Students can rely on these Class 8 Science Notes Chapter 2 The Invisible Living World Beyond Our Naked Eye Class 8 Notes for complete exam preparation.
Class 8 Science Chapter 2 The Invisible Living World Beyond Our Naked Eye Notes
Class 8 Science Curiosity Chapter 2 Notes
Class 8 Science Chapter 2 Notes – The Invisible Living World Beyond Our Naked Eye Notes Class 8
→ Cell The basic unit of life that makes up all living organisms.
→ Multicellular Organism An organism made up of many cells that work together, like plants and animals.
→ Unicellular Organism An organism made up of only one cell, like bacteria and protozoa.
→ Nucleus The central part of a cell that controls its activities and contains genetic material.
→ Cytoplasm The jelly-like fluid inside a cell where most life processes take place.
→ Cell Membrane The thin outer layer of a cell that controls what enters and leaves the cell.
→ Tissue A group of similar cells that work together to perform a specific function.
→ Decomposition To break down dead plants, animals, or waste materials into simpler substances with the help of microorganisms.
→ Manure A dark, nutrient-rich material made from decayed plant or animal waste, used to improve soil fertility.
→ Rhizobium A type of helpful bacteria that lives in root nodules of legume plants and fixes nitrogen from the air.
→ Microalgae Microscopic, plant-like organisms that live in water and produce oxygen through photosynthesis.
→ Nerve cells are specialized to transmit signals quickly across the body and have a distinctive structure with long extensions. What is the shape best used to describe these cells?
→ Muscle cells have a distinct elongated shape that allows them to contract and relax efficiently to help movement. What is the typical shape of muscle cells described as?
→ Inside the cell, there is a jelly-like fluid that fills the space between the nucleus and the cell membrane, serving as the site where most cellular activities and chemical reactions take place. What is this substance called?
→ Plant cells contain a special organelle filled with chlorophyll that captures sunlight to produce food through the process of photosynthesis. What is this organelle called?
→ When many similar cells group together to perform a specific task, what do they form?
→ To observe the cells of microorganisms clearly, which device is necessary that magnifies tiny structures?
→ Which microorganism is used in baking to ferment dough by producing gas that makes it rise?
→ Certain bacteria live in nodules on legume roots and help convert nitrogen from the air into a usable form for plants. What are these bacteria called?
→ Microalgae are used by humans as health supplements. Name a microalgae that is called superfood and is also a good source of vitamin B12.
Cell
All living organisms are made up of cells, the basic units of life. A single cell can perform all essential life functions.
On the basis of number of cells, the organisms can be of following types
- Unicellular organisms like bacteria and protozoa carry out all functions within a single cell.
- Multicellular organisms like plants and animals have many cells, each performing specialised roles.
Among organisms classified on the basis of the number of cells, bacteria and fungi are two major groups differ not only in the number of cells but also in the structural organisation of their cells. - Bacterial cells lack a proper nucleus; their genetic material lies in a region called the nucleoid.
- Fungal cells have a cell membrane and wall but no chloroplasts.
Discovery of cell
For a long time tiny things remained unseen because the human eye can only detect objects above a certain size. The invention of lenses i.e., curved pieces of glass that magnify the small objects led to the development of microscopes. These tools allowed scientists to explore the microscopic world. Scientists have also developed a simple, foldable paper microscope called a foldscope. Although a foldscope does not show as much detail as laboratory microscope, it helps more people explore and understand the microscopic world.
In 1665, an English scientist named Robert Hooke published a book called Micrographia in which he showed detailed drawings of tiny things that people had never seen before. Using a simple microscope, Robart Hooke observed a thin slice of cork and noticed that it was made up of many tiny, empty compartments.
These compartments looked like the structure of a honeycomb. He drew what he saw and named each compartment a cell. This was the first time the term cell was used in science to describe the basic unit of life. Around the same time in 1660s, a Dutch scientist named Antonie van Leeuwenhoek improved the design of microscopes by making better lenses.
His microscopes could magnify objects more clearly. He was the first to observe and describe tiny living organisms such as bacteria and blood cells. Because of his important contributions, he is known as the Father of Microbiology.
Variation in Shape and Size of Cells
Some cells are round, some are flat, and others are tube-like. Cells in the body come in different shapes and sizes. For example, muscle cells are spindle-shaped, while nerve cells are long with branches.
The shape of a cell is closely related to the role it performs in the body.
Some examples of different types of cells are given below
- Cheek cells : These cells are thin and flat. They form a protective lining on the inner surface of the cheek.
- Nerve cells (neurons) : These are long and branched. Their shape helps them carry messages quickly to different parts of the body.
- Plant cells : These cells come in various shapes- rectangular, elongated, oval, or even tube-like. Some plant cells form long tubes that help in the transport of water throughout the plant.
- Cells in the digestive system : Different parts of the digestive system are made up of different types of cells.
For example
- Muscle cells in the food pipe These are spindle-shaped, thin, and flexible. They help push food down to the stomach by contracting and relaxing in a wave-like manner.
- Muscle cells in the stomach wall These help churn the food during digestion.
- Cells in the stomach lining These produce digestive juices and acid needed to break down food.
Structure of Cell
A cell is composed of three main parts, i.e. the cell membrane, the cytoplasm, and the nucleus. The thin outer layer that surrounds the cell and separates it from other cells is known as cell membrane. It is porous and allows useful substances to enter and waste to exit the cell. The cytoplasm is jelly-like substance between the cell membrane and the nucleus.
It contains important, pompounds and is the site for most life processes. The nucleus, found in the center of the cell, controls all activities of the cell and is also covered by a thin membrane. Plant and animal cells share these basic parts but also have some differences.
Structure of plant cell v/s animal cell
All living organisms are made up of cells, but the structure of plant and animal cells differs in many ways to suit their specific roles. While both share some common features, certain organelles are unique to each, reflecting their different life processes.
- Plant cells have an additional outer layer called the cell wall, which gives shape and support to the cell. They also contain plastids, which are tiny rod-shaped structures.
- Some plastids, like chloroplast, contains chlorophyll, which makes them green and helps in photosynthesis. Other plastids help in storing substances in non-green parts of the plant.
- Another feature of plant cells is a large, empty-looking space called a vacuole. It stores important substances, removes waste, and helps maintain the shape of the cell. This gives strength and support to the plant.
- In contrast, animal cells do not have a cell wall or plastids. Vacuoles are usually absent or very small. When present, they store substances dissolved in water.
In both plants and animals, different cell parts work together to perform specific functions. But cells do not work alone, they are part of a bigger system that builds the body of a living organism in an organized way.
Level of Organisation in the Living Organisms
Living organisms are organized in a systematic way, where different parts work together to perform specific functions. This organization begins with the cell, which is the basic unit of life.
- Similar cells group together to form tissues, and different tissues combine to form organs, such as the heart in animals or the leaf in plants.
- Several organs then work together as organ systems. For example, the digestive system in animals.
- All the organ systems together make up a complete organism, like a plant, an animal, or a human being.
- This levels of organization in a living body can be summarized as:
Cell → Tissue → Organ → Organ system → Organism
The life of a complex living organism starts from a single cell called an egg. This cell has the special ability to divide again and again, forming a complete body made up of many cells. Organisms made of many cells are known as multicellular organisms. Animals, including humans, and plants are examples of multicellular organisms.
The yolk (the yellow part of an egg) 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.
Microorganisms
Some living organisms are made up of only one or a few cells. These are very small and cannot be seen with the naked eye. They are called microorganisms. Some microorganisms, such as bacteria and Amoeba, single cell and are known as unicellular. Others, like certain fungi and algae, are multicellular. The list of some of these organisms with their unique features are tabulated below.
Organisms present in pond water
Organisms present in soil suspension
These kind of microorganisms are found all around, like in water, soil, and even inside the human body.
Some of them thrive in extreme conditions, such as hot springs, freezing cold regions, or dry deserts, while others grow well in moderate climates. Many microorganisms also live inside our bodies, especially in the gut.
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.
Different Role of Microorganisms
Microorganisms perform several useful functions. They help in food production, maintain soil fertility, clean the environment, and support the health of humans and animals.
(i) In Decomposition and Recycling
Microorganisms play an important role in breaking down dead plants, animals, fruit peels, and leaves into simpler substances. This process produces dark, nutrient-rich material called manure, which helps improve soil fertility. Manure formation occurs at optimal temperature and appropriate moisture level.
Piles of decaying leaves or kitchen waste gradually disappear because microbes such as bacteria and fungi decompose them, returning valuable nutrients to the soil. This natural recycling process supports plant growth and helps to keep the environment clean
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’.
Some microorganisms, like certain bacteria and fungi, can also break down waste in places where there is no oxygen, such as compost pits or waste tanks. While doing this, they release gases (biogas) like methane and carbon dioxide. Methane can be used as fuel for cooking, heating, producing electricity, and even running vehicles.
(ii) As Food
Microorganisms are used in making many food items that we use every day. Their activity helps improve taste, texture, and quality. Some common examples include
- Curd It contains helpful bacteria. One of them is Lactobacillus, which helps turn milk into curd. These bacterium feeds on the sugar in milk (lactose), multiplies and ferment milk to form curd. They produce lactic acid which makes curd sour. It also makes dishes like idlis and bhaturas soft and spongy when added to their batter.
- Dough for Baking Yeast a microorganism that produces carbon dioxide during respiration. The gas forms bubbles in the dough, making it rise and become fluffy. This process is used in making breads, cakes, and pastries.
(iii) As Biofertilizers
Microorganisms can be grouped into different types like protozoa, fungi, bacteria, and some algae. Among them, certain bacteria such as Rhizobium are especially useful for farming.
These bacteria live in small swollen structures called nodules on the roots of legume plants like peas, beans, and lentils.
Rhizobium bacteria absorb nitrogen from the air and convert it into a form that plants can use.
This helps the plants grow well without the need for chemical fertilizers. Because of this, farmers often grow legumes in rotation with other crops. It naturally adds nitrogen to the soil and keeps it rich for future crops.
(iv) As Oxygen Producer and Biofuel Sources
Microalgae are tiny plant-like organisms that live in water, soil, air, and even on trees. They use sunlight to make their own food and release oxygen in the process. In fact, microalgae produce more than half of the Earth’s oxygen.
They are rich in nutrients and serve as food for many aquatic animals. Some, like Spirulina, Chlorella, and Diatoms, are also used by humans as health supplements and in medicines.
Diatoms are tiny algae with glass-like shells made of silica that live in water and produce oxygen through photosynthesis, forming the base of many aquatic food chains. When they die, their shells settle and form a soft rock called diatomaceous earth, which has many uses, like it’s a natural insect killer, helps filter water, and is used in products like toothpaste and paints. Despite being microscopic, diatoms and their remains play a huge role in both nature and everyday human life.
Microalgae help clean water and are also used to produce biofuel. However, pollution, climate change, and the loss of natural habitats are causing their numbers to decline.
Protecting microalgae is essential for maintaining a healthy environment and preserving the Earth’s oxygen balance.
Spirulina
Spirulina, a microalga, 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 has a lot of protein, more than 60 per cent of its body weight and only a small amount of fat and sugar.
Now a days, farming of Spirulina is becoming a feasible livelihood opportunity. You can grow Spirulina easily by following these steps
- Set a clear glass tank in a bright place away from direct sunlight.
- Cover the tank with a shade net, or keep the tank at a place with moderate temperature conditions.
- Fill the tank with pond water.
- Add living Spirulina collected from a pond.
- Stir the growing Spirulina twice a week.
- After 3 – 6 weeks, Spirulina may be harvested from the tank by filtering it through a fine cloth.