Brief Summary
Alright kiddos, let's quickly recap Chapter 2 of Class 8 Science, "The Invisible Living World Beyond Our Naked Eye." We're talking cells, microscopes, microorganisms, and how they all connect to us and the environment.
- We'll explore the history of microscopes and cell discovery.
- We'll understand cell structure, types, and organization.
- We'll learn about microorganisms, their roles, and benefits.
Introduction
The chapter starts with questions about the invisible world of microbes, viruses, and bacteria. If these became visible, how would our perspective on size, complexity, and life change? How do these tiny beings interact? The human eye can only see objects above a certain size, leaving many tiny things unknown for a long time. People discovered curved glass could magnify small things, leading to lenses. These lenses, shaped like lentil seeds (thick in the middle, thin at the edges), improved over time, resulting in microscopes. The invention of the microscope opened a fascinating hidden world filled with tiny living creatures.
One Shot Revision
We've already learned about the variety of living beings like plants and animals, differing in shape, size, and color. Some are tiny, some are huge, like whales to hummingbirds. They differ not only in structure but also in features, like breathing and eating habits. All these living beings, plants or animals, are called organisms. Have you ever noticed the smallest organism around you visible to the naked eye? Some might say an ant or mosquito. Some people use reading glasses to see better, correcting their vision. Magnifying glasses also help observe things by making them bigger.
An activity involves filling a round-bottom flask with water, covering the mouth, and looking at letters through it. The letters appear larger because the flask acts like a magnifying glass. Using a real magnifying glass to look at an ant allows you to see its body details more clearly. For a long time, people were curious to explore tiny organisms but couldn't see them with their naked eyes. The discovery of the microscope opened this world.
In 1665, scientist Robert Hooke published "Micrographia," with detailed drawings of tiny things seen through a microscope that magnified 200-300 times. He looked at a thin slice of cork and saw it was made of many small, empty spaces, which he called "cells." This was the first time the word "cell" was used in science to describe the basic unit of life. Around 1660, Anton van Leeuwenhoek made better lenses and more powerful microscopes. He was the first to see and describe tiny living things like bacteria and blood cells, earning him the title "Father of Microbiology." He discovered living cells around 1674.
All living beings are made of cells. An activity involves taking an onion bulb, cutting it into pieces, and using forceps to remove a thin, transparent layer called the onion peel. This peel is placed in a petri dish with safranin for 30 seconds to stain it pink, making it easier to see. The peel is then transferred to another petri dish with water to remove excess safranin. Carefully, the onion peel is transferred to a rectangular slide, ensuring no folds or cracks, and glycerin is added to prevent drying. A cover slip is placed on top, avoiding bubbles, and a blotting paper removes extra glycerin.
Under a microscope, the onion peel shows nearly rectangular structures, similar to a brick wall. These are the cells of the onion peel, compactly arranged without space. Plant cells also have a cell wall. Different plants' leaves can be observed to see their cell structures. Animals are also made of cells. For this activity, the inner lining of the cheek is scraped with a blunt toothpick, and the material is placed on a slide with water. The material is spread, and methylene blue is added for staining. After a minute, glycerin is added, and a cover slip is placed, avoiding bubbles.
Under a microscope, polygonal-shaped cells are visible. These cheek cells make up the inner lining of the mouth. Similarities and differences can be observed between onion peel cells and human cheek cells. Cells have three main parts: a thin outer lining, a central region, and a nucleus. The outer lining is the cell membrane, separating one cell from another. The round structure in the middle is the nucleus, also covered by a thin membrane. The space between the cell membrane and the nucleus is filled with cytoplasm. Plant cells have an additional layer outside the cell membrane called the cell wall.
The cell membrane contains the cytoplasm and nucleus, separating cells. It is porous, allowing essential substances in and waste out, and is selectively permeable. Cytoplasm contains components like carbohydrates, proteins, fats, and minerals, and is a jelly-like matrix where most life processes occur. The nucleus regulates processes and controls activities, including growth and genetic information. The cell wall, present only in plant cells, provides rigidity and strength. Plant cells are compactly arranged for rigidity.
Plant cells also have tiny rod-shaped structures called plastids, including chloroplasts, which contain chlorophyll for photosynthesis. Chlorophyll is a green pigment that makes plant parts green. Non-green parts store substances. Plant cells also have a large, empty-looking space called a vacuole, which stores substances and helps remove waste, maintaining the cell's shape. Animal cells have small or absent vacuoles. Mitochondria, the powerhouses of the cell, provide energy. Cells have other structures like Golgi bodies and lysosomes, which are studied in higher classes. Cells are not just bags of liquid but have structures with special functions, keeping organisms alive.
Cells have a basic structure, but plant and animal cells have different shapes and structures. Different animal cells also vary in structure and shape. Muscle cells are spindle-shaped, while nerve cells are long and branched. Some cells are round, others are long and thin. The number of cells varies in different organisms. The unique shape, size, and structure of cells depend on their function. Inner cheek cells are thin and flat to form a protective layer. Nerve cells, or neurons, are elongated and branched to transmit messages quickly.
Plant cells also vary, with rectangular, elongated, oval, or tube-like shapes. Some plants form long tubes to carry water, like xylem. Different parts of the digestive system are made of different types of cells. Muscle cells in the food pipe contract and relax to push food down the stomach. This movement is possible because muscle cells are thin, flexible, and spindle-shaped. The stomach also has different cells for different functions, like muscle cells for mixing food and cells that produce digestive juices and acids.
The shape and structure of cells are fixed because their function is specialized, allowing the organ to perform its task.
Website Overview
All living organisms are complex. Humans have digestive, respiratory, and excretory systems. This complexity starts with cells, the basic unit of life. A group of similar cells forms a tissue. Different tissues form an organ. Organs working together form an organ system. All organ systems together form an organism, plant or animal. The levels of organization are cell, tissue, organ, organ system, and organism.
The life of complex living organisms begins with a simple cell called an egg. When a female egg and male sperm meet, they form a single cell, which divides and develops into a baby. Even complex organisms start from a single cell. The ostrich egg yolk is the largest single cell. The longest single cell is the neuron, or nerve cell. The eggs of many organisms can divide repeatedly to form a complete living being made of many cells. These are called multicellular organisms, including animals, humans, and plants.
The largest cell is the ostrich egg yolk cell. The smallest cell is mycoplasma. The longest cell is the nerve cell, or neuron. The ostrich egg yolk, the yellow part, is a single cell and the largest in the living world, measuring 130-170 mm in diameter. The white part and shell are non-cellular materials for nourishment and protection.
Some living organisms are made of one cell (unicellular) or a few cells. These are so small they can't be seen with the naked eye and are called microorganisms. Micro means very small. Examples include bacteria and amoeba, which are single-celled. Fungi and algae are made of a few cells and are also microorganisms. Microorganisms are everywhere: water, soil, air, and even inside our bodies.
To observe microorganisms, we need a microscope to magnify their size. Scientists have created a low-cost, foldable paper microscope called a Foldscope. It doesn't provide high resolution but makes the microscopic world accessible. Microbes can look different from plant and animal cells. An activity involves taking pond water, placing a drop on a slide, and observing it under a microscope. Tiny organisms are visible.
Another activity involves soil suspension. Moist soil is mixed with water, and the mixture is left undisturbed. A drop of the water is placed on a slide and observed under a microscope. Small moving organisms are visible, indicating that soil also contains a variety of tiny creatures. These tiny creatures can't be seen with the naked eye, so they are called microorganisms or microbes. They are found everywhere.
Grade 8 students performed these activities and collected information from libraries and the internet. They recorded their data, identifying protozoa, algae, fungi, and bacteria. Protozoa are primitive animals. Amoeba is unicellular, moves with pseudopodia, and has an irregular shape. Paramecium is also single-celled and moves with cilia. Algae can be single-celled or multicellular, look green due to chlorophyll, and move with flagella.
Soil suspension contains fungi, algae, and bacteria. Fungi have branched filaments called hyphae and no chlorophyll. They have sac-like structures with spores. Bread mold and brush-like fungi are examples. Algae in soil suspension are spherical and have chlorophyll. Bacteria can be spherical, rod-shaped, comma-shaped, or spiral-shaped. They may have long hair-like structures for movement.
Microorganisms are everywhere and can only be seen with a microscope. A microscope magnifies objects 100-400 times. Microorganisms are small but play a crucial role in our lives. Viruses are different from normal cells and are acellular. They contain DNA or RNA and a protein coating. Viruses multiply inside living cells, infecting plant, animal, and bacterial cells.
Have you ever seen a lemon, tomato, or orange rot after being left outside? This happens because they are infected by microbes, which are found everywhere. You can use a Foldscope or microscope to explore surfaces of leaves, stems, and roots. Microorganisms are diverse and can be found in extreme conditions. Some live inside our bodies, especially in our gut, aiding digestion.
Like plants and animals, microorganisms have different shapes, sizes, and structures. Pickles and murabbas don't spoil because they contain salt and sugar, which act as preservatives by absorbing moisture, preventing microbial growth. The diversity of microorganisms plays a role in our daily life and helps clean the environment. Microbes help clean the environment. An activity involves filling a container with garden soil and adding fruit and vegetable peels, covering them with soil. After two to three weeks, the peels turn into a dark-colored material called manure, which is rich in nutrients and increases soil fertility.
Soil contains microorganisms, including fungi and bacteria, which decompose plant waste into manure. Gardeners collect dry leaves and plant waste to make natural manure. Ancient texts, particularly the Vedas, refer to tiny entities called "Krimi," which can be visible or invisible. These texts mention their beneficial and harmful effects. The Atharvaveda also references Krimi. Decaying plants and fallen leaves disappear over time because microorganisms break them down into simpler substances, returning nutrients to the soil. Microorganisms also decompose bodies of dead animals, helping to recycle waste and return nutrients to nature.
Manure formation requires optimal temperature and moisture. Microbes need favorable conditions to work effectively. Microbes, like bacteria and fungi, decompose plant and animal waste into nutrient-rich manure, recycling materials. These bacteria also decompose animal waste like dung, maintaining cleanliness. What would happen if microorganisms didn't exist on Earth? There would be piles of dead bodies, dung, and waste.
Microbes also help produce biogas. Many organisms, like bacteria and fungi, live in places without oxygen. Some bacteria can decompose plant and animal waste, releasing a mixture of gases, including carbon dioxide and methane. Methane, or biogas, can be used as fuel for cooking, electricity, and vehicles.
Dr. Anand Mohan Chakraborty was a scientist who created a bacterium in 1971 that could break down oil spills. This discovery was patented in 1980, giving him copyright protection. His work showed how microorganisms could be used to solve environmental problems like pollution. What other problems can be solved with the help of microorganisms?
Microorganisms also help in our kitchen. An activity involves taking two bowls, adding flour to both. Yeast is added to one bowl, and both are mixed. One bowl is mixed with warm water and a pinch of sugar, while the other is mixed with normal water. Both are left to rest. The bowl with yeast rises and becomes fluffy, while the other remains the same. The yeast also produces a different smell.
Yeast is a microorganism from the fungi family that grows well in warm conditions. Like other organisms, yeast respires and breaks down food to release energy. Sugar acts as food for the yeast, and warm water helps it grow. The yeast releases carbon dioxide, causing the dough to rise. It also produces a small amount of alcohol, causing the change in smell. This property of yeast is used in making bread and cakes.
In addition to yeast, some bacteria, like lactobacillus, help in fermentation. This is used in making idli and dosa batter. An activity involves taking two glass bowls, adding lukewarm milk to one and cold milk to the other. A spoon of curd is added to both and mixed. One bowl is kept in a warm place, and the other is kept in the fridge. After a few hours, the milk in the warm bowl turns into curd, while the milk in the cold bowl does not.
The curd contains lactobacillus bacteria, which convert the lactose sugar in milk into lactic acid, giving the curd its sour taste. These bacteria grow well in warm conditions. We can categorize microorganisms into different categories, such as protozoa, fungi, bacteria, and algae. Rhizobium is a bacterium found in the root nodules of leguminous plants like beans, peas, and lentils.
Rhizobium traps nitrogen from the air and converts it into a form that plants can use for growth. This increases the nitrogen content in the soil, increasing soil fertility. This process is called biological nitrogen fixation. The symbiosis is beneficial for both the plant and the bacteria. Farmers rotate leguminous crops with other crops to improve soil fertility.
Microalgae are tiny helpers in water. Microalgae are microscopic, plant-like organisms found in water, soil, air, and on trees. They are important because they use sunlight to make their own food through photosynthesis, releasing oxygen. Microalgae produce more than half of the Earth's oxygen. They are also nutrient-rich and a good food source for aquatic animals. Examples include spirulina, chlorella, and diatoms, which humans use as health supplements.
Microalgae are also used for cleaning water and as biofuel. However, pollution, climate change, and habitat destruction are reducing microalgae populations. We need to conserve these tiny organisms to maintain the Earth's oxygen balance. Spirulina is a microalga considered a superfood due to its health benefits. It is a good source of vitamin B12 and protein, with low fat and sugar content. It also contains antioxidants.
Farming spirulina is becoming a feasible livelihood opportunity. A clear glass tank is placed in a bright location without direct sunlight and covered with a shade net. Pond water is added, and living spirulina is collected and added to the tank. The tank is stirred twice a week. After three to six weeks, the spirulina can be harvested using a fine cloth. Conserving microalgae ensures food security and provides a source of livelihood.
The bodies of all living organisms are made up of tiny building blocks called cells. A cell has different components that perform different functions. Multicellular organisms are made of many cells, which perform specialized functions. Cells cooperate to increase survival chances. Some microorganisms, like bacteria and protozoa, are made of a single cell (unicellular). All life processes occur within that one cell. Other microbes, like algae and fungi, can be unicellular or made of a few cells. Yeast is unicellular, while fungus is multicellular.
Animal cells have a cell membrane, nucleus, and cytoplasm. Plant cells have a cell wall, cell membrane, nucleus, cytoplasm, vacuoles, and plastids (including chloroplasts). Fungi have a cell wall but no chloroplasts, so they cannot perform photosynthesis. Bacteria do not have a well-defined nucleus but have a nucleoid. This feature differentiates them from other microorganisms.
Cells have other components that are studied in higher classes. Cells are the smallest living things with all life processes occurring within them, making them the basic fundamental unit of life. For observing subcellular components, we need microscopes with high magnification, such as electron microscopes, which can magnify objects up to 1 million times. All living beings, including microorganisms, are made of one or more cells. Cells have different shapes, sizes, and structures. Plant and animal cells also have differences. Understanding these differences helps us learn how organisms function differently.
