Consider a wilted lettuce leaf placed in water. Within an hour it becomes firm and noticeably larger. Its fresh mass has risen, but nothing has been built — only water has entered the vacuoles. Dry the leaf in an oven to remove all water and its mass is unchanged. Because there is no permanent increase in dry mass, this is not growth.

Contrast this with a germinating seedling. As it photosynthesises it converts carbon dioxide and water into new carbohydrate, protein and lipid. Oven-dry it and the mass has genuinely risen. That is growth. This is exactly why the syllabus wording is permanent increase in size and dry mass — the second half of the phrase does the real work, and dropping it loses the mark.

The Cambridge and AQA syllabuses summarise living organisms using the seven characteristics above (MRS GREN). The Edexcel specification lists an eighth characteristic alongside them: living organisms control their internal conditions.

A mammal that keeps its core body temperature close to 37°C whether the air around it is cold or hot is showing this characteristic; so is any cell that keeps its internal chemistry stable enough for its enzymes to keep working. The detailed mechanisms — kidneys, temperature control in the skin, and control of blood glucose by insulin and glucagon — are developed fully in the Homeostasis chapter later in the course. Here, the point is narrower: for Edexcel, controlling internal conditions is recognised as one of the basic characteristics that define a living organism, alongside nutrition, respiration, excretion, sensitivity, movement, reproduction and growth.

The mule is the trap in this table. An individual mule cannot reproduce, yet it is unquestionably alive. The characteristic of reproduction applies to the kind of organism, not to every individual: worker bees, mules and post-reproductive humans are all living. Do not use an individual's infertility as an argument that it is not alive — but do use it as evidence about species boundaries, which is the subject of the next section.

Common misconception: that respiration is the movement of air in and out of the lungs.

Breathing (properly called ventilation) is a physical process that moves air; respiration is a set of chemical reactions inside cells that releases energy from nutrient molecules. Plants respire without breathing. A single-celled organism respires without any breathing apparatus at all. In a definition question, the words chemical reactions, in cells and release energy are what earn the mark.

Domestic dogs range from a chihuahua to a great dane, differing enormously in size, shape and coat. They are nevertheless a single species, Canis lupus familiaris, because any two of them can in principle produce fertile puppies. Conversely, two moths that look identical to the eye may belong to separate species if they cannot interbreed successfully.

The lesson for exam answers: reproductive evidence outweighs visual evidence. If a question gives you breeding data, use it. Shared habitat is not evidence of the same species — a pond contains many species — and geographical separation does not by itself create a new one.

You are not examined on resolving these cases, but knowing that the definition has limits is what distinguishes a strong answer on why classification uses DNA evidence as well as breeding evidence.

Question. Four big cats are named Panthera leo (lion), Panthera tigris (tiger), Panthera pardus (leopard) and Acinonyx jubatus (cheetah). Which is the odd one out, and why?

Notice what the name did not tell you: nothing about habitat, size or colour. A binomial records a classification decision — it does not by itself prove an evolutionary relationship, which requires the evidence discussed in Section 7.

Common misconception: that sapiens is the name of our species.

The species is Homo sapiens — both words together. The second word is not unique: Bellis perennis (daisy) and Lolium perenne (ryegrass) share a very similar second element while being entirely unrelated. Always quote both words.

The hierarchy is not merely filing. Because each group is contained entirely within the group above it, knowing an organism's position tells you everything about it that is true of every larger group. Identify an organism as a mammal and you have already established that it is a chordate, an animal, and a eukaryote — without a single further observation. This is what makes classification a predictive tool rather than a catalogue.

Every organism's characteristics are encoded in the sequence of bases in its DNA. When a species splits into two, each descendant line accumulates changes to its base sequence independently. The longer two lines have been separate, the more differences build up.

This gives a direct, quantitative test of relatedness: the more similar the base sequences of two organisms, the more recent their common ancestor. Unlike a judgement about body shape, it produces a number.

Question. A section of the same gene is sequenced in four species. Using species A as the reference, the number of differing bases is: $B=2,C=9,D=24$. Place B, C and D in order of how closely each is related to A, and justify your answer.

Mark-scheme note. The justification must refer to a more recent common ancestor. Writing only 'B has more similar DNA' restates the data without explaining it and does not gain the reasoning mark.

Common misconception: that similar DNA means humans evolved from chimpanzees.

Similar base sequences indicate a shared ancestor, not descent from one another. Humans and chimpanzees both descend from a common ancestral species that is now extinct and was neither a human nor a chimpanzee. Both lines have been changing for the same length of time since the split.

Protoctists are best understood as the kingdom defined by exclusion. If an organism is a eukaryote and is clearly not an animal, plant or fungus, it is a protoctist.

Common misconception: that fungi are plants because they are rooted in place and are sold as vegetables.

Fungi have no chlorophyll and cannot photosynthesise, so they are not autotrophic. Their cell walls are made of chitin, not cellulose. They feed saprotrophically, secreting digestive enzymes onto their food and absorbing the soluble products — digestion happens outside the organism. Both the wall material and the mode of nutrition place them in a separate kingdom.

Edexcel asks for the same diagnostic features as above, plus a characteristic body plan, a storage carbohydrate and named examples for each group. It also asks for the term that links several of the kingdoms together: pathogen.

Viruses are named examples too, and Edexcel expects you to link each one to the disease or effect it causes: the tobacco mosaic virus infects tobacco plants and discolours their leaves by preventing chloroplasts from forming; the influenza virus causes 'flu'; and HIV is the virus that can lead to AIDS. All three follow the general viral pattern from the section below — no cellular structure, a protein coat, and one type of nucleic acid (DNA or RNA) — and can only reproduce inside a living host cell.

Question. An animal has a backbone, dry scaly skin, breathes using lungs and lays soft-shelled eggs on land. Its body temperature changes with its surroundings. Identify its group and give two features that rule out the alternatives.

Common misconception: that any large animal living in the sea is a fish.

Whales and dolphins have lungs, not gills; hair rather than scales; give birth to live young; and feed them on milk. Every diagnostic feature places them with the mammals. Their fish-like shape is an adaptation to moving through water that evolved independently — a resemblance of function, not of ancestry.

In a photograph, count legs first — it separates all four groups on its own in most cases. Two cautions: some legs may be hidden beneath the body, so count on both sides and double the visible number if the image is symmetrical; and an insect's first pair of legs is sometimes held forward and mistaken for antennae. Antennae are thin, unjointed at the tip and used for sensing, never for walking.

Common misconception: that all small crawling arthropods are insects.

A spider has 8 legs, 2 body parts and no antennae. An insect has 6 legs, 3 body parts and 1 pair of antennae. Both are arthropods, but they belong to different groups within it — arachnids and insects respectively.

You will rarely be shown a seed cut open, so use the features that travel with it. Parallel leaf veins and fibrous roots accompany a single cotyledon; a network of veins and a tap root accompany two. Grasses and cereals are the monocotyledons you are most likely to meet; most broad-leaved garden plants and trees are dicotyledons.

Viruses sit precisely on the boundary of the definition of life, which is what makes them a good test of whether you understand it. They possess genetic material and they reproduce — two of the seven characteristics — but they possess no metabolism, and both of those characteristics are exercised only by hijacking a genuinely living cell. Because the seven characteristics are not all satisfied by the virus itself, it falls outside the definition, and therefore outside the classification system built on it.

Question. Use the key below to identify a leaf that is simple, has a smooth edge and is broader than it is long.

Method note. Record the route you took (1 → 3 → 4) as you go. If you reach a name that plainly does not match the specimen, the error is almost always a single wrong turn, and a written route lets you find it without starting again.

Question. Construct a dichotomous key to separate a beetle, a spider, a crab and a snail.

Common misconception: that a key works as long as it asks about differences between the organisms.

A key fails if a step's alternatives are not exhaustive. 'Is it red or is it blue?' leaves a green organism with nowhere to go, and 'does it have wings or 6 legs?' sends a winged 6-legged insect down two routes at once. Each step must send every remaining organism down exactly one branch.

Classification systems are models of relationships, not permanent labels chosen by appearance alone. Visible features remain useful for identifying organisms, but they can be misleading when unrelated organisms face similar environmental pressures. A whale and a fish both have streamlined bodies because moving through water selects for a similar shape; DNA evidence shows that a whale is much more closely related to other mammals.

Scientists therefore compare several kinds of evidence. Similar DNA base sequences, shared structural features and similar development can all support a close evolutionary relationship. When new evidence is found, a classification may be refined. This is a strength of science: the model is updated to fit better evidence.

A dichotomous key uses the same evidence-based habit on a smaller scale. Each paired choice must be observable, mutually exclusive and lead to the next useful distinction. Avoid choices such as ‘large’ unless a measurement or clear comparison is provided; a key works only when two people examining the same organism can reach the same outcome.

Question. A student claims that a car is alive because it moves, uses fuel and gives out waste gases. Explain why a car is not a living organism.

Model answer. A living organism must show all seven characteristics. A car does not grow, because it shows no permanent increase in size and dry mass. It does not reproduce, because it cannot make more of its own kind. It does not respire, because it has no cells in which chemical reactions release energy from nutrient molecules — burning fuel is a combustion reaction, not respiration. Its movement and waste output are produced by an engine, not by metabolism.

Marking. One mark for each characteristic correctly identified as absent with a reason (max 3), and one mark for the principle that all seven must be shown. Simply listing 'it does not grow or reproduce' without reasons scores 2 of 4.

Question. An organism is multicellular, has cell walls made of chitin, has no chlorophyll, and digests dead leaves by secreting enzymes onto them and absorbing the products. (a) Name its kingdom. (b) Give two features from the description that support your answer. (c) Name the type of nutrition shown. (d) Explain why it is not classified as a plant.

Marking. (a) 1 mark. (b) 1 mark per correct feature, max 2. (c) 1 mark. (d) 1 mark. Note that part (d) requires a comparison with plants — answering 'because it is a fungus' is circular and scores zero.

Question. Scientists compare a gene shared by four bird species. The percentage of bases identical to species W is: $X=98$%, $Y=91$%, $Z=74$%. (a) Which species is most closely related to W? (b) Explain how the data support your answer. (c) State one advantage of using DNA evidence rather than physical features.

Marking. (a) 1 mark. (b) 1 mark for quoting the figure, 1 mark for linking greater similarity to a more recent common ancestor. (c) 1 mark for either advantage.

Question. Construct a dichotomous key to separate four leaves: A is needle-shaped; B is broad with a toothed edge; C is broad with a smooth edge and one main vein; D is broad with a smooth edge and several main veins.

Marking. 1 mark for each step with two genuinely opposite alternatives, and 1 mark for a key in which all four leaves are correctly identified by exactly one route. Four organisms, three steps — the count checks out.