Class 8 Science NCERT Notes- Chapter 2: The Invisible Living World, Beyond Our Naked Eye (PDF | MindMap | Q&A | Quizes)

Here is a set of detailed revision notes based on Chapter 2, “The Invisible Living World: Beyond Our Naked Eye,” from the NCERT Class 8 Science book, followed by questions and answers to support student revision.

The Invisible Living World: Study Notes

Introduction to the Invisible World

• The human eye can only see objects above a certain size, leaving many small living things hidden.
• The invention of lenses and later the microscope enabled people to view these tiny organisms.
• Robert Hooke (1665) first used the word “cell” after observing cork under a microscope.
• Antonie van Leeuwenhoek built more advanced microscopes and discovered microorganisms like bacteria and blood cells, earning the title “Father of Microbiology”.

What is a Cell?

• All living things are built from cells.
• Basic cell structure includes:
  • Cell membrane: a thin outer covering that separates a cell from others and allows the exchange of materials.
  • Nucleus: controls cell activities and growth.
  • Cytoplasm: a jelly-like substance where life processes occur.
  • Cell wall: an extra, tough outer layer in plant cells for support and rigidity (not present in animal cells).
  • Vacuole: a large storage space in plant cells (smaller or absent in animal cells).
  • Plastids (in plants): include chloroplasts for photosynthesis.

Types and Variation of Cells

• Cells can differ in shape, size, and function.
  • Example: Muscle cells are spindle-shaped for contraction; nerve cells are long with branches for message transmission.
• Groups of similar cells form tissues; tissues form organs; organs combine into organ systems, making a complete organism.
• Multicellular organisms (e.g., humans, plants) begin life as a single cell (egg).

Microorganisms

• Microorganisms (or microbes) are organisms too small to see without a microscope, found everywhere: water, soil, air, and even inside living bodies.
  • Unicellular: Made of one cell (e.g., bacteria, Amoeba).
  • Multicellular: Some fungi and algae.
  • Viruses: Microscopic and acellular, can reproduce only inside a living cell.
• Types of microorganisms include bacteria, protozoa, fungi, and algae.

Observing Microbes

• Microorganisms can be observed by:
  • Examining pond water or soil suspension under a microscope.
  • Using special low-cost microscopes (foldscope) to explore microbial life.

Role of Microorganisms in Daily Life

• Decomposers: Fungi and bacteria break down dead plants and animals into simpler substances, forming manure and returning nutrients to the soil.
• Nitrogen fixation: Rhizobium bacteria in root nodules of legumes convert atmospheric nitrogen to a form plants can use, enriching soil naturally.

Microorganisms in Food

• Fermentation: Yeast (a fungus) and bacteria like Lactobacillus help in:
  • Making bread, idli, dosa, and curd.
  • Yeast produces carbon dioxide (makes dough rise) and alcohol (fermentation smell).
  • Lactobacillus ferments milk sugar into lactic acid (curd formation).

Microalgae

• Microalgae like Spirulina and Chlorella are nutritious, provide oxygen, and help clean water, and are used as human supplements.

Key Differences in Cells

Feature	Plant Cell	Animal Cell	Bacterial Cell
Cell Wall	Present	Absent	Present
Nucleus	Present (well-defined)	Present (well-defined)	Absent (nucleoid region)
Chloroplast	Present (some)	Absent	Absent
Vacuole	Large	Small or absent	Absent
Shape	Usually rectangular	Various	Various
Example	Onion cell	Cheek cell	Bacteria

Summary Points

• All living things are made of cells—the fundamental unit of life.
• Microorganisms are present in all environments and perform essential functions like decomposition and nitrogen fixation.
• Beneficial microbes help produce food, recycle nutrients, and clean the environment; some, however, can cause disease.
• Viruses differ from other microbes as they can only reproduce inside living hosts.

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Questions & Answers for Practice

Short Answers

Q1: Who is known as the Father of Microbiology? Why?
A1: Antonie van Leeuwenhoek is called the Father of Microbiology because he built better microscopes and was the first to observe and describe microorganisms such as bacteria and blood cells.

Q2: What are the main parts of a typical plant cell?
A2: The main parts are the cell wall, cell membrane, cytoplasm, nucleus, vacuole, and chloroplasts.

Q3: How do bacteria in root nodules help plants?
A3: Rhizobium bacteria in root nodules convert atmospheric nitrogen into forms plants can absorb, enriching the soil and supporting plant growth without the need for chemical fertilizers.

Q4: Why do bread and curd rise/change texture during preparation?
A4: Bread rises because yeast ferments sugar, producing carbon dioxide gas bubbles; curd forms as Lactobacillus bacteria ferment milk sugar into lactic acid, thickening and souring the milk.

Q5: What key feature distinguishes bacterial cells from plant and animal cells?
A5: Bacterial cells lack a well-defined nucleus and instead have a nucleoid region; they may also have a cell wall but no chloroplasts or vacuoles like plant cells.

Important Questions

Q6: List two functions of microorganisms in soil.
A6: Microorganisms decompose dead matter, converting it into nutrient-rich manure, and fix nitrogen, making it available to plants.

Q7: What is fermentation? Name one microorganism involved.
A7: Fermentation is the process by which microorganisms like yeast or bacteria convert sugars into gases (like carbon dioxide) or acids (like lactic acid), used in making bread and curd.

Q8: Why is Spirulina called a superfood?
A8: Spirulina is rich in protein (over 60%), vitamins (including B12), and is low in fat and sugar, making it a highly nutritious supplement.

Q9: Explain why curd forms faster in warm conditions.
A9: The bacteria involved in curdling milk (Lactobacillus) multiply and ferment lactose faster at warm temperatures, leading to quicker curd formation.

Q10: What do you call organisms made of only one cell? Give two examples.
A10: They are called unicellular organisms. Examples: Bacteria and Amoeba.

Mind-map of Chapter 2: The Invisible Living World: Beyond Our Naked Eye

Here is a quiz based on Chapter 2, “The Invisible Living World: Beyond Our Naked Eye,” from the NCERT Class 8 Science textbook.

Quiz: The Invisible Living World: Beyond Our Naked Eye

Multiple Choice & Short Answer Questions

1. Who coined the term ‘cell’ in science and what did he observe under the microscope?
   a) Antonie van Leeuwenhoek
   b) Robert Hooke
   c) Louis Pasteur
   d) Alexander Fleming
2. What is the function of the cell wall in plant cells?
   a) Controls cell growth
   b) Provides rigidity and strength
   c) Produces energy
   d) Carries messages
3. Microorganisms that can be seen only with a microscope are called _________.
4. Which microorganism is used in making bread fluffy?
   a) Bacteria
   b) Virus
   c) Yeast
   d) Algae
5. Name any one beneficial role of Rhizobium bacteria in soil.
6. What makes curd sour when left at room temperature for several hours or days?
   a) Production of carbon dioxide
   b) Growth of Lactobacillus bacteria
   c) Addition of yeast
   d) High temperature only
7. True or False: All microorganisms are harmful to living beings.
8. Define unicellular organism and give one example from the chapter.
9. List out the correct sequence of level of organisation in living beings:
   a) Organism → Organ → Cell → Tissue → Organ system
   b) Cell → Organ → Tissue → Organ system → Organism
   c) Cell → Tissue → Organ → Organ system → Organism
   d) Tissue → Cell → Organ system → Organism → Organ
10. Which pigment gives green colour to plant cells and helps in photosynthesis?
    a) Hemoglobin
    b) Chlorophyll
    c) Melanin
    d) Carotene

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Answer Key

1. b) Robert Hooke – He observed small empty spaces in a cork and called them ‘cells’.
2. b) Provides rigidity and strength.
3. Microorganisms or microbes.
4. c) Yeast.
5. Converts atmospheric nitrogen into forms plants can use (nitrogen fixation).
6. b) Growth of Lactobacillus bacteria.
7. False – Many are beneficial in decomposition, food, and soil fertility.
8. Organisms made of a single cell; Example: Amoeba or bacteria.
9. c) Cell → Tissue → Organ → Organ system → Organism.
10. b) Chlorophyll.

Briefing: The Invisible Living World – Beyond Our Naked Eye

This briefing summarizes key themes and important facts based on Chapter 2, “The Invisible Living World: Beyond Our Naked Eye,” from the NCERT Class 8 Science book, focusing on the discovery of the microscopic world, the fundamental unit of life (the cell), and the diverse roles of microorganisms in our environment and daily lives.

I. Unveiling the Invisible World: The Power of Lenses and Microscopes

For a long time, the human eye’s limitations meant that “many tiny things around us remained unknown.” The invention and improvement of lenses and microscopes were pivotal in revealing this hidden realm.

• Early Discoveries: People initially found that a “curved piece of glass could make small things look bigger,” leading to the term “lens” (shaped like a lentil seed).
• The “invention of the microscope opened a fascinating hidden world filled with tiny living creatures.”
• Key Pioneers: Robert Hooke (1665): Published “Micrographia,” detailing observations through his microscope, which magnified objects “200 to 300 times bigger.” He was the first to observe and coin the term “cell” after seeing “many small, empty spaces” in a thin slice of cork that reminded him of a honeycomb.
• Antonie van Leeuwenhoek (1660s): A Dutch scientist who “made better lenses” and was the “first person to clearly see and describe tiny living things like bacteria and blood cells.” He is recognized as the “Father of Microbiology.”

II. The Cell: The Basic Unit of Life

The fundamental concept established is that “All living beings are made up of cells.” The document details the basic structure of cells and highlights variations.

• Core Components of a Cell:Cell Membrane: The “thin outer lining” that encloses the cytoplasm and nucleus, separating one cell from another. It is “porous and allows the entry of materials essential for life processes and the exit of waste material.”
• Cytoplasm: “The space between the cell membrane and nucleus” filled with other cell components and compounds like “carbohydrates, proteins, fats, and mineral salts.” “Most of the life processes take place within the cytoplasm.”
• Nucleus: A “small round structure inside” the cell, covered by a thin membrane, which “regulates all activities that occur within the cell” and “regulates growth.”
• Variations in Cell Structure: Plant Cells: Possess an “extra outer layer called the cell wall,” which “provides rigidity and strength to plants.” They also contain “plastids” (some like chloroplasts have chlorophyll for photosynthesis) and a large “vacuole” for storage, waste disposal, and maintaining shape.
• Animal Cells: Generally “not present” with large vacuoles, or if present, they are “usually small.” Animal cells lack cell walls and chloroplasts.
• Bacterial Cells: Do not have a “well-defined nucleus and a nuclear membrane,” instead possessing a “nucleoid.”
• Cell Diversity and Function: Cells vary significantly in “shape, size, and structure,” which are directly related to their “specific functions.”
• Examples: Muscle cells are spindle-shaped for contraction, nerve cells are long and branched to carry messages, and flat cheek cells form protective linings.

III. Levels of Organization in Living Organisms

Living organisms exhibit a hierarchical organization, building from the most basic unit to a complex being.

1. Hierarchy:Cell: The “basic unit of life.”
2. Tissue: “A group of similar cells forms a type of tissue.”
3. Organ: “Different tissues are organised to form an organ.”
4. Organ System: “Several organs work together to form an organ system that performs a major function of the body.”
5. Organism: “All the organ systems together make up a complete organism.”

• Multicellular Organisms: Organisms like humans, animals, and plants are “made up of many cells.” Their life begins with a “single cell— ‘egg’” which “divides repeatedly to form a complete living being.” The ostrich egg’s yolk is highlighted as the “largest known cell” (130-170 mm in diameter).

IV. The World of Microorganisms: Ubiquitous and Diverse

Microorganisms are tiny living beings, often invisible to the naked eye, found almost everywhere and exhibiting significant diversity.

• Definition: “Some living organisms are made up of just one or very few cells. They are so small that they can not be seen with the naked eye. These are called microorganisms.”
• Visibility: Require microscopes (including low-cost foldscopes) for observation, as they “magnifies their size and makes them visible to us.”
• Types and Examples:Unicellular: Bacteria and Amoeba (made of just one cell).
• Multicellular: Some fungi and algae (have many cells).
• Categories: Protozoa, algae, fungi, and bacteria.
• Viruses: Described as “microscopic and acellular,” they “multiply when they enter a living cell” and “may infect plants, animals, or bacterial cells and may cause a disease.”
• Habitat: Found “all around us — in water, soil, air, and even inside our body!” They can survive in “extreme climatic conditions, such as hot water springs and snow cold zones as well as at moderate temperatures.”

V. Beneficial Roles of Microorganisms in Our Lives

Microorganisms, despite their small size, play “an important role in our lives,” particularly in environmental processes and food production.

• Environmental Cleanup (Recycling Nutrients): Microorganisms, especially “fungi and bacteria, act on the plant waste and slowly break it down into simpler, nutrient-rich manure.”
• They “decompose bodies of dead animals,” thereby “help recycle the waste and return important nutrients to nature.”
• Biogas Production: Some bacteria and fungi in oxygen-free environments decompose waste, releasing “a mixture of gases containing carbon dioxide, and a high proportion of another gas, methane,” which is used as fuel.
• Oil Spill Cleanup: Dr. Ananda Mohan Chakrabarty developed a bacterium to “break down oil spills, helping to clean the environment.”
• Food Production: Fermentation (Yeast): Yeast, a type of fungus, respires and produces “carbon dioxide,” which creates “bubbles that makes the dough soft and fluffy” in breads, cakes, and pastries. It also produces a “small amount of alcohol.”
• Fermentation (Bacteria): Bacteria like Lactobacillus ferment milk to form curd by feeding on lactose and producing “lactic acid.” Other bacteria aid in fermenting batter for idli, dosa, and bhatura.
• Agriculture and Soil Fertility: Nitrogen Fixation: Certain bacteria, like Rhizobium, live in “root nodules” of legumes (beans, peas, lentils) and “trap nitrogen from the air and make it useful for the plants,” reducing the need for chemical fertilizers.
• Health and Biofuel: Microalgae: These “microscopic plant-like organisms” produce “more than half of the Earth’s oxygen supply” through photosynthesis. They are nutrient-rich food sources for aquatic animals and can be used as “health supplements and medicines” (e.g., Spirulina, Chlorella, Diatoms) and to “make biofuel.”
• Gut Bacteria: Our intestine contains “many bacteria that help in digestion.”

VI. Conclusion

The document effectively illustrates that the “invisible living world” is incredibly complex and vital. From the historical breakthrough of microscopic observation to the intricate cellular structures and the diverse, often beneficial, roles of microorganisms, the source underscores the interconnectedness of all life and the fundamental importance of the cell as its basic building block. Understanding these tiny entities is crucial for comprehending biological processes, maintaining environmental health, and developing new solutions to global challenges.