Microorganisms Class 8 Notes: NCERT Structure, Diseases & Best Online Science Classes Portal

The microscopic world around us is teeming with life forms invisible to the naked eye. These tiny living structures are known as microorganisms or microbes. While some play an indispensable role in making curd, baking bread, and producing life-saving medicines, others act as pathogens that cause severe health issues.

For Class 8 students looking to secure top marks in school exams or build a solid baseline for secondary school science, mastering the classification of microbes, the history of antibiotics, and disease transmission routes is essential. This comprehensive guide breaks down the core groups of the microscopic kingdom.

Human Disease	Causative Microorganism	Mode of Transmission	Essential Preventive Measures
Tuberculosis (TB)	Bacterium (Mycobacterium tuberculosis)	Air / Droplets (Coughing, sneezing)	Isolate the patient; BCG vaccination at an early age.
Measles	Virus	Air / Direct Contact	Maintain personal hygiene; timely MMR vaccination.
Chickenpox	Virus (Varicella-Zoster)	Air / Direct Physical Contact	Keep the patient's personal items strictly separate.
Cholera	Bacterium (Vibrio cholerae)	Contaminated Water / Food	Drink boiled drinking water; consume properly cooked food.
Typhoid	Bacterium (Salmonella typhi)	Contaminated Water	Maintain clean sanitation; get typhoid vaccinations.
Hepatitis-B	Virus (Hepatitis B Virus)	Body Fluids / Infected Blood	Use sterilized needles; get vaccinated against Hepatitis B.

1. What are Protozoa? Classification and Locomotion

Protozoa are a group of single-celled, eukaryotic microorganisms that exhibit animal-like characteristics. They are heterotrophic, meaning they cannot manufacture their own food and must capture external organic material. They are primarily found in moist or aquatic environments like ponds, lakes, rivers, and damp soils.

Because they must hunt for food, protozoa are classified based on their specialized locomotory organs (the structures they use to move around):

Classification of Protozoa

Amoeboid Protozoans (Using Pseudopodia): 

These organisms have flexible, ever-changing cell bodies. They extend temporary cytoplasmic projections called pseudopodia (false feet) to creep along surfaces and engulf food particles.

Example: Amoeba, Entamoeba histolytica.

Ciliated Protozoans (Using Cilia): 

The bodies of these protozoans are covered in thousands of tiny, hair-like structures called cilia. The coordinated, rhythmic beating of these cilia moves the organism forward and sweeps nutrient particles directly into its cellular mouth.

Example: Paramecium.

Flagellated Protozoans (Using Flagella): 

These microbes possess long, whip-like structures called flagella. By lashing the flagellum back and forth, the cell can swim efficiently through liquid mediums. Many flagellated protozoans live as parasites.

Example: Euglena, Trypanosoma (which causes sleeping sickness).

Sporozoans (No Specialized Locomotory Organs): 

These are exclusively parasitic protozoa that do not possess any visible locomotive structures in their adult phase. They form tiny, resilient spores to transfer from one host organism to another.

Example: Plasmodium (the notorious malarial parasite).

2. Algae: Characteristics, Pigments, and Types

Algae are simple, plant-like organisms that contain chlorophyll. Unlike terrestrial plants, they have simple, undifferentiated bodies called a thallus—meaning they lack true roots, stems, or leaves. They are autotrophic, running solar-powered photosynthesis to generate their own sugars, and live predominantly in moist or aquatic habitats.

Important Characteristics of Algae:

Cellular Structure: Can exist as microscopic unicellular cells (like Chlamydomonas) or massive multicellular colonies and filaments (like Spirogyra and seaweeds).

Cell Walls: Composed of a rigid layer of cellulose.ff

Storage: Primarily store energy reserves in the form of starch.

Major Types of Algae

Algae are grouped into three primary classes based on the dominant photosynthetic pigments contained within their cells:

Green Algae (Chlorophyceae): 

Contain abundant amounts of chlorophyll a and b, giving them a bright grass-green color. They are common in freshwater bodies.

Examples: Chlamydomonas, Spirogyra, Volvox.

Brown Algae (Phaeophyceae): 

Possess a brown-olive shade due to large concentrations of xanthophyll and a unique pigment called fucoxanthin. They are almost exclusively marine seaweeds.

Examples: Fucus, Laminaria, Sargassum.

Red Algae (Rhodophyceae): 

Characterized by a brilliant red tint produced by a specialized masking pigment called phycoerythrin, allowing them to photosynthesize at great ocean depths.

Examples: Gelidium, Gracilaria (used to harvest commercial agar).

3. Structure and Nutrition in Fungi: Mycelium and Hyphae

Fungi represent a distinct kingdom of heterotrophic eukaryotic organisms that lack chlorophyll, meaning they are completely incapable of photosynthesis. Instead, they survive through saprophytic nutrition (absorbing dissolved organic matter from decaying leaves, wood, and organic waste) or parasitic relationships. Common examples include mushrooms, molds, and unicellular yeast.

What is Mycelium and Hyphae?

With the exception of unicellular yeasts, the structural body of a fungus is built out of a delicate, microscopic network of threads.

Hypha (plural: Hyphae):

 A hypha is a thin, transparent, elongated thread-like tubular filament. The walls are reinforced with a tough structural polymer called chitin. These tubes contain cytoplasm and nuclei, and grow continuously from their tips.

Mycelium (plural: Mycelia): 

When thousands of individual hyphae grow, branch out, and intertwine into a dense, felt-like vegetative mat, the entire network is called a mycelium. The mycelium spreads directly through food sources (like stale bread or forest soil), secreting external enzymes to digest nutrients before absorbing them through the hyphal walls.

4. Antibiotics: The Discovery of Penicillin and Their Importance

An antibiotic is a specialized chemical substance produced by certain beneficial microorganisms (mainly fungi and bacteria) that can kill or stop the growth of harmful, disease-causing bacteria inside an infected host.

The Historic Discovery of Penicillin

Before the mid-20th century, simple bacterial infections were frequently fatal. The medical landscape changed entirely in 1928 due to a famous chance observation by Scottish scientist Alexander Fleming:

Fleming was conducting research on a disease-causing bacterium named Staphylococcus.

He noticed that one of his unwashed culture plates had grown a contamination patch of green mold called Penicillium notatum.

Upon closer inspection under his microscope, Fleming observed a clear zone around the mold where the bacteria could not survive.

He deduced that the mold was secreting a chemical substance that actively destroyed the bacterial cell walls. This extracted substance was named Penicillin, becoming the world's very first commercial life-saving antibiotic.

Importance and Precautions of Antibiotics

Antibiotics are highly effective weapons against bacterial illnesses like tuberculosis, cholera, and typhoid. However, they must be used responsibly:

Ineffective Against Viruses: Antibiotics target specific bacterial structures (like cell walls). Therefore, they are completely useless against viral infections like the common cold, flu, or chickenpox.

The Danger of Misuse: Taking antibiotics without a doctor's prescription or terminating a course early allows tough bacteria to survive. These survivors mutate, developing antibiotic resistance, turning minor bugs into untreatable "superbugs."

5. Microorganisms and Human Diseases: 

Transmission Routes

Pathogenic microbes utilize various transmission routes to move from infected individuals to fresh hosts. Understanding these vectors is a frequent focus of school biology exams.

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