CBSE Class 8 Science Notes Chapter 2 PDF Download

CBSE Class 8 Science Notes Chapter 2 The Invisible Living World  Beyond Our Naked Eye introduces students to the hidden world of microorganisms that cannot be seen without a microscope. This chapter helps learners understand how bacteria, viruses, fungi, and other microbes exist all around us in air, water, food, and even inside living organisms.

 These notes explain their structure, types, roles (both useful and harmful), and their impact on human life, health, and the environment in a simple and exam-friendly manner.

What are Microorganisms?

Microorganisms are microscopic living entities present everywhere around us—in the air we breathe, the water we drink, the soil beneath our feet, and even inside our bodies.

• Discovery: The credit for discovering this hidden world goes to scientists like Antonie van Leeuwenhoek, who first observed microbes under a simple microscope.

• Habitats: Microbes are highly resilient. They can survive in all types of environments, ranging from ice-cold climates to hot springs, and from dry deserts to marshy lands.

Major Groups of Microorganisms

Microorganisms are broadly classified into five major groups based on their cellular structures and characteristics:

A. Bacteria

• Single-celled (unicellular) prokaryotic organisms.

• They come in various shapes: spherical (cocci), rod-shaped (bacilli), and spiral (spirilla).

• Examples: Lactobacillus, Rhizobium, E. coli.

B. Fungi

• Plant-like organisms that lack chlorophyll and cannot perform photosynthesis. They can be unicellular or multicellular.

• They feed on dead and decaying organic matter (saprophytes).

• Examples: Yeast, Bread mould, Penicillium, Mushrooms.

C. Protozoa

• Unicellular eukaryotic organisms that are mostly aquatic and display animal-like behaviour (movement and capturing food).

• Examples: Amoeba, Paramecium, Plasmodium (the malarial parasite).

D. Algae

• Simple, plant-like organisms containing chlorophyll. They are autotrophic and perform photosynthesis.

• Examples: Chlamydomonas and Spirogyra are algae. Blue-green algae (cyanobacteria) are photosynthetic bacteria and are discussed separately in relation to nitrogen fixation.

E. Viruses

• Viruses are unique because they are considered a borderline link between living and non-living things.

• They are inert outside a host body but reproduce rapidly once inside the cells of a host organism (plant, animal, or bacterium).

• Examples: Influenza virus, HIV, Bacteriophage, Coronavirus.

Friendly Microorganisms: The Good Side

Microbes are not just entities that cause sickness; many play a vital role in our daily lives, industries, and ecosystems.

I. Food Industry

• Making Curd: The bacterium Lactobacillus promotes the formation of curd from milk by multiplying and converting lactose sugar into lactic acid.

• Baking: Yeast reproduces rapidly and produces carbon dioxide gas during respiration. This gas bubbles out, causing dough to rise, making bread, cakes, and pastries fluffy.

II. Commercial and Industrial Uses

• Fermentation: Microbes are used for the large-scale production of alcohol, wine, and acetic acid (vinegar). Yeast converts natural sugars present in grains like barley, wheat, and rice into alcohol—a process discovered by Louis Pasteur.

III. Medicinal Applications

• Antibiotics: Medicines manufactured from fungi and bacteria that kill or stop the growth of disease-causing microbes. Penicillin was the first antibiotic discovered by Alexander Fleming.

• Vaccines: When a dead or weakened microbe is introduced into a healthy body, the body produces antibodies to fight it off. The body remembers how to fight the microbe if it attacks again, providing immunity. Vaccines prevent diseases like cholera, tuberculosis, smallpox, and hepatitis.

IV. Environmental and Agricultural Uses

• Cleaning the Environment: Decomposers like bacteria and fungi break down dead organic waste from plants and animals into simple substances, recycling nutrients back into the soil.

• Increasing Soil Fertility: Certain bacteria and blue-green algae fix atmospheric nitrogen into nitrogenous compounds, enriching the soil and increasing its fertility.

Harmful Microorganisms: The Bad Side

Microorganisms that cause diseases in living organisms are called pathogens.

I. Diseases in Humans

Pathogens can enter our bodies through air, water, food, or direct contact. Diseases that can spread from an infected person to a healthy person are called communicable diseases.

• Airborne: Common cold, Tuberculosis, Chickenpox.

• Waterborne: Cholera, Typhoid.

• Carriers/Vectors: Insects or animals that carry pathogens without suffering from the disease themselves. For example, the female Anopheles mosquito carries the malaria parasite (Plasmodium), and the female Aedes mosquito acts as a carrier for the Dengue virus.

II. Diseases in Animals and Plants

• Animals: Anthrax is a dangerous human and cattle disease caused by a bacterium. Foot and mouth disease of cattle is caused by a virus.

• Plants: Microbes reduce crop yields. Examples include Citrus canker (bacterial), Rust of wheat (fungal), and Yellow vein mosaic of bhindi/okra (viral).

Food Preservation

Since microbes spoil our food, producing foul smells and toxic substances, food preservation is critical to prevent food poisoning. Common methods include:

1. Chemical Method: Adding preservatives like salt and edible oils to check microbial growth. Sodium benzoate and sodium metabisulphite are standard chemical preservatives.

2. Preservation by Common Salt: Used for meat, fish, amla, raw mangoes, and tamarind.

3. Preservation by Sugar: Reduces moisture content, which inhibits the growth of bacteria in jams, jellies, and squashes.

4. Heat and Cold Treatments: Boiling kills many microorganisms, while refrigeration keeps temperatures low, slowing down microbial reproduction.

5. Pasteurization: Milk is heated to about 70°C for 15 to 30 seconds and then suddenly chilled. This process kills microbes without changing the flavor of the milk (discovered by Louis Pasteur).

6. Storage and Packing: Airtight sealed packets prevent micro-organism attacks.

The Nitrogen Cycle

Atmospheric nitrogen (78%) cannot be taken directly by plants and animals. The Nitrogen Cycle ensures a constant circulation of nitrogen through the atmosphere, soil, and living organisms:

1. Nitrogen Fixation: Rhizobium bacteria (living in the root nodules of leguminous plants like peas and beans) and blue-green algae convert atmospheric nitrogen into usable nitrogen compounds in the soil. Lightning strikes also fix nitrogen.

2. Assimilation: Plants absorb these nitrogen compounds from the soil to form plant proteins. Animals eat these plants to gain protein.

3. Excretion & Decomposition: When plants and animals die, fungi and bacteria in the soil convert nitrogenous wastes back into simple compounds for reuse.

4. Denitrification: Certain specialized bacteria convert some compounds back into nitrogen gas, releasing it into the atmosphere, keeping the total percentage of atmospheric nitrogen constant.

The Invisible Living World: Beyond Our Naked Eye PDF Download

Below is the PDF link to download CBSE Class 8 Science Notes Chapter 2 The Invisible Living World: Beyond Our Naked Eye. Students can use this study material for quick revision, understanding microorganisms, and strengthening NCERT concepts for exams.

How to Prepare CBSE Class 8 Science Notes Chapter 2 Effectively?

Since CBSE Class 8 Science Notes Chapter 2 – The Invisible Living World: Beyond Our Naked Eye is based on microorganisms and their effects on life, students should focus on understanding concepts rather than rote memorisation.

• Understand the concept of microorganisms and their types (bacteria, fungi, protozoa, algae, viruses).

• Learn where microorganisms are found in everyday life (air, water, soil, inside the body)

• Focus on the difference between useful and harmful microorganisms with real-life examples.

• Revise how microorganisms help in food production (curd, bread) and medicine (antibiotics).

• Understand how disease-causing microbes spread and basic prevention methods.

• Learn key methods of protection like hygiene, vaccination, clean water, and proper cooking.

• Practice NCERT diagrams and examples related to microbes and infections.

• Revise classification tables to remember types and functions easily.