism, so removing them is egestion.
Writing the binomial name with both parts capitalised or in the wrong order; it is genus then species, and only the genus takes a capital letter.
Calling a spider an insect; spiders are arachnids with eight legs, two body parts and no antennae.
Treating viruses as living cells; they have no cytoplasm or ribosomes and belong to no kingdom.
Study tips
Learn the seven definitions word for word: mark schemes reward "permanent increase in size and dry mass", not just "getting bigger".
When constructing a key, use only observable, fixed features (number of legs, presence of wings) and make each pair of statements truly contrasting.
Evaluate the idea
Classification by visible features is quick and needs no laboratory, but DNA base-sequence comparison is more reliable because similar features can evolve separately in unrelated groups.
Reasoning
Common ancestor → similar DNA base sequences → similar proteins and features → placed in the same genus, class or kingdom
Summary
MRS GREN: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition, each with a precise definition.
A species produces fertile offspring; the binomial name is written Genus species.
Classification reflects evolutionary relationships; DNA base sequences give the most accurate picture (Extended).
Kingdoms, five vertebrate classes and four arthropod groups are told apart by fixed observable features; all five kingdoms, and viruses as non-cells, are Extended.
Check your understanding
Question 1: A permanent increase in size and dry mass is the definition of…
1. reproduction
2. growth
3. nutrition
Question 2: An arthropod with two body parts, eight legs and no antennae is…
1. an insect
2. an arachnid
3. a crustacean