Grade 8 Ch 2 The Invisible World of Science (Notes)

 

Ch 2: The Invisible Living World: Beyond Our Naked Eye

1. Introduction

• Human eyes cannot see very tiny organisms.
• Use of lenses → magnifying glasses → microscopes → discovery of invisible living world.
• Microscopes magnify objects 100-400 times.

2. Early Discoveries

    Robert Hooke (1665):

• Published Micrographia.
• Observed cork cells → looked like honeycomb.
• Coined the term “cell”.

  Antonie van Leeuwenhoek (1660s):

• Made powerful lenses.
• First to describe bacteria and blood cells.
• Known as Father of Microbiology.

3. What is a Cell?

• Basic unit of life.
• Cells differ in shape, size, structure.

Observation:

• Onion peel cells (rectangular, with cell wall).
• Human cheek cells (polygonal, no cell wall).

All cells have three basic parts:

1. Cell Membrane:

    §  Thin, flexible layer forming the boundary of the cell.

    §  Separates one cell from another.

    §  Porous, allowing certain materials to enter or exit; keeps harmful substances out and retains usefull          molecules.

2. Cytoplasm:

     §  Jelly-like substance filling most of the cell inside the membrane.

     §  Contains dissolved nutrients like carbohydrates, proteins, fats, and mineral salts.

     §  Most cellular life processes occur here.

3. Nucleus:

     §  Spherical or oval structure, usually located centrally.

     §  Controls all cell activities, including growth and division.

     §  Surrounded by its own thin membrane.

Plant cells have extra parts:

1. Cell Wall (in plants only):

    §  A tough, outer layer surrounding the cell membrane.

    §  Provides rigidity, strength, and protection to plant cells.

    §  Makes cells appear firm and compactly arranged.

2. Plastids (including chloroplasts, in plants only):

    §  Tiny rod-shaped structures within plant cells.

    §  Chloroplasts contain chlorophyll and enable photosynthesis.

    §  Other plastids help store food and substances.

 3. Vacuole:

    §  Large, clear cavity in plant cells; smaller or absent in animal cells.

    §  Stores water, nutrients, waste, and helps maintain cell shape and turgidity

                         

 

                          

    4. Mitochondria:

• Produces energy for cell.

4. Variation in Cell Shape

1. Animal cells:

• Muscle cell → spindle-shaped, helps in movement.
• Nerve cell → long, branched, carries messages quickly.
• Cheek cell → thin, flat, protective lining.
  
  

2.Plant cells:

• Rectangular, oval, elongated, tube-like.
• Some form 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.

5. Levels of Organisation in Living Beings:

The structure of every living organism-from tiny plants to complex animals-is organized in a system and hierarchical way. This organisation allows the body to function efficiently, as each level is built from the previous, simpler level.

                               

                                           Level of Organisation

 1. Cell - The Basic Unit of Life

   §  The cell is the smallest, most fundamental unit of all living beings.

   §  Just as a brick is the basic building block of a wall, a cell is the building block of life.

   §  Each cell performs all the basic processes necessary for life, such as taking in nutrients, producing    energy, and reproducing.

2. Tissue-Group of Similar Cells

   §  A tissue is a group of similar cells that work together to perform a specific function.

   §  Muscle tissue (made of muscle cells for movement), nerve tissue (made of nerve cells for message transmission), etc.

  §  Tissues provide structure and support to organs and help in carrying out specialized tasks.

3. Organ-Structure Formed by Different Tissues

  §  An organ is formed when different types of tissues combine and work together to perform a particular function.

  §  Heart (pumps blood), stomach (helps in digestion), leaf (photosynthesis in plants).

  §  Each organ has a specific structure and role in the body.

4. Organ System-Group of Organs for Major Functions

  §  An organ system is a group of organs that work together to perform a major life function.

  §  Digestive system (mouth, stomach, intestines, etc.), respiratory system (lungs, windpipe), circulatory system (heart, blood vessels).

  §  Each organ system takes care of major tasks, ensuring the survival and wellbeing of the organism.

5. Organism-Complete Living Being

  §  All the organ systems together make up a complete, multicellular organism-like a plant, animal, or  human.

  §  The organism is able to perform all the functions of life: growth, development, response to the  environment, and reproduction.

1. Cell → basic unit.
2. Tissue → group of similar cells.
3. Organ → made of tissues.
4. Organ system → group of organs.
5. Organism → complete living being.

Summary Table: Levels of Organisation

Level                Description/Definition                                           Example

Cell                  Basic unit of life                                                 Muscle cell, nerve cell

Tissue              Group of similar cells                                         Muscle tissue, nervous tissue

Organ              Structure formed by different tissues                 Heart, stomach, leaf

Organ system  Group of organs working for a major function  Digestive system, circulatory system

Organism         All organ systems combined as a living being   Plant, human, animal (dog, bird, etc.)

 

• Unicellular organisms: Made of one cell (e.g., Amoeba, bacteria).
• Multicellular organisms: Many cells (e.g., plants, animals).
• Life starts from a single egg cell.
• Example: Ostrich egg yolk → largest known cell.

What Are Microorganisms?

 Some living organisms are so incredibly small that they are invisible to the naked eye. Unlike the plant   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.

A 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.

Microbes as a Source of Biogas

  §  Some bacteria and fungi can grow in places where there is no oxygen com

  §  These microbes decompose plant and methane and carbon dioxide

  §  Methane (biogas) is an important renewable fund for cooking heating electricity and even running  vehicles

  §  Real Life Example: Dr. Anand Mohan Chakrabarty developed a special bacterium that can break oil  spills gats (patented in 1980) showing how microbes can help solve real-world pollution problem.

Microorganisms in Food Preparation

  Fermentation and Rising of Dough

  §  Yeast (fungus) is used to make dough for tread 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.

 Why Is the Cell Considered the Basic Unit of Life?

 The cell is 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 respon 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.

 

Question Answer 

Question 1. Various parts of a cell are given below. Write them in the appropriate places in the following diagram.

1. Nucleus
2. Cytoplasm
3. Chloroplast
4. Cell wall
5. Cell membrane
6. Nucleoid

Answer:

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 setup in a warm place, away from sunlight.

(i) What do you predict will happen after 3-4 days? She observed that the balloon attached to test tube B was inflated. What can be a possible explanation for this?

(a) Water evaporated in test tube B and filled the balloon with the water vapour.
(b) The warm atmosphere expanded the air inside test tube B, which inflated the balloon.
(c) Yeast produced a gas inside test tube B, which inflated the balloon.
(d) Sugar reacted with warm air, which produced gas, eventually inflating the balloon.
(ii) She took another test tube, 1/4 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?
Answer:
(i) (c) Yeast produced a gas inside the test tube B, which inflated the balloon.
Explanation: The yeast fermented the sugar, releasing carbon dioxide gas (CO2), which inflated the balloon.
(ii) She wants to test whether the gas produced is carbon dioxide (CO2). If the lime water turns milky, it confirms the presence of CO2, because CO2 reacts with lime water to form calcium carbonate.

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?

Answer: Beans are leguminous crops that form a symbiotic relationship with Rhizobium bacteria present in their root nodules. These bacteria fix nitrogen from the atmosphere into the soil, providing a natural source of nitrogen. Therefore, the second farmer does not need to add nitrogen-rich fertiliser.

Question 4. Snehal dug two pits, A and B, in her garden. In pit A, she put fruit and vegetable peels and mixed them 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?
Answer: She is trying to test the effectiveness of composting.

• In Pit A, the mixture of green waste (fruit/vegetable peels) and dry leaves provides the correct carbon-nitrogen balance needed for decomposition.
• Pit B lacks this balance and will decompose more slowly and may smell. This experiment shows that decomposing agents like microorganisms work better when both carbon-rich and nitrogen-rich materials are present.

Question 5. Identify the following micro-organisms:
(i) I live in every kind of environment, and inside your gut.
(ii) I make bread and cakes soft and fluffy.
(iii) I live in the roots of pulse crops and provide nutrients for their growth.
Answer:
(i) Bacteria
(ii) Yeast
(iii) Rhizobium

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

Answer: Set up 3 bread slices:

Slice A: Warm, moist environment (near sink)

Slice B: Dry environment (sealed container)

Slice C: Cold environment (refrigerator)

After 3 days, observe: Slice A will have maximum microbial (fungal) growth.
Conclusion: Microorganisms grow best when temperature, moisture, and air are optimal.

Question 7. Take 2 slices of bread. Place one slice on a plate near the sink. Place the other slice in the refrigerator. Compare after three days. Note your observations. Give reasons for your observations.
Answer:

• A bread slice near the sink will show fungal growth (mold).
• Slices in the refrigerator will likely have no or slower growth.

Reason: Warm and moist conditions near the sink promote microbial growth, while cold temperatures in the refrigerator inhibit it.

Question 8. A student observes that when curd is left out for a day, it becomes more sour. What are two possible explanations for this observation?

Answer:

• Microorganisms (lactic acid bacteria) continue to grow and multiply, producing more lactic acid, which makes the curd more sour.
• Warmer temperature speeds up bacterial activity, increasing acid production and sourness.

Question 9. Observe the set-up given in Figure and answer the following questions.

(i) What happens to the sugar solution in flask A?

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

(in) What would happen if yeast were not added to flask A?

Answer:
(i) The yeast ferments the sugar in the warm solution, producing carbon dioxide gas and a small amount of alcohol.
(ii) Lime water in test tube B turns milky. This happens because CO2 produced in flask A travels through the delivery tube into flask B and reacts with lime water, confirming the presence of carbon dioxide.
(iii) Fermentation would not occur, so no carbon dioxide would be produced. As a result, lime water would remain clear in test tube B.