The Invisible Living World: Beyond Our Naked Eye

Hello, my dear students! Welcome to today's science lesson. I am so happy to see you all here, ready to explore something truly fascinating. Today, we are going to learn about a chapter that will completely change the way you look at the world around you. We are going to discover "The Invisible Living World: Beyond Our Naked Eye." Isn't that exciting? Just imagine — there is an entire world of living things that exists all around us, but we cannot see them without special tools. So, let's begin our journey into this amazing hidden world!

Now, students, have you ever wondered what you might see if the invisible world around you became visible? Think about it. 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'? Have you thought about how these tiny living beings interact with each other? These are some really interesting questions to ponder over, and by the end of this chapter, you will be able to answer them all by yourself.

Let's start with something very simple. The human eye can only see objects that are above a certain size. For a long time, many tiny things around us remained unknown because we simply could not see them. But then, something wonderful happened. Long ago, people discovered that a curved piece of glass could make small things look bigger. This was a revolutionary discovery! The piece of glass was shaped like a lentil seed — thick in the middle and thin at the edge — hence they called it a lens. Can you imagine a lens? Yes, just like a masoor dal or a moong dal seed. Over time, lenses were improved to become more powerful. Each new tool, from simple magnifying glasses to microscopes, helped humans see what their eyes could not. The invention of the microscope opened a fascinating hidden world filled with tiny living creatures. We will explore some of these life forms in this chapter.

Students, you have already learnt about the amazing variety of living beings in your previous classes. Just look around — there are so many beautiful plants and animals! They are of all shapes, sizes, and colours. Some living beings are tiny, while others are really big. They differ not only in their structure but also in many other features. All these living beings, whether plants or animals, are called organisms.

Now, let me ask you a question. Have you ever noticed the smallest organism around you that is visible to the naked eye? Think about it — how small a thing can your eyes actually see? You might have seen some people using reading glasses. How does it help them see better? Or what happens when we use a magnifying glass to observe something? Let me tell you, a magnifying glass makes things appear larger than they actually are. That is why we can see small details more clearly when we use one.

Now, let's do a very interesting activity together. This is Activity 2.1. Take a round-bottom flask made up of glass. Fill it with water. Close the mouth of the flask with a cork. Now, place the flask on an open book and look at the letters through it. Do you notice something interesting? The letters appear larger when seen through the flask! This happens because the flask filled with water acts like a magnifying glass. Now, use a real magnifying glass to look at small organisms, like an ant. Were you able to see the details of its body more clearly? I am sure you must have noticed how much clearer the ant's body parts looked under the magnifying glass. This is the power of a lens!

For a long time, students, people were curious to explore the tiny organisms around them, but they could not see them with their naked eyes. So, how did we finally discover this invisible world? Do you know which scientific discovery helped us see the tiny world for the first time? Let me tell you about two very important scientists who changed the way we see the world.

In 1665, a scientist named Robert Hooke published a book called Micrographia. He was a careful observer, and a skilled artist. In this book, he showed detailed drawings of tiny things that people had never seen before — things he saw using a tool we now call a microscope. His microscope made things look 200 to 300 times bigger than what one could see with the unaided eye. One day, he looked at a thin slice of cork and saw it was made of many small, empty spaces. These compartments reminded him of a honeycomb. He drew what he saw and called each small space a cell. This was the first time the word cell was used in science to describe the basic unit of life. Can you imagine, students? The term "cell" was first used to describe the tiny compartments in a piece of cork! This is truly a landmark discovery in the history of science.

Around the same time in the 1660s, another scientist named Antonie van Leeuwenhoek, who was from Holland, made better lenses that allowed him to build more useful microscopes. He was the first person to clearly see and describe tiny living things like bacteria and blood cells. Because of this, he is known as the Father of Microbiology. Isn't that wonderful? These two scientists opened up an entirely new world for us to explore!

Now, let's move on to a very important concept — What is a Cell?

All living beings are made up of cells. You might wonder what cells actually look like. Let us take a closer look at the basic structure of a cell using a microscope. We are going to do Activity 2.2, which is a teacher demonstration activity. Pay attention, because this is very important!

First, take an onion bulb from your kitchen or garden and wash it thoroughly with water. Cut the onion bulb vertically into pieces. Take one piece of onion and pull out the thin, transparent layer from its inner surface with the help of forceps. This layer is called the onion peel. Place the peel in a petri dish containing a few drops of safranin, which is a red-coloured stain, for 30 seconds. This will give a pinkish colour to the cells and help us see them clearly. With the help of a thin brush, transfer the onion peel to another petri dish containing water to rinse the peel and remove extra stain. Now, carefully place the stained onion peel on the glass slide using a thin brush, ensuring it does not break or fold. Put a drop of glycerin over the onion peel on the slide. The glycerin will prevent drying of the cells and improve clarity for better visualisation of cells. Slowly place a coverslip over the peel using a needle, such that no air bubbles get trapped. Use blotting paper to gently wipe off any extra glycerin around the edges of the coverslip. Now, observe the slide under a microscope. Compare what you see with the figures given in your textbook.

What did you observe? You will see nearly rectangular structures under the microscope. These are the cells of the onion peel, which are closely arranged without any space between them. Try to observe the peels of the leaves of different plants around you. You will find that all plants are made up of cells. What do you think the body of an animal is made of? Yes, that's right — animals are also made up of cells!

Now, let's do another activity to see animal cells. This is Activity 2.3. First, rinse your mouth with clean water. Use the blunt end of a clean toothpick, and gently scrape the inside of your cheek. Place the scraped material in a drop of water on a clean glass slide and spread it evenly. Add a drop of methylene blue, which is a blue-coloured stain, over the material on the slide. Adding stain improves the visibility of the material under the microscope by increasing contrast. After one minute, add a drop of glycerin over the material on the slide to prevent the cells from drying. Now, carefully place a clean coverslip on the material, and remove the excess glycerin from the edges of the coverslip using blotting paper. Observe the slide under a microscope and draw what you see in your notebook.

What did you observe? You will see a polygon-shaped structure. These are cheek cells, which form the inner lining of your mouth. Now, what similarities and differences did you observe between the cells of onion peel in Activity 2.2 and human cheek cells in Activity 2.3? Let's think about this together.

In the onion peel, you saw rectangular cells, while in the cheek, you saw polygon-shaped cells. Both have a similar basic structure, but there are some differences too. We will learn about these differences in detail.

Now, students, you have observed that cells have three main parts. The outer layer is called the cell membrane. The round structure in the middle is the nucleus, which is also covered by a thin membrane. The space between the cell membrane and nucleus is filled with cytoplasm. These three — cell membrane, cytoplasm, and nucleus — are the basic parts of a cell. Some cells, like onion peel cells, have an extra outer layer called the cell wall. What is the importance of these structures in a cell? What functions do they perform? Are these functions important for the maintenance of life? Let me explain each of these parts in detail.

The cell membrane encloses the cytoplasm and nucleus. The cell membrane separates one cell from another. It is porous and allows the entry of materials essential for life processes and the exit of waste material. Think of it like the boundary of your school — it separates the school from the outside world, but still allows students and teachers to come in and go out.

Cytoplasm contains other components of the cell and compounds, such as carbohydrates, proteins, fats, and mineral salts. Most of the life processes take place within the cytoplasm. This is like the playground where all the activities happen!

The nucleus regulates all activities that occur within the cell. It also regulates growth. The nucleus is like the principal of the school who makes all the important decisions!

The cell wall in the plant cell provides rigidity and strength to plants. This is why all cells are arranged compactly with each other and look firm in structure. The cell wall is like the walls of a house that give it shape and support.

Now, let's go a step further. Cells in all parts of a plant have tiny rod-shaped structures called plastids. Some plastids, like chloroplasts, contain chlorophyll, which makes them green and helps in photosynthesis. In non-green parts, they help in the storage of substances. Plant cells also have a large, empty-looking space called a vacuole. This helps the plant cell store important substances, get rid of waste, and maintain the shape of the cell. This gives strength and support to the plant. In animal cells, vacuoles are usually not present, or if present, they are usually small. These small vacuoles store certain substances dissolved in water. So, a cell is not just a simple bag of liquid — it is a complex structure made up of many different parts, each with its own special function to allow the cell, and in turn the entire organism to work.

We have now understood the basic structure of a cell. And we now also understand that plant and animal cells differ in shape and structure.

Now, let me ask you a question. Do different animal cells also vary in their shape and structure? The answer is yes! Let me show you two different types of cells found in humans. Look at the muscle cell and the nerve cell shown in your textbook. What are the similarities and differences you see in them?

A muscle cell is shaped like a spindle, while a nerve cell is very long and has branches. Similarly, some cells are round in shape, while others are long and thin. The number of cells also varies in different organisms. Why do cells look so different from each other? Does the shape and structure of a cell relate to its function? Yes, it does! The unique shape, size, and structure of cells help them carry out their specific functions.

You observed in Activity 2.3 that inner cheek cells are thin and flat. They form a protective lining on the inner surface of the cheek. Nerve cells, also known as neurons, carry messages in our body. The elongated shape and branched structure help them reach different parts of the body and pass on messages quickly. Similarly, plant cells also show variation. In plants, too, cells may be rectangular, elongated, oval, or even tube-like. Some plant cells form long tubes that help carry water throughout the plant.

You have already studied the digestive system in Grade 7. Different parts of the digestive system are made up of different types of cells. A group of muscle cells are present in the food pipe. These cells contract and relax in a wave-like manner, pushing the food down to the stomach. This movement is possible because muscle cells are thin, flexible, and spindle-shaped. The stomach also has different types of cells for performing different functions. Muscle cells in the stomach wall help churn the food. Other cells in the inner lining of the stomach produce digestive juices and acid that help break down the food. All these cells work together to make digestion possible.

So, students, to summarize what we have learned so far: cells are the basic building blocks of all living organisms. They have different parts that perform different functions. The shape and size of cells are related to the work they do. Now, let's move on to the next important concept.

What are the levels of organisation in the body of a living organism?

The body of a living organism is organised in a complex way. A cell is the basic unit of life, just like a brick is the basic unit of a wall. A group of similar cells forms a type of tissue. Different tissues are organised to form an organ. Several organs work together to form an organ system that performs a major function of the body. All the organ systems together make up a complete organism — like a plant or an animal.

So, the levels of organisation are: Cell → Tissue → Organ → Organ System → Organism.

These levels of organisation help us understand how simple building blocks like cells come together to form a complex living being.

The life of complex living organisms begins with a single cell — the egg. The egg of any organism has an amazing ability to divide repeatedly to form a complete living being made up of many cells. Such living beings are called multicellular organisms. Animals, including humans, and plants are all examples of multicellular organisms.

Now, here's something really interesting for you! Did you know that the yolk of an ostrich egg is a single cell — the largest known cell in the living world? It measures about 130 mm to 170 mm in diameter! The egg contains extra non-cellular material: a shell for protection and a white liquid that nourishes the cell during its continued development. Isn't that amazing? Just think about it — one single cell can be so big that you can actually see it with your naked eye!

Now, let's talk about microorganisms. Some living organisms are made up of just one or very few cells. They are so small that they cannot be seen with the naked eye. These are called microorganisms. Some microorganisms, like bacteria and Amoeba, are made of just one cell, which means they are unicellular. Others, like some fungi and algae, have many cells, which means they are multicellular. They are found all around us — in water, soil, air, and even inside our body! But what do their cells look like? Are they like the plant and animal cells we just learnt about, or are they different? To observe the cells of a microorganism, again, we need to use a microscope which magnifies their size and makes them visible to us. Scientists have also created a low-cost and foldable paper microscope. Such foldable paper microscope may not provide the same level of details like high-powered laboratory microscopes. However, these make microscopic world accessible to many people.

Let us now take a closer look at the fascinating world of microbes. We are going to do two activities to observe microorganisms.

Activity 2.4: Let us observe pond water or stagnant water. Take a container and collect pond or stagnant water in it with the help of your teacher or elder. Use a dropper and place a drop of pond or stagnant water on a microscope slide. Put a coverslip and observe it under the microscope. Observe the tiny organisms found in the pond or stagnant water.

Activity 2.5: Let us observe soil suspension. Take a beaker and collect some moist soil in it from the nearby field or garden. Do not touch the soil with your bare hands — use a spoon or gloves. Pour some water into the beaker and stir it with a glass rod. The liquid, which may look dirty, has very fine particles of soil, and is called soil suspension. Keep it aside for some time and let the mixture settle. Use a dropper and take a drop of water from the top layer. Place the drop on a microscope slide. Cover it gently with a coverslip and observe it under the microscope.

You may observe small moving organisms similar to those you saw in Activity 2.4. This indicates that even soil suspension contains a variety of tiny creatures that cannot be seen with the unaided eye.

The tiny creatures that cannot be seen with the naked eye are called microorganisms or microbes. The word "micro" means very small, and "organisms" means living beings. So, microorganisms are very small living beings.

Now, let me tell you about what a group of students observed when they performed these activities. They identified the microorganisms as protozoa, algae, fungi, and bacteria. In pond water, they observed Amoeba, which is a protozoa. Amoeba is a single cell, moves, and has an irregular shape. They also observed Paramecium, which is also a protozoa. It is a single cell that moves from one place to another, and its movement takes place with the help of specialised structures. They also saw algae, which are single cell, look green because of the presence of green pigment, and move with the help of specialised structures.

In soil suspension, they observed bread mould, which is a fungus. It has branched filaments without chlorophyll and has a sac-like structure. They also saw another type of mould, which has a brush-like structure. They saw algae, which are spherical and have chlorophyll, a green pigment. And they saw bacteria, which can be spherical, comma-shaped, spiral or rod-shaped. Bacteria have one long hair-like structure and many small hair-like projections around the cell.

Now, students, you must understand that microorganisms are everywhere, and we can only see them with a microscope — a device that magnifies them 100 to 400 times. Though microorganisms are small in size, they play an important role in our lives.

Before we move on, let me tell you about one more type of microorganism — viruses. Viruses are microscopic and acellular. Viruses multiply when they enter a living cell. They may infect plants, animals, or bacterial cells and may cause a disease. We will learn more about diseases caused by microbes in the next chapter.

Now, let's discuss how we are connected to microbes. Can we find microorganisms in other places, too?

Have you ever seen a lemon, tomato, orange, or any other food item rot after being left outside for some time? If yes, you may have noticed a powdery or cotton-like growth on them. This happens because they have been infected by microbes. But where did these microbes come from? How did they come in contact with the food? This happens because microorganisms can be found everywhere, be it in water, soil, air, or even in some food items.

You can use a microscope to explore surfaces of leaves, stems, roots, or any other part to see them. Like plants and animals, microorganisms also show great diversity. Some of them can even be found in extreme climatic conditions, such as hot water springs and snow cold zones as well as at moderate temperatures. You already know some of these organisms live inside our bodies, especially in our gut! You have studied in the chapter "Life Processes in Animals" in Curiosity, Grade 7 that our intestine has many bacteria that help in digestion. Like plants and animals, microorganisms vary in shape, size, and structure. You would have observed microorganisms of different shapes — spherical, rod-shaped, or irregular.

Now, let's understand how the diversity of microorganisms plays a role in our daily life and how they help clean the environment.

Let us do Activity 2.7. Take an empty container and fill it halfway with garden soil. Add some fruit and vegetable peels to the container. Thereafter, put a layer of soil on it and leave it aside. After 2–3 weeks, observe the changes that have taken place. Do you observe any difference in the contents of the container?

You may find that peels of fruits and vegetables have turned into a dark-coloured material. This is manure, which is rich in nutrients and helps increase the fertility of the soil. But how did the peels of fruits and vegetables turn into manure?

In Activity 2.6, you saw that soil contains various kinds of microorganisms. Some of these microorganisms, like fungi and bacteria, act on the plant waste and slowly break it down into simpler, nutrient-rich manure. You may have seen gardeners in your school or in a field near your house collecting dry leaves and plant waste and putting them into pits. Do you now understand why they do this? It is to make natural manure.

This is truly our scientific heritage! Ancient Indian texts, particularly the Vedas, have references of the word "Krimi" which means different tiny entities including "Drishya" (visible) and "Adrishya" (invisible). Various Vedic texts mention their beneficial and harmful effects. Atharvaveda also refers to "Krimi".

If you look around carefully, you might see decaying plants and fallen leaves stored in a container or lying in the garden, disappear after some time from the surroundings. This is because microorganisms break down the complex substances of fallen leaves into simpler substances that are rich in nutrients — a process called decomposition. These nutrients go back to the soil and help plants grow better. Microorganisms also decompose bodies of dead animals. So, microbes help recycle the waste and return important nutrients to nature. Manure formation occurs at optimal temperature and appropriate moisture level.

Isn't it interesting? By now, you must have understood that bacteria and some fungi are types of microorganisms that play an important role in our lives. And guess what, these helpful bacteria can also decompose animal wastes like dung!

From Activity 2.7, we can also infer that microorganisms not only help in plant growth, but also clean our environment by breaking down waste. Now, think what would have happened if microorganisms did not exist on Earth? There would be no decomposition, and waste would keep piling up everywhere! That would be terrible, right?

Now, let me tell you about another amazing thing — microbes as a source of biogas. Many microorganisms, like bacteria and fungi, live in an oxygen-free environment. Some of these bacteria have the ability to decompose plant and animal waste present in the environment or household wastewater. During the process, they release a mixture of gases called biogas, primarily composed of carbon dioxide and a high proportion of another gas, methane. This gas has been used as a fuel source for cooking, heating, generating electricity, and even to run vehicles. In fact, India has a long history of biogas production. One of our oldest biogas plants was set up in the late 1850s. There is a Biogas Program initiated by the Ministry of New and Renewable Energy, Government of India.

Now, let me tell you about a famous scientist. Ananda Mohan Chakrabarty was a scientist who studied bacteria. In 1971, he developed a special bacterium that could break down oil spills, helping to clean the environment. His discovery received a patent in 1980. A patent is a copyright given to a person so that no one else can copy, use or sell his or her invention without permission. His work showed how microorganisms could be used to solve environmental problems like pollution. He is remembered for his contributions to science and for protecting the environment using microbes.

Now, let's discuss how the diversity of microorganisms helps in our kitchen. Let's do Activity 2.8. Take two bowls, A and B. In each, take 200 grams of flour, which is atta or maida, and add a pinch of sugar. Now, in bowl A, add a small amount of yeast powder and mix it well with the flour. In bowl B, do not add any yeast, so that we can compare the results of the two bowls. Knead the flour of the two bowls with warm water to make soft dough. Cover the dough with a damp cloth and keep it in a warm place. Observe both the bowls after 4–5 hours.

Did you find any change in the volume, smell, or texture of the dough? If not, leave the dough for some more time. After some time, you may notice that the dough in bowl A, where yeast was added, has risen slightly, become fluffy, and has a different smell compared to the dough kneaded without yeast. Why does this happen? What is the role of yeast? Why did we add sugar and warm water to the flour?

Yeast is a type of microorganism. It belongs to a group of microorganisms called fungi. Yeast grows well in warm conditions. You may recall from the chapter "Life Process in Animals" in Grade 7 that, like other organisms, yeast also respires and breaks down food to release energy for their growth and carry out life processes. During this process, carbon dioxide is released, which forms bubbles that make the dough soft and fluffy. Yeast also produces a small amount of alcohol during this process, which gives the dough a slightly different smell. This special property of yeast is used in the process of making breads, cakes, and more! In addition to yeast, some bacteria, such as Lactobacillus, help in fermentation of batter for making idli and dosa, and dough for making bhatura.

Now, let's do Activity 2.9 to understand curd formation. Take two small glass bowls and label them A and B. Pour lukewarm milk in bowl A, and cold milk in bowl B. Now, add a small spoonful of curd to each bowl and mix well using a spoon. Cover both bowls. Keep bowl A in a warm place and bowl B in a cool place, like a refrigerator, for a few hours or overnight. Observe the changes in the glass bowls.

You will observe that in bowl A, the milk has turned into curd after a few hours and has become little sour. Whereas in bowl B, the milk has not curdled, but it might be a little sour. Do you know why this happens? The curd contains several types of bacteria. One of them is Lactobacillus. This bacterium feeds on the sugar in the milk, which is called lactose, multiplies, and ferments the milk to form curd. Instead of producing alcohol like yeast, these bacteria produce lactic acid, which makes curd sour. These bacteria grow well in warm conditions. That is why curd is formed in bowl A but not in bowl B.

Now, let's talk about root nodules. Some bacteria, such as Rhizobium, form the swollen regions called nodules and live in them. Roots of certain legumes, such as beans, peas, and lentils, have root nodules that contain Rhizobium bacteria. These bacteria trap nitrogen from the air and make it useful for the plants. This helps plants grow better without chemical fertilisers. That is why farmers grow legumes in rotation with other crops. This naturally increases the nitrogen in the soil and keeps it healthy for the next crop.

Now, let's learn about amazing microalgae — tiny helpers in water. Microalgae are microscopic plant-like organisms that live in water, soil, air, and even on trees. They make their own food using sunlight. While doing this, they also release oxygen and produce more than half of the Earth's oxygen supply. They are rich in nutrients and serve as a food source for many aquatic animals. Some, like Spirulina, Chlorella, and Diatoms, are also used by humans as health supplements and medicines. Microalgae also help in cleaning water and are used to make biofuel.

However, pollution, climate change, and habitat destruction are threatening microalgal diversity and abundance. It is important to conserve these tiny organisms to protect the environment and maintain oxygen balance on Earth.

Now, here's something interesting about Spirulina. Spirulina is a microalga and is called a superfood because of its health benefits. Spirulina is also a good source of vitamin B12, which is essential for our body. It is rich in protein — more than 60 per cent of its body weight — and only a small amount of fat and sugar. Nowadays, farming of Spirulina is becoming a feasible livelihood opportunity. You can grow Spirulina easily by following some simple steps.

Now, let's answer the question: Why is cell considered to be a basic unit of life?

The body of all living organisms are made up of tiny building blocks called cells. A single cell contains various components that help organisms perform various functions. The bodies of all plants and animals are made up of many cells. Therefore, they are called multicellular organisms. In multicellular organisms, cells carry out specialised functions individually but also cooperate with each other to increase the chance of survival.

Some microorganisms, such as bacteria and protozoa, are made up of just one cell. These are called unicellular organisms. They carry out all the functions necessary for their survival in a single cell. Other microbes, like algae and fungi, are made up of one or more cells. For example, yeast is a unicellular fungus while mould is a multicellular fungus.

Like animal and plant cells, the cells of microorganisms are also surrounded by a cell membrane. Cells of fungi additionally have a cell wall, but they do not have chloroplasts, so they cannot make their own food through photosynthesis. Bacteria do not have a well-defined nucleus and a nuclear membrane. Instead, they have a nucleoid. This feature distinguishes them from cells of yeast, protozoa, algae, fungi, plants, and animals.

We have only looked at a few basic cell structures here. The cell has other components about which you will learn in higher classes. For observing subcellular components, we need microscopes with high magnification. An electron microscope magnifies the cell about 10,00,000 times, where we can see more structures present in a cell.

By now, you must have understood that all living beings, including microorganisms, are made up of one or more cells. Their cells differ in size, shape, and structure. Plant and animal cells also have some differences. Understanding these differences helps us learn how these organisms function differently.

In this chapter, we have learnt about the beneficial microorganisms. However, there are some microbes that cause diseases in plants and animals, including humans. We will learn about some of the diseases caused by microbes in the next chapter.

Now, let's solve the questions given in the "Keep the curiosity alive" section.

Question 1: Various parts of a cell are given below. Write them in the appropriate places in the following diagram. The parts are: Nucleus, Cytoplasm, Chloroplast, Cell wall, Cell membrane, Nucleoid.

Let me explain where each of these parts goes. The cell membrane is the outer covering that surrounds the cell. The cell wall is the outer layer in plant cells, which provides rigidity and strength. Inside the cell membrane, we have the cytoplasm, which is the jelly-like substance that fills the cell. The nucleus is the round structure in the middle that controls all activities of the cell. In plant cells, we also have chloroplasts, which are green structures that contain chlorophyll for photosynthesis. In bacterial cells, instead of a well-defined nucleus, we have a nucleoid, which is the region where the genetic material is located.

Question 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.

Part (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?

The correct answer is (c) — Yeast produced a gas inside the test tube B which inflated the balloon. This is because yeast is a microorganism that performs respiration. When yeast is added to sugar solution in warm conditions, it breaks down the sugar and releases carbon dioxide gas. This gas fills the balloon and inflates it.

Part (ii): She took another test tube, one-fourth 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 find out if the gas produced by yeast is carbon dioxide. When carbon dioxide is passed through lime water, it turns the lime water milky. This is a test for carbon dioxide gas.

Question 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?

The reason is that bean plants have root nodules that contain Rhizobium bacteria. These bacteria can trap nitrogen from the air and convert it into a form that plants can use. This process is called nitrogen fixation. So, bean crops do not need additional nitrogen fertiliser because they get nitrogen from the air with the help of Rhizobium. This is also why farmers grow legumes in rotation with other crops — they help naturally enrich the soil with nitrogen.

Question 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 whether the presence of dried leaves, which contain carbon and provide food for microorganisms, helps in faster decomposition of waste. Dried leaves act as food for microorganisms and help them grow and multiply faster, which speeds up the decomposition process. So, we can say that she is testing the role of carbon source in decomposition.

Question 5: Identify the following microorganisms:

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

This is bacteria. Bacteria are found everywhere — in water, soil, air, and even inside our digestive system where they help in digestion.

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

This is yeast. Yeast is a fungus that produces carbon dioxide gas during fermentation, which makes bread and cakes soft and fluffy.

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

This is Rhizobium. Rhizobium bacteria live in the root nodules of leguminous plants like peas, beans, and lentils, and help fix nitrogen from the air into the soil.

Question 6: Design an experiment to test that microorganisms need optimal temperature, air, and moisture for their growth.

To test this, we can take three sets of conditions. For temperature, we can keep three samples of dough or milk at different temperatures — one in the refrigerator (cold), one at room temperature, and one in a warm place. We can observe which one shows the most microbial activity. For air, we can keep one sample in an airtight container and another in an open container. For moisture, we can keep one sample dry and another moist. By comparing the results, we can show that microorganisms need optimal temperature, air, and moisture for their growth.

Question 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.

The slice kept near the sink will likely develop mould faster because the sink area is warm and moist, which are ideal conditions for microbial growth. The slice kept in the refrigerator will develop mould slower because the cold temperature slows down microbial growth. This shows that microorganisms need warm and moist conditions for growth.

Question 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?

One possible explanation is that the lactic acid bacteria in the curd continue to grow and produce more lactic acid, making it sourter. Another possible explanation is that other microorganisms from the air might have entered the curd and caused further fermentation, producing more acid.

Question 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, which contains yeast, the sugar solution will be fermented by yeast. Yeast will break down the sugar and produce carbon dioxide gas and alcohol.

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

Test tube B contains lime water. The carbon dioxide gas produced in flask A will pass through the delivery tube into test tube B. The lime water will turn milky because carbon dioxide reacts with lime water to form calcium carbonate, which makes it milky.

(iii) What would happen if yeast was not added in flask A?

If yeast was not added, there would be no fermentation. No carbon dioxide gas would be produced, and the lime water in test tube B would not turn milky.

Now, let's do a quick recap of everything we have learned in this chapter.

We started by learning about how humans discovered the invisible world of tiny organisms through the invention of the microscope. We learned about Robert Hooke, who first used the term "cell" to describe the basic unit of life, and Antonie van Leeuwenhoek, who is known as the Father of Microbiology because he was the first to see bacteria and blood cells clearly.

We learned that all living beings are made up of cells. We did activities to observe onion peel cells and human cheek cells. We learned about the three main parts of a cell: the cell membrane, cytoplasm, and nucleus. We learned that plant cells have an additional cell wall, chloroplasts, and large vacuoles, while animal cells usually have small vacuoles or none at all.

We learned that cells differ in shape and size depending on their function. For example, muscle cells are spindle-shaped, nerve cells are long and branched, and cheek cells are flat and polygonal.

We learned about the levels of organisation in living organisms: cell → tissue → organ → organ system → organism.

We learned about microorganisms — tiny living beings that cannot be seen with the naked eye. We learned about different types of microorganisms: bacteria, protozoa, fungi, and algae. We learned that some microorganisms are unicellular, while others are multicellular.

We learned about how microorganisms are useful to us. They help in cleaning the environment by decomposing waste. They help in making biogas. They help in making food like bread, cakes, idli, dosa, and curd. They help in nitrogen fixation in the soil through Rhizobium. They produce oxygen through microalgae.

We learned that microorganisms can be found everywhere — in water, soil, air, and even inside our bodies. We learned about the importance of conserving microorganisms.

We learned why the cell is considered the basic unit of life — because all living beings are made up of cells, and cells carry out all the life processes.

We also learned about some not-so-nice aspects of microorganisms — they can cause diseases, which we will learn more about in the next chapter.

This brings us to the end of our lesson. I hope you all now have a clear understanding of the invisible living world beyond our naked eye. Remember, there is a whole world of tiny organisms living all around us, and they play a very important role in our lives. Some help us in many ways, like in making food and cleaning the environment, while others can cause diseases. It is important for us to understand them better so that we can harness the beneficial ones and protect ourselves from the harmful ones.

Thank you for listening so patiently. Keep curious, keep exploring, and never stop asking questions! See you in the next class. Goodbye, students!