Notes Chapter 2 Science Curiosity Class 8 CBSE Board

The Invisible Living World: Beyond Our Naked Eye

Introduction

• Our eyes can only see objects above a certain size. Tiny living things, too small to see, remained hidden until special tools were invented.
• A lens (a curved piece of glass, thick in the middle and thin at the edges) was used to make small things look bigger.
• Over time, lenses improved, leading to the invention of microscopes, which revealed a hidden world of tiny creatures called organisms.

The Discovery of the Microscopic World

Magnifying Glass: A simple tool that makes small objects look bigger. For example, looking at an ant through a magnifying glass shows its body details clearly.

Microscope: A powerful tool that magnifies objects 200-300 times or more, allowing us to see tiny organisms.

Robert Hooke (1655):

• Published a book called Micrographia.
• Used a microscope to observe a thin slice of cork and saw tiny, empty spaces that looked like a honeycomb.
• Named these spaces cells, the first time this term was used in science.

Antonie van Leeuwenhoek:

• Made better lenses and microscopes.
• First to see and describe tiny living things like bacteria and blood cells.
• Known as the Father of Microbiology.

What is a Cell?

• Cells are the basic building blocks of all living organisms (plants, animals, and microorganisms).
• All organisms, whether tiny or large, are made of cells.
• Cells can only be seen under a microscope because they are very small.

Structure of a Cell

1. Main Parts of a Cell:

Cell Membrane: A thin outer layer that surrounds the cell, separating it from other cells. It is porous, allowing essential materials to enter and waste to exit.

Cytoplasm: The jelly-like substance inside the cell where most life processes happen. It contains components like carbohydrates, proteins, fats, and minerals.

Nucleus: A small, round structure inside the cell, covered by a thin membrane. It controls all cell activities and regulates growth.

2.Plant Cells:

• Have an extra outer layer called the cell wall, which gives strength and rigidity.
• Contain plastids (e.g., chloroplasts with chlorophyll for photosynthesis, making them green).
• Have a large vacuole to store substances, remove waste, and maintain cell shape.

3.Animal Cells:

• Do not have a cell wall or chloroplasts.
• May have small vacuoles for storing substances.

4.Bacterial Cells:

• Have a cell wall but no well-defined nucleus. Instead, they have a nucleoid (a region where genetic material is stored).

Activity 2.2: Observing Onion Peel Cells

Steps:

1. Take an onion bulb, cut it, and peel off a thin, transparent layer (onion peel).

2.Place the peel in a petri dish with safranin (a red stain) for 30 seconds to color the cells.

3.Rinse the peel in water to remove extra stain.

4.Place the peel on a glass slide, add a drop of glycerin (to prevent drying), and cover with a coverslip.

5.Observe under a microscope.

Observation: Onion peel cells are rectangular, closely packed with no spaces, and have a cell wall, cell membrane, cytoplasm, and nucleus.

Activity 2.3: Observing Human Cheek Cells

Steps:

1. Rinse your mouth with water.

2.Gently scrape the inside of your cheek with a toothpick.

3.Place the scraped material in a drop of water on a slide and spread it.

4.Add a drop of methylene blue (a blue stain) to improve visibility.

5. Add glycerin, place a coverslip, and observe under a microscope.

Observation: Cheek cells are polygon-shaped, with a cell membrane, cytoplasm, and nucleus. Unlike plant cells, they lack a cell wall and chloroplasts.

Differences Between Plant and Animal Cells

Variation in Cell Shape and Structure

Cells have different shapes and structures based on their functions:

• Muscle Cells: Spindle-shaped, help in movement (e.g., pushing food in the digestive system).
• Nerve Cells (Neurons): Long and branched, carry messages across the body.
• Plant Cells: Can be rectangular, elongated, oval, or tube-like (e.g., tube-like cells carry water in plants).

The shape and structure of a cell are related to its specific function.

Levels of Organization in Living Organisms

• Cell: The basic unit of life (like a brick in a wall).
• Tissue: A group of similar cells performing a specific function.
• Organ: Different tissues working together (e.g., stomach).
• Organ System: Several organs working together (e.g., digestive system).
• Organism: All organ systems together form a complete living being (e.g., a human or plant).
• Example: In the digestive system, muscle cells in the food pipe push food, while stomach cells produce digestive juices to break it down.

Multicellular vs. Unicellular Organisms

1. Multicellular Organisms: Made of many cells (e.g., plants, animals, humans).

• Cells have specialized functions but work together for survival.
• Example: The ostrich egg yolk is the largest known single cell (130-170 mm in diameter).

2.Unicellular Organisms: Made of one cell (e.g., bacteria, Amoeba).

• The single cell performs all life functions.

Microorganisms

Microorganisms (or microbes) are tiny organisms invisible to the naked eye, seen only under a microscope (100-400 times magnification).

Types of microorganisms:

1.Bacteria: Unicellular, have a cell wall, no defined nucleus (nucleoid instead).

2.Protozoa: Unicellular, move and feed in water or soil.

3.Fungi: Unicellular (e.g., yeast) or multicellular (e.g., mould), have a cell wall but no chloroplasts.

4.Algae: Unicellular or multicellular, make food using sunlight (photosynthesis).

5.Viruses: Acellular (not made of cells), multiply only inside a host cell, can cause diseases.

Activity 2.4: Observing Pond Water

Collect pond or stagnant water, place a drop on a slide, cover with a coverslip, and observe under a microscope.

Observation: Tiny moving organisms (e.g., protozoa, algae) are visible.

Activity 2.5: Observing Soil Suspension

Mix moist soil with water, let it settle, take a drop from the top layer, and observe under a microscope.

Observation: Small moving organisms like those in pond water, showing microbes live in soil too.

Role of Microorganisms in Our Lives

1. Cleaning the Environment

Activity 2.7: Mix fruit/vegetable peels with soil, cover, and observe after 2-3 weeks.

• Result: Peels turn into dark, nutrient-rich manure due to microbes (fungi, bacteria) breaking them down.
• Role: Microbes decompose plant and animal waste, recycling nutrients back to the soil, helping plants grow.

Example: Gardeners collect dry leaves in pits to make natural manure.

Ancient Indian texts (e.g., Vedas) mention tiny entities called Krimi (visible and invisible) and their beneficial/harmful effects.

2. Microbes as a Source of Biogas

• Some bacteria live in oxygen-free environments and decompose waste, producing biogas (a mix of methane and carbon dioxide).
• Biogas is used for cooking, heating, electricity, and running vehicles.
• Dr. Ananda Mohan Chakrabarty: Developed a bacterium in 1971 to break down oil spills, helping clean the environment.

3. Microorganisms in Food

Activity 2.8: Yeast in Dough:

• Mix flour, sugar, and yeast in bowl A; only flour in bowl B. Knead with warm water, cover, and observe after 4-5 hours.
• Result: Dough in bowl A rises, becomes fluffy, and smells different due to yeast (a fungus).
• Why: Yeast respires, breaking down sugar to release carbon dioxide (makes dough fluffy) and a little alcohol (gives smell).
• Uses: Yeast is used to make bread, cakes, and pastries. Bacteria like Lactobacillus help make idli, dosa, and bhatura.

Activity 2.9: Curd Formation:

• Add curd to lukewarm milk (bowl A) and cold milk (bowl B). Keep A warm, B cool, and observe after a few hours.
• Result: Milk in bowl A turns into sour curd; bowl B remains mostly unchanged.
• Why: Lactobacillus bacteria in curd feed on milk sugar (lactose), producing lactic acid, which curdles milk. They grow better in warm conditions.

Why Pickles Don’t Spoil: High salt or sugar in pickles/murabbas prevents microbe growth.

4. Microorganisms in Agriculture

Rhizobium Bacteria:

• Live in root nodules of legumes (e.g., beans, peas).
• Trap nitrogen from the air and convert it into a form plants can use, improving soil fertility.
• Farmers rotate legume crops to naturally enrich soil with nitrogen.

5. Microalgae: Tiny Helpers in Water

Microalgae: Tiny plant-like organisms in water, soil, or air that make food via photosynthesis.

Roles:

• Produce over half of Earth’s oxygen.
• Serve as food for aquatic animals.
• Used as health supplements (e.g., Spirulina, Chlorella) and in medicines.
• Help clean water and produce biofuel.

Spirulina Farming:

• Grow in a glass tank with pond water, away from direct sunlight.
• Stir twice a week, harvest after 3-6 weeks by filtering.
• Spirulina is a superfood with high protein and vitamin B12.