Chapter 2 Notes: The Invisible Living World: Beyond Our Naked Eye

How We Discovered the Tiny World

• The human eye cannot see very small objects without help.
• A lens is a curved piece of glass — thick in the middle, thin at the edge — that makes small things look bigger.
• Robert Hooke (1665) used a microscope to look at cork and saw tiny empty spaces he called cells — the first time this word was used in science. He published his findings in a book called Micrographia.
• Antonie van Leeuwenhoek built better microscopes and was the first to clearly see bacteria and blood cells — earning him the title Father of Microbiology.

💡 A water-filled round-bottom flask acts like a magnifying glass and makes letters in a book appear larger!

What Is Inside a Cell?

All living beings are made up of cells. Every cell has three basic parts:

Part	Function
Cell Membrane	Outer boundary; porous; controls what enters and exits the cell
Cytoplasm	Jelly-like space between membrane and nucleus; where most life processes happen
Nucleus	Round structure in the centre; regulates all cell activities and growth

Plant Cells Have Extra Parts:

• Cell Wall — extra rigid outer layer giving strength and shape
• Chloroplasts — contain chlorophyll; help in photosynthesis; make plant cells green
• Vacuole — large space storing water, nutrients, and waste; gives support to the plant

Animal Cells:

• No cell wall, no chloroplasts
• Vacuoles are absent or very small

🔬 Onion peel cells look rectangular and compact. Human cheek cells look flat and polygon-shaped.

Why Do Cells Have Different Shapes?

The shape and structure of a cell is related to the function it performs.

• Cheek cells — flat and thin; form a protective lining inside the mouth
• Muscle cells — spindle-shaped; can contract and relax to move food or body parts
• Nerve cells (Neurons) — very long with branches; carry messages quickly to different parts of the body
• Plant tube cells — long and tube-like; carry water throughout the plant

💡 Different cells do different jobs — this is called specialisation.

From Cell to Organism

Living organisms are organised in a hierarchy from simple to complex:

Cell → Tissue → Organ → Organ System → Organism

• Cell — basic unit of life
• Tissue — group of similar cells working together (e.g., muscle tissue)
• Organ — different tissues organised together (e.g., stomach)
• Organ System — several organs working together (e.g., digestive system)
• Organism — complete living being (e.g., a human, a plant)

Living beings made of many cells are called multicellular organisms (e.g., plants, animals, humans).

🥚 Fun Fact: The yolk of an ostrich egg is a single cell — the largest known cell — measuring 130–170 mm in diameter!

What Are Microorganisms?

Microorganisms (microbes) are living beings so small they cannot be seen with the naked eye — we need a microscope to observe them.

Types of Microorganisms:

Type	Example	Unicellular/Multicellular
Bacteria	Lactobacillus, Rhizobium	Unicellular
Protozoa	Amoeba, Paramecium	Unicellular
Fungi	Yeast (unicellular), Mould (multicellular)	Both
Algae	Spirulina, Chlorella	Both
Viruses	(special — acellular)	—

Where Are They Found?

• Water, soil, air, food, and inside our bodies (gut bacteria help in digestion)
• Some survive in extreme conditions — hot springs, snow, or salty environments

Bacteria Are Special:

• Bacteria do not have a well-defined nucleus — instead they have a nucleoid region where genetic material is located.
• This is what makes bacteria different from all other cells.

⚠️ Viruses are acellular (not made of cells) and can only multiply inside a living host cell.

How Do Microbes Help Clean the Environment?

Decomposition:

• Bacteria and fungi break down dead plants, fallen leaves, and animal waste into simpler, nutrient-rich substances.
• This process is called decomposition.
• The nutrients return to the soil and help new plants grow — this is called recycling of nutrients.

Manure Formation:

• Fruit and vegetable peels + soil microbes → Manure (dark, nutrient-rich material)
• Manure increases soil fertility naturally.
• Decomposition works best at optimal temperature and moisture.

Biogas:

• Some bacteria decompose plant and animal waste without oxygen, releasing a gas mixture called biogas.
• Biogas is mainly methane and carbon dioxide and is used as a fuel for cooking, heating, and generating electricity.

🌱 Without microorganisms, dead waste would pile up everywhere and nutrients would never return to the soil!

How Do Microbes Help in the Kitchen?

Yeast and Bread/Dough:

• Yeast is a unicellular fungus that grows well in warm conditions.
• It breaks down sugar and releases carbon dioxide gas, which makes dough rise and become fluffy.
• It also produces a small amount of alcohol, giving dough a slightly different smell.
• Used in making bread, cakes, idli, dosa, and bhatura.

Lactobacillus and Curd:

• Lactobacillus is a bacterium found in curd.
• It feeds on lactose (milk sugar), multiplies, and produces lactic acid, which turns milk into curd and makes it sour.
• It grows best in warm conditions — that is why curd sets in a warm place but not in a refrigerator.

Rhizobium and Nitrogen:

• Rhizobium bacteria live in root nodules of legumes (beans, peas, lentils).
• They trap nitrogen from the air and convert it into a form useful for plants.
• This is why legume farmers do not need to add nitrogen fertiliser — Rhizobium does the job naturally!

🍞 Cause & Effect: Yeast + sugar + warmth → CO₂ gas produced → bubbles form in dough → dough rises and becomes soft and fluffy.

What Are Microalgae?

• Microalgae are microscopic, plant-like organisms found in water, soil, air, and on trees.
• They make their own food using sunlight (photosynthesis) and release oxygen.
• They produce more than half of Earth's oxygen supply!
• Examples: Spirulina, Chlorella, Diatoms

Uses of Microalgae:

• Food source for aquatic animals
• Used as health supplements and medicines for humans
• Help in cleaning water
• Used to make biofuel

Spirulina — A Superfood:

• Rich in protein (over 60% of body weight), vitamins (especially B12), and minerals
• Low in fat and sugar
• Can be grown at home in a glass tank with pond water

⚠️ Pollution and climate change are threatening microalgal diversity — conserving them is important for oxygen balance on Earth.

Cell Comparison Table

Feature	Plant Cell	Animal Cell	Bacterial Cell
Cell Wall	✅ Present	❌ Absent	✅ Present
Cell Membrane	✅ Present	✅ Present	✅ Present
Nucleus	✅ Well-defined	✅ Well-defined	❌ Absent (has Nucleoid)
Chloroplast	✅ Present	❌ Absent	❌ Absent
Vacuole	✅ Large	❌ Small/Absent	❌ Absent
Cytoplasm	✅ Present	✅ Present	✅ Present

Summary of Levels of Organisation:

Cell → Tissue → Organ → Organ System → Organism

Key Microorganism Roles:

• 🍞 Yeast → makes bread fluffy (releases CO₂)
• 🥛 Lactobacillus → makes curd (produces lactic acid)
• 🌱 Rhizobium → fixes nitrogen in legume roots
• 🌿 Bacteria/Fungi → decompose waste, make manure
• ☀️ Microalgae → produce oxygen, source of nutrition