Chapter 2- The Invisible Living World: Beyond our Naked Eye

1. Introduction: The Hidden Universe Around Us

Students, imagine for a moment that you have a superpower—super vision that acts like a powerful zoom lens. If you looked at your clean hands, a drop of clear water, or even the soil in your garden with this vision, you would be shocked. Why? Because the world is not empty. It is crowded with millions of tiny living beings that are bustling with activity, eating, moving, and growing right under our noses.

This is what we call the Invisible Living World. For thousands of years, humans had no idea this world existed because our eyes have a limit. We can only see objects that are above a certain size (macroscopic). Anything smaller than that (microscopic) remains hidden from our naked eye. In this chapter, we are going to use the tool of science—the microscope—to cross this barrier and explore the tiny organisms that shape our planet.

2. The Journey of Discovery: From Lenses to Microscopes

The discovery of this invisible world didn’t happen overnight. It started with a simple observation: a curved piece of glass can make small things look bigger. Ancient people noticed that glass shaped like a lentil seed (thick in the middle, thin at the edges) had magnifying properties. This is why we call it a “lens” (named after the lentil).

Scientists realized that if one lens could magnify a little, combining two or more lenses could magnify a lot! This led to the invention of the microscope—an instrument that allows us to see things far smaller than what our eyes can detect.

Two Heroes of Microscopy

We must remember two important names in this history:

• Robert Hooke (1665): He was an English scientist with a deep curiosity. He built his own microscope and decided to look at a thin slice of cork (the dead bark of a tree). Under the microscope, he saw a structure that looked like a honeycomb—thousands of tiny, empty boxes packed together. He called these boxes “Cells” (from the Latin word cella, meaning a small room). Although he was looking at dead plant cells, his discovery gave biology its most important concept: the cell is the building block of life.
• Antonie van Leeuwenhoek (1660s): Around the same time, a Dutch scientist named Leeuwenhoek improved lens-making techniques. He created simple microscopes with very high magnification. Unlike Hooke, Leeuwenhoek looked at living samples like pond water and tooth scrapings. He was the first human to see living cells—tiny creatures swimming and moving. Because of this, he is honored as the Father of Microbiology.

Figure-1: Robert Hooke used a primitive microscope to observe cork cells, which looked like the compartments of a honeycomb.

3. What Is a Cell?

Just as a large school building is constructed from thousands of individual bricks, every living organism—from the smallest ant to the largest banyan tree—is made of cells. The cell is defined as the basic structural and functional unit of life.

• Structural Unit: It gives the organism its shape and structure.
• Functional Unit: It performs all the life processes (like digestion, respiration, growth) needed to keep the organism alive.

Are all cells microscopic?
Most are, but not all! The largest known single cell in the world is the egg of an ostrich. It measures about 130 mm to 170 mm in diameter. The yellow yolk inside the egg is a single giant cell, surrounded by white albumen and a shell for protection. This proves that cells come in many sizes.

4. Peeking Inside: Plant vs. Animal Cells

To understand how cells work, we need to look inside them. In our science lab, we usually perform two activities to compare the cells of plants and animals.

Activity 1: The Onion Peel (Plant Cell)

When we peel a thin, transparent layer from an onion bulb, stain it with Safranin (a red dye), and view it under a microscope, we see a very organized pattern. The cells look like bricks in a wall. They are rectangular and arranged tightly without any gaps. The boundary of each cell is very thick and distinct.

Activity 2: The Human Cheek Cell (Animal Cell)

When we gently scrape the inside of our cheek with a toothpick and stain the material with Methylene Blue, the view is different. We see irregular, polygon-shaped cells that are not arranged in strict rows. They look softer and more flexible than the onion cells.

Figure-2: Left: Plant cells (Onion) are rigid and rectangular. Right: Animal cells (Cheek) are irregular and flexible.

5. Detailed Structure of a Cell

A cell is not just a bag of liquid. It is a complex “tiny factory” with specific parts (organelles) that perform specific jobs. Let’s study the main components:

1. Cell Membrane (The Gatekeeper)

This is the thin, delicate outer skin of the cell. It holds the cell together and separates it from its neighbors. Crucially, it is porous (has tiny holes). It acts like a security guard, allowing essential materials like food and oxygen to enter while letting waste products exit. Both plant and animal cells have this.

2. Cell Wall (The Armor – PLANTS ONLY)

Plant cells have an extra, thick, and rigid layer outside the cell membrane called the Cell Wall.

Why do plants need this? Animals can move to find shelter from rain, heat, or wind. Plants cannot; they are fixed in the soil. The cell wall provides the strength and stiffness plants need to withstand environmental stress and stand upright. Animal cells do not need this, so they lack a cell wall.

3. Nucleus (The Brain)

Floating in the center of the cell is a dense, round structure called the Nucleus. It is the most important part. It controls and regulates all activities of the cell, such as eating, growing, and dividing. Without a nucleus, the cell would die. It is separated from the rest of the cell by a nuclear membrane.

4. Cytoplasm (The Workspace)

The space between the cell membrane and the nucleus is filled with a jelly-like fluid called Cytoplasm. This is where the cell’s “work” happens. It contains water, proteins, fats, and other organelles.

5. Vacuoles (The Storage Tanks)

These are sac-like structures used for storage.

In Plants: Plant cells usually have one gigantic vacuole that occupies the center of the cell. It stores water and nutrients and pushes the nucleus to the side. It helps keep the cell rigid.

In Animals: Vacuoles are either absent or very small and temporary.

6. Plastids (The Kitchen – PLANTS ONLY)

Have you wondered why plants are green? Plant cells contain small colored bodies called Plastids. The most important ones are Chloroplasts, which contain the green pigment Chlorophyll. These are the kitchens of the cell where photosynthesis happens. Animals don’t make their own food, so they don’t have plastids.

Figure-3: Note the Cell Wall, Chloroplasts, and large Vacuole in the Plant Cell (Right) which are missing in the Animal Cell (Left).

6. Cell Diversity and Organization

Shape and Function: Not all cells look the same.

– Nerve Cells (Neurons): Long and branched to carry messages quickly.

– Muscle Cells: Spindle-shaped to contract and help us move.

– Skin Cells: Flat to cover and protect.

Levels of Organization:

In multicellular organisms (like humans), cells work in teams:

Cell (Worker) → Tissue (Team) → Organ (Department) → Organ System (Division) → Organism (Company).

7. Microorganisms: The Invisible Population

Now that we know about cells, let’s talk about organisms that are often made of just one single cell. These are called Microorganisms or Microbes. They are everywhere—in the deepest oceans, high in the atmosphere, inside hot springs, and even inside your gut!

The 5 Major Groups of Microbes

Scientists classify them based on their features:

• 1. Bacteria: The simplest unicellular organisms. They are unique because they do not have a well-defined nucleus (the genetic material just floats in a region called the nucleoid). They come in shapes like rods, spirals, and spheres. Example: Lactobacillus.
• 2. Fungi: These can be unicellular (like Yeast) or multicellular (like Bread Mould, Mushrooms). They are plant-like but lack chlorophyll, so they feed on dead or decaying matter.
• 3. Protozoa: These are unicellular “animal-like” hunters. They can move and capture food. Example: Amoeba (uses false feet called pseudopodia) and Paramecium (uses hair-like cilia to swim).
• 4. Algae: These are “plant-like” microbes found in water. They contain chlorophyll and make their own food via photosynthesis. Example: Spirogyra (green filaments) and Chlamydomonas.
• 5. Viruses (The Tricky Ones): Viruses are tiny agents that are on the borderline of living and non-living. They are acellular (not cells). Outside a body, they act like dead particles. But once they enter a living host (plant, animal, or bacteria), they hijack the host’s machinery to multiply. They cause diseases like flu, Polio, and COVID-19.

Figure-4: The diverse family of microorganisms. Notice how Bacteria lack a nucleus, while viruses look very mechanical.

8. Friend or Foe? The Role of Microbes

Microbes are not just germs. In fact, without them, life on Earth would collapse. They are our invisible friends in three major areas:

A. In the Food Industry (Kitchen Helpers)

Curd Formation: A bacterium called Lactobacillus turns milk into curd. It multiplies in warm milk, eats the lactose sugar, and produces lactic acid. This acid makes the milk thick and sour (curd).

Baking and Fermentation: We use Yeast (a fungus) to make bread, cakes, and idlis. When yeast is added to dough containing sugar, it starts respiring rapidly. It produces bubbles of Carbon Dioxide (CO₂) gas. These bubbles get trapped in the dough, causing it to rise (swell up). When baked, the gas escapes, leaving the bread soft and spongy. This process of breaking down sugar into alcohol and CO₂ is called Fermentation.

B. In Agriculture (Soil Fertility)

Plants need Nitrogen to grow, but they can’t take it from the air. A friendly bacterium called Rhizobium lives in the roots of leguminous plants (pulses like peas, beans, grams). They form small bumps called root nodules. Inside these nodules, Rhizobium traps nitrogen from the air and converts it into a natural fertilizer for the soil. This helps farmers get a good harvest without using chemicals.

Figure-5: Root nodules are the homes of nitrogen-fixing bacteria. They act as natural fertilizer factories for the soil.

C. In the Environment (The Cleaners)

Microbes are nature’s recyclers. Bacteria and fungi act as decomposers. They feed on dead plants, animal waste, and vegetable peels, breaking them down into simple substances. This process turns waste into nutrient-rich manure. Without microbes, the earth would be piled high with dead matter!

D. Oxygen Production

Microscopic algae (like phytoplankton) floating in oceans produce more than 50% of the oxygen we breathe. They are essential for life on Earth.

9. Indian Scientific Heritage

Our ancestors in India were aware of the invisible world long before modern science. Ancient texts like the Vedas mention tiny organisms called ‘Krimi’. They categorized them into ‘Drishya’ (visible) and ‘Adrishya’ (invisible) and knew that some caused diseases while others were part of nature’s balance.

Practice Set (CBSE Pattern)

A. Very Short Answer Questions

Q1: Who discovered the cell and in which year?

Answer: Robert Hooke discovered the cell in 1665.

Q2: Name the bacterium that converts milk into curd.

Answer: Lactobacillus.

Q3: Which gas makes bread and cake dough rise?

Answer: Carbon Dioxide (CO₂).

Q4: What is the full form of the book ‘Micrographia’?

Answer: It is the book written by Robert Hooke where he published his drawings of cells.

B. Short Answer Questions

Q1: Why are viruses called “borderline” organisms?

Answer: Viruses are considered borderline because they behave like non-living things (like dust or crystal) when they are outside a body. However, once they enter a living host (plant or animal), they become active and start reproducing like living beings.

Q2: Explain the function of the Cell Wall. Why is it absent in animals?

Answer: The cell wall provides rigidity, structural strength, and protection against variations in temperature, wind, and moisture. It is found in plants because they are stationary (cannot move) and need this extra shield. Animals can move to find shelter, so they do not need a rigid cell wall; they only have a flexible cell membrane.

Q3: How do microorganisms clean the environment?

Answer: Microorganisms like bacteria and fungi act as decomposers. They break down complex organic waste (dead plants, animal dung, vegetable peels) into simple, harmless substances. This converts waste into useful manure and prevents the accumulation of garbage in the environment.

C. Long Answer Questions

Q1: Describe the structure of a plant cell with special emphasis on the parts not found in animal cells.

Answer: A plant cell has three specific parts that are usually absent in animal cells:

1. Cell Wall: A thick, rigid outer layer made of cellulose that gives the cell a fixed rectangular shape.

2. Chloroplasts (Plastids): Green-coloured organelles containing chlorophyll, which is essential for photosynthesis (making food).

3. Large Vacuole: A massive central sac filled with cell sap that pushes the nucleus to the side and maintains the cell’s turgidity (stiffness).

Apart from these, it has common parts like the Cell Membrane, Cytoplasm, and Nucleus.

Q2: “Yeast is a baker’s friend.” Explain the scientific process behind this statement.

Answer: Yeast is used in baking because of a process called fermentation. When yeast is mixed with flour, warm water, and sugar, it feeds on the sugar and respires anaerobically (without oxygen). This reaction releases bubbles of Carbon Dioxide (CO₂) gas. These gas bubbles get trapped inside the sticky dough, causing it to expand and rise. When the bread is baked, the heat kills the yeast and evaporates the alcohol, but the holes left by the gas bubbles make the bread porous, soft, and fluffy.

D. Case-Based Question

Case Study: A farmer, Ramu, grows wheat in his field every year. He notices his soil quality is dropping. His friend advises him to grow a crop of beans or peas for one season before planting wheat again.

Q1: Why did the friend suggest growing beans or peas?

Answer: Beans and peas are leguminous crops. Their roots have nodules containing Rhizobium bacteria.

Q2: How will this help the soil?

Answer: The Rhizobium bacteria will fix nitrogen from the air and store it in the soil. When these plants decay, the nitrogen enriches the soil, acting as a natural fertilizer for the next wheat crop.

E. Assertion–Reason

Assertion: An onion peel cell has a regular, brick-like shape.

Reason: It possesses a cell wall that provides rigidity and a fixed shape.

Answer: Both Assertion and Reason are true, and the Reason is the correct explanation for the Assertion. The cell wall prevents the plant cell from changing its shape.