Four eukaryotic groups all have a nucleus
All four eukaryotic groups have a true nucleus. PLANTS: multicellular; chloroplasts; cellulose wall; store starch/sucrose (maize, peas). ANIMALS: multicellular; no chloroplasts; no cell wall; store glycogen (humans, insects). FUNGI: mycelium of hyphae; chitin wall; no photosynthesis; saprotrophic nutrition; store glycogen (Mucor, yeast). PROTOCTISTS: single-celled — Amoeba (animal-like), Chlorella (plant-like, chloroplasts); Plasmodium causes malaria.
Bacteria (prokaryotes) have no true nucleus
Bacteria are the prokaryotes at 4BI1. What sets them apart from eukaryotes: NO membrane-bound nucleus — the genetic material is a single circular chromosome of DNA free in the cytoplasm; plasmids (small rings of DNA) are also present; a cell wall (not cellulose or chitin), a cell membrane and cytoplasm. Named examples: Lactobacillus bulgaricus (rod-shaped; converts lactose to lactic acid in yoghurt) and Pneumococcus (spherical; the pathogen causing pneumonia).
Viruses are non-living particles, not cells
Statement 1.4 has two parts. PATHOGENS cause disease and include fungi, bacteria (Pneumococcus — pneumonia), protoctists (Plasmodium — malaria) and viruses (HIV — AIDS). VIRUSES are NOT living — they are particles, not cells, with only (1) a protein coat and (2) ONE type of nucleic acid, DNA or RNA (never both); no nucleus, cytoplasm or membrane. They reproduce only inside a host cell. Named viruses: tobacco mosaic virus (leaf discolouration), influenza (flu) and HIV (AIDS).
Drawn from real examiner reports.
Viruses are not living cells
The spec states explicitly that viruses are non-living PARTICLES, not cells. Do not call a virus a 'microorganism' or say it is 'alive'. It has no cellular structure — only a protein coat and one type of nucleic acid. Because it performs none of the characteristics of life independently, antibiotics cannot kill it.
Bacteria have no nucleus
The prokaryote/eukaryote difference is whether the genetic material sits in a nucleus. Bacteria (prokaryotes) have NO nucleus — their single circular chromosome of DNA lies free in the cytoplasm, with plasmids. Saying 'bacteria have a small nucleus' is wrong: they have none at all. Eukaryotes (plants, animals, fungi, protoctists) do have a true nucleus.
Chitin vs cellulose cell walls
Cell-wall materials are routinely swapped. Plant cell walls are made of CELLULOSE; fungal cell walls are made of CHITIN. Bacteria have a wall too, but it is neither cellulose nor chitin (its composition is not required at 4BI1). Animals have NO cell wall. Writing 'fungi have cellulose walls' or 'plants have chitin' loses the mark.
Glycogen, not starch, in animals and fungi
Carbohydrate stores are easily confused. Plants store carbohydrate as STARCH (or sucrose); animals AND fungi both store it as GLYCOGEN. Writing 'fungi store starch' or 'animals store starch' is wrong. Only plants (and some protoctists such as Chlorella) photosynthesise, because only they have chloroplasts; fungi never do.
A virus has DNA or RNA, not both
A virus contains ONE type of nucleic acid — either DNA OR RNA, never both. Writing 'viruses contain DNA and RNA' is a marked error. Its only other structural feature is a protein coat; there is no nucleus, membrane, cytoplasm or cell wall. For example, influenza and HIV are both RNA viruses.
Not every microbe is a pathogen
A pathogen is any agent that causes disease — it can be a fungus, bacterium, protoctist or virus. But not every member of those groups is a pathogen: Mucor is a saprotrophic fungus, and Lactobacillus bulgaricus is a beneficial bacterium used to make yoghurt. Do not assume 'bacterium' or 'fungus' automatically means harmful.
Build a five-group comparison table
For classification questions, build a table with five rows — plants, animals, fungi, protoctists, bacteria — and columns for nucleus, cell wall, photosynthesis, cell number and carbohydrate store. Filling each cell stops you swapping group terms like chitin and cellulose.
Answer virus questions with two features
For a virus question, do not use the cell-group table — give the two-element list instead: (1) a protein coat and (2) one type of nucleic acid, DNA or RNA. Add 'non-living particle, no cellular structure' if the question asks why a virus is not a living organism.
Learn the named spec examples
Many marks come from naming spec organisms. Learn: maize, peas/beans (plants); humans, insects (animals); Mucor, yeast (fungi); Amoeba, Chlorella, Plasmodium (protoctists); Lactobacillus bulgaricus, Pneumococcus (bacteria); tobacco mosaic virus, influenza, HIV (viruses).
The living world is divided into eukaryotes (cells with a true nucleus) and prokaryotes (cells without a true nucleus). Viruses are neither — they are non-living particles.
| Feature | Plants | Animals | Fungi | Protoctists |
|---|---|---|---|---|
| True nucleus | Yes | Yes | Yes | Yes |
| Cell wall | Yes — cellulose | No | Yes — chitin | Varies |
| Chloroplasts / photosynthesis | Yes | No | No | Some (e.g. Chlorella) |
| Multicellular or unicellular | Multicellular | Multicellular | Usually multicellular (hyphae); yeast is unicellular | Unicellular |
| Carbohydrate store | Starch or sucrose | Glycogen | Glycogen | Varies |
| Named spec examples | Maize; peas or beans | Humans (mammals); housefly and mosquito (insects) | Mucor; yeast | Amoeba; Chlorella; Plasmodium |
Plants are multicellular; their cells contain chloroplasts (enabling photosynthesis) and have cellulose cell walls; they store carbohydrate as starch or sucrose.
Animals are multicellular; they have no chloroplasts and no cell walls; they usually have nervous co-ordination and can move from place to place; they store carbohydrate as glycogen.
Fungi cannot photosynthesise; their body is usually a mycelium of thread-like hyphae that contain many nuclei; their cell walls are made of chitin; they feed by saprotrophic nutrition — they secrete digestive enzymes outside their body onto food material and then absorb the soluble products; they store carbohydrate as glycogen. Note: both animals and fungi store glycogen.
Protoctists are microscopic and unicellular. Some (e.g. Amoeba) resemble animal cells; others (e.g. Chlorella) have chloroplasts and resemble plant cells. Plasmodium is a protoctist pathogen that causes malaria.
Bacteria are prokaryotic: their cells have no membrane-bound nucleus. The genetic material is a single circular chromosome of deoxyribonucleic acid (DNA) that lies free in the cytoplasm. Bacteria also contain plasmids (small circles of DNA). They have a cell wall, a cell membrane and cytoplasm.
| Feature | Bacteria (prokaryote) | Eukaryote (e.g. plant or animal) |
|---|---|---|
| Nucleus | No true nucleus | True membrane-bound nucleus |
| DNA arrangement | Circular chromosome, free in cytoplasm; plasmids also present | Linear chromosomes inside nucleus |
| Cell wall | Present (not cellulose or chitin) | Present in plants (cellulose) and fungi (chitin); absent in animals |
Named spec examples: Lactobacillus bulgaricus (rod-shaped; used to convert lactose to lactic acid in yoghurt production) and Pneumococcus (spherical; pathogen causing pneumonia).
Viruses are not living organisms — they do not carry out the characteristics of life independently. They are particles, not cells, and have no cellular structure.
Structure of a virus:
Viruses can only reproduce by invading a living host cell and using the host's cellular machinery.
Named spec examples and what they cause:
| Virus | Disease caused | Notes |
|---|---|---|
| Tobacco mosaic virus (TMV) | Discolouration of tobacco plant leaves | Prevents chloroplast formation in leaves |
| Influenza virus | Influenza ('flu) | RNA virus |
| HIV (human immunodeficiency virus) | AIDS | RNA virus; attacks the immune system |
A pathogen is an organism or agent that causes disease. Pathogens include:
State TWO structural features of a virus.
Bacteria are prokaryotic organisms.
State three features of a bacterial cell that distinguish it from a eukaryotic cell. (3 marks)
