1. Characteristics and classification of living organisms

A shoot bending toward light shows sensitivity and movement, while its permanent increase in dry mass shows growth. An Amoeba engulfing a food particle shows nutrition and movement. Germinating seeds release energy by respiration even before green leaves develop.

Respiration is a set of chemical reactions in cells, not breathing. Excretion removes metabolic waste such as carbon dioxide or excess substances; egestion removes undigested food and is not one of the seven characteristics.

Plants carry out all seven characteristics. Plant movement may involve the growth or position of part of the plant rather than movement of the whole organism from place to place.

Choose features that are observable or measurable and apply them consistently. Place organisms together only when the stated shared feature is actually present; one superficial resemblance may be outweighed by several stronger differences.

Viruses could be grouped by shape, size, type of genetic material or host. Animals might first be grouped by a shared structural feature and then separated into smaller groups using further differences.

Both parts matter: the adults must be able to reproduce with one another, and their offspring must be fertile—able to reproduce in turn. Similar appearance alone does not establish that organisms belong to the same species.

A donkey and a zebra can produce a zedonk, but the zedonk is infertile. Producing an offspring is therefore not enough: the fertility condition shows that the parents are different species.

Members of one species need not look identical. Individuals can vary in colour, size, sex or age while still belonging to the same species.

Pan troglodytes belongs to the genus Pan. Panthera leo and Panthera tigris share the genus Panthera but have different complete binomial names and are different species.

After the genus has been written in full, it may be abbreviated where the meaning is unambiguous, for example Falco peregrinus then F. peregrinus.

To construct a key, choose one identifiable feature at each branch and write mutually exclusive alternatives, such as ‘three pairs of legs’ versus ‘more than three pairs of legs’. Keep both statements about the same feature and make every specimen follow exactly one route.

For a spider: ‘three pairs of legs’ is false, so follow ‘more than three pairs’; ‘four pairs of legs’ is true, so the key reaches the spider group. The conclusion follows from the whole route, not from guessing the picture.

On a classification or evolutionary tree, a branching point represents a common ancestor. Two groups whose branches meet at a more recent branching point share a more recent common ancestor and are more closely related.

Trace each pair of branches backwards until they meet. Compare the positions of those meeting points; the pair with the most recent meeting point is the closest relationship shown by the tree.

Classifications can change when new anatomical, biochemical or molecular evidence gives a better account of ancestry. The aim is not to preserve an old grouping but to represent the best-supported relationship.

DNA comparison can distinguish organisms that look similar and provides a large set of precisely measurable features. Reference sequences can also be used to identify an unknown specimen or strain.

For sequences ATGCC and ATACC, compare position by position: four bases match and one differs. Repeating this across a suitable sequence provides evidence that can be compared between several organisms.

After lineages separate, mutations can accumulate independently. A recent split leaves less time for differences to build up, so corresponding DNA sequences tend to remain more alike.

When given several sequences, align them, count differences for each requested pair, and rank the pairs. Use the smallest difference count to identify the closest pair; use the largest to identify the most distant pair.

A multicellular organism with cellulose cell walls and chloroplasts belongs to the plant kingdom. A multicellular organism whose cells lack walls and chloroplasts and which feeds on other organisms belongs to the animal kingdom.

An animal with an exoskeleton, three body regions and three pairs of jointed legs is an insect. An animal with a backbone, moist unscaled skin and eggs laid in water is an amphibian.

When evidence is mixed, discuss both support and contradiction. A single shared feature, such as the same number of teeth or the ability to swim, is weaker than a whole matching feature set.

First ask whether a nucleus is present. No nucleus indicates a prokaryote. With a nucleus, use multicellularity, cell-wall material, chloroplasts and feeding method to separate plants, animals, fungi and protoctists.

A unicellular organism with a nucleus and chloroplasts is a photosynthetic protoctist, not a prokaryote. A multicellular organism with non-cellulose walls that secretes digestive enzymes and absorbs products is a fungus.

For a visible specimen, leaf width and venation are often the fastest evidence: narrow parallel-veined leaves support monocot; broad branching-veined leaves support dicot. Fronds and spores support fern.

A plant with narrow leaves and parallel veins is classified as a flowering plant and, more specifically, a monocotyledon. A plant with fronds and spores but no flowers is a fern.

An answer should pair identity with evidence: ‘monocotyledon because its leaves are narrow and have parallel veins’, not just the group name.

In a simple virus diagram, an outer shell is the protein coat and the strand or material inside is the genetic material. Both are present in all viruses within the syllabus scope.

A virus is not classified as a prokaryote. A bacterial cell has cytoplasm, a cell membrane and other cellular structures; the syllabus virus model is limited to genetic material and a protein coat.