Patterns in Life: Diversity and Classification · Lesson 5 of 13
Kingdom Protista — Unicellular Eukaryotes
“Protists and fungi prove that being tiny does not make anyone easy to classify.”
• Describe the defining cellular organisation of protists. • Compare autotrophic and heterotrophic protists. • Explain how a hay infusion can reveal microscopic life. • Distinguish Protista from Fungi using organisation, cell wall and nutrition. • Describe the ecological and practical importance of fungi.
A drop of pond water may look empty, yet under a microscope it can become a moving community. Amoeba changes its shape, Paramecium moves using many tiny cilia and Euglena travels using a flagellum. Each complete organism consists of one cell, but unlike bacteria, that cell contains a membrane-bound nucleus.
A eukaryote is an organism whose cell or cells contain a membrane-bound nucleus.
A protist is a predominantly unicellular eukaryotic organism placed in Kingdom Protista.
Protists are microscopic and highly diverse. They commonly live in water or moist places. Some contain chlorophyll and prepare food through photosynthesis, while others obtain food from surrounding material or other organisms. Euglena demonstrates why one visible characteristic cannot define an entire kingdom: it can photosynthesise in light but may feed heterotrophically when light is unavailable.
| Protist | Movement or feature | Nutrition highlighted |
|---|---|---|
| Amoeba | Changes shape and forms temporary projections | Heterotrophic |
| Paramecium | Moves with numerous cilia | Heterotrophic |
| Euglena | Moves with a flagellum | May be autotrophic or heterotrophic |
| Chlamydomonas | Unicellular form with chlorophyll | Autotrophic |
Protists are important in aquatic food chains. Photosynthetic forms produce food and oxygen, providing resources for small aquatic animals. Other protists help decompose material and contribute to nutrient cycling. Their ecological importance is therefore much greater than their size suggests.
Activity: Let us Make
A hay infusion creates conditions in which microscopic organisms can multiply. Place a small amount of cut grass, straw or fodder in a clean bottle, fill it with stagnant or pond water, cover the mouth with muslin cloth and leave it undisturbed for about a week. A drop collected carefully from the infusion can then be mounted on a clean slide and observed under a microscope.
Problem
How should a moving organism in the prepared slide be studied?
- 1.Begin with a low magnification to locate movement in a wider field.
- 2.Observe the outline of the organism and whether it changes shape.
- 3.Look for cilia, a flagellum or temporary projections used in movement.
- 4.Compare the observation with illustrations of common protists.
- 5.Record only visible evidence instead of naming the organism from one uncertain feature.
A hay infusion may smell unpleasant and can contain many microorganisms. Wear appropriate protective equipment, avoid direct contact and dispose of the material only through the approved laboratory procedure.
Kingdom Fungi — Multicellular, Heterotrophic Eukaryotes with a Cell Wall
A piece of moist bread left in warm conditions may develop a spreading mould. What appears to be a coloured patch is formed by fine fungal filaments. Together, these filaments make a network called mycelium. The network spreads through or over the food and absorbs nutrients.
Mycelium is the network formed by the fine filaments of a fungus.
A saprophyte obtains nutrients from dead and decaying organic matter.
Fungi are eukaryotes and are mostly multicellular. Their cells possess walls made of chitin. They lack chlorophyll and do not manufacture food through photosynthesis. Instead, they release substances onto organic material and absorb the resulting nutrients. Many fungi are saprophytes, some form mutually beneficial associations and others live as parasites.
| Characteristic | Protista | Fungi |
|---|---|---|
| Organisation | Mostly unicellular | Mostly multicellular; yeast is unicellular |
| Cell type | Eukaryotic | Eukaryotic |
| Cell wall | May be absent or may contain cellulose | Contains chitin |
| Nutrition | Autotrophic or heterotrophic | Heterotrophic by absorption |
| Common habitat | Water or moist places | Commonly warm, moist places with organic material |
| Ecological role | Producers, food-chain members or decomposers | Major decomposers; also symbiotic or parasitic forms |
Fungi break complex organic matter into simpler substances that become available in the soil. Without fungal decomposition, dead leaves, twigs and organisms would accumulate, and the recycling of minerals would slow. Fungi therefore contribute directly to soil fertility and ecological balance.
Fungi reproduce sexually and asexually, often through spores. Yeast is an important exception to the usual multicellular fungal body. It is unicellular, but its eukaryotic organisation, chitin wall and method of nutrition support its placement in Fungi. Aspergillus and Penicillium are used in the production of useful enzymes and antibiotics, while mushrooms are visible fungi that reproduce by spores.
Problem
Yeast has only one cell. Why is it not placed with typical protists?
- 1.Cell number is an important criterion but is not used alone.
- 2.Yeast is a eukaryote, as are protists and other fungi.
- 3.Its cell wall contains chitin.
- 4.It obtains nutrients in the characteristic fungal manner.
- 5.The combined evidence supports its placement in Fungi.
Bridging Science and Society
Many communities possess detailed traditional knowledge for distinguishing edible and poisonous wild mushrooms. Some mushrooms are used as food and others in traditional practices. Correct identification is essential because outwardly similar forms may not be equally safe. Cultivation of suitable mushrooms can provide a livelihood because it needs limited space, relatively low investment and a short growing period.
Scientific and experienced local identification is essential. A lesson diagram or a single visible feature is not sufficient evidence that a wild mushroom is safe to eat.
Quiz
Which description best matches Eukaryote?
Which description best matches Protist?
Which term matches this description: A eukaryote is an organism whose cell or cells contain a membrane-bound nucleus.
Which term matches this description: A protist is a predominantly unicellular eukaryotic organism placed in Kingdom Protista.
Which statement is a key takeaway from this lesson?
Practice Problems
- Explain why Amoeba belongs to Protista rather than Monera.
- Compare a photosynthetic protist with a saprophytic fungus.
- Describe how fungal decomposition supports plant growth.
- Use three criteria to explain why yeast belongs to Fungi.
Key Takeaways
• Protists are predominantly unicellular eukaryotes. • Protists may prepare food or obtain it heterotrophically. • Fungi possess chitin walls and absorb nutrients. • Most fungi are multicellular, while yeast is an important unicellular exception. • Protists and fungi both contribute to food chains and nutrient cycling.