NCERT Solutions for Class 8 Science Chapter 2 – The Invisible Living World: Beyond Our Naked Eye
Class 8 Science Chapter 2 the Invisible living world PDF
What is a Cell?
Cell is the basic structural and functional unit of
life. Every living organism, whether it is a plant, animal, or microorganism,
is made up of one or more cells.
Definition: A cell is the smallest unit of a living
organism that can perform all the basic life processes.
Timeline of
Cell Discovery
Basic Parts of a Cell
Every typical cell has three main parts:
➣ Cell Membrane - Protects the
cell and controls the movement of substances in and out.
➣ Cytoplasm - Jelly-like substance
where most life processes take place.
➣ Nucleus - Controls all the
activities and growth of the cell.
Plant cells have one additional part:
Cell Wall - Provides
strength, support, and rigidity to the plant cell.
1. Cell Membrane : -
Definition - The cell membrane is the thin outer covering of the cell.
Functions : -
➣ Encloses the cytoplasm and
nucleus.
➣ Separates one cell from another.
➣ Is porous, allowing:
⦾ Entry of oxygen, water, and nutrients.
⦾ Exit of carbon dioxide and waste materials.
➣ Protects the cell.
Remember: Cell membrane = Protective covering + Controls movement of substances
2. Cytoplasm : -
Definition - The cytoplasm is the jelly-like material
present between the cell membrane and the nucleus.
It contains:
⦾
Carbohydrates
⦾
Proteins
⦾
Fats
⦾
Mineral salts
⦾
Other cell components (organelles)
Functions : -
➣ Site of most life processes.
➣ Supports the cell organelles.
➣ Stores important substances.
Remember: Most activities of the cell occur in the
cytoplasm.
3. Nucleus : -
Definition - The nucleus is the round structure
present inside the cell.
Functions : -
➣ Controls all activities of the
cell.
➣ Regulates cell growth.
➣ Acts as the control centre of
the cell.
Remember: Nucleus = Brain of the cell
Cell Wall (Only in Plant Cells) : -
Definition - The cell wall is the rigid outer layer
present outside the cell membrane in plant cells.
Functions : -
➣ Gives strength to the plant.
➣ Provides rigidity.
➣ Maintains the shape of the cell.
➣ Helps plant cells remain compact
and firm.
Remember: Cell wall is absent in animal cells.
Besides the cell membrane, cytoplasm, nucleus, and
cell wall, plant cells contain plastids and a large vacuole.
i. Plastids : -
Definition - Plastids are tiny rod-shaped cell
organelles found only in plant cells.
Types and Functions : -
1. Chloroplasts
➣ Chloroplasts are a type of
plastid.
➣ They contain chlorophyll, the
green pigment.
➣ Chlorophyll gives plants their
green colour.
➣ It helps plants prepare food by
photosynthesis.
2. Other Plastids
➣ Found in the non-green parts of
plants.
➣ Store food and other important
substances.
Functions of Plastids : -
➣ Chloroplast - Contains
chlorophyll and performs photosynthesis.
➣ Other Plastids - Store food and
other substances.
Remember: Chloroplast = Green colour + Photosynthesis
ii. Vacuole : -
Definition - A vacuole is a large, empty-looking sac
found inside plant cells.
Functions of Vacuole : -
➣ Stores water, food, and other
important substances.
➣ Stores waste materials before
removal.
➣ Maintains the shape of the cell.
➣ Provides strength and support to
the plant cell.
Remember: Vacuole = Storage + Shape + Support
Vacuoles in Animal Cells
➣ Animal cells usually do not have
vacuoles.
If present, they are:
⦿ Small in size.
⦿ Store certain substances dissolved in water.
iii. Mitochondria : -
Definition - Mitochondria are important cell organelles present in both plant and animal cells.
Functions of
⦿
Release energy from food.
⦿
Supply energy needed for various life processes.
Remember: Mitochondria = Powerhouse of the Cell
Variation in Shape and Structure of Cells
Cells differ in:
⦾
Shape
⦾
Size
⦾
Structure
The shape of a cell depends on the function it
performs.
i. Muscle Cell : -
Shape - Spindle-shaped.
Function
⦾
Contracts and relaxes.
⦾
Helps in body movement.
⦾
Pushes food through the food pipe.
ii. Nerve Cell (Neuron) : -
Shape - Very long.
Highly branched.
Function
⦾
Carries messages throughout the body.
⦾
Connects different body parts.
iii. Cheek Cells : -
Shape - Thin and flat.
Function
⦾
Form a protective lining inside the mouth.
iv. Plant Cells : -
Plant cells may be:
⦾
Rectangular
⦾
Oval
⦾
Elongated
⦾
Tube-like
⦿
Tube-like Cells - Transport water throughout the plant.
Cells in the Digestive System : - Different types of
cells perform different functions.
i. Food Pipe
⦿
Contains spindle-shaped muscle cells.
⦿
Push food towards the stomach by wave-like contractions (peristalsis).
ii. Stomach - Contains different cells:
⦿
Muscle cells churn food.
⦿
Gland cells produce digestive juices.
⦿
Acid-producing cells help digest food.
What Are the Levels of Organisation in the Body of
a Living Organism?
Levels of Organisation in Living Organisms
⦿
The body of every living organism is organised in a systematic and complex way.
⦿
The cell is the basic structural and functional unit of life, just as a brick
is the basic unit of a wall.
⦿ A
group of similar cells performing the same function forms a tissue.
⦿
Different types of tissues combine to form an organ.
⦿
Several organs work together to perform a major body function and form an organ
system.
⦿
All the organ systems together make up a complete organism, such as a plant or
an animal.
⦿
The sequence of organisation in living organisms is:
Cell → Tissue → Organ → Organ System → Organism
⦿
These different levels of organisation help simple cells work together to form
complex living beings.
Multicellular Organisms
⦿
The life of every multicellular organism begins with a single cell called the
egg.
⦿
The egg divides repeatedly to produce millions of cells.
⦿
These cells gradually develop into tissues, organs, organ systems, and finally
a complete organism.
⦿
Organisms made up of many cells are called multicellular organisms.
⦿
Humans, animals, and plants are all examples of multicellular organisms.
Interesting Fact
⦿
The ostrich egg yolk is the largest known living cell in the world.
⦿ It
measures about 130–170 mm in diameter.
⦿
The egg also contains a shell for protection and a white liquid (albumen) that
nourishes the developing cell.
What Are Microorganisms?
Microorganisms
⦿
Some living organisms are so small that they cannot be seen with the naked eye.
⦿
These tiny organisms are called microorganisms or microbes.
⦿
Microorganisms are found everywhere—in air, water, soil, and even inside the
bodies of plants and animals.
Types of Microorganisms : -
⦿
Some microorganisms consist of only one cell and are called unicellular
organisms.
⦿
Bacteria and Amoeba are examples of unicellular microorganisms.
⦿
Some microorganisms consist of many cells and are called multicellular
organisms.
⦿
Certain fungi and algae are examples of multicellular microorganisms.
Observation of Microorganisms : -
⦿
Microorganisms are too small to be seen without special instruments.
⦿ A
microscope is used to observe their cells and study their structure.
⦿
Scientists have also developed a low-cost foldable paper microscope.
⦿
Although a paper microscope is less powerful than a laboratory microscope, it
makes the microscopic world accessible to many people.
⦿
Microscopes help scientists understand the fascinating world of microorganisms
and their importance.
Table 2.2: Organisms Present in Soil Suspension
How Are We Connected to Microbes?
Microorganisms Around Us -
⦿
Microorganisms are present everywhere—in air, water, soil, food, and even
inside our bodies.
⦿
When fruits or vegetables are left outside for a long time, they may develop a
cotton-like or powdery growth due to microorganisms.
⦿ These
microorganisms come from the surrounding environment and grow on food under
favourable conditions.
⦿
Microorganisms also live on the surfaces of leaves, stems, roots, and other
plant parts.
⦿
Some microorganisms can survive in extreme environments, such as hot springs
and snow-covered regions.
⦿
Many useful bacteria live in our intestine (gut) and help in the digestion of
food.
⦿
Microorganisms show great diversity in shape, size, and structure, including
spherical, rod-shaped, spiral, comma-shaped, and irregular forms.
Microorganisms as Nature's Cleaners : -
Decomposition and Manure Formation
⦿
Soil contains many useful microorganisms such as bacteria and fungi.
⦿
These microorganisms break down dead plants, fruit peels, vegetable peels, and
other organic waste into simpler substances.
⦿
This process is called decomposition.
⦿
The decomposed material forms manure, which is rich in nutrients.
⦿
Manure improves the fertility of the soil and helps plants grow better.
⦿
Gardeners collect dry leaves and plant waste to prepare natural manure through
decomposition.
⦿
Microorganisms also decompose the bodies of dead animals and animal waste like
dung.
⦿ By
decomposing waste materials, microorganisms recycle nutrients back into nature.
⦿
Manure is formed best under proper temperature and moisture conditions.
⦿
Without microorganisms, dead plants, animals, and wastes would accumulate on
Earth, making the environment dirty.
Microbes as a Source of Biogas : -
⦿
Some bacteria and fungi live in oxygen-free (anaerobic) conditions.
⦿
These microorganisms decompose plant waste, animal waste, and household
wastewater.
⦿
During decomposition, they produce a mixture of gases called biogas.
⦿
Biogas mainly contains methane and carbon dioxide.
⦿
Methane is an important fuel used for:
➣ Cooking
➣ Heating
⦿
Generating electricity
⦿
Running vehicles
⦿
Biogas is an eco-friendly and renewable source of energy.
Ananda Mohan Chakrabarty (1938–2020)
⦾ He
was an Indian-American scientist who studied bacteria.
⦾ In
1971, he developed a bacterium capable of breaking down oil spills.
⦾
His invention helped in cleaning environmental pollution.
⦾ In
1980, he received a patent for this discovery.
⦾
His work showed that microorganisms can be used to solve environmental
problems.
Microorganisms and Food : -
Role of Yeast
⦾
Yeast is a useful microorganism belonging to the fungi group.
⦾ It
grows best in warm conditions.
⦾
Yeast respires by breaking down sugar and releases carbon dioxide.
⦾
The carbon dioxide forms tiny bubbles that make the dough soft, fluffy, and
rise.
⦾
Yeast also produces a small amount of alcohol, giving the dough a
characteristic smell.
⦾
Yeast is used in making:
➣ Bread
➣ Cakes
➣ Pizza
➣ Bakery products
Role of Lactobacillus
⦾
Lactobacillus is a useful bacterium found in curd.
⦾ It
converts the milk sugar (lactose) into lactic acid.
⦾
Lactic acid makes the milk sour and changes it into curd.
⦾
Lactobacillus grows well in warm conditions.
⦾
That is why curd forms easily in warm places but not in cold places.
Fermentation
⦾
Fermentation is the process in which microorganisms convert sugars into useful
products.
⦾
Yeast carries out fermentation to produce carbon dioxide and alcohol.
⦾
Lactobacillus carries out fermentation to produce lactic acid.
⦾
Fermentation is used in preparing:
➣ Idli
➣ Dosa
➣ Bhatura
➣ Bread
➣ Cakes
➣ Curd
Rhizobium and Nitrogen Fixation
⦾
Rhizobium is a beneficial bacterium.
⦾ It
lives in the root nodules of leguminous plants such as:
➣ Peas
➣ Beans
➣ Lentils
➣ Cowpea
⦾
Rhizobium converts atmospheric nitrogen into a form that plants can use.
⦾
This process is called nitrogen fixation.
⦾
Nitrogen fixation naturally increases soil fertility.
⦾
Farmers grow leguminous crops in rotation with other crops to improve soil
fertility and reduce the need for chemical fertilizers.
Amazing Microalgae – Tiny Helpers
Microalgae : -
⦾
Microalgae are tiny, plant-like organisms found in water, soil, air, and on
trees.
⦾
They prepare their own food by photosynthesis.
⦾
During photosynthesis, they release oxygen.
⦾
More than 50% of Earth's oxygen is produced by microalgae.
⦾
They are rich in nutrients and serve as food for many aquatic animals.
Some useful microalgae include:
➣ Spirulina
➣ Chlorella
➣ Diatoms
These microalgae are used as:
➣ Health supplements
➣ Medicines
➣ Biofuel production
➣ Water purification
Importance of Conserving Microalgae
⦾
Pollution, climate change, and habitat destruction are reducing the number of
microalgae.
⦾
Conserving microalgae helps:
➣ Maintain oxygen balance.
➣ Improve food security.
➣ Protect aquatic ecosystems.
➣ Support sustainable livelihoods.
Ever Heard Of? – Spirulina
⦾
Spirulina is known as a superfood.
⦾ It
contains:
➣ More than 60% protein
➣ Vitamin B12
➣ Very little fat and sugar
➣ Spirulina farming provides
employment opportunities.
⦾ It
can be grown easily in tanks using pond water under moderate temperature
conditions.
⦾
Spirulina is harvested after 3–6 weeks by filtering it through a fine cloth.
Q - Why is the Cell Called the Basic Unit of Life?
⦿
The bodies of all living organisms are made up of tiny building blocks called
cells.
⦿ A
cell is the basic structural and functional unit of life.
⦿
Every cell contains different components that perform various life processes.
⦿
Cells carry out all the essential activities needed for the survival of an
organism.
⦿
Therefore, the cell is known as the basic unit of life.
Multicellular Organisms
⦿
Most plants and animals are made up of many cells.
⦿
Such organisms are called multicellular organisms.
⦿ In
multicellular organisms, different cells perform specialised functions.
⦿
These specialised cells work together in a coordinated manner.
⦿
Cooperation among cells increases the organism's efficiency and chances of
survival.
⦿
Examples of multicellular organisms include humans, animals, and plants.
Unicellular Organisms
⦿
Some microorganisms consist of only one cell.
⦿
Such organisms are called unicellular organisms.
⦿ A
single cell performs all life processes, including nutrition, respiration,
growth, reproduction, excretion, and movement.
Examples of unicellular organisms are:
➣ Bacteria
➣ Protozoa (Amoeba, Paramecium)
➣ Yeast
➣ Fungi
⦿
Fungi may be unicellular or multicellular.
⦿
Yeast is a unicellular fungus.
⦿
Mould is a multicellular fungus.
Cells of Microorganisms : -
⦿
Like plant and animal cells, the cells of microorganisms are also surrounded by
a cell membrane.
⦿ Fungal
cells have an additional cell wall.
⦿
Fungi do not contain chloroplasts, so they cannot prepare their own food by
photosynthesis.
⦿
Bacterial cells do not have a true nucleus.
⦿
Instead of a well-defined nucleus, bacteria possess a nucleoid.
⦿
The nucleoid is a region containing the bacterial genetic material.
⦿
The absence of a nuclear membrane distinguishes bacteria from:
➣ Plants
➣ Animals
➣ Fungi
➣ Algae
➣ Protozoa
➣ Yeast
Microscopes and Cell Study : -
⦿
Ordinary microscopes show only the basic structure of a cell.
⦿ To
observe the tiny structures inside a cell, high-powered microscopes are
required.
⦿ An
electron microscope can magnify objects up to 10,00,000 (one million) times.
⦿ Electron microscopes help scientists study the detailed internal structure of cells.