The Invisible Living World: Beyond Our Naked Eye Chapter Notes | Science Curiosity Class 8 - New NCERT PDF Download

Variation in Shape and Structure of Cells

Cells within living organisms can have different shapes, sizes, and structures, depending on their role and location.

• Muscle cells: Spindle-shaped (tapered ends), designed for contracting and relaxing to enable movement.
  
• Nerve cells (neurons): Long and branched, suitable for carrying messages quickly across the body.
  Nerve Cell
• Cheek cells: Thin and flat, forming a protective lining.
• Plant cells/ Onion Cells: May be rectangular, elongated, oval, or tube-shaped; some form long tubes for transporting water.
  
  

Why Do Cells Differ in Shape and Structure?

The unique shape and size of each cell type help it perform specific functions for the organism.

• Nerve cells must reach distant parts, so they are long and extended.
• Muscle cells contract and relax, so their spindle shape helps this motion.
• Plant tube cells transport water, so they are elongated and tube-like.

Role of Different Cells in Body Functions

• In humans, muscle cells in the digestive tract move food by contracting in waves.
• The stomach contains muscle cells for churning food and specialized cells for producing digestive juices and acids.
• In plants, tube-like cells help move water and nutrients up the stem and into leaves.

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

What Are Microorganisms?

Some living organisms are so incredibly small that they are invisible to the naked eye. Unlike the plants, animals, and cells we've seen before, these tiny forms of life require special tools just to be observed.

Microorganisms (or microbes):

• Living beings made up of just one cell (unicellular) or only a few cells.
• They are so tiny that we cannot see them without the aid of a microscope.
• The word "microorganism" comes from "micro" (very small) and "organism" (living being).

Key Points About Microorganisms

• Size: Too small to see without a microscope; invisible to unaided eyes.
• Cellularity: Some (like bacteria and Amoeba) are unicellular (single-celled). Others (like some fungi and algae) can be multicellular (made of many cells) but still remain extremely small.
• Everywhere: in water, soil, air, on and inside our bodies, and in extreme places too.
  

How Do We See Microorganisms?

Microscopes are needed to observe the cells of microorganisms. Microscopes enlarge (magnify) the image of microbes so they become visible.

Types of Microscopes:

• Laboratory Microscopes: High-powered, show clear details but are expensive.
• Foldscope: A low-cost, foldable paper microscope that allows more people to study microbes, though it may not show as much detail as advanced microscopes.

Are Microorganisms the Same as Plant or Animal Cells?

• Like plant and animal cells, microbial cells may have a cell membrane, cytoplasm, and sometimes a nucleus.
• However, there are many differences in size, structure, and the way they perform life processes.
• Not all microbes have the same features as plant or animal cells; some can be quite unique.

Where Do We Find Microorganisms?

• In Water: Lakes, rivers, oceans, ponds, even in a drop of water!
• In Soil: The earth is full of bacteria, fungi, and protozoa.
• In Air: Some float in the atmosphere.
• Inside Living Beings: Many live in our intestines or on our skin—helping or sometimes causing illness.

Why Are Microorganisms Important?
Microbes play crucial roles in nature—including recycling nutrients, decomposing waste, supporting plant growth, and even digesting food in our bodies!

How Are We Connected to Microbes?

• Microorganisms (microbes) are found all around us—not just in laboratories or textbooks, but in every corner of our daily lives.
• You may have noticed when fruits like lemons, tomatoes, or oranges are left out, they sometimes develop a powdery or cotton-like growth.
• This is microbial growth (usually fungi); the food has been infected by microbes.
• These microbes reach food from the air, water, soil, or even by landing on the food surface.

Where Can Microorganisms Be Found?

• On food: Responsible for spoilage and rotting (mouldy bread, spoiled fruits).
• On plant surfaces: Leaves, stems, and roots all have microbial residents.
• In water, soil, and air: Microbes thrive in ponds, rivers, soil, and even float in the air.
• Inside living beings: The human body (especially the intestine) is home to many bacteria that help with vital functions like digestion.
• Extreme environments: Some microbes live in very hot springs, icy cold zones, or salty lakes—showing their incredible adaptability and diversity.

Microbial Diversity

• Microbes come in many shapes: spherical, rod-shaped, spiral, and irregular.
• Like animals and plants, they also vary in size, structure, and function.
• This diversity allows them to survive everywhere and play different roles.

Key Players in Cleaning the Environment

Microbes, especially bacteria and fungi, break down dead plant and animal matter, turning it into simpler substances (decomposition).

• This process is what turns fallen leaves and fruit peels into manure (compost), enriching the soil for healthy plant growth.
• Ancient texts (the Vedas) recognized both visible and invisible "tiny entities" (Krimi), mentioning their helpful and harmful effects—showing the long-standing appreciation of microbes.

Manure Formation

• When organic waste (like fruit and vegetable peels) is left in moist soil for a few weeks, microbes decompose it, forming dark-coloured, nutrient-rich manure.
  
• This process needs optimal temperature and moisture.
• The nutrients released go back into the soil, supporting new plant growth.
• Microbes also decompose animal waste (like dung), cleaning up the environment naturally.
• They even break down dead animal bodies, ensuring nature recycles its resources.
• Without microbes, waste and dead matter would accumulate and the recycling of nutrients would stop.

Why Don’t Microbes Spoil Pickles and Murabbas?

• Preservation with Salt and Sugar: Pickles and murabbas are made with high concentrations of salt or sugar. 
• These act as preservatives and prevent the growth of microbes, so the food does not spoil easily.

Mango Pickle

Microbes as a Source of Biogas

• Some bacteria and fungi can grow in places where there is no oxygen (anaerobic conditions).
• These microbes decompose plant and animal waste, releasing gases—mainly methane and carbon dioxide.
• Methane (biogas) is an important renewable fuel used for cooking, heating, electricity, and even running vehicles.
• Real-Life Example: Dr. Ananda Mohan Chakrabarty developed a special bacterium that can break down oil spills (patented in 1980), showing how microbes can help solve real-world pollution problems.

Microorganisms in Food Preparation

Fermentation and Rising of Dough

• Yeast (a fungus) is used to make dough rise for breads, cakes, and some Indian foods:
• When mixed with flour and warm water, yeast ferments sugars, releasing carbon dioxide (which forms bubbles, making the dough soft and fluffy) and a little alcohol (which adds to the smell).
• Bowl experiment: Dough with yeast becomes fluffy and airy; without yeast, it stays dense.

Formation of Curd and Other Fermented Foods

• Lactobacillus (a type of bacteria) is used in curd formation:
• Added to warm milk, it multiplies and converts sugars (lactose) into lactic acid, making the milk sour and thick (curd).
• This needs a warm environment to work well.
• Warm milk forms curd quickly; cold milk does not.
• Bacteria like Lactobacillus and yeast help in fermentation for foods like idli, dosa, and bhatura.
• All these organisms make our food tastier, more nutritious, and sometimes easier to digest.

Nitrogen Fixation

• Rhizobium bacteria live in the root nodules of legumes (peas, beans).
• These bacteria convert nitrogen from the air into forms plants can use, improving soil fertility naturally (without chemical fertilizers).
• This is why farmers grow legumes in crop rotation.

Amazing Microalgae: Tiny Helpers in Water

Microalgae are microscopic, plant-like organisms found in water, soil, air, and even on tree bark.

• They perform photosynthesis (making food from sunlight) and release oxygen—more than half of the Earth’s oxygen comes from microbes like these.
• They are a major food source for aquatic animals.
• Some, like Spirulina, Chlorella, and Diatoms, are used as dietary supplements and medicines for humans.

Other Benefits

• Microalgae help in cleaning water and are being developed as a source of biofuel (clean energy).
• However, they are threatened by pollution, climate change, and loss of habitat.
• Conserving microalgae is important for maintaining oxygen supply, food security, and the health of aquatic ecosystems.

Example: Spirulina

• Known as a "superfood" because it is rich in protein (over 60%) and vitamin B12, while being low in fat and sugar.
• Spirulina can be easily farmed in tanks with pond water, moderate temperature, and sunlight.
• Spirulina farming is a growing livelihood option for communities.

Question for Chapter Notes: The Invisible Living World: Beyond Our Naked Eye

Try yourself:

How do microbes help in food preparation?

• A.

  By decomposing waste
• B.

  By making food salty
• C.

  By fermenting sugars
• D.

  By adding color to food

Explanation

Microbes play a key role in food preparation by fermenting sugars.

For example:

• Yeast helps dough rise by converting sugars into carbon dioxide.
• Lactobacillus bacteria turn milk into curd by producing lactic acid.

This fermentation process makes food tastier and easier to digest.

Report a problem

Cancel Report

View Solution

Why Is the Cell Considered the Basic Unit of Life?

The cellis called the basic unit of life because it is the smallest structure that can carry out all the functions necessary for survival. All living organisms—plants, animals, and microorganisms—are made up of cells.

Multicellular organisms (plants and animals):

• The bodies of all plants and animals are made up of many cells so, they are called Multicellular organisms.
• These cells are specialized to perform different functions (e.g., skin cells, muscle cells, nerve cells).
• Cells cooperate and communicate with each other to keep the whole organism alive.
• Each type of cell plays a unique role, but all are essential for survival.
• Examples of multicellular organisms are: Plants, animals, humans.

Unicellular organisms (bacteria, protozoa, some fungi and algae):

• They are made up of only one single cell.
• This single cell performs all life processes—nutrition, movement, reproduction, growth, and response to the environment.
• Examples of unicellular organisms are: Bacteria, amoeba, yeast.

Examples of Microbial Organization

• Bacteria and protozoa are usually unicellular (single-celled).
• Some algae and fungi can be unicellular or multicellular:
• Yeast: A unicellular fungus (lives as single cells).
• Moulds: Multicellular fungi (made of many cells).

Components and Structure of Cells

• All cells are typically surrounded by a cell membrane (keeps contents in and controls movement of substances).
• Fungal cells have an extra cell wall (for protection and shape), but unlike plant cells, they don’t have chloroplasts and cannot make their own food.
• Bacterial cells are different: They do not have a well-defined nucleus or a nuclear membrane. Instead, their genetic material is found in a region called the nucleoid.
• In plant and animal cells, the nucleus is clearly defined and surrounded by a membrane.

Special Features in Cells

• There are many parts inside cells (called organelles), each with a special function.
• To see even smaller details, you need stronger microscopes:
• Electron microscopes can magnify cells up to 10,00,000 times (revealing structures not seen with ordinary microscopes).

Diversity in Cells

• Cells vary in size, shape, and structure based on their function and the organism they belong to.
• Even plant and animal cells are different in how they look and what structures they contain.