By the end of this lesson
- Describe the common features of animals, plants, fungi and protoctists.
- Use cell structure and nutrition to identify an unfamiliar eukaryote.
- Explain why one visible feature is not enough for classification.
BIG QUESTION
Why is a fungus not a plant, even though neither one walks away?
Classification depends on several independent features: cell structure, mode of nutrition, organisation and storage molecules.
What all four groups share
Animals, plants, fungi and protoctists are eukaryoteAn organism whose cells have a nucleus and membrane-bound organelles, such as plants, animals, fungi and protoctists.. Their cells contain genetic material inside a membrane-bound nucleus and have cytoplasm, a cell membrane, ribosomes and membrane-bound organelles such as mitochondria. This separates them from prokaryotic bacteria.
The groups are distinguished by combinations of features. A cell wall alone does not identify a plant because fungi also have walls. Photosynthesis alone is not enough to describe every plant cell because root cells normally lack chloroplasts. Use structure, nutrition, organisation and storage together.
Animals and plants
animalA multicellular eukaryote that feeds on other organisms (heterotrophic), has no cell walls or chloroplasts and may store carbohydrate as glycogen. are multicellular heterotrophs. Their cells lack cell walls and chloroplasts, and carbohydrates are stored as glycogen. Nervous coordination and muscles allow rapid, coordinated movement in most animals. Humans and houseflies are course examples.
plantA multicellular eukaryote that photosynthesises using chloroplasts, has cellulose cell walls and stores carbohydrate as starch or sucrose. are multicellular autotrophs with cellulose cell walls. Photosynthetic tissues contain chloroplasts, and carbohydrates are commonly stored as starch or transported as sucrose. Maize and peas are course examples. Do not claim that every plant cell contains chloroplasts: underground root cells usually do not.
Fungi
fungusA eukaryote that cannot photosynthesise, has cell walls made of chitin and usually a body of thread-like hyphae; it feeds by extracellular digestion (saprotrophic nutrition). are eukaryotic heterotrophs with cell walls made of chitin, not cellulose. They do not contain chlorophyll. Many consist of microscopic threads called hyphaOne of the thin, thread-like filaments that make up the body of a fungus.; a network of hyphae forms a myceliumThe network of hyphae that forms the body of a fungus.. Some fungi, including yeast, are unicellular.
A saprotrophic fungus secretes digestive enzymes onto material outside its body. Large insoluble molecules are digested into small soluble products, which are then absorbed. Fungi store carbohydrate as glycogen. Mucor, yeast and Trichophyton show that one group can contain decomposers, useful microorganisms and pathogens.
Protoctists are diverse
protoctistA eukaryote, usually single-celled, that does not fit the plant, animal or fungus groups; some photosynthesise (like plants) and some feed (like animals). are eukaryotes, commonly microscopic and single-celled, that do not fit the animal, plant or fungal feature combinations used in this course. They are not all nutritionally alike.
Amoeba ingests food and is animal-like. Chlorella has chloroplasts and photosynthesises, so it is plant-like. Plasmodium is a parasitic protoctist that causes malaria. These examples show why “all protoctists photosynthesise” is incorrect.
Classification is a model
The five-group framework is useful for the features required in this specification. Modern biological classification also uses molecular evidence and evolutionary relationships, so group boundaries can be revised as evidence improves.
In an exam, classify from the evidence provided. Name the observed feature, connect it to a group, and check that other features support rather than contradict the conclusion.
Four eukaryotic groups compared
Read across each row: no single column should be used alone.
| Group | Cell and body organisation | Nutrition, storage and examples |
|---|---|---|
| Animals | Multicellular; no cell wall or chloroplasts. | Heterotrophic; glycogen; human and housefly. |
| Plants | Multicellular; cellulose walls; chloroplasts in photosynthetic tissues. | Autotrophic; starch or sucrose; maize and pea. |
| Fungi | Chitin walls; usually hyphae forming a mycelium; yeast is unicellular. | Saprotrophic; glycogen; Mucor, yeast and Trichophyton. |
| Protoctists | Usually microscopic and single-celled; eukaryotic but varied. | Autotrophic, heterotrophic or parasitic; Chlorella, Amoeba and Plasmodium. |
Language that earns precision
- eukaryote
- An organism whose cells contain genetic material inside a membrane-bound nucleus.
- heterotroph
- An organism that obtains organic nutrients from other organisms or their products.
- autotroph
- An organism that manufactures organic nutrients from inorganic materials using an energy source.
- saprotrophic nutrition
- External digestion followed by absorption of soluble products.
- hypha
- A thread-like fungal structure; many hyphae form a mycelium.
- protoctist
- A member of a diverse eukaryotic group represented here by Amoeba, Chlorella and Plasmodium.
Common misconceptions
Claim: Every cell in a plant has chloroplasts.
Correction: Only photosynthetic cells normally contain chloroplasts; most root cells do not.
Claim: Fungi are plants because they have cell walls.
Correction: Fungal walls contain chitin, fungi lack chlorophyll and obtain nutrients heterotrophically.
Claim: All protoctists photosynthesise.
Correction: Chlorella photosynthesises, Amoeba ingests food and Plasmodium is parasitic.
Claim: All fungi are multicellular.
Correction: Many fungi form hyphae, but yeasts are unicellular fungi.
Match the answer to the command word
Give the relevant observable or structural features without explaining their cause.
Write linked similarities and differences using the same feature for both groups.
Select the group and support it with evidence from the information given.
Recall, apply, then justify
Recall, apply, then justify (continued)
State two features of a typical plant that distinguish it from a typical animal.
Hint
Use cell structures or carbohydrate storage.
Suggested marking points
- Any two from: cellulose cell walls; chloroplasts in photosynthetic tissues; starch storage rather than glycogen.
Explain how a saprotrophic fungus obtains nutrients.
Hint
Your answer needs secretion, digestion outside the organism and absorption.
Suggested marking points
- Digestive enzymes are secreted onto food material.
- Large insoluble molecules are digested outside the fungus into small soluble molecules.
- The soluble products are absorbed into fungal hyphae.
A microscopic eukaryote contains chloroplasts. Explain why this alone does not prove it is a plant.
Hint
Name another group and the additional evidence needed.
Suggested marking points
- Some protoctists, such as Chlorella, contain chloroplasts.
- The organism could therefore be a photosynthetic protoctist.
- Evidence such as multicellularity, cellulose walls and the full plant feature combination is needed.
Compare fungi with plants.
Hint
Pair each difference directly: wall material, nutrition and storage.
Suggested marking points
- Both are eukaryotic and have cell walls.
- Plant walls contain cellulose whereas fungal walls contain chitin.
- Plants photosynthesise whereas fungi are heterotrophic and often saprotrophic.
- Plants commonly store starch whereas fungi store glycogen.
An unfamiliar organism is multicellular, has cell walls and stores glycogen. Decide which group is most likely and describe one further test.
Planning hint
Use the combination of wall plus storage, then seek evidence about wall material or nutrition.
What a strong design includes
- Fungus is the most likely group.
- Glycogen storage supports fungi rather than plants.
- Test for chitin, absence of chlorophyll, hyphae, or external digestion.
- Keep the conclusion provisional until additional evidence is obtained.