MRS GREN: but with the definitions that score
Naming the seven characteristics gets you nothing at IGCSE. The marks are in the definitions, several of which have trap wording:
- Movement: an action by an organism or part of an organism causing a change of position or place.
- Respiration: the chemical reactions in cells that break down nutrient molecules and release energy. Never write 'breathing'. That is gas exchange, and the mark scheme explicitly rejects it.
- Sensitivity: the ability to detect and respond to changes in the internal or external environment (stimuli).
- Growth: a permanent increase in size and dry mass. 'Permanent' and 'dry mass' are both paying words: a plant cell swelling with water is not growth.
- Reproduction: the processes that make more of the same kind of organism.
- Excretion: the removal of the waste products of metabolism and substances in excess of requirements. Egestion (removing undigested food) is NOT excretion. Confusing the two is one of the most reliably penalised errors in the whole syllabus.
- Nutrition: the taking in of materials for energy, growth and development.
Binomial naming: two rules, one mark, frequently lost
The binomial system, defined as an internationally agreed system in which the scientific name of an organism is made up of two parts: the genus and the species. The formatting rules are marked strictly:
- Genus starts with a capital letter; species is all lower case: Homo sapiens, never 'homo Sapiens' or 'Homo Sapiens'.
- In print it is italic; handwritten, it should be underlined.
Examiners also test the logic: two organisms in the same genus are more closely related than two organisms merely in the same family. The modern justification for classification is that it reflects evolutionary relationships, and at Extended level the paying point is that DNA base sequences are used. Organisms with more similar base sequences are more closely related.
The five kingdoms and the features that distinguish them
You need the distinguishing features, stated as contrasts:
| Kingdom | Features that score |
|---|---|
| Animal | Multicellular; no cell walls; no chlorophyll; feed on organic substances |
| Plant | Multicellular; cellulose cell walls; photosynthesise using chlorophyll |
| Fungus | Cell walls not made of cellulose; do not photosynthesise; feed by saprophytic or parasitic nutrition; usually multicellular (hyphae). Yeast is the unicellular exception |
| Prokaryote | Unicellular; no nucleus (circular DNA, plasmids); cell wall not cellulose |
| Protoctist | The 'everything else' kingdom. Mostly unicellular with a nucleus; some photosynthesise (e.g. Chlorella), some feed like animals (e.g. Amoeba) |
Viruses sit outside all five kingdoms: the paying statement is that they are not living because they can only replicate inside host cells and carry out none of the seven life processes themselves. Their structure is just a protein coat and genetic material. Two marks, fixed wording.
Dichotomous keys: a procedure worth 3-4 marks every time
Key questions are pure technique. The method: start at statement 1 for EVERY organism, follow only the branch that matches, and record the route. When asked to construct a key, use paired contrasting statements about visible features (leaf shape, number of legs, wings present/absent). Never behaviour, colour that varies, or size words like 'big', which examiners reject as not reliably observable. Each numbered pair must split the remaining organisms into two groups until every organism is identified alone. Practise two or three from past papers and this is a guaranteed bank of marks. The question format never changes.