This MCQ module is based on: Types of Microorganisms
TOPIC 4 OF 50
Types of Microorganisms
🎓 Class 8
Science
CBSE
Theory
Ch 2 — Microorganisms: Friend and Foe
⏱ ~30 min
🌐 Language: [gtranslate]
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Myaischool Lt Science Assessment
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[myaischool_lt_science_assessment grade_level="class_8" science_domain="biology" difficulty="basic"]
Microorganisms Are Everywhere
The invisible world we began exploring in Part 1 is far larger than you might imagine. Microorganisms (also called microbes) are found virtually everywhere — floating in the air, dissolved in water, buried deep in soil, and even living inside your own body. They are so incredibly small that a microscope magnifying 100 to 400 times is needed just to see them.
Despite their tiny size, microorganisms play enormous roles in the world. Some help us make food, some keep soil fertile, and some cause diseases. To understand them better, scientists have grouped them into five major categories: Bacteria, Fungi, Algae, Protozoa, and Viruses.
Activity 2.4 — Preparing a Hay/Grass Infusion
L3 Apply
Think first: If you soak dried grass or hay in water for several days, will living organisms eventually appear in the water? Where would they come from?
- Take a handful of dried grass or hay and place it in a glass jar or beaker.
- Add enough water to fully cover the grass. Leave the jar open (or loosely covered) in a warm spot for 3 to 5 days.
- After a few days, the water may appear slightly cloudy or have a faint smell.
- Take a single drop of this "infusion" water and place it on a glass slide with a cover slip.
- Observe under a microscope at different magnifications. Draw what you see.
What you should see: The water now teems with microscopic life! You should spot Paramecium (slipper-shaped organisms covered with tiny hair-like cilia, gliding smoothly), Amoeba (shapeless blobs that constantly change form using arm-like extensions called pseudopodia), and possibly other protists. These microorganisms were either already present as dormant forms on the hay or entered from the air and multiplied rapidly in the nutrient-rich water.
Table 2.1 — Common Organisms Found in Pond Water L1 Remember
| Organism | Shape & Appearance | Key Feature | Movement |
|---|---|---|---|
| Amoeba | Irregular, blob-like; constantly changing shape | Pseudopodia (false feet) — temporary arm-like extensions of the body | Crawls slowly by extending pseudopodia |
| Paramecium | Slipper-shaped (elongated oval) | Covered with hundreds of tiny hair-like cilia | Swims swiftly by beating cilia in coordinated waves |
| Euglena | Spindle-shaped (pointed at both ends) | Has a single whip-like flagellum at one end; also has chloroplasts | Pulls itself forward by rotating its flagellum |
| Spirogyra | Long green filament (chain of cells) | Spiral-shaped chloroplast visible inside each cell | Does not move actively; floats in water |
Activity 2.5 — Examining Soil Under the Microscope
L3 Apply
Think first: Soil seems lifeless compared to pond water. Do you think soil also contains microscopic organisms? What types might live there?
- Take a small amount of garden or field soil (about a tablespoon) and mix it into a glass of water.
- Stir well and let the mixture settle for a few minutes. The heavier particles will sink to the bottom.
- Carefully pour off or filter the upper liquid (filtrate) into another glass using a cloth or filter paper.
- Place a drop of this filtrate on a glass slide, add a cover slip, and observe under the microscope.
- Record what you see — look for different shapes: round, rod-like, branched filaments, etc.
What you should see: Soil is far from lifeless! Under the microscope, you should be able to find: Bacteria (tiny spherical, rod-shaped, or spiral organisms), Fungi (branched thread-like filaments, sometimes with bead-like or brush-like structures at their tips), and Algae (small round green cells). Soil is one of the richest habitats for microorganisms on Earth.
Table 2.2 — Common Organisms Found in Soil L1 Remember
| Organism | Shape & Appearance | Key Feature |
|---|---|---|
| Bread Mould (Fungi) | Branched filaments without green colour; sac-like structures at tips | Grows on bread, decaying matter; absorbs nutrients from dead organic material |
| Mould (Penicillium-type) | Branched filaments; brush-like clusters of spores at tips | Often blue-green; source of the antibiotic penicillin |
| Algae | Spherical, small green cells | Contains green pigment (chlorophyll); makes its own food via photosynthesis |
| Bacteria | Spherical, comma-shaped, spiral, or rod-shaped | Some have a long hair-like flagellum; others have many short hair-like projections (pili) |
Viruses — A Special Category
Viruses occupy a unique position in biology. They are much smaller than bacteria and have a fundamentally different nature: they are acellular, meaning they are not made of cells at all. A virus is essentially a tiny packet of genetic material (DNA or RNA) wrapped in a protein coat.
Here is what makes viruses truly unusual: they cannot reproduce on their own. A virus must enter a living cell — whether it belongs to a plant, animal, or even a bacterium — and hijack that cell's machinery to make copies of itself. Outside a living cell, a virus is essentially inert, like a lifeless particle. This is why scientists debate whether viruses should be classified as "living" or "non-living."
Key Distinction: Bacteria, fungi, algae, and protozoa are all made of cells and can carry out life processes independently. Viruses are NOT made of cells and can only multiply inside a living host cell. This makes viruses fundamentally different from all other microorganisms.
Classification of Microorganisms
Scientists classify the vast diversity of microorganisms into five major groups based on their structure, how they obtain food, and how they reproduce:
Preview — Useful Microbes: Not all microorganisms are harmful! Many are incredibly useful. In Part 3, you will discover how bacteria help make curd from milk, how yeast makes bread fluffy, and how certain soil bacteria naturally fertilise the ground for farmers. The microscopic world is full of tiny helpers!
Competency-Based Questions
Ravi's teacher asks students to prepare two slides — one from pond water and one from soil suspension — and list all the different types of organisms they can see. Ravi observes a slipper-shaped organism covered with tiny hair-like structures in the pond water, and tiny rod-shaped organisms in the soil suspension.
Q1. L2 Understand The slipper-shaped organism Ravi observed in pond water is most likely:
Answer: C. Paramecium. Paramecium is characteristically slipper-shaped (elongated oval) and is covered with hundreds of tiny hair-like structures called cilia, which it uses for swimming. Amoeba is irregular and blob-like, Euglena is spindle-shaped with a flagellum, and Spirogyra is a green filamentous algae.
Q2. L1 Remember Fill in the blank: Rod-shaped bacteria are called __________, while spherical bacteria are called __________.
Answer: Bacillus (or bacilli); Coccus (or cocci). The four common bacterial shapes are: coccus (sphere), bacillus (rod), vibrio (comma), and spirillum (spiral).
Q3. L4 Analyse Compare Euglena with Spirogyra. Both are found in pond water and both contain chloroplasts. Yet scientists classify them differently. Explain one key structural difference that justifies this. (Short Answer — 2 marks)
Answer: Although both Euglena and Spirogyra contain chloroplasts and can photosynthesize, they differ fundamentally in structure: Euglena is a single-celled organism with a flagellum that allows it to move actively, and it can also feed on other organisms when light is unavailable (making it part protozoan, part algae). Spirogyra is a filamentous algae — a chain of many connected cells — that does not move actively and is entirely plant-like. Euglena is classified as a protist/protozoa, while Spirogyra is classified as an algae.
Q4. L5 Evaluate True or False: "Viruses are living organisms because they can reproduce." Evaluate this statement critically. (3 marks)
Answer: False (or at least highly debatable). While viruses can reproduce, they cannot do so independently. They must enter a living host cell and use that cell's machinery to make copies of themselves. Outside a host cell, viruses show no characteristics of life — they do not metabolise, grow, or respond to stimuli. They lack cells, which are considered the basic unit of life. For these reasons, most scientists consider viruses to be on the boundary between living and non-living, and they are often described as "obligate intracellular parasites" rather than true living organisms.
Q5. L3 Apply A student prepares a hay infusion and observes it after 4 days. She sees blob-like organisms that keep changing shape and move by extending arm-like projections. Which organism is she observing, and what is the scientific name for these arm-like extensions? (Short Answer — 2 marks)
Answer: The organism is Amoeba. The arm-like extensions are called pseudopodia (literally "false feet"). Amoeba uses pseudopodia both for movement (crawling slowly across surfaces) and for capturing food by surrounding and engulfing it.
Assertion-Reason Questions
Assertion (A): Viruses are classified differently from bacteria, fungi, algae, and protozoa.
Reason (R): Viruses are acellular (not made of cells) and can only reproduce inside a living host cell.
Answer: A. Both statements are true. The Reason correctly explains the Assertion — viruses are placed in a separate category precisely because they lack cellular structure and cannot reproduce independently, unlike bacteria, fungi, algae, and protozoa which are all cellular organisms.
Assertion (A): Spirogyra is classified as an algae even though it looks like a plant.
Reason (R): Spirogyra lacks true roots, stems, and leaves, and exists as a simple chain of cells with spiral-shaped chloroplasts.
Answer: A. Both are true and R explains A correctly. Spirogyra contains chlorophyll and can photosynthesise (which is why it resembles a plant), but it is classified as an algae because it has no differentiated plant organs — it is simply a filament of identical cells, each with a distinctive spiral chloroplast.
Assertion (A): Amoeba moves by extending pseudopodia, while Paramecium moves by beating cilia.
Reason (R): Both Amoeba and Paramecium are protozoa, but they have evolved different structures for locomotion.
Answer: A. Both statements are true. The Reason correctly explains why both organisms, despite belonging to the same broad group (protozoa), use different locomotion methods — Amoeba's pseudopodia are temporary cytoplasmic extensions, while Paramecium's cilia are permanent hair-like structures covering its body.
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