Class 8 chapter 2 Invisible world

 

Probe and Ponder: Invisible Living Beings

 What are Invisible Living Beings?

These are tiny organisms called microorganisms that cannot be seen with naked eyes.
They can only be observed using a microscope.

 Types of Microorganisms

• Bacteria 
• Fungi 
• Protozoa
• Algae

 Helpful Microorganisms 

• Help in making curd, bread, cheese
• Used in medicines (like antibiotics)
• Help in decomposition (cleaning environment)
• Improve soil fertility

Harmful Microorganisms 

• Cause diseases like:
  • Cholera
  • Tuberculosis
• Spoil food
• Can infect plants and animals

How do we study them? 

• Using instruments like a microscope
• Scientists observe their shape, movement, and behavior

 Discovery of the Invisible World

Humans discovered a hidden world of tiny objects using optical instruments like lenses and microscopes.

 Lenses 

Definition:
A lens is a transparent curved piece of glass or plastic that bends light.

Types of Lenses:

• Convex Lens (Converging lens)
• Brings light rays together
• Used to magnify objects
• Concave Lens (Diverging lens)
  • Spreads light rays
  • Used in spectacles for certain vision problems

 Lenses work on the principle of Refraction

Magnifying Glass 

Definition:
A magnifying glass is a simple device with a convex lens used to see small objects clearly.

Features:

• Makes objects appear bigger and clearer
• Used by students, scientists, and detectives

Example:

• Reading small letters
• Observing insects

Microscope 

Definition:
A microscope is an instrument used to see very tiny objects that cannot be seen with naked eyes.

Types:

• Simple Microscope → uses one lens
• Compound Microscope → uses multiple lenses (more powerful)

What can we see?

• Bacteria 
• Cells
• Tiny structures

Importance of Discovery 

• Helped scientists discover microorganisms
• Improved medicine and healthcare
• Helped understand cells and life processes

Robert Hooke

 Who was he?

• An English scientist (17th century)
• One of the first scientists to study tiny structures

 Famous Work:

• Wrote a book called Micrographia

 Important Discovery:

• Observed cork under a microscope
• Saw tiny box-like structures and named them “cells”

 This was the first discovery of cells.

Antonie van Leeuwenhoek

Who was he?

• A Dutch scientist
• Known as the “Father of Microbiology”

 Contribution:

• Made powerful microscopes by himself
• First to observe living microorganisms

 What did he see?

• Bacteria 
• Protozoa
• Tiny living organisms in water

Why are they important?

• Helped discover the invisible world of cells and microbes
• Laid the foundation for modern biology and medicine

Cell:-

Definition:
A cell is the smallest unit of life that can perform all basic life processes.

Cells are called the building blocks of life because all living organisms are made up of cells.

Observation of Cells

1. Onion Peel Cells

• A thin layer from an onion can be observed under a microscope.
• Cells appear rectangular and arranged like a grid.
• A cell wall is clearly visible.

2. Cheek Cells

• Taken from the inner lining of the human cheek.
• Cells appear round or irregular in shape.
• Do not have a cell wall, only a cell membrane.

Basic Parts of a Cell

• Cell membrane: Controls entry and exit of substances
• Cytoplasm: Jelly-like substance where activities occur
• Nucleus: Controls cell functions and contains genetic material

Variation in Shape and Structure of Cells:-

Cells are not all the same. Their shape and structure vary depending on the function they perform.

1. Muscle Cells

• Muscle cells are long and cylindrical in shape.
• They have the ability to contract and relax, which helps in movement.
• Found in muscles like arms, legs, and heart.

Types of Muscle Cells:

• Striated muscles – Found in skeletal muscles (voluntary movement)
• Smooth muscles – Found in internal organs (involuntary movement)
• Cardiac muscles – Found in the heart (rhythmic contraction)

Function:

• Help in body movement and pumping of blood.

2. Nerve Cells (Neurons)

• Nerve cells are long and branched.
• They have three main parts:
  • Cell body
  • Dendrites (receive signals)
  • Axon (transmits signals)

Special Feature:

• Can carry messages (nerve impulses) quickly from one part of the body to another.

Function:

• Control and coordinate body activities.

Levels of Organisation

Cell → Tissue → Organ → Organ System → Organism

Living organisms are highly organized. Smaller units combine to form larger and more complex structures.

Tissue

• A tissue is a group of similar cells working together to perform a specific function.

Examples:

• Muscle tissue – helps in movement
• Nervous tissue – carries messages
• Plant tissues – xylem, phloem

Organ

• An organ is made up of different types of tissues working together.

Examples:

• Heart – pumps blood
• Lungs – help in breathing
• Leaf (in plants) – performs photosynthesis

Organ System

• An organ system is a group of organs working together to perform a major function.

Examples:

• Digestive system – breaks down food
• Respiratory system – helps in breathing
• Circulatory system – transports blood

Organism

• An organism is a complete living being made up of organ systems.

Examples:

• Humans
• Animals
• Plants

Microorganisms

Microorganisms (or microbes) are very tiny living organisms that cannot be seen with the naked eye. They can be seen using a microscope.

Types Based on Cell Number

1. Unicellular Microorganisms

• Made up of only one cell
• That single cell performs all life functions (nutrition, respiration, reproduction)

Examples:

• Protozoa – Amoeba, Paramecium
• Bacteria – Lactobacillus, E. coli
• Some Algae – Chlamydomonas

2. Multicellular Microorganisms

• Made up of many cells
• Different cells perform different functions

Examples:

• Some Algae – Spirogyra
• Fungi – Mushroom, Bread mold

1. Protozoa

• Unicellular organisms
• Mostly found in water
• Example: Amoeba. 

2. Algae

• Can be unicellular or multicellular
• Make their own food by photosynthesis
• Found in water or moist places
• Example: Chlamydomonas, Spirogyra

3. Fungi

• Mostly multicellular (some unicellular like yeast)
• Cannot make their own food
• Grow on dead and decaying matter
• Example: Mushroom, Yeast

4. Bacteria

• Unicellular
• Found everywhere (air, water, soil, inside body)
• Some are useful, some are harmful

5. Viruses

• Very tiny and non-living outside host
• Show life only inside a living cell
• Cause diseases (like cold, flu)

How Are We Connected to Microbes?

Microorganisms are everywhere—around us, inside us, and even in the food we eat. Our connection with microbes can be understood in three main areas:

1. Microbes in the Environment

Microbes play a huge role in maintaining balance in nature.

• Decomposition: Bacteria and fungi break down dead plants and animals into simpler substances. This recycles nutrients back into the soil.
• Nitrogen Cycle: Certain bacteria help convert nitrogen into forms plants can use.
• Oxygen Production: Some algae produce oxygen through photosynthesis.

 Without microbes, waste would pile up and ecosystems would collapse.

 2. Microbes in Food

Microorganisms are widely used in preparing many foods.

• Curd/Yogurt: Made by bacteria like Lactobacillus
• Bread: Yeast (a fungus) helps dough rise
• Cheese & Pickles: Prepared using microbes

 Microbes help in fermentation, which improves taste, texture, and preservation of food.

3. Microbes in Health

Microbes affect our health in both positive and negative ways.

 Useful Roles:

• Help in digestion (good bacteria in our intestine)
• Used to make medicines like Penicillin
• Help in boosting immunity

Harmful Roles:

• Cause diseases like
• Tuberculosis
• Cholera
• COVID-19

Key Players in Cleaning the Environment :

• Decomposition:
  Microbes (bacteria, fungi) break down dead plants and animals into simple substances, recycling nutrients.
• Manure:
  Microorganisms convert organic waste into compost/manure, which improves soil fertility.
• Biogas:
  Bacteria decompose waste (like dung) to produce biogas (methane) used as fuel; leftover slurry is used as manure.

Our Scientific Heritage

1. ‘Krimi’ in the Vedas

• In ancient Indian texts like the Vedas and Ayurveda, the term “Krimi” was used for tiny living organisms.
• These texts recognized that such organisms could cause diseases and also exist in the environment.
• This shows early awareness of microorganisms long before modern microbiology developed.

2. Ananda Mohan Chakrabarty and Oil-Spill Bacteria

• Ananda Mohan Chakrabarty developed a genetically engineered bacterium capable of breaking down crude oil.
• This bacterium is used to clean oil spills in oceans, a process known as bioremediation.
• His work was a major achievement in biotechnology and environmental protection.