Class 8 Science Chapter 2 – The Invisible Living World: Beyond Our Naked Eye from NCERT Curiosity with easy Notes and complete Book Solutions. Get simple explanations, important concepts, questions and answers, and revision material for effective learning, practice and exam preparation.
The Invisible Living World:
Beyond Our Naked Eye
Probe and Ponder
Answer: We might see many tiny organisms such as bacteria, microscopic fungi and other microbes.
Answer: It would show us that very tiny living things can be complex and important.
Answer: Yes. Microorganisms can interact with each other, plants, animals and their environment.
Answer: Example: How do microorganisms help plants and animals?
Chapter Summary
The human eye cannot see extremely small objects. Lenses and microscopes help us observe the tiny world.
Robert Hooke observed tiny compartments in cork and called them cells. Antonie van Leeuwenhoek later observed tiny living organisms such as bacteria.
All living organisms are made of one or more cells. Cells have different structures and functions.
Very small living organisms that cannot be seen with the naked eye are called microorganisms or microbes.
Microorganisms can be useful. They help in decomposition, food fermentation, curd formation, nitrogen fixation, biogas production and environmental cleaning.
Some microorganisms can also be harmful and cause diseases.
1. From Lenses to Microscopes
Lens
A curved piece of glass that can make small objects appear larger.
Magnifying Glass
It uses a lens to make small objects appear bigger and easier to observe.
Microscope
A microscope is an instrument that magnifies very small objects so that we can observe them.
Robert Hooke
In 1665, Robert Hooke published Micrographia and used a microscope to observe cork.
Antonie van Leeuwenhoek
He made better lenses and observed tiny living organisms such as bacteria. He is known as the Father of Microbiology.
Cell
Hooke saw small compartments in cork and called them cells. The cell is the basic unit of life.
2. 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.
| Cell Part | Easy Function |
|---|---|
| Cell membrane | Surrounds the cell and controls the entry and exit of materials. |
| Cytoplasm | Contains cell components and is the place where many life processes occur. |
| Nucleus | Controls the activities and growth of the cell. |
| Cell wall | Gives plant cells strength, support and rigidity. |
| Chloroplast | Contains chlorophyll and helps plants perform photosynthesis. |
| Vacuole | Stores substances and helps maintain the shape of plant cells. |
3. Plant Cells and Animal Cells
| Plant Cell | Animal Cell |
|---|---|
| Has a cell wall. | Does not have a cell wall. |
| Usually has chloroplasts. | Does not have chloroplasts. |
| Usually has a large vacuole. | Vacuoles are usually absent or small. |
| Often has a more fixed shape. | Can have different shapes. |
4. Variation in Shape and Structure of Cells
Cells have different shapes and structures because they perform different functions.
Cheek Cells
They are thin and flat and form a protective lining inside the cheek.
Nerve Cells
They are long and branched so that they can carry messages through the body.
Muscle Cells
They are spindle-shaped and help in contraction and movement.
Plant Cells
Plant cells can be rectangular, elongated, oval or tube-like depending on their function.
5. Levels of Organisation in the Body
Living organisms are organised from simple building blocks into complex structures.
6. What Are Microorganisms?
Microorganisms are very small living organisms that cannot usually be seen with the naked eye.
Bacteria
Tiny organisms. Many are useful, while some can cause diseases.
Protozoa
Usually single-celled organisms such as Amoeba and Paramecium.
Fungi
Microorganisms such as yeast and mould. Some are useful in food production and decomposition.
Algae
Some algae are microscopic and can make their own food using sunlight.
7. Unicellular and Multicellular Organisms
| Type | Meaning | Examples |
|---|---|---|
| Unicellular | Made up of one cell. | Bacteria, Amoeba, yeast |
| Multicellular | Made up of many cells. | Plants, animals, humans |
8. Viruses
Viruses are microscopic and acellular. They multiply only when they enter a living cell. They may infect plants, animals or bacterial cells and can cause diseases.
9. Microorganisms and the Environment
Microorganisms are found almost everywhere—in soil, water, air, food and even inside our bodies.
Some bacteria and fungi break down dead plants, animals and other waste into simpler substances.
The nutrients released during decomposition return to the soil and help plants grow.
10. Microbes as a Source of Biogas
Some microorganisms live in oxygen-free conditions and break down plant and animal waste.
During this process they release biogas. Biogas contains mainly carbon dioxide and a high proportion of methane.
Biogas can be used for cooking, heating, generating electricity and running vehicles.
11. Microorganisms and Food
Yeast
Yeast is a fungus. It produces carbon dioxide while breaking down food. The gas makes dough soft and fluffy.
Lactobacillus
This bacterium helps convert milk into curd by producing lactic acid.
Fermentation
Microorganisms help change food substances into new products such as curd and fermented batter.
Rhizobium
Rhizobium lives in root nodules of legumes and helps make nitrogen available to plants.
12. Amazing Microalgae
Microalgae are microscopic plant-like organisms found in water, soil, air and even on trees.
They make their own food using sunlight and release oxygen. They also form an important food source for aquatic animals.
Some microalgae, such as Spirulina, Chlorella and Diatoms, are used by humans as health supplements and medicines.
Microalgae can also help clean water and can be used to make biofuel.
Keep the Curiosity Alive
1. Various parts of a cell are given below. Write them in the appropriate places in the following diagram.
Answer: The parts are: Nucleus, Cytoplasm, Chloroplast, Cell wall, Cell membrane and Nucleoid. Their positions depend on the type of cell shown in the diagram.
2. Aanandi took two test tubes and marked them A and B. She put two spoonfuls of sugar solution in each of the test tubes. In test tube B, she added a spoonful of yeast. Then she attached two incompletely inflated balloons to the mouth of each test tube. She kept the set-up in a warm place, away from sunlight.
Answer: Test tube B contains yeast, which can produce a gas when it acts on sugar.
(i) What do you predict will happen after 3–4 hours? She observed that the balloon attached to test tube B was inflated. What can be a possible explanation for this?
Answer: (c) Yeast produced a gas inside the test tube B which inflated the balloon.
(a) Water evaporated in test tube B and filled the balloon with the water vapour.
Answer: Not correct. The balloon is inflated mainly because yeast produces carbon dioxide gas.
(b) The warm atmosphere expanded the air inside the test tube B, which inflated the balloon.
Answer: Not correct. The important cause is the gas produced by yeast.
(c) Yeast produced a gas inside the test tube B which inflated the balloon.
Answer: Correct. Yeast produces carbon dioxide during its activity.
(d) Sugar reacted with warm air, which produced gas, eventually inflating the balloon.
Answer: Not correct. Yeast is responsible for producing the gas.
(ii) She took another test tube, 1/4 filled with lime water. She removed the balloon from test tube B in such a manner that the gas inside the balloon did not escape. She attached the balloon to the test tube with lime water and shook it well. What do you think she wants to find out?
Answer: She wants to test whether the gas produced by yeast is carbon dioxide. Carbon dioxide turns lime water milky.
3. A farmer was planting wheat crops in his field. He added nitrogen-rich fertiliser to the soil to get a good yield of crops. In the neighbouring field, another farmer was growing bean crops, but she preferred not to add nitrogen fertiliser to get healthy crops. Can you think of the reasons?
Answer: Bean plants have root nodules containing Rhizobium bacteria. These bacteria help make nitrogen available to the plant.
4. Snehal dug two pits, A and B, in her garden. In pit A, she put fruit and vegetable peels and mixed it with dried leaves. In pit B, she dumped the same kind of waste without mixing it with dried leaves. She covered both the pits with soil and observed after 3 weeks. What is she trying to test?
Answer: She is trying to compare how mixing dried leaves with waste affects decomposition and manure formation.
5. Identify the following microorganisms:
(i) I live in every kind of environment, and inside your gut.
Answer: Bacteria.
(ii) I make bread and cakes soft and fluffy.
Answer: Yeast.
(iii) I live in the roots of pulse crops and provide nutrients for their growth.
Answer: Rhizobium.
6. Design an experiment to test that microorganisms need optimal temperature, air, and moisture for their growth.
Answer: Keep similar food samples under different conditions: warm/cold, moist/dry and with/without air. Observe their growth after a few days and compare the results.
7. Take 2 slices of bread. Place one slice in a plate near the sink. Place the other slice in the refrigerator. Compare after three days. Note your observations. Give reasons for your observations.
Answer: The bread near the sink is likely to develop mould faster because it is warmer. The refrigerator slows microorganism growth.
8. A student observes that when curd is left out for a day, it becomes more sour. What can be two possible explanations for this observation?
Answer:
1. Bacteria continue producing lactic acid.
2. Warm conditions allow the bacteria to remain active.
9. Observe the set-up given in Fig. 2.15 and answer the following questions.
(i) What happens to the sugar solution in flask A?
Answer: Yeast acts on the sugar and produces carbon dioxide and other products.
(ii) What do you observe in test tube B after four hours? Why do you think this happened?
Answer: The lime water becomes milky because carbon dioxide reacts with lime water.
(iii) What would happen if yeast was not added in flask A?
Answer: Little or no carbon dioxide would be produced by fermentation, so the lime water would not become milky in the same way.
Discover, Design, and Debate
India has a long history of biogas production. One of our oldest biogas plant was set up in late 1850s. Find out about the Biogas Program initiated by the Ministry of New and Renewable Energy, Government of India.
Answer: The programme promotes biogas as a useful renewable source of energy made from organic waste.
Fermented food items like fermented soya bean and fermented bamboo shoots are consumed as traditional food in some parts of India. With the help of your parents and teachers, list some traditional food items from your area that utilise the process of fermentation. Investigate the ingredients used in the preparation of these fermented food items; the method of preparing them; the microorganism responsible for the fermentation of the food, and the cultural and nutritional importance of the fermented food.
Answer: Examples include idli, dosa, dhokla and curd. Different microorganisms help in their fermentation. Fermentation improves taste and can also improve digestibility.
Study the different parts of a macro fungus mushroom using a magnifying glass and microscope. Take the help of students from senior classes and explore the internal structure of different parts of mushrooms under the microscope in your school laboratory.
Answer: Observe the cap, stalk and other visible parts using a magnifying glass and microscope. Record the structures you observe in your notebook.
Interact with an entrepreneur and learn the steps for cultivation of mushroom.
Answer: Learn about preparing the growing material, adding mushroom culture, maintaining suitable temperature and moisture, and harvesting the mushrooms.
Prepare Some Questions Based on Your Learnings
Why?
Example: Why do microorganisms grow faster in warm and moist conditions?
When?
Example: When does yeast make dough rise?
How long?
Example: How long does it take for bread mould to appear?
Where?
Example: Where can microorganisms be found?
Why not?
Example: Why do some microorganisms not grow well in very cold conditions?
Reflect on the Questions Framed by Your Friends
Think carefully about each question. Use your observations, experiments and knowledge from the chapter to find the answer.
Science grows when questions lead to investigation.
Snapshots — Quick Revision
- Microorganisms are small organisms and cannot usually be seen with the unaided eye.
- Microorganisms can live in many environments, including inside plants and animals.
- They may be unicellular or multicellular.
- Bacteria and protozoans are generally unicellular.
- Fungi may be unicellular or multicellular.
- Plants and animals are multicellular.
- The cell is the basic unit of life.
- A typical cell has a cell membrane, cytoplasm and nucleus.
- Plant, fungal and bacterial cells have a cell wall.
- Bacteria do not have a well-defined nucleus.
- Cell shape and structure are related to the function of the cell.
- Bacteria, fungi and protozoa are different types of microorganisms.
- Viruses are microscopic and reproduce inside host cells.
- Microorganisms can be useful as well as harmful.
- Some microorganisms decompose plant and animal waste.
- Rhizobium helps increase nitrogen availability in soil.
- Yeast is used in making bread, cakes and other foods.
- Lactobacillus helps in curd formation.
- Microalgae are important producers of oxygen and can have many useful applications.
Important Keywords
| Word | Easy Meaning |
|---|---|
| Lens | A curved transparent object that can magnify objects. |
| Microscope | An instrument used to see very small objects. |
| Cell | The basic unit of life. |
| Microorganism | A very small living organism. |
| Unicellular | Made of one cell. |
| Multicellular | Made of many cells. |
| Decomposition | Breaking down complex waste into simpler substances. |
| Fermentation | A process in which microorganisms change food substances. |
| Microalgae | Microscopic plant-like organisms. |
| Nucleoid | The region containing the genetic material in a bacterial cell. |
Keep the Curiosity Alive!
Look closely, ask questions, observe carefully and investigate the world around you.
Tiny organisms may be invisible to our eyes, but they can have a huge impact on life!
Prepare with this NCERT Curiosity worksheet for Chapter 2 The worksheet includes important questions and revision exercises to help students understand key concepts, practise what they have learned, and prepare effectively for class tests and exams.
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