1.2The Nature and Variety of Living Organisms

Variety of living organisms — Edexcel Modular - OL Revision Notes & Topical Questions

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Lesson summary

Variety of living organisms Edexcel O Level Modular Biology notes: describe the common features shown by eukaryotic organismsA pathogen is a microorganism that causes disease.Bacteria contain plasmids and a circular chromosome.describe the common features shown by eukaryotic orga…

Variety of living organisms Edexcel O Level Modular Biology: what to do next
Prokaryote
Protoctists
Give / State / Name
Evaluate / Assess
Keywords the mark scheme looks for in this lesson

Section 1 of 19Process & sequence

Organism
Nucleus?
Prokaryote or eukaryote
Kingdom features

Pathogens

Fungi – athlete’s foot

Bacteria – Pneumococcus

Protoctist – Plasmodium (malaria)

Virus – HIV, influenza

Section 2 of 19Keep going

Variety of living organisms Edexcel O Level Modular Biology: what to do next

These Variety of living organisms Edexcel O Level Modular Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Variety of living organisms Edexcel O Level Modular Biology understanding with the practice questions below, then reread any Variety of living organisms Edexcel O Level Modular Biology point you were unsure about before moving on.

Practice: Variety of living organisms Edexcel O Level Modular Biology practice questions

Previous lesson: 1.1 Characteristics of living organisms — Notes

Next lesson: 2.1 Level of organisation — Notes

Course: Edexcel Modular - OL — all units

Official syllabus: Pearson Edexcel International GCSEs

Section 3 of 19Fungi

Fungi are eukaryotes with a distinct, membrane-bound nucleus.

They have cell walls (made of chitin, not cellulose), do not photosynthesise, and do not have nervous coordination.

Saprobionts

Most feed on other living or dead organisms; those that feed on dead organic matter are called saprobionts or decomposers.

They are usually multicellular (for example Mucor) but some, like yeast, are single-celled.

Hyphae

Multicellular fungi are made of thread-like hyphae containing many nuclei, organised into a network called a mycelium.

Some fungi store carbohydrate as glycogen.

Examples: moulds, mushrooms and yeasts.

Section 4 of 19Protoctists

The protoctists are a very diverse group, with features different from animals, plants and fungi.

They are mostly single-celled eukaryotes.

Examples include amoeba, Paramecium, Plasmodium and Chlorella.

Chlorella contains chloroplasts and photosynthesises; amoeba feeds by engulfing food; Plasmodium is a parasite.

You should be able to recognise, draw and interpret images of cells, so practise drawing and labelling fungal and protoctist cells.

Section 5 of 19Bacteria (prokaryotes)

Prokaryotes differ from the other four groups, which are all eukaryotes. Bacteria are single-celled, have no nucleus, and are very small. Features: cell wall, cell membrane, cytoplasm, plasmids and a circular chromosome of DNA. Most feed on other living or dead organisms. Some photosynthesise. Reproduction is asexual by binary fission, producing exact copies of the parent cell.

Section 6 of 19Viruses

Viruses are small particles, always smaller than bacteria. They are parasitic and can reproduce only inside living cells. They infect every type of living organism. They have no cellular structure but a protein coat and contain one type of nucleic acid, DNA or RNA.

Section 7 of 19Pathogens

A pathogen is an organism that causes disease in another organism. Groups that include pathogens: bacteria, fungi, protoctists and viruses. Example: Pneumococcus is a spherical bacterium that causes pneumonia. Bacterial pathogens do not always infect cells; they may remain in body cavities.

Pathogen	Group	Disease
Pneumococcus	Bacterium	Pneumonia
Plasmodium	Protoctist	Malaria
HIV / influenza virus	Virus	AIDS / flu
Some fungi	Fungus	Plant diseases, athlete's foot

Section 8 of 19Comparison table of the four non-plant, non-animal groups

Feature	Fungi	Protoctists	Bacteria	Viruses
Cells?	Yes, eukaryotic	Yes, eukaryotic	Yes, prokaryotic	No
Nucleus	Yes	Yes	No	No
Number of cells	Single or many	Mostly single	Single	None
Reproduction	Spores, budding	Binary fission and others	Binary fission	Only inside host cells
Examples	Mucor, yeast	Amoeba, Chlorella	Pneumococcus	Influenza, HIV, tobacco mosaic virus

Fungi digest food outside their cells by secreting enzymes (extracellular digestion) and absorb the soluble products, which is why they are good decomposers in soil. Antibiotics kill bacteria but have no effect on viruses, because viruses have no cell structure and use the host cell's own machinery.

Applying the groups to examples

Use the key features to place an unfamiliar organism.

A single-celled organism with a nucleus and chloroplasts that lives in a pond is a protoctist such as Chlorella.

A mould growing on bread with white threads and black spore cases is a fungus (Mucor).

A rod-shaped organism with no nucleus and a cell wall is a bacterium.

A particle that infects tobacco leaves and has a protein coat but no cell structure is a virus.

For disease: malaria is passed by mosquito bites carrying the protoctist, whereas pneumonia and many food-poisoning illnesses are bacterial.

Section 9 of 19Compare

Groups of living organisms
Group	Cells	Cell wall	Nutrition	Storage	Example
Plants	Multicellular	Cellulose	Photosynthesis	Starch, sucrose	Maize, pea
Animals	Multicellular	None	Feed on organisms	Glycogen	Human, housefly
Fungi	Mostly multicellular (yeast single)	Chitin	Saprotrophic	Glycogen	Mucor, yeast
Bacteria	Single (prokaryote)	Yes (not cellulose)	Some photosynthesise	—	Lactobacillus, Pneumococcus
Protoctists	Mostly single	Some	Varies	—	Amoeba, Chlorella
Viruses	Not cells	Protein coat	Parasitic	—	Influenza, HIV, TMV

Section 10 of 19Worked examples

Describe the structure of a multicellular fungus. [2]2 marks
Name two examples of protoctists. [2]2 marks
State three features of viruses. [3]3 marks
What is a saprobiont? [1]1 mark

Section 11 of 19Pathogens and disease

A pathogen is an infectious agent that causes disease in a host. Pathogens include certain bacteria, fungi and protoctists, as well as viruses. Most members of these groups are not necessarily disease-causing; the term pathogen applies to those that cause disease.

Examples:

Examples Pneumococcus is a bacterium associated with pneumonia; some fungi cause plant diseases; the protoctist Plasmodium causes malaria and is transmitted by mosquitoes. Match each named disease to its pathogen group rather than assuming all infections are bacterial.

Section 12 of 19Why viruses are different

Why viruses are different

Viruses are much smaller than bacteria and are not cells. A virus contains genetic material (DNA or RNA) enclosed in a protein coat. It cannot carry out the full set of life processes independently and can reproduce only after entering a living host cell.

Antibiotics target bacterial structures or processes. They do not kill viruses, so identifying the type of pathogen matters when discussing treatment and control.

Section 13 of 19Tobacco mosaic virus

Tobacco mosaic virus

Tobacco mosaic virus (TMV) infects plants, including some crop species. Infected leaves develop a patchy mosaic pattern because chloroplast function is disrupted. With less effective photosynthesis, growth and crop yield can fall.

TMV spreads through contact with infected plants or contaminated material and can persist in the growing environment. Good field hygiene and resistant crop varieties help limit spread; there is no simple cure for an infected plant.

Section 14 of 19HIV and influenza

HIV and influenza

HIV infects cells of the immune system.

An early flu-like illness may be followed by a long period with few obvious symptoms.
If the immune system becomes severely damaged, the late stage is called AIDS and the person becomes vulnerable to other infections.
HIV can pass through sexual contact, contaminated blood or shared needles, and from mother to child.
Antiretroviral treatment can control viral replication and greatly slow progression.

Influenza virus infects cells lining the airways and causes a contagious respiratory illness. Typical symptoms include fever, body aches and fatigue. In an exam answer, describe both the affected tissue and the resulting symptoms, rather than simply naming the virus.

Section 15 of 19Check your understanding

Question:

Question Why can TMV lower a crop's yield? Answer: Damaged chloroplast function reduces photosynthesis, so the plant makes less glucose for growth and produces a smaller yield.

Question:

Question Why would an antibiotic not cure influenza? Answer: Influenza is caused by a virus, while antibiotics act on bacterial targets.

Section 16 of 19Key terms

Term	Meaning
Hyphae	Thread-like structures of a fungus.
Mycelium	The network of hyphae.
Saprobiont	An organism that feeds on dead matter.
Protoctist	A diverse group of eukaryotes, e.g. amoeba.
Binary fission	Asexual division of bacteria into two.
Parasite	An organism that lives on or in a host and harms it.

Section 17 of 19Syllabus checklist

Syllabus points

describe the common features shown by eukaryotic organisms

plants
animals
fungi and protoctists

describe the common features shown by prokaryotic organisms such as bacteria

understand the term pathogen and know that pathogens may include fungi, bacteria, protoctists or viruses

Section 18 of 19Command terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Section 19 of 19Exam technique

Saying viruses are the smallest cells.

Saying fungi have cellulose walls.

Saying all protoctists are harmful.

Saying hyphae have a single nucleus.

Last updated: 11 October 2026