Class 8 › Science › Chapter 2
The Invisible Living World: Beyond Our Naked Eye
Microscopes and the discovery of cells, structure and types of cells, levels of organisation, microorganisms, and how microbes help us (decomposition, food, nitrogen, oxygen).
1. Seeing the Invisible: Lens and Microscope
- The human eye can see only objects above a certain size. A curved piece of glass, thick in the middle and thin at the edge like a lentil seed, makes small things look bigger; it is called a lens.
- Improved lenses gave magnifying glasses and then microscopes, which opened up the hidden world of tiny living creatures.
- Activity 2.1: a round-bottom flask filled with water and corked acts like a magnifying glass; letters seen through it look larger.
- All living beings, plants or animals, are called organisms.
Robert Hooke (1665)
Named the 'cell'
Published Micrographia with detailed drawings. His microscope magnified 200–300 times. In a thin slice of cork he saw many small empty compartments like a honeycomb and called each a cell: first use of the word for the basic unit of life.
Antonie van Leeuwenhoek (1660s)
Father of Microbiology
Dutch scientist who made better lenses and microscopes; first to clearly see and describe tiny living things like bacteria and blood cells.
- School microscopes magnify about 100–400 times; an electron microscope magnifies about 10,00,000 times and shows more cell structures.
- A low-cost foldable paper microscope makes the microscopic world accessible to many people (less detail than lab microscopes).
2. What Is a Cell?
Cell: the basic structural unit of all living beings (like a brick in a wall). Closely packed rectangular cells of onion peel look like a brick wall.
Schematic animal and plant cells (not to scale).
| Part | Function |
|---|---|
| Cell membrane | Outer layer enclosing cytoplasm and nucleus; separates one cell from another; porous: lets in materials needed for life and lets waste out |
| Cytoplasm | Fills the space between membrane and nucleus; contains carbohydrates, proteins, fats, mineral salts; most life processes occur here |
| Nucleus | Round structure in the middle, covered by a thin membrane; regulates all cell activities and growth |
| Cell wall (plant cells; also fungi, bacteria) | Extra outer layer; gives rigidity and strength, so cells are compact and firm |
| Plastids / chloroplasts (plants) | Chloroplasts contain chlorophyll (green; photosynthesis); non-green plastids store substances |
| Vacuole | Large in plant cells: stores substances, removes waste, maintains cell shape. Usually absent or small in animal cells |
Activities: observing cells
Onion peel (Activity 2.2): peel thin layer from inner surface → stain with safranin (30 s) → rinse → mount on slide with glycerin (prevents drying, improves clarity) → lower a coverslip with a needle (no air bubbles) → blot extra glycerin → observe. Result: nearly rectangular cells, closely arranged, no gaps.
Cheek cells (Activity 2.3): scrape inside of cheek gently → stain with methylene blue (increases contrast) → glycerin and coverslip → observe. Result: polygon-shaped cells forming the lining of the mouth.
| Onion peel (plant) cell | Cheek (animal) cell | |
|---|---|---|
| Shape | Nearly rectangular | Polygonal / irregular |
| Cell wall | Present | Absent |
| Common parts | Cell membrane, cytoplasm, nucleus | |
3. Variation in Shape and Structure of Cells
| Cell | Shape | Function it suits |
|---|---|---|
| Muscle cell | Spindle-shaped, thin, flexible | Contracts and relaxes in a wave-like way, e.g. pushes food down the food pipe; churns food in the stomach wall |
| Nerve cell (neuron) | Very long, branched | Reaches different parts of the body and passes on messages quickly |
| Cheek / lining cell | Thin and flat | Forms a protective lining |
| Stomach lining cells | – | Produce digestive juices and acid |
| Some plant cells | Long tubes | Carry water through the plant |
- The shape, size and structure of a cell are related to its function. Plant cells may be rectangular, elongated, oval or tube-like.
- Different types of cells work together to make processes like digestion possible.
4. Levels of Organisation
Levels of organisation in a multicellular organism.
| Level | Meaning |
|---|---|
| Cell | Basic unit of life |
| Tissue | A group of similar cells |
| Organ | Different tissues organised together |
| Organ system | Several organs working together for a major body function |
| Organism | All organ systems together: a complete plant or animal |
- A complex organism starts as a single cell (egg) that divides repeatedly. Organisms made of many cells are multicellular (animals, humans, plants).
- In multicellular organisms cells do specialised jobs and also cooperate to improve survival.
- Largest known cell: the yolk of an ostrich egg, about 130–170 mm in diameter (the shell and white are non-cellular material).
5. What Are Microorganisms?
Microorganisms (microbes): living organisms made of one or very few cells, too small to be seen with the naked eye (micro = very small). Found in water, soil, air, food and even inside our bodies. Seen only with a microscope.
| Microbe | Cells | Features / where found |
|---|---|---|
| Bacteria | Unicellular | Spherical, comma, spiral or rod-shaped; no well-defined nucleus (nucleoid); some have a long hair-like structure and small projections |
| Protozoa (Amoeba, Paramecium) | Unicellular | Amoeba: moving, irregular shape. Paramecium: moves with specialised structures. Pond water |
| Algae | Single or many cells | Green because of chlorophyll; some move using specialised structures |
| Fungi | Yeast: unicellular; moulds: multicellular | Branched filaments, no chlorophyll (bread mould with sac-like tips, other mould with brush-like tips); soil |
- Pond water (Activity 2.4) and soil suspension (Activity 2.5: soil stirred in water, drop from top layer after settling) both show small moving organisms. Use a spoon or gloves for soil.
- Viruses are microscopic and acellular; they multiply only after entering a living cell (of plants, animals or bacteria) and may cause disease.
- Microbes show great diversity and can live in extreme places such as hot springs and snow-cold regions; gut bacteria help in digestion.
6. How We Are Connected to Microbes: Cleaning the Environment
- Food spoilage: fruits and vegetables left out grow a powdery or cotton-like layer because microbes from air, water and soil infect them.
- Pickles and murabbas do not spoil because high concentration of salt or sugar (preservatives, with spices) does not allow microbes to grow.
Decomposition and manure (Activity 2.7)
Set-up: container half filled with garden soil, fruit and vegetable peels, another layer of soil; observe after 2–3 weeks.
Result: peels turn into dark manure, rich in nutrients, which raises soil fertility.
Why: soil bacteria and fungi break complex substances of plant waste into simpler nutrient-rich substances: decomposition. They also decompose dead animals and dung, so microbes recycle waste and return nutrients. Manure forms best at optimal temperature and moisture.
Biogas
Methane from waste
Bacteria living in an oxygen-free environment decompose plant and animal waste and wastewater, releasing biogas: mainly CO2 with a high proportion of methane. Used as fuel for cooking, heating, electricity and vehicles.
Be a scientist
Ananda Mohan Chakrabarty
(1938–2020) developed (1971) a bacterium that could break down oil spills; patent granted in 1980. A patent stops others from copying, using or selling an invention without permission.
- Our scientific heritage: the Vedas (incl. Atharvaveda) mention Krimi: tiny entities, Drishya (visible) and Adrishya (invisible), with beneficial and harmful effects.
7. Microorganisms and Food
Yeast (fungus)
Bread, cakes, idli, dosa
Respires and breaks down sugar for energy in warm conditions, releasing carbon dioxide (bubbles make dough soft and fluffy, so it rises) and a little alcohol (changes the smell).
Lactobacillus (bacterium)
Curd, idli/dosa batter, bhatura
Feeds on milk sugar (lactose), multiplies and ferments milk to curd, producing lactic acid (not alcohol), which makes curd sour. Grows well in warm conditions.
Kitchen activities
Activity 2.8: two bowls of flour with a pinch of sugar; yeast only in bowl A; knead with warm water, cover with damp cloth, keep warm 4–5 hours. Bowl A dough rises, becomes fluffy and smells different (bowl B is the control for comparison). Sugar is food for yeast; warmth helps it grow.
Activity 2.9: lukewarm milk (bowl A, kept warm) vs cold milk (bowl B, kept cool), each with a spoon of curd. Only A curdles; B may turn slightly sour but does not set.
- Fermentation by microbes gives traditional foods (e.g. fermented soya bean, bamboo shoots).
- Microbes need suitable temperature, air and moisture to grow; bread left near a sink spoils faster than bread kept in a refrigerator.
8. Rhizobium and Microalgae
Rhizobium
Nitrogen fixers in root nodules
Live in swollen root nodules of legumes (beans, peas, lentils, cowpea). They trap nitrogen from the air and make it usable for plants, so legumes grow without chemical nitrogen fertiliser. Farmers grow legumes in rotation to raise soil nitrogen for the next crop.
Microalgae
Tiny helpers in water
Microscopic plant-like organisms in water, soil, air and on trees. Make food by photosynthesis and produce more than half of Earth's oxygen; food for aquatic animals; used as supplements and medicines (Spirulina, Chlorella, diatoms); clean water; make biofuel.
- Spirulina: a 'superfood'; over 60% protein by weight, good source of vitamin B12, little fat and sugar. It can be grown in a clear glass tank of pond water in bright, shaded, moderate-temperature conditions, stirred twice a week, and harvested after 3–6 weeks by filtering through fine cloth.
- Pollution, climate change and habitat destruction threaten microalgae; conserving them protects the environment and Earth's oxygen balance.
9. Why Is the Cell the Basic Unit of Life?
| Cell type | Membrane | Cell wall | Nucleus | Chloroplast |
|---|---|---|---|---|
| Animal | Yes | No | Yes (with nuclear membrane) | No |
| Plant | Yes | Yes | Yes | Yes |
| Fungal (incl. yeast, mould) | Yes | Yes | Yes | No (cannot make own food) |
| Bacterial | Yes | Yes | No well-defined nucleus; has a nucleoid | No |
- All living things, including microorganisms, are made of one or more cells, which differ in size, shape and structure.
- Unicellular: bacteria, protozoa, yeast. They carry out all life functions in one cell. Multicellular: plants, animals, moulds.
- Some microbes cause diseases in plants and animals (next chapter).
10. Quick Revision
| Concept | Key point |
|---|---|
| Lens | Thick in the middle, thin at edge; magnifies |
| Hooke / Leeuwenhoek | Named 'cell' (cork, 1665) / saw bacteria and blood cells, Father of Microbiology |
| Basic cell parts | Cell membrane, cytoplasm, nucleus (+ cell wall in plants) |
| Levels | Cell → Tissue → Organ → Organ system → Organism |
| Microbes | Bacteria, protozoa, fungi, algae (viruses are acellular) |
| Decomposition | Bacteria and fungi make manure, return nutrients |
| Yeast vs Lactobacillus | CO2 + alcohol (dough) vs lactic acid (curd) |
| Rhizobium | Fixes nitrogen in legume root nodules |
| Microalgae | More than half of Earth's oxygen |
Exercise hints
From 'Keep the curiosity alive'
Q2(i): (c) yeast produced a gas (CO2) that inflated the balloon; lime water turning milky tests for CO2. Q3: bean plants have Rhizobium in root nodules, so they need no nitrogen fertiliser. Q5: (i) bacteria, (ii) yeast, (iii) Rhizobium.
Remember
Key numbers
Hooke: 1665, 200–300× · School microscope: 100–400× · Electron microscope: about 10,00,000× · Ostrich egg yolk: 130–170 mm · Spirulina: over 60% protein.
Common mistakes:
• Saying yeast makes lactic acid; yeast makes CO2 and alcohol, Lactobacillus makes lactic acid.
• Calling viruses cells or microorganisms with a nucleus; they are acellular and multiply only inside a host.
• Saying bacteria have no cell wall or no genetic material; they have a wall and a nucleoid (no well-defined nucleus).
• Saying all microorganisms are harmful.
• Saying animal cells have a cell wall or chloroplasts.
• Saying fungi make food by photosynthesis; they lack chloroplasts.
• Saying Amoeba is multicellular; it is a single-celled protozoan.
• Saying yeast makes lactic acid; yeast makes CO2 and alcohol, Lactobacillus makes lactic acid.
• Calling viruses cells or microorganisms with a nucleus; they are acellular and multiply only inside a host.
• Saying bacteria have no cell wall or no genetic material; they have a wall and a nucleoid (no well-defined nucleus).
• Saying all microorganisms are harmful.
• Saying animal cells have a cell wall or chloroplasts.
• Saying fungi make food by photosynthesis; they lack chloroplasts.
• Saying Amoeba is multicellular; it is a single-celled protozoan.