Notes Chapter 2 Science Curiosity Class 8 CBSE Board

Notes For All Chapters – Science Curiosity Class 8

The Invisible Living World: Beyond Our Naked Eye

1. Introduction

• Human eyes cannot see very tiny organisms.
• Use of lenses → magnifying glasses → microscopes → discovery of invisible living world.
• Microscopes magnify objects 100-400 times.

2. Early Discoveries

Robert Hooke (1665):

• Published Micrographia.
• Observed cork cells → looked like honeycomb.
• Coined the term “cell”.

Antonie van Leeuwenhoek (1660s):

• Made powerful lenses.
• First to describe bacteria and blood cells.
• Known as Father of Microbiology.

3. What is a Cell?

• Basic unit of life.
• Cells differ in shape, size, structure.

Observation:

• Onion peel cells (rectangular, with cell wall).
• Human cheek cells (polygonal, no cell wall).

3.1 Parts of a Cell

1. Cell membrane:

• Thin outer covering, porous.
• Controls entry/exit of substances.

2.Cytoplasm:

• Jelly-like material.
• Contains nutrients and other cell parts.
• Life processes occur here.

3.Nucleus:

• Round central structure.
• Controls cell functions and growth.

4.Cell wall (only in plants):

• Extra covering outside cell membrane.
• Provides rigidity and strength.

5.Plastids (in plants):

• Chloroplasts (contain chlorophyll, photosynthesis).
• Storage plastids (store substances).

6.Vacuole:

• Large in plants (stores water, waste, nutrients, maintains shape).
• Small in animals.

7.Mitochondria:

• Produces energy for cell.

4. Variation in Cell Shape

1. Animal cells:

• Muscle cell → spindle-shaped, helps in movement.
• Nerve cell → long, branched, carries messages quickly.
• Cheek cell → thin, flat, protective lining.

2.Plant cells:

• Rectangular, oval, elongated, tube-like.
• Some form tubes for transporting water.

5. Levels of Organisation in Living Beings

1. Cell → basic unit.
2. Tissue → group of similar cells.
3. Organ → made of tissues.
4. Organ system → group of organs.
5. Organism → complete living being.

• Unicellular organisms: Made of one cell (e.g., Amoeba, bacteria).
• Multicellular organisms: Many cells (e.g., plants, animals).
• Life starts from a single egg cell.
• Example: Ostrich egg yolk → largest known cell.

6. Microorganisms (Microbes)

• Very small organisms → cannot be seen without microscope.
• Types: Protozoa, Algae, Fungi, Bacteria, Viruses.
• Found in water, soil, air, food, even inside our bodies.

6.1 Examples

• Pond water: Amoeba, Paramecium, Algae.
• Soil suspension: Fungi (bread mould, other moulds), Algae, Bacteria.
• Viruses: Not cellular, reproduce only inside a host. Cause diseases.

7. Microorganisms Around Us

• Grow on rotten fruits/vegetables.
• Pickles and murabba are safe → salt/sugar act as preservatives.
• Survive in extreme conditions (hot springs, snow, etc.).
• Some live inside our intestine → help in digestion.

8. Role of Microorganisms

(a) Cleaning Environment

• Decompose waste (plant/animal) → manure (nutrient-rich).
• Recycle nutrients, clean environment.
• Some bacteria decompose waste without oxygen → produce biogas (methane).
• Dr. Ananda Mohan Chakrabarty (1971): Developed bacteria to clean oil spills.

(b) In Food

1. Yeast (fungus):

• Makes dough soft & fluffy (releases CO₂ + alcohol).
• Used in bread, cakes, pastries.

2.Lactobacillus (bacterium):

• Converts milk to curd (produces lactic acid).
• Helps ferment dosa/idli batter.

3. Rhizobium (bacterium):

• Lives in legume root nodules.
• Fixes nitrogen → increases soil fertility.

(c) Microalgae

• Microscopic, plant-like organisms.
• Make food via photosynthesis, release oxygen.
• Produce more than half of Earth’s oxygen.
• Food source for aquatic animals.
• Examples: Spirulina, Chlorella, Diatoms.
• Uses: health supplements, medicines, cleaning water, making biofuel.
• Spirulina: Superfood, rich in protein, Vitamin B12.

9. Cell – The Basic Unit of Life

• All living beings made of cells.
• Unicellular: One cell performs all functions (e.g., Amoeba, yeast).
• Multicellular: Specialised cells perform different functions but work together.
• Bacterial cells: Have cell wall, cell membrane, nucleoid (not true nucleus).
• Fungal cells: Have cell wall, but no chloroplasts.
• Plant/Animal cells: Differ in cell wall, vacuole, and plastids.

10. Snapshots (Summary Points)

• Microorganisms are very small, seen only with microscope.
• Types: bacteria, protozoa, fungi, algae, viruses.
• Cell is the basic unit of life.
• Cells differ in shape, size, structure.
• Plant cells have cell wall, chloroplast, large vacuole; animals do not.
• Bacteria have no true nucleus.
• Microbes may be useful (decompose waste, make food, fix nitrogen) or harmful (cause disease).
• Viruses multiply only inside host organisms.