SK Tuitions • Class 8 Science Notes
The Invisible Living World: Beyond Our Naked Eye
Complete notes, definitions, microorganism classification, uses of microbes, pathogens, viruses, question answers, HOTS and interactive quiz.
Chapter Overview
This chapter introduces us to the hidden world of tiny living beings that cannot be seen with the naked eye. With the help of lenses, magnifying glasses and microscopes, humans discovered cells and microorganisms. The chapter explains the structure of cells, differences between plant, animal, fungal and bacterial cells, types of microorganisms, their usefulness in food, agriculture and environment, and why viruses are different from other microorganisms.
1. Discovery of the Invisible World
Role of Lens and Microscope
The human eye can see only objects above a certain size. A curved glass lens makes small objects appear bigger. Later, magnifying glasses and microscopes helped scientists observe tiny living organisms.
Important Scientists
- Robert Hooke: In 1665, he observed thin slices of cork and used the word cell.
- Antonie van Leeuwenhoek: He made better lenses and first clearly observed tiny living things like bacteria and blood cells. He is called the Father of Microbiology.
2. What is a Cell?
A cell is the basic structural and functional unit of life. All living organisms are made up of one or more cells. Some organisms have only one cell, while plants and animals are made up of many cells.
Main Parts of a Typical Cell
| Cell Part | Meaning / Structure | Function |
|---|---|---|
| Cell membrane | Thin outer boundary of the cell | Separates one cell from another and allows entry of useful materials and exit of waste. |
| Cytoplasm | Jelly-like substance between cell membrane and nucleus | Contains cell components and substances like proteins, fats, carbohydrates and mineral salts. |
| Nucleus | Round structure generally present near the centre | Controls cell activities and regulates growth. |
| Cell wall | Extra outer covering in plant, fungal and bacterial cells | Provides rigidity, shape and strength. |
| Chloroplast | Green plastid containing chlorophyll | Helps plants and microalgae prepare food by photosynthesis. |
| Vacuole | Empty-looking space inside the cell | Stores substances, removes waste and helps maintain cell shape. |
| Nucleoid | Region containing genetic material in bacteria | Acts as control region in bacteria because bacteria do not have a well-defined nucleus. |
3. Plant Cell and Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell membrane | Present | Present |
| Cell wall | Present | Absent |
| Chloroplast | Present in green parts | Absent |
| Vacuole | Usually large | Absent or very small |
| Shape | Usually regular and firm | Usually irregular or flexible |
Onion Peel Cell and Cheek Cell
- Onion peel cells are nearly rectangular and closely arranged like bricks in a wall.
- Human cheek cells are polygon-shaped and form the inner lining of the mouth.
- Both contain cell membrane, cytoplasm and nucleus.
- Onion peel cells have a cell wall, but cheek cells do not.
4. Variation in Shape and Structure of Cells
Cells differ in shape, size and structure because they perform different functions. The structure of a cell is related to the work it performs.
Nerve Cell
Long and branched. It carries messages quickly to different parts of the body.
Muscle Cell
Spindle-shaped, thin and flexible. It helps in contraction and relaxation.
Cheek Cell
Thin and flat. It forms a protective lining inside the mouth.
5. Levels of Organisation in Living Organisms
A group of similar cells forms a tissue. Different tissues form an organ. Several organs work together to form an organ system. All organ systems together form a complete organism.
6. What are Microorganisms?
Microorganisms or microbes are very small living organisms that cannot be seen with the naked eye. They can be observed only with the help of a microscope. They are found in air, water, soil, food, extreme environments and even inside our body.
Where are Microorganisms Found?
- Pond water and stagnant water
- Soil and soil suspension
- Air and food items
- Inside human and animal bodies, especially in the gut
- Extreme places like hot water springs and snow-covered areas
7. Classification of Microorganisms
| Type of Microorganism | Examples | Structure / Features | Important Role |
|---|---|---|---|
| Protozoa | Amoeba, Paramecium | Mostly unicellular; Amoeba has irregular shape; Paramecium moves using specialised structures. | Live in water and moist places; some are useful while some may cause diseases. |
| Algae / Microalgae | Spirulina, Chlorella, Diatoms | Plant-like; contain chlorophyll; make their own food using sunlight. | Produce oxygen, serve as food for aquatic animals, help clean water and may be used for biofuel. |
| Fungi | Yeast, bread mould, mould | Yeast is unicellular; mould is multicellular; fungi have cell wall but no chloroplast. | Used in baking and fermentation; also help in decomposition. |
| Bacteria | Lactobacillus, Rhizobium | Unicellular; may be spherical, rod-shaped, comma-shaped or spiral; no well-defined nucleus. | Curd formation, nitrogen fixation, digestion, decomposition and biogas production. |
| Viruses | Disease-causing viruses | Microscopic and acellular; multiply only inside a living host cell. | Can infect plants, animals and bacteria and may cause diseases. |
8. Why is Virus Different?
Viruses are different from other microorganisms because they are acellular, which means they are not made up of a complete cell like bacteria, fungi, algae or protozoa. They do not grow and reproduce on their own. They multiply only after entering a living host cell.
Other Microorganisms
- Made up of one or more cells
- Have cell structures
- Can perform life processes
Viruses
- Acellular
- Active only inside host cells
- May infect plants, animals or bacteria
9. Useful Microorganisms in Different Fields
| Field | Microorganism | Use |
|---|---|---|
| Food industry | Yeast | Makes bread, cakes and dough soft and fluffy by releasing carbon dioxide during respiration. |
| Indian foods | Lactobacillus and other bacteria | Help in fermentation of idli, dosa and bhatura batter. |
| Curd formation | Lactobacillus | Converts milk sugar lactose into lactic acid, making milk turn into curd. |
| Agriculture | Rhizobium | Lives in root nodules of leguminous plants and fixes nitrogen, increasing soil fertility. |
| Environment | Bacteria and fungi | Decompose plant and animal waste into manure and recycle nutrients. |
| Energy | Anaerobic bacteria and fungi | Decompose waste in oxygen-free conditions and produce biogas rich in methane. |
| Water and oxygen balance | Microalgae | Produce oxygen, clean water and support aquatic food chains. |
| Health and nutrition | Spirulina | Used as a nutrient-rich health supplement; rich in protein and vitamin B12. |
| Pollution control | Special bacteria | Some bacteria can help break down oil spills and clean polluted environments. |
10. Microorganisms and Food
Yeast and Dough
Yeast is a fungus. In warm conditions, it breaks down sugar and releases carbon dioxide. This gas forms bubbles in the dough, making it soft and fluffy. Yeast also produces a small amount of alcohol, which gives dough a slightly different smell.
Lactobacillus and Curd
Curd contains bacteria such as Lactobacillus. These bacteria feed on lactose, the sugar present in milk, and produce lactic acid. This acid makes the milk sour and turns it into curd. Warm conditions help these bacteria grow.
Food Spoilage and Preservation
Microorganisms grow on food and may cause rotting, seen as powdery or cotton-like growth on fruits. Pickles and murabbas do not spoil easily because salt and sugar act as preservatives. High concentration of salt or sugar prevents the growth of microorganisms.
11. Microorganisms in Agriculture and Environment
Decomposition
Bacteria and fungi break down fallen leaves, fruit peels, vegetable waste and dead bodies into simpler substances. This process is called decomposition.
Manure Formation
Microorganisms convert plant waste into dark, nutrient-rich manure. This increases soil fertility. Manure formation needs suitable temperature and moisture.
Nitrogen Fixation
Rhizobium bacteria live in root nodules of legumes such as peas, beans and lentils. They trap nitrogen from air and make it useful for plants.
Biogas
Some microorganisms decompose plant and animal waste in oxygen-free conditions and release biogas, which contains methane and carbon dioxide.
12. Pathogens
Some microorganisms are harmful because they cause diseases in humans, animals and plants. Such disease-causing microorganisms are called pathogens.
| Term | Meaning | Important Point |
|---|---|---|
| Pathogen | A disease-causing microorganism | May infect humans, animals or plants. |
| Infection | Entry and multiplication of harmful microbes inside a living body | Can lead to disease symptoms. |
| Host | Living organism in which a microbe lives or multiplies | Viruses multiply only inside host cells. |
| Food spoilage | Rotting of food due to growth of microbes | Can be reduced by preservatives like salt and sugar. |
13. Definitions Table
| Term | Definition |
|---|---|
| Organism | Any living being, whether plant, animal or microorganism. |
| Cell | The basic structural and functional unit of life. |
| Microscope | An instrument that magnifies very small objects and helps us see microorganisms and cells. |
| Microorganism | A very small organism that cannot be seen with the naked eye. |
| Unicellular organism | An organism made up of only one cell. |
| Multicellular organism | An organism made up of many cells. |
| Tissue | A group of similar cells performing a specific function. |
| Organ | A body part made of different tissues working together. |
| Organ system | A group of organs working together to perform a major body function. |
| Fermentation | A process in which microorganisms break down food substances and produce useful products. |
| Decomposition | Breakdown of dead plants, animals and wastes into simpler substances by microbes. |
| Manure | Nutrient-rich material formed by decomposition of organic waste. |
| Biogas | A mixture of gases produced by microbes during decomposition of waste in oxygen-free conditions. |
| Microalgae | Microscopic plant-like organisms that make food using sunlight. |
| Pathogen | A disease-causing microorganism. |
| Acellular | Not made up of cells; viruses are acellular. |
14. Question and Answers
Q1. Why can we not see microorganisms with the naked eye?
Answer: Microorganisms are extremely small in size. They can be seen only with the help of a microscope, which magnifies them.
Q2. Why is the cell called the basic unit of life?
Answer: The cell is called the basic unit of life because all living organisms are made up of cells, and each cell performs life processes necessary for survival.
Q3. What are the three main parts of a typical cell?
Answer: The three main parts of a typical cell are cell membrane, cytoplasm and nucleus.
Q4. How are onion peel cells different from human cheek cells?
Answer: Onion peel cells are rectangular, closely arranged and have a cell wall. Human cheek cells are polygon-shaped and do not have a cell wall.
Q5. What are microorganisms?
Answer: Microorganisms are tiny living organisms that cannot be seen with the naked eye. They include bacteria, fungi, protozoa and some algae.
Q6. How does yeast make dough soft and fluffy?
Answer: Yeast breaks down sugar and releases carbon dioxide gas. The gas forms bubbles in the dough, making it rise and become soft and fluffy.
Q7. Why does milk turn into curd in warm conditions?
Answer: Curd contains Lactobacillus bacteria. In warm conditions, these bacteria multiply and convert lactose into lactic acid, which turns milk into curd.
Q8. How do microorganisms help in cleaning the environment?
Answer: Microorganisms decompose plant and animal waste into simpler substances. This recycles nutrients and helps clean the environment.
Q9. Why are legumes useful for soil fertility?
Answer: Legumes have root nodules containing Rhizobium bacteria. These bacteria trap nitrogen from air and convert it into a form useful for plants.
Q10. Why are viruses considered different from other microorganisms?
Answer: Viruses are acellular and cannot multiply on their own. They reproduce only inside the living cells of a host organism.
15. High Level Thinking Questions
Q1. What would happen if microorganisms did not exist on Earth?
Answer: Dead plants, animals and wastes would not decompose properly. Nutrients would not return to the soil, soil fertility would decrease, and natural recycling would be disturbed.
Q2. Why does a farmer grow legumes in rotation with other crops?
Answer: Legumes contain Rhizobium bacteria in their root nodules. These bacteria fix nitrogen and improve soil fertility, reducing the need for nitrogen fertilisers for the next crop.
Q3. A dough with yeast rises, but dough without yeast does not. Explain.
Answer: Yeast respires and breaks down sugar, releasing carbon dioxide gas. The gas forms bubbles and increases the volume of dough. Without yeast, this gas is not produced, so the dough does not rise.
Q4. Why does bread kept near a sink spoil faster than bread kept in a refrigerator?
Answer: The area near a sink is usually moist and warm, which supports microbial growth. The refrigerator has low temperature, which slows the growth of microorganisms.
Q5. Why can bacteria survive as unicellular organisms but humans cannot?
Answer: A bacterium can perform all life processes within one cell. Humans are multicellular and require specialised cells, tissues, organs and organ systems to perform complex functions.
Q6. Why is microalgae conservation important?
Answer: Microalgae produce a large amount of oxygen, support aquatic food chains, help clean water and can be used for food supplements and biofuel. Their loss would disturb environmental balance.
Quick Revision Box
- Microscope helped humans discover cells and microorganisms.
- Robert Hooke first used the word cell after observing cork.
- Antonie van Leeuwenhoek is called the Father of Microbiology.
- Cell membrane, cytoplasm and nucleus are basic parts of a cell.
- Plant cells have cell wall, chloroplasts and large vacuoles.
- Bacteria do not have a well-defined nucleus; they have a nucleoid.
- Microorganisms include bacteria, fungi, protozoa and some algae.
- Viruses are acellular and multiply only inside living host cells.
- Yeast makes dough soft and fluffy by releasing carbon dioxide.
- Lactobacillus converts milk into curd.
- Rhizobium fixes nitrogen in legume root nodules.
- Bacteria and fungi decompose waste and help clean the environment.
Interactive Quiz
Choose the correct answer and click “Submit Quiz” to check your score.
FAQs
What is the main idea of this chapter?
The chapter explains cells, microorganisms, their usefulness, harmful effects, and why viruses are different from other microbes.
Are all microorganisms harmful?
No. Many microorganisms are useful. They help in curd formation, baking, decomposition, nitrogen fixation, biogas production and oxygen production.
Why do we need a microscope?
We need a microscope to observe cells and microorganisms that are too small to be seen with the naked eye.
What is the difference between bacteria and viruses?
Bacteria are unicellular organisms, while viruses are acellular and multiply only inside a living host cell.
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