The Invisible Living World: Beyond Our Naked Eye — Class 8 Science
Curiosity · Chapter 2
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6- Mathematics
- Science
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- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
2- Mathematics
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Cell
- The basic structural and functional unit of all living organisms, first named by Robert Hooke.
- Cell membrane
- The porous outer boundary of a cell that controls entry of useful materials and exit of wastes.
- Nucleus
- The structure that regulates the activities and growth of a cell; absent as a well-defined body in bacteria.
- Microorganism
- A tiny living being, such as bacteria, fungi, protozoa, or some algae, visible only under a microscope.
- Unicellular and multicellular
- Unicellular organisms are made of a single cell; multicellular organisms are made of many cells.
- Decomposition
- The breaking down of dead plant and animal matter by microbes into simpler, nutrient-rich substances.
Extra questions & answers
Who first used the word "cell" and how?
Robert Hooke, in 1665, while observing a thin slice of cork under his microscope, where the empty compartments reminded him of a honeycomb.
Why is Antonie van Leeuwenhoek called the Father of Microbiology?
He built improved microscopes and was the first to clearly see and describe tiny living things like bacteria and blood cells.
Name the three basic parts present in a typical cell.
The cell membrane, the cytoplasm, and the nucleus.
How does a plant cell differ from an animal cell?
A plant cell has an extra rigid cell wall, plastids such as chloroplasts, and usually a large vacuole, which are generally absent or small in animal cells.
What are the levels of organisation in a multicellular organism?
Cell, tissue, organ, organ system, and organism.
How does the shape of a nerve cell suit its function?
Its long, branched shape lets it reach different parts of the body and pass messages quickly.
How do microorganisms help clean the environment?
Fungi and bacteria decompose plant and animal waste into simpler nutrient-rich substances such as manure, recycling nutrients back to the soil.
What is the role of Rhizobium bacteria in root nodules?
They trap nitrogen from the air and make it usable for legume plants, increasing soil fertility naturally.
How is yeast useful in making bread?
Yeast respires and releases carbon dioxide, which forms bubbles that make the dough soft and fluffy, along with a little alcohol.
How are viruses different from other microorganisms?
Viruses are acellular and can multiply only after entering a living host cell, unlike bacteria and protozoa which carry out life processes on their own.
Probe and ponder
Have you ever wondered what you might see if the invisible world around you became visible?
If the invisible world became visible we would see millions of tiny living beings everywhere - bacteria, fungi, algae and protozoa in water, soil, air, on our skin and even inside our body. We would also see the basic building blocks of all life, the cells, and how even a drop of pond water is full of moving single-celled organisms. It would show us that what looks empty or clean is actually full of life.
How do you think your observation of this hidden world might change the way you think about size, complexity, or even what counts as ‘living’?
Seeing the hidden world would change our idea that only big things are important or complex. We would learn that even a very tiny single cell can carry out all life processes like nutrition, respiration, growth and reproduction. It would show that complexity does not depend on size and that even microscopic organisms are truly living beings because they are made of cells and show characteristics of life.
Have you thought how these tiny living beings interact with each other?
Yes, tiny beings interact in many ways. Some help each other, like Rhizobium bacteria living in root nodules of legumes and giving nitrogen to the plant while getting food and shelter. Some decompose waste together to recycle nutrients. Others compete for food or cause harm by causing spoilage and disease. Like larger organisms, they form communities and depend on each other and on their environment.
Share your questions
Some questions students can share are: How small is the smallest cell? Are viruses living? Why do we need a microscope to see microbes? How do microbes help in making curd and bread? Can microbes be useful and harmful at the same time?
Keep the curiosity alive
1. Various parts of a cell are given below. Write them in the appropriate places in the following diagram. Nucleus Cytoplasm Chloroplast Cell wall Cell membrane Nucleoid
The diagram is a Venn diagram comparing a plant cell, an animal cell and a bacterial cell. Common to all three cells: Cell membrane and Cytoplasm are present in all. Common to plant and animal cells only: Nucleus is present in both plant and animal cells but not in bacteria. Only in Plant Cell: Chloroplast is found only in plant cells. Cell wall is present in plant and bacterial cells but not in animal cells, so it goes in the overlapping part of plant and bacterial circles. Only in Bacterial Cell: Nucleoid is present only in bacteria instead of a true nucleus. Only in Animal Cell has no unique structure from the list, but in many textbook versions animal cell is shown without cell wall and chloroplast.
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. (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? (a) Water evaporated in test tube B and filled the balloon with the water vapour. (b) The warm atmosphere expanded the air inside the test tube B, which inflated the balloon. (c) Yeast produced a gas inside the test tube B which inflated the balloon. (d) Sugar reacted with warm air, which produced gas, eventually inflating the balloon.
After 3 to 4 hours the balloon on test tube B will inflate and become tight while the balloon on test tube A will remain as before. The correct explanation is (c) Yeast produced a gas inside the test tube B which inflated the balloon. Yeast is a fungus which ferments sugar in warm conditions, respires anaerobically and releases carbon dioxide gas. The carbon dioxide fills the balloon. Option (a) is wrong because both test tubes have water, (b) is wrong because both were kept in the same warm place, and (d) is wrong because sugar alone does not react with air to produce gas.
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. (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?
She wants to test which gas was produced by the yeast and collected in the balloon. By passing the gas into lime water she can confirm if it is carbon dioxide. If the lime water turns milky, it proves the gas is carbon dioxide, confirming that yeast produces carbon dioxide during fermentation of sugar.
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?
Wheat does not have nitrogen-fixing bacteria, so it needs nitrogen from fertiliser to grow well. Beans are leguminous plants whose roots have nodules containing Rhizobium bacteria. These bacteria trap nitrogen gas from the air and convert it into a form the plant can use, naturally increasing soil fertility. Therefore the bean farmer does not need to add extra nitrogen fertiliser, and growing beans also enriches the soil for the next crop.
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?
She is trying to test how decomposition and manure formation happens and what helps it. She is comparing whether mixing dry leaves with kitchen waste speeds up decomposition by microbes. She is also testing that microorganisms need air, moisture and the right mix of materials to decompose waste into nutrient-rich manure. Pit A with mixed waste is expected to form better, darker manure faster because the mixture provides balanced carbon and nitrogen and better aeration for bacteria and fungi.
5. Identify the following microorganisms: (i) I live in every kind of environment, and inside your gut.
Bacteria. Bacteria are found everywhere - in water, soil, air, hot springs, snow and inside the human body. Helpful bacteria in our intestine help in digestion.
5. Identify the following microorganisms: (ii) I make bread and cakes soft and fluffy.
Yeast. Yeast is a unicellular fungus that ferments sugar and releases carbon dioxide. The gas bubbles make dough rise and make bread, cakes, bhatura and idli soft and fluffy.
5. Identify the following microorganisms: (iii) I live in the roots of pulse crops and provide nutrients for their growth.
Rhizobium. It is a bacterium that lives in the root nodules of pulses like beans, peas and lentils. It fixes atmospheric nitrogen and provides it to the plant, helping growth and increasing soil fertility.
6. Design an experiment to test that microorganisms need optimal temperature, air, and moisture for their growth.
Take three sets of bread slices or moist soil with waste. Keep one set in warm, moist and open to air, second set in refrigerator cold, third set dried completely and kept airtight. Keep warmth and moisture optimal only for the first set. Observe after 3-4 days for mould growth or decomposition. Only the warm, moist, aerated sample will show rapid growth of microbes and decomposition, proving that optimal temperature, air and moisture are needed for microbial growth.
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.
After three days the bread near the sink will show white, green or black cottony mould and a bad smell, while the bread in the refrigerator will remain mostly fresh with little or no growth. This is because near the sink there is warmth, moisture and air which are favourable for fungi and bacteria to grow and spoil food. In the refrigerator the low temperature slows microbial growth so spoilage is much less.
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?
Two explanations are: 1) Lactobacillus bacteria present in curd continue to grow at warm room temperature, ferment lactose sugar in milk and produce more lactic acid which makes curd more sour. 2) Other microbes from the air may also grow in the curd when left open and produce acids during their growth, increasing sourness.
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?
In flask A the yeast ferments the sugar solution. Yeast breaks down sugar for energy and produces carbon dioxide and a little alcohol. The solution becomes bubbly due to gas formation and may produce a yeasty smell.
9. Observe the set-up given in Fig. 2.15 and answer the following questions. (ii) What do you observe in test tube B after four hours? Why do you think this happened?
After four hours the lime water in test tube B turns milky or white. This happens because carbon dioxide produced in flask A travels through the tube into lime water. Carbon dioxide reacts with lime water to form calcium carbonate which is milky, proving that fermentation releases carbon dioxide.
9. Observe the set-up given in Fig. 2.15 and answer the following questions. (iii) What would happen if yeast was not added in flask A?
If yeast was not added, no fermentation would occur. No carbon dioxide would be produced, so there would be no bubbles in flask A and the lime water in test tube B would remain clear and not turn milky.
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.
The Ministry of New and Renewable Energy runs the National Biogas Programme to promote family-size and community biogas plants. The programme provides subsidy and training to set up plants that use cattle dung, kitchen waste and plant waste. Bacteria decompose the waste in the absence of air and produce biogas which contains methane and carbon dioxide. The gas is used for cooking, lighting and electricity, and the leftover slurry is used as rich manure. This helps clean the environment, reduces dependence on firewood and provides renewable energy.
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.
Examples from different regions are idli and dosa batter, curd, dhokla, bhatura, appam, and fermented soya bean called kinema or bamboo shoot pickle. For idli, rice and black gram are soaked, ground and left warm to ferment by Lactobacillus and yeast, which makes the batter rise and become nutritious and easily digestible. Curd is made by Lactobacillus converting milk sugar to lactic acid. These foods are culturally important, have enhanced taste, better nutrition, longer shelf life and provide beneficial probiotics.
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.
A mushroom has a cap, gills on the underside of the cap, and a stalk. Under a magnifying glass the gills appear as thin plates that produce spores. Under the microscope the gills show many filamentous hyphae and spores. The internal filaments are the fungal body that absorbs nutrients. This shows that even a visible fungus is made of many microscopic thread-like cells.
Interact with an entrepreneur and learn the steps for cultivation of mushroom.
Steps for mushroom cultivation are: 1) Prepare compost or paddy straw as growing medium and sterilise it. 2) Inoculate with mushroom spawn which contains fungal mycelium. 3) Keep bags in a cool, moist and dark place for mycelium to spread. 4) Maintain moisture and air circulation. 5) After 2-3 weeks small button mushrooms appear and can be harvested. Mushroom farming is a good livelihood because it needs little land, recycles agricultural waste and provides protein-rich food.