IGCSE Biology Characteristics & classification notes

Anything that is alive does seven things. Examiners ask for the exact definitions, so learn each one as a sentence rather than a single word. The seven characteristics are:

Two of these cause the most trouble. Excretion must involve a waste product of metabolism — carbon dioxide from respiration and urea from the breakdown of excess amino acids are the classic examples. Passing out undigested food (faeces) is egestion, not excretion, because the food was never part of any cell’s chemical reactions. And growth is not just getting bigger: the definition insists on a permanent increase in size and dry mass, which is why a tree taking up water weighs more but has not necessarily grown until its dry mass increases.

In an unfamiliar context, work through MRS GREN one letter at a time and quote the phrase in the question back at the mark scheme. If a stem says "releases energy in its cells", the answer is respiration; if it says "responds to a change in light", the answer is sensitivity; if it says "the numbers of that organism increase", the answer is reproduction.

With millions of organisms on Earth, biologists sort them into groups by the features they share — this is classification. The smallest of these groups is the species: a group of organisms that can reproduce to produce fertile offspring. The fertility part matters. A horse and a donkey can mate and produce a mule, but mules are sterile, so horses and donkeys remain separate species.

Each species is given a scientific name using the binomial system — an internationally agreed system in which the name has two parts: the genus and the species. Humans are *Homo sapiens*: *Homo* is the genus (shared with extinct relatives) and *sapiens* is the species. Written conventions: the genus starts with a capital letter, the species name is all lower case, and both parts are italicised in print (or underlined by hand).

Supplement: the groups of a classification system are not arbitrary — they aim to reflect evolutionary relationships. Modern classification uses the sequences of bases in DNA: organisms that share a more recent common ancestor (are more closely related) have more similar base sequences than organisms that share only a distant ancestor. A protein’s amino acid sequence can be compared in the same way, because the sequence of amino acids is itself inherited from the DNA code.

A labelled diagram you should be able to picture: the classification hierarchy as an upside-down tree. The trunk is the kingdom; each fork narrows the group — kingdom, phylum, class, order, family, genus, species — so a species is one leaf at the tip and the genus is the small cluster of twigs it sits on.

At the top of the classification hierarchy sit the kingdoms. You must know the features used to separate animals from plants at Core, and Supplement adds the full five kingdoms: animals, plants, fungi, prokaryotes and protoctists.

The animal kingdom is divided first into animals with backbones (vertebrates) and animals without. The five vertebrate classes, and the features that place an animal in each, are:

Fish

Scales, gills for gas exchange, fins, eggs fertilised in water; cold-blooded. Example: salmon.

Amphibians

Moist, scaleless skin; eggs laid and fertilised in water; larvae (tadpoles) aquatic; cold-blooded. Example: frog.

Reptiles

Dry, scaly skin; leathery, shelled eggs laid on land; cold-blooded. Examples: snake, lizard.

Birds

Feathers, beak, two legs and two wings; hard-shelled eggs; warm-blooded. Example: eagle.

Mammals

Fur or hair; young fed on milk from mammary glands; mostly live young; warm-blooded. Example: bat.

The arthropods all share a hard exoskeleton, jointed legs and a segmented body, but the four groups are separated by their legs, antennae and body sections:

Supplement: within the plant kingdom, you need to separate ferns from the flowering plants. Ferns have no flowers or seeds and reproduce with spores; flowering plants produce seeds inside ovaries formed from flowers, and split into monocotyledons (one seed leaf, strap-shaped leaves with parallel veins, flower parts in multiples of 3 — grasses, lilies) and dicotyledons (two seed leaves, broad leaves with a network of veins, flower parts in multiples of 4 or 5 — beans, roses). A labelled leaf-vein comparison is a favourite diagram: draw two simple leaves, one with lines running side-by-side from base to tip, one with a branching net.

Supplement: viruses are not placed in any kingdom. They are not cells at all — a virus is a protein coat enclosing genetic material (DNA or RNA), far smaller than a bacterial cell, and it can only reproduce inside a living host cell. That minimal structure is exactly what the syllabus wants: protein coat + genetic material.

A dichotomous key identifies organisms step by step. *Dichotomous* means "cut in two": every step offers exactly two contrasting statements, and each statement sends you either to a named organism or to the next step. Keys are built from features you can see — number of legs, presence of wings, shape of leaves — never from microscopic details you would not have in the field.

Picture the key as a branching diagram: a single line at the top splits into two arrows; one arrow ends in a name, the other splits again. A textbook draws this as a tree; an exam answer writes it as numbered pairs.

1. 1Pick one feature

   Choose a feature that splits the set roughly in half — legs, antennae, wings, leaves, veins.

2. 2Write the two halves

   Each statement must be checkable in one look: "4 pairs of legs" vs "not 4 pairs of legs".

3. 3Repeat within each branch

   Keep splitting until every branch ends in exactly one organism.

4. 4Test the key

   Trace each organism from the start. If two end at the same name, add another step.

When using a key, read both statements at a step before moving, and follow only the one that is true. Mark schemes award for usable contrasting pairs and for each correct identification, so a key that works is worth the same whether or not it matches the examiner’s.