(b) Variety of living organisms
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
1.2Eukaryotic organism featuresDescribe the common features of eukaryotic organisms: plants, animals, fungi and protoctists.Plants are multicellular; their cells contain chloroplasts, carry out photosynthesis and have cellulose cell walls; they store carbohydrate as starch or sucrose. Examples include cereals such as maize and herbaceous legumes such as peas or beans.Animals are multicellular; their cells lack chloroplasts and cell walls and cannot photosynthesise; they usually have nervous co-ordination, can move from place to place and often store carbohydrate as glycogen. Examples include mammals such as humans and insects such as houseflies and mosquitoes.Fungi cannot photosynthesise; their bodies are usually mycelia of thread-like, multinucleate hyphae, although some are single-celled; their cell walls contain chitin; they use saprotrophic nutrition by secreting digestive enzymes and absorbing the products, and may store glycogen. Examples include Mucor and yeast.Protoctists are microscopic single-celled organisms. Amoeba has animal-cell-like features, Chlorella has chloroplasts and plant-like features, and the pathogen Plasmodium causes malaria.1.3Prokaryotic organism featuresDescribe the common features of prokaryotic organisms such as bacteria.Bacteria are microscopic and single-celled. They have a cell wall, cell membrane, cytoplasm and plasmids, but no nucleus; their DNA forms a circular chromosome. Some photosynthesise, but most feed on living or dead organisms. Examples include rod-shaped Lactobacillus bulgaricus, used to make yoghurt, and spherical Pneumococcus, which causes pneumonia.1.4PathogensUnderstand that a pathogen causes disease and may be a fungus, bacterium, protoctist or virus.Viruses are non-living particles smaller than bacteria. They are parasites that reproduce only inside living cells and can infect every type of organism. They have no cellular structure, but have a protein coat and contain either DNA or RNA. Examples include tobacco mosaic virus, which prevents chloroplast formation and discolours tobacco leaves; influenza virus; and HIV, which causes AIDS.
Distinguish the four eukaryotic groups
Plants, animals, fungi and protoctists are eukaryotic groups, but their organisation, nutrition, cell coverings and carbohydrate stores distinguish them.
| Group | Organisation and nutrition | Cell covering and carbohydrate store | Syllabus examples |
|---|---|---|---|
| plants | multicellular; chloroplasts enable photosynthesis | cellulose cell walls; starch or sucrose | maize; peas or beans |
| animals | multicellular; no chloroplasts, so cannot photosynthesise; usually have nervous coordination and locomotion | no cell walls; often glycogen | humans; houseflies or mosquitoes |
| fungi | usually a mycelium of thread-like, multinucleate hyphae; some are single-celled; cannot photosynthesise | chitin cell walls; may store glycogen | Mucor; yeast |
| protoctists | microscopic and single-celled; features vary between examples | Amoeba is animal-cell-like; Chlorella has chloroplasts and is plant-like | Amoeba; Chlorella; Plasmodium |
A fungus feeds saprotrophically: it secretes digestive enzymes onto food, digestion occurs outside the fungus, and the soluble products are absorbed. This explains how a non-photosynthetic mycelium obtains nutrients without ingesting food.
A feature of one group is not universal to all eukaryotes. Yeast is a single-celled fungus, and protoctists do not all look or feed alike. Plasmodium is a pathogenic protoctist that causes malaria.
Recognise a prokaryotic bacterial cell
Bacteria are microscopic, single-celled prokaryotes: their genetic material is not enclosed in a nucleus.
| Feature | What is present in a bacterium |
|---|---|
| boundary | a cell wall outside a cell membrane |
| cell contents | cytoplasm, but no nucleus |
| main genetic material | one circular chromosome of DNA |
| additional genetic material | small DNA circles called plasmids |
| nutrition | some photosynthesise; most feed on living or dead organisms |
Lactobacillus bulgaricus is rod-shaped and is used to make yoghurt. Pneumococcus is spherical and is a pathogen that causes pneumonia. Shape or use can help identify an example, but the prokaryotic cell plan defines the group.
A bacterial cell is not empty because it lacks a nucleus: it still has cytoplasm, a cell membrane and DNA. A bacterial cell wall does not make the organism a plant, and a plasmid is additional DNA rather than the main circular chromosome.
Distinguish pathogens and viruses
A pathogen is an agent that causes disease. Pathogens may be fungi, bacteria, protoctists or viruses, but not every member of those groups is pathogenic.
| Type | Relevant distinction | Syllabus example |
|---|---|---|
| fungus | a living eukaryotic organism; some species cause disease | pathogen group named in the syllabus |
| bacterium | a living, single-celled prokaryote; some species cause disease | Pneumococcus causes pneumonia |
| protoctist | a living, single-celled eukaryote; some species are parasitic | Plasmodium causes malaria |
| virus | a non-living particle with no cellular structure | tobacco mosaic virus, influenza virus and HIV |
A virus is smaller than a bacterium. It has a protein coat containing one type of nucleic acid, either DNA or RNA, and can reproduce only inside living cells. It can infect every type of organism.
Tobacco mosaic virus prevents chloroplast formation and discolours tobacco leaves; influenza virus causes flu; HIV causes AIDS. A virus does not become a cell when it enters a host—the host cell supplies the living machinery needed for viral reproduction.