The Invisible Living World: Beyond Our Naked EyeClass 8 Science Notes

The Invisible Living World: Beyond Our Naked Eye

The human eye can only see objects above a certain size. This limitation meant that many tiny things around us remained unknown for a long time. However, people discovered that a curved piece of glass, called a lens, could make small things look bigger. Lenses were shaped like lentil seeds—thick in the middle and thin at the edge. As lenses improved, so did our ability to see smaller objects. The invention of the microscope opened a fascinating hidden world filled with tiny living creatures.

All living beings, whether plants or animals, are called organisms. Organisms come in all shapes, sizes, and colors. Some are tiny, while others are enormous.

Activity 2.1: Let Us Observe

A round-bottom flask filled with water acts like a magnifying glass, making letters appear larger when viewed through it. Similarly, a real magnifying glass allows you to see the details of small organisms, like an ant, more clearly.

For a long time, people were curious about the tiny organisms around them but couldn't see them with their naked eyes. The scientific discovery that helped us see this invisible world for the first time was the microscope.

Ever Heard Of...

In 1665, a scientist named Robert Hooke published a book called Micrographia. Hooke was a careful observer and a skilled artist. In his book, he showed detailed drawings of tiny things that people had never seen before, using a microscope.

Hooke's microscope magnified objects 200 to 300 times larger than what the unaided eye could see. He looked at a thin slice of cork and saw it was made of many small, empty spaces resembling a honeycomb. He 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.

Around the same time, in the 1660s, Antonie van Leeuwenhoek, a Dutch scientist, 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.

What Is a Cell?

All living beings are made up of cells. To understand what cells look like, we can take a closer look at their basic structure using a microscope.

Activity 2.2: Let Us Study a Cell (Teacher Demonstration Activity)

This activity involves observing onion peel cells under a microscope.

1. Take an onion bulb and cut it into pieces.
2. Pull out a thin, transparent layer (the onion peel) from its inner surface.
3. Place the peel in a petri dish with safranin (a red-colored stain) for 30 seconds to give the cells a pinkish color, making them easier to see.
4. Rinse the peel in water to remove extra stain.
5. Carefully place the stained onion peel on a glass slide using a thin brush, ensuring it does not break or fold.
6. Put a drop of glycerin over the onion peel to prevent drying and improve clarity.
7. Slowly place a coverslip over the peel using a needle to avoid air bubbles.
8. Use blotting paper to wipe off extra glycerin.
9. View the slide under a microscope or foldscope.

Under the microscope, you will observe nearly rectangular structures. These are the cells of the onion peel, closely arranged without any space between them. All plants are made up of cells.

Activity 2.3: Let Us Investigate

This activity involves observing human cheek cells under a microscope.

1. Rinse your mouth with clean water.
2. Gently scrape the inside of your cheek with the blunt end of a clean toothpick.
3. Place the scraped material in a drop of water on a clean glass slide and spread it evenly.
4. Add a drop of methylene blue (a blue-colored stain) to improve visibility by increasing contrast.
5. After one minute, add a drop of glycerin to prevent the cells from drying.
6. Carefully place a clean coverslip on the material and remove excess glycerin with blotting paper.
7. Observe the slide under a microscope and draw what you see.

You will observe polygon-shaped structures. These are cheek cells, which form the inner lining of your mouth.

Cells have three main parts:

• A thin outer lining called the cell membrane.
• A central region called the nucleus, covered by a thin membrane.
• The space between the cell membrane and nucleus, filled with cytoplasm.

Some cells, like onion peel cells, have an extra outer layer called the cell wall.

The cell membrane encloses the cytoplasm and nucleus, separating one cell from another. It is porous, allowing the entry of essential materials and the exit of waste.

Cytoplasm contains other components of the cell and compounds like carbohydrates, proteins, fats, and mineral salts. Most life processes take place within the cytoplasm.

The nucleus regulates all activities within the cell, including growth.

The cell wall in plant cells provides rigidity and strength.

A Step Further

Plant cells have tiny rod-shaped structures called plastids. Some plastids, like chloroplasts, contain chlorophyll, which makes them green and helps in photosynthesis. Plant cells also have a large, empty-looking space called a vacuole, which stores important substances, gets rid of waste, and maintains the cell's shape. Animal cells usually have small or no vacuoles.

Variation in Shape and Structure of Cells

Different animal cells vary in their shape and structure. A muscle cell is shaped like a spindle, while a nerve cell is very long and has branches. The shape and structure of a cell relate to its function.

The inner cheek cells are thin and flat, forming a protective lining. Nerve cells, also known as neurons, carry messages throughout the body. Their elongated shape and branched structure help them reach different parts of the body and pass on messages quickly. Plant cells can be rectangular, elongated, oval, or tube-like. Some form long tubes that carry water throughout the plant.

In the digestive system, muscle cells in the food pipe contract and relax, pushing food down to the stomach. Muscle cells are thin, flexible, and spindle-shaped, making this movement possible. The stomach also has different types of cells for different functions.

What Are the Levels of Organisation in the Body of a Living Organism?

The body of a living organism is organized in a complex way:

1. Cell: The basic unit of life.
2. Tissue: A group of similar cells.
3. Organ: Different tissues organized together.
4. Organ System: Several organs working together to perform a major function.
5. Organism: All the organ systems together.

The levels of organization are: Cell → Tissue → Organ → Organ System → Organism.

The life of complex living organisms begins with a single cell, the egg. The egg divides repeatedly to form a complete living being made up of many cells. These living beings are called multicellular organisms. Animals, including humans, and plants are examples of multicellular organisms.

Ever Heard Of...

The yolk of an ostrich egg is a single cell—the largest known cell in the living world—measuring 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.

What Are 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 (unicellular). Others, like some fungi and algae, have many cells (multicellular). They are found all around us—in water, soil, air, and even inside our bodies. To observe the cells of a microorganism, we need to use a microscope, which magnifies their size. Scientists have also created a low-cost, foldable paper microscope called a foldscope.

Activity 2.4: Let Us Observe Pond Water/ Stagnant Water

Collect pond or stagnant water. Place a drop on a microscope slide, cover it with a coverslip, and observe it under a microscope or foldscope. Observe the tiny organisms found in the water.

Activity 2.5: Let Us Observe Soil Suspension

1. Collect some moist soil in a beaker using a spoon or gloves.
2. Pour water into the beaker and stir it with a glass rod to create a soil suspension.
3. Let the mixture settle.
4. Take a drop of water from the top layer, place it on a microscope slide, and cover it with a coverslip.
5. Observe it under the microscope.

You may observe small moving organisms similar to those seen in pond water. These tiny creatures that cannot be seen with the naked eye are called microorganisms (micro means very small; organisms means living beings) or microbes.

Activity 2.6: Let Us Study

Students observed pond water and soil suspension and identified microorganisms as protozoa, algae, fungi, and bacteria.

Microorganisms are everywhere, and we can only see them with a microscope, which magnifies them 100 to 400 times. Although small, they play an important role in our lives.

Ever Heard Of...

Viruses are microscopic and acellular. Viruses multiply when they enter a living cell. They may infect plants, animals, or bacterial cells and may cause disease.

How Are We Connected to Microbes?

Microorganisms can be found everywhere—in water, soil, air, and even in some food items.

Microorganisms show great diversity and can be found in extreme climatic conditions, such as hot water springs and snow-cold zones, as well as at moderate temperatures. Some live inside our bodies, especially in our gut, helping in digestion.

Microorganisms vary in shape, size, and structure, appearing spherical, rod-shaped, or irregular.

Key Players in Cleaning the Environment

Microorganisms help clean the environment.

Activity 2.7: Let Us Do

1. Take an empty container and fill it halfway with garden soil.
2. Add some fruit and vegetable peels, then cover with a layer of soil.
3. Leave it aside for 2-3 weeks.

The peels of fruits and vegetables will turn into a dark-colored material called manure, which is rich in nutrients and increases soil fertility. This happens because microorganisms, like fungi and bacteria, act on the plant waste and break it down into simpler, nutrient-rich manure.

Microorganisms break down decaying plants and fallen leaves into simpler substances rich in nutrients. These nutrients go back to the soil and help plants grow better. Microorganisms also decompose bodies of dead animals. Thus, microbes help recycle waste and return important nutrients to nature. Manure formation occurs at optimal temperature and appropriate moisture levels.

Bacteria and some fungi play an important role in our lives by decomposing plant and animal waste.

Microorganisms not only help in plant growth but also clean our environment by breaking down waste.

Our Scientific Heritage

Ancient Indian texts, particularly the Vedas, have references to 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’.

A Step Further

Microbes as a Source of Biogas

Many microorganisms, like bacteria and fungi, live in an oxygen-free environment. Some of these bacteria decompose plant and animal waste, releasing a mixture of gases containing carbon dioxide and methane. Methane has been used as a fuel source for cooking, heating, generating electricity, and even running vehicles.

Be a Scientist

Dr. Ananda Mohan Chakrabarty (1938-2020) 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/her invention without permission. His work showed how microorganisms could be used to solve environmental problems like pollution.

Microorganisms and Food

Microorganisms also play a role in food preparation.

Activity 2.8: Let Us Perform

1. Take two bowls, A and B.
2. In each, take 200 g of flour and add a pinch of sugar.
3. In bowl A, add a small amount of yeast powder and mix it well.
4. In bowl B, do not add any yeast.
5. Knead the flour of both bowls with warm water to make soft dough.
6. Cover the dough with a damp cloth and keep it in a warm place.
7. Observe both bowls after 4-5 hours.

The dough in bowl A, where yeast was added, will have risen slightly, become fluffy, and have a different smell compared to the dough in bowl B.

Yeast is a type of microorganism belonging to the fungi group. Yeast grows well in warm conditions. It respires and breaks down food to release energy. During this process, carbon dioxide is released, forming bubbles that make the dough soft and fluffy. Yeast also produces a small amount of alcohol, giving the dough a slightly different smell. This property of yeast is used in 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.

Activity 2.9: Let Us Prepare

1. Take two small glass bowls, A and B.
2. Pour lukewarm milk in bowl A and cold milk in bowl B.
3. Add a small spoonful of curd to each bowl and mix well.
4. 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.
5. Observe the changes in the glass bowls.

In bowl A, the milk will turn into curd after a few hours and become a little sour. In bowl B, the milk will not curdle, but it might be a little sour.

Curd contains several types of bacteria, one of which is Lactobacillus. This bacterium feeds on the sugar in the milk (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.

Microorganisms can be categorized into different groups, such as protozoa, fungi, bacteria, and some algae. Some bacteria, such as Rhizobium, live in root nodules of certain legumes, such as beans, peas, and lentils. These bacteria trap nitrogen from the air and make it useful for the plants, helping them grow better without chemical fertilizers. Farmers grow legumes in rotation with other crops to naturally increase nitrogen in the soil.

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 used by humans as health supplements and medicines. Microalgae also help in cleaning water and are used to make biofuel.

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

Ever Heard Of...

Spirulina, a microalga, is called a superfood because of its health benefits. Spirulina is a good source of vitamin B12 and has a lot of protein (more than 60 percent of its body weight) and only a small amount of fat and sugar.

Farming of Spirulina is becoming a feasible livelihood opportunity.

Why Is Cell Considered to Be a Basic Unit of Life?

The body of all living organisms is 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 (many-celled) organisms. In multicellular organisms, cells carry out specialized 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 (single-celled) 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 mold 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 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.

Cells of microorganisms are also surrounded by a cell membrane. Fungi cells additionally have a cell wall, but they do not have chloroplasts and cannot make their own food through photosynthesis. Bacteria lack a well-defined nucleus and have a nucleoid.

Electron microscopes can magnify cells about 1,000,000 times, allowing us to see more structures present in a cell.

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 learned about beneficial microorganisms. However, some microbes cause diseases in plants and animals, including humans.