1 The nature and variety of living organisms
- Syllabus
- 2024
- Section
- 1
- Level
- —
Exam analysis
Most tested topics
(a) Characteristics of living organisms
- Syllabus
- 2024
- Topic
- —
- Level
- —
Recognise the shared characteristics of living organisms
Living organisms carry out a shared set of life processes. No single process is enough on its own; the complete pattern distinguishes living activity.
| Characteristic | What it means | Observable example |
|---|---|---|
| nutrition | obtaining materials needed for energy, growth and repair | a plant makes sugars; an animal feeds |
| respiration | releasing usable energy from nutrients in cells | energy supports muscle contraction or active transport |
| excretion | removing waste products made by metabolism | carbon dioxide is removed from an animal |
| response | detecting and reacting to a change in the surroundings | a shoot grows towards light |
| movement | changing position of the whole organism or part of it | an animal walks; a plant shoot bends |
| control of internal conditions | keeping the internal environment within workable limits | body temperature is regulated |
| reproduction | producing new organisms | cells or organisms form offspring |
| growth and development | permanent increase in size plus changes towards a mature form | a seedling increases in mass and forms new tissues |
The processes support one another: nutrition supplies raw materials, respiration releases energy, growth builds new material, and internal control keeps cellular conditions suitable for these reactions.
Respiration is a chemical process in cells, not simply breathing. Excretion removes metabolic waste; removal of undigested food is egestion. Movement does not require an organism to travel from place to place, because part of an organism can move or grow in a new direction.
(b) Variety of living organisms
- Syllabus
- 2024
- Topic
- —
- Level
- —
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.