1b Variety of Living Organisms¶
Part of 1 The Nature and Variety of Living Organisms.
Organisms are grouped by their cell features, the way they obtain food and their body plan. The key distinctions are between eukaryotes, prokaryotes and viruses, and between the four main eukaryotic groups: plants, animals, fungi and protoctists.
What You Need to Learn¶
Further detail: Pearson Edexcel International GCSE Biology specification.
The features that identify plants, animals, fungi and protoctists, the structure of bacteria as prokaryotes, and the structure and behaviour of viruses. Pathogens from each group, and how they cause disease, are included.
Eukaryotic Groups¶
Eukaryotic cells keep their genetic material inside a nucleus and have membrane-bound organelles. The four groups differ in the features below.
| Feature | Plants | Animals | Fungi | Protoctists |
|---|---|---|---|---|
| Body | Multicellular | Multicellular | Hyphae forming a mycelium, or single cells (yeast) | Mostly single-celled |
| Cell wall | Cellulose | None | Chitin | Varies |
| Chloroplasts | Yes | No | No | Some (Chlorella) |
| Carbohydrate store | Starch or sucrose | Glycogen | Glycogen | Varies |
| Nutrition | Photosynthesis | Feed on other organisms | Saprotrophic | Varies |
| Examples | Maize, peas | Humans, flies | Mucor, yeast | Amoeba, Chlorella |
Plants make their own food. Chlorophyll in the chloroplasts absorbs light energy for photosynthesis, the cellulose wall gives structural strength, and a permanent vacuole holds cell sap and helps keep the cell rigid.
Animals have no wall and no chloroplasts, so they cannot photosynthesise and must feed on other organisms. Most co-ordinate their movement with a nervous system.
Fungi are built from thread-like hyphae, and the hyphae together form a mycelium. Each hypha contains many nuclei. Fungi feed saprotrophically: they secrete digestive enzymes onto their food, which breaks it into soluble molecules, and then absorb those molecules. Yeast is an unusual fungus because it is single-celled.
Protoctists are a mixed group of mostly microscopic, single-celled organisms that do not fit the other three. Amoeba lives in pond water and has animal-like cells. Chlorella has chloroplasts and behaves like a plant cell.
Exam technique
A question asking how fungi differ from plants wants specific features: a chitin wall and not a cellulose one, no chloroplasts, and nutrition by extracellular digestion and absorption. "Fungi are not plants" scores nothing by itself.
Prokaryotes¶
Prokaryotic cells have no nucleus and no membrane-bound organelles. Bacteria are the prokaryotes in this course. A bacterial cell has a cell wall, a cell membrane and cytoplasm. Its DNA is a circular chromosome lying free in the cytoplasm, and many bacteria also carry small extra loops of DNA called plasmids.
Bacteria are single-celled and very small, and they come in different shapes. Lactobacillus bulgaricus is rod-shaped and is used to make yoghurt. Pneumococcus is spherical and causes pneumonia. Most bacteria feed on living or dead organic matter, and some can photosynthesise.
Bacteria are structurally simpler than eukaryotic cells, even though they carry out the same life processes.
| Feature | Eukaryotic cell | Bacterial cell |
|---|---|---|
| Nucleus | Present | Absent |
| Membrane-bound organelles | Present | Absent |
| DNA | Linear chromosomes in the nucleus | Circular chromosome in the cytoplasm, plus plasmids |
| Typical size | Larger | Much smaller |
Pathogens and Viruses¶
A pathogen is a microorganism or other infectious agent that causes disease. Every group in this topic contains pathogens.
Viruses are much smaller than bacteria, and they have no cellular structure. Each consists of one type of nucleic acid, either DNA or RNA, inside a protein coat. For this reason they are not usually counted as living organisms. Viruses are obligate parasites: they can reproduce only inside a living host cell. A virus takes over the cell's machinery to make many copies of itself, and the cell then bursts and releases the new particles. They can infect every type of organism. Tobacco mosaic virus, influenza and HIV are the examples to know.
| Group | Example | How it causes disease |
|---|---|---|
| Bacteria | Salmonella (food poisoning) | Reproduce rapidly by binary fission, and often release toxins that damage host cells |
| Viruses | Influenza, HIV | Reproduce inside host cells, which burst |
| Protoctists | Plasmodium (malaria) | Parasites that live in animal hosts |
| Fungi | Athlete's foot | Spores spread through the air or by contact, and fungicides treat it |
Relative Size of Cells, Bacteria and Viruses¶
Typical eukaryotic cells are tens of micrometres across, bacteria are a few micrometres, and viruses are smaller again, measured in tens to hundreds of nanometres. The interactive below keeps that scale gap in view. Its deeper zoom levels show only how quickly size keeps shrinking below the cell. Open full interactive.
Worked example: placing an unknown organism
A single-celled organism has a cell wall, circular DNA lying free in the cytoplasm and no nucleus.
No nucleus means it is a prokaryote. A prokaryote with a cell wall and circular DNA is a bacterium. It is not a protoctist, which would have a nucleus, and it is not a virus, which has no cytoplasm or cell wall at all.
Common Confusions¶
- Fungi vs plants: Fungal cells can have walls like plant cells, but fungi do not photosynthesise and their walls are made of chitin and not cellulose.
- Viruses vs bacteria: Viruses are much smaller and have no cell structure. Antibiotics act on bacteria and have no effect on viruses.
- Yeast is a fungus: It is single-celled, but it is still a fungus and not a protoctist.
- Protoctists are not a type of plant, animal or fungus: They are a varied group that does not fit the other categories.
Check Yourself¶
- State two features that all eukaryotic cells have and bacterial cells lack.
- Give two ways in which fungi differ from plants.
- Describe the structure of a virus.
- Explain why a virus can reproduce only inside a living cell.
- A student is given a cell with a nucleus, a cellulose wall and chloroplasts. Which group does it belong to, and which feature rules out a fungus?
- A patient with a viral infection asks for antibiotics. Explain why they will not work.
Answers
- A nucleus enclosing the genetic material, and membrane-bound organelles.
- Any two of: fungal walls are made of chitin and plant walls of cellulose; fungi have no chloroplasts; fungi feed saprotrophically and plants photosynthesise; fungal bodies are made of hyphae.
- A single type of nucleic acid, either DNA or RNA, enclosed in a protein coat. There is no cell membrane, cytoplasm or other cell structure.
- A virus has no cell machinery of its own. It must enter a host cell and use that cell's machinery to make copies of itself.
- It is a plant cell. The chloroplasts rule out a fungus, which has none (and a cellulose wall also rules it out, since fungal walls are chitin).
- Antibiotics act on bacteria. A virus has no cell structure of its own for the antibiotic to target and reproduces inside the host's cells.
Key Terms¶
- Eukaryote: an organism whose cells contain genetic material enclosed in a nucleus.
- Prokaryote: an organism made of cells that do not contain a nucleus.
- Protoctist: a mostly single-celled eukaryotic organism that does not fit into the plant, animal or fungal groups.
- Hypha: a thread-like fungal filament containing many nuclei.
- Mycelium: the network of hyphae that forms the body of many fungi.
- Saprotrophic nutrition: feeding by extracellular digestion followed by absorption of the soluble products.
- Pathogen: an organism or agent that causes disease.
- Virus: a non-cellular infectious particle made of nucleic acid inside a protein coat.
- Plasmid: a small circular loop of DNA found in bacterial cells, separate from the main chromosome.
- Binary fission: asexual reproduction in bacteria in which a single cell divides to produce two identical cells.
- Chitin: the material from which fungal cell walls are made.