The Invisible Living World: Beyond Our Naked Eye

1. The Invisible Living World

Our naked eyes cannot see extremely tiny objects. The invention of the microscope made it possible to see very small living organisms.

• A lens makes small objects appear larger.
• A microscope magnifies very tiny objects.
• Tiny living organisms that cannot normally be seen with naked eyes are called microorganisms or microbes.

2. Discovery of Cells

Robert Hooke

• In 1665, Robert Hooke observed a thin slice of cork using a microscope.
• He saw many tiny box-like compartments.
• He called these compartments cells.

Antonie van Leeuwenhoek

• He developed better microscopes.
• He was among the first to clearly observe tiny living organisms such as bacteria.
• He is known as the Father of Microbiology.

3. What is a Cell?

A cell is the basic structural and functional unit of life.

All living organisms are made up of one or more cells.

A cell can be compared to a brick in a wall:

Brick → Wall
Cell → Living organism

4. Basic Parts of a Cell

A typical cell has three main parts:

(a) Cell Membrane

• It forms the outer boundary of the cell.
• It separates one cell from another.
• It allows useful substances to enter and waste substances to leave.

(b) Cytoplasm

• Jelly-like material present between the cell membrane and nucleus.
• It contains different cell components and substances such as proteins, fats, carbohydrates and mineral salts.
• Most life processes occur in the cytoplasm.

(c) Nucleus

• It controls and regulates the activities of the cell.
• It also helps regulate the growth of the cell.

A typical cell contains a cell membrane, cytoplasm and nucleus, while plant, fungal and bacterial cells also possess a cell wall. Bacteria do not have a well-defined nucleus.

5. Plant Cell

A plant cell generally contains:

• Cell wall
• Cell membrane
• Cytoplasm
• Nucleus
• Chloroplasts
• Large vacuole

Cell Wall

Provides strength, rigidity and support to the plant cell.

Chloroplast

• Contains the green pigment chlorophyll.
• Helps plants carry out photosynthesis.

Vacuole

• Stores important substances.
• Helps remove/store waste.
• Helps maintain the shape of the cell.

6. Animal Cell

An animal cell mainly has:

• Cell membrane
• Cytoplasm
• Nucleus

Animal cells do not have a cell wall or chloroplasts. Their vacuoles, if present, are generally smaller than those in plant cells.

7. Plant Cell vs Animal Cell

8. Shape of Cells and Their Functions

Cells have different shapes because they perform different functions.

Nerve Cell

• Very long and branched.
• Carries messages from one part of the body to another.

Muscle Cell

• Spindle-shaped, thin and flexible.
• Contracts and relaxes to produce movement.

Cheek Cells

• Thin and flat.
• Form a protective lining inside the mouth.

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

9. Levels of Organisation

Living organisms are organised from simple units to complex structures:

Cell → Tissue → Organ → Organ System → Organism

Example:

Muscle cell → Muscle tissue → Stomach → Digestive system → Human being

• Similar cells form a tissue.
• Different tissues form an organ.
• Several organs working together form an organ system.
• All organ systems together form an organism.

10. Unicellular and Multicellular Organisms

Unicellular Organisms

Organisms made up of only one cell.

Examples:

• Bacteria
• Amoeba
• Paramecium
• Yeast

A single cell performs all necessary life functions.

Multicellular Organisms

Organisms made up of many cells.

Examples:

• Humans
• Animals
• Plants
• Some fungi

11. What are Microorganisms?

Microorganisms or microbes are organisms that are so small that they usually cannot be seen with naked eyes.

They can be found in:

• Air
• Water
• Soil
• Food
• Plants and animals
• Inside our body
• Even extreme environments

Some microorganisms are unicellular, while others are multicellular.

Main groups studied in the chapter:

• Bacteria
• Fungi
• Protozoa
• Algae

12. Examples of Microorganisms

Amoeba

• Protozoan
• Unicellular
• Irregular shape
• Can move

Paramecium

• Protozoan
• Unicellular
• Moves using specialised structures.

Algae

• Some contain chlorophyll.
• Can make their own food by photosynthesis.

Fungi

Examples:

• Yeast
• Bread mould

Bacteria

May have different shapes such as:

• Spherical
• Rod-shaped
• Spiral
• Comma-shaped

The chapter shows several microorganisms found in pond water and soil suspension and explains that microscopes are needed to observe them.

13. Viruses

Viruses are:

• Microscopic
• Acellular, meaning they are not made of cells.
• Able to multiply only after entering a living host cell.
• Capable of infecting plants, animals and bacteria.

14. Microorganisms as Environment Cleaners

Microorganisms such as bacteria and fungi break down dead plants, animals and organic waste.

This process is called:

Decomposition

During decomposition:

Dead plants/waste → Microorganisms act on them → Simpler substances → Nutrients return to soil

This:

• Cleans the environment.
• Recycles nutrients.
• Improves soil fertility.
• Helps form manure.

15. Microbes and Biogas

Some microorganisms break down plant and animal waste in the absence of oxygen.

This produces biogas, which contains a high proportion of methane along with gases such as carbon dioxide.

Biogas can be used for:

• Cooking
• Heating
• Generating electricity
• Running vehicles

16. Microorganisms in Food

Yeast

Yeast is a type of fungus.

It breaks down sugar and releases:

• Carbon dioxide
• A small amount of alcohol

The carbon dioxide bubbles make dough:

• Rise
• Become soft
• Become fluffy

Therefore, yeast is useful in making:

• Bread
• Cakes
• Pastries
• Bhatura and other fermented foods

17. Lactobacillus and Curd

Lactobacillus is a useful bacterium.

It helps:

• Convert milk into curd.
• Ferment certain foods.

Warm conditions generally favour curd formation.

18. Nitrogen Fixation

Some useful microorganisms live in the root nodules of leguminous plants, such as:

• Peas
• Beans
• Lentils

They trap/fix nitrogen from the air and make it available to plants.

This helps:

• Increase soil fertility.
• Reduce the need for nitrogen fertilisers in leguminous crops.

19. Bacterial Cell

Bacteria have:

• Cell membrane
• Cytoplasm
• Cell wall
• Nucleoid

Bacteria do not have a well-defined nucleus surrounded by a nuclear membrane. Instead, their genetic material occurs in a region called the nucleoid.

⭐ QUICK REVISION

Cell → Basic unit of life
Nucleus → Controls cell activities
Cell membrane → Controls movement of materials in and out
Cytoplasm → Site of most life processes
Cell wall → Gives strength and rigidity
Chloroplast → Photosynthesis
Vacuole → Storage and maintaining cell shape
Microorganisms → Tiny organisms invisible to naked eye
Yeast → Fungus used in fermentation
Lactobacillus → Helps make curd
Decomposition → Breakdown of complex dead matter into simpler substances
Nucleoid → Nuclear region of bacterial cell
Virus → Acellular; multiplies only inside a living cell

PART B — BACK EXERCISE: “KEEP THE CURIOSITY ALIVE”

The textbook back exercise contains Questions 1–9 on pages 25–26.

Q1. Various parts of a cell are given below. Write them in the appropriate places in the diagram.

Given: Nucleus, Cytoplasm, Chloroplast, Cell wall, Cell membrane, Nucleoid.

Answer:

Common to all three cells:

• Cell membrane
• Cytoplasm

Only in Plant Cell:

• Chloroplast

Only in Bacterial Cell:

• Nucleoid

Common to Plant and Bacterial Cells:

• Cell wall

Plant and Animal Cells:

• Nucleus

Note: A bacterial cell does not have a well-defined nucleus; it has a nucleoid instead.

Q2. Aanandi took two test tubes A and B containing sugar solution. She added yeast only to test tube B and attached balloons.

(i) Why did the balloon attached to test tube B inflate?

Answer: Correct option is (c) Yeast produced a gas inside test tube B which inflated the balloon.

Yeast breaks down sugar during fermentation and releases carbon dioxide gas. The gas collects inside the balloon and inflates it.

(ii) She transferred the gas to a test tube containing lime water. What did she want to find out?

Answer: She wanted to test whether the gas produced by yeast was carbon dioxide.

Carbon dioxide turns lime water milky. Therefore, if the lime water becomes milky, it confirms the presence of carbon dioxide.

The experimental question and setup are shown on page 25 of the textbook.

Q3. Why does a farmer growing wheat add nitrogen-rich fertiliser, while a farmer growing beans may not need to add it?

Answer:

Wheat plants need nitrogen from the soil and cannot fix atmospheric nitrogen themselves. Therefore, nitrogen-rich fertilisers may be added for better growth.

Bean plants are leguminous plants. Their root nodules contain useful nitrogen-fixing microorganisms. These microorganisms trap nitrogen from the air and make it available to the plant.

Therefore, bean crops may need less external nitrogen fertiliser.

Q4. Snehal dug two pits A and B. In pit A, she mixed fruit and vegetable peels with dried leaves. In pit B, she used the same waste without dried leaves. What is she trying to test?

Answer:

Snehal is trying to test how the presence of dried leaves affects the decomposition of organic waste and manure formation.

She can compare the amount and rate of decomposition in the two pits after three weeks.

Microorganisms such as bacteria and fungi break down plant waste into simpler, nutrient-rich material called manure.

Q5. Identify the following microorganisms.

(i) I live in every kind of environment, and inside your gut.

Answer: Bacteria

Bacteria occur in many environments, and useful bacteria also live inside our intestine.

(ii) I make bread and cakes soft and fluffy.

Answer: Yeast

Yeast releases carbon dioxide during fermentation, which makes dough rise and become soft and fluffy.

(iii) I live in the roots of pulse crops and provide nutrients for their growth.

Answer: Nitrogen-fixing bacteria living in the root nodules of leguminous/pulse plants.

The textbook asks these three identifications in Question 5.

Q6. Design an experiment to test that microorganisms need optimal temperature, air and moisture for their growth.

Answer:

We can perform an experiment using four similar pieces of bread.

• Keep Bread A moist, exposed to air and at a warm/room temperature.
• Keep Bread B moist but in a refrigerator to test the effect of low temperature.
• Keep Bread C dry but at a suitable temperature to test the need for moisture.
• Keep Bread D with limited exposure to air while keeping other conditions suitable.

Observe all samples for a few days.

Observation:

Bread kept under suitable temperature, moisture and air conditions should show faster microbial/mould growth.

Conclusion:

Microorganisms grow best when they receive suitable environmental conditions such as optimal temperature and appropriate moisture, with air requirements depending on the microorganism.

Q7. Take two slices of bread. Keep one near the sink and the other in the refrigerator. Compare after three days. Give reasons.

Answer:

After three days:

Bread near the sink:
It is likely to show more mould/microbial growth, especially because the surroundings are generally warmer and moist.

Bread in the refrigerator:
It will show less or slower microbial growth.

Reason:

Microorganisms generally grow faster under suitable warm and moist conditions. The low temperature inside a refrigerator slows down their growth.

Q8. A student observes that curd left out for a day becomes more sour. Give two possible explanations.

Answer:

Two possible explanations are:

1. Lactobacillus bacteria continue to grow and ferment the milk components, producing more acid. This increases the sour taste.
2. Warm conditions favour bacterial activity, so curd kept outside becomes sour faster than curd kept in a refrigerator.

Lactobacillus is used in curd formation and food fermentation.

Q9. Observe the experimental setup in Fig. 2.15.

The setup contains warm sugar solution + yeast in flask A, connected to lime water in test tube B.

(i) What happens to the sugar solution in flask A?

Answer:
Yeast uses/breaks down the sugar during fermentation. In this process, it releases carbon dioxide gas and a small amount of alcohol.

(ii) What do you observe in test tube B after four hours? Why?

Answer:
The lime water in test tube B turns milky.

Reason:

Yeast in flask A produces carbon dioxide gas. This gas passes through the tube into the lime water.

Carbon dioxide reacts with lime water and makes it milky, confirming the presence of carbon dioxide.

(iii) What would happen if yeast were not added to flask A?

Answer:
If yeast were not added:

• Fermentation would not occur in the expected way.
• Carbon dioxide would not be produced by yeast.
• Therefore, the lime water in test tube B would not turn milky due to yeast fermentation.

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