Eukaryotes have a nucleus; prokaryotes (bacteria) do not
Eukaryotes (plants, animals, fungi, protoctists) have a true nucleus; prokaryotes (bacteria) have none — their DNA is a circular chromosome free in the cytoplasm, plus small rings called plasmids. Signature features: plants photosynthesise and have cellulose cell walls; animals have no wall and no chloroplasts; fungi feed saprotrophically and have chitin walls; protoctists are mostly single-celled, some animal-like (Amoeba), some plant-like (Chlorella).
Viruses are acellular and need a host to reproduce
Viruses are not cells (acellular) and are much smaller than bacteria. A virus is only a protein coat around ONE type of nucleic acid — either DNA or RNA, never both. It has no cytoplasm, nucleus, ribosomes or means of releasing energy, so it cannot carry out any life process on its own; it reproduces only by infecting a living host cell and using its machinery. Named examples: tobacco mosaic virus (mottles leaves), influenza (flu) and HIV (causes AIDS).
A pathogen is a microorganism that causes disease
Pathogen = a microorganism that causes disease. The mark-scheme form has two elements: it is a microorganism (or virus particle) AND it causes disease — "a germ" is too vague. Pathogens come from four groups: fungi (athlete's foot), bacteria (Pneumococcus causing pneumonia), protoctists (Plasmodium causing malaria) and viruses (HIV causing AIDS). Being a pathogen describes what an organism does, not its kingdom: most bacteria and fungi are harmless or even useful.
Drawn from real examiner reports.
Pathogen needs both elements
The definition has two elements: a pathogen is (1) a microorganism (or virus particle) AND (2) it causes disease. "A germ", "bacteria in your body" or "something harmful" miss one or both and score zero. Do not restrict pathogens to bacteria — fungi, bacteria, protoctists and viruses can all be pathogens. Learn the exact wording; examiners penalise everyday paraphrases.
Bacteria have no nucleus
Bacteria are prokaryotes: they have no nucleus. Their genetic material is a single circular chromosome free in the cytoplasm, together with small extra rings of DNA called plasmids. Writing that a bacterium "has a nucleus", or that its DNA is enclosed in a nuclear membrane, is a marked error — that describes a eukaryote such as a plant, animal, fungus or protoctist.
Cellulose vs chitin vs no wall
Match the wall to the group: plant cell walls are cellulose, fungal cell walls are chitin, and animal cells have no cell wall at all. A frequent slip is labelling a fungal wall "cellulose", or giving an animal cell a wall. Bacteria do have a cell wall, but it is not made of cellulose — do not borrow the plant answer for them.
Not all protoctists are animal-like
Protoctists are a mixed group. Some are animal-like (e.g. Amoeba, which engulfs food); others contain chloroplasts and are plant-like (e.g. Chlorella); some are pathogens (e.g. Plasmodium, which causes malaria). Assuming every protoctist behaves like an animal, or that none can photosynthesise, loses marks in classification questions.
Plant virus is TMV, not Mucor
The named plant virus is the tobacco mosaic virus (TMV); it mottles/discolours the leaves, so there is less chlorophyll and less photosynthesis. A common wrong answer is Mucor — but Mucor is a fungus, not a virus. Learn the named examples exactly: naming the wrong type of organism scores zero even when it sounds plausible.
Jun 2023 Paper 1B: many candidates could not name the plant virus (TMV); a common wrong answer was Mucor, which is a fungus.
Virus vs bacterium: be structural
When a question asks for STRUCTURAL differences between a virus and a bacterium, give structural points, not functional ones. "Bacteria can respire but viruses cannot" is functional and scores nothing. Structural: a virus is only a protein coat around nucleic acid, with no cytoplasm and no cell wall; a bacterium has a cell wall, cytoplasm and plasmids, and no protein coat.
Run a group feature checklist
For "common features of group X" or "which group is this organism" questions, run a fixed checklist: single- or multicellular? cell wall (of what)? chloroplasts? nucleus? how does it feed? The list gives enough distinct features to score every mark.
Match describe vs explain
Read the command word. "Describe" wants the features listed accurately; "explain" wants a reason — e.g. "explain why a virus is not a cell" needs "it has no cytoplasm or organelles and cannot carry out life processes on its own", not just a list of parts.
Name the one decisive feature
For "which group is this organism" questions, quote the single feature that fixes the group rather than listing everything — e.g. "cell wall of chitin -> fungus" or "no nucleus -> bacterium". One decisive feature per organism is faster and avoids self-contradiction.
Living organisms are sorted into groups that share common features. The big split is between eukaryotes (cells with a nucleus) and prokaryotes (cells with no nucleus). Viruses sit outside this scheme because they are not cells at all.
Give the common features of PLANTS.
Maize is a cereal plant. Give three features that are common to all plants. (3 marks)
