"The Fundamental Unit Of Life" Class 9

THE CELL
the fundamental unit of life

CELLS WERE FIRST DISCOVERED BY
HIM IN 1665
ROBERT HOOKE

ROBERT OBSERVED :
CORK COMES FROM THE BARK
OF THE TREE
HONEY COMB

ROBERT CALLED
 THESE LITTLE COMPARTMENTS AS
CELLS.
 CELL IS A LATIN WORD FOR ‘ A LITTLE
ROOM’.

ONION PEEL
1)THESE STRUCTURES LOOK
SIMILAR TO EACH OTHER.
2) TOGETHER THEY FORM A BIG
STRUCTURE LIKE AN ONION
BULB.
3) THE CELLS OF AN ONION
PEEL WILL ALL LOOK
SAME,REGARDLESS OF THE
SIZE OF THE ONION.
4) THESE SMALL STRUCTURES
ARE THE BASIC BUILDING
UNITS OF THE ONION BULB AND
ARE CALLED CELLS.

UNICELLULAR ORGANISMS
 ORGANISMS THAT HAVE A SINGLE
CELL.
AMOEBA
PARAMOECIUM
CHLAMYDOMONAS

MULTICELLULAR ORGANISMS
 ORGANISMS HAVING MORE THAN
ONE CELL TO PERFORM VARIOUS
FUNCTIONS.
ANIMALS
PLANTS
FUNGI

DIFFERENT CELLS
BONE CELL
OVUM
SPERM BLOOD CELL
NERVE CELL FAT CELL

CELL
 A cell is the smallest unit that is capable of
performing life functions.

CELL
 THE SHAPE AND SIZE OF CELLS ARE RELATED TO THE
SPECIFIC FUNCTION THEY PERFORM.
 EACH LIVING CEL LHAS THE CAPACITY TO PERFORM
CERTAIN BASIC FUNCTIONS.
 CELL CONTAINS A SPECIFIC COMPONENT CALLED
ORGANELLES.
 EACH KIND OF CELL ORGANELLE PERFORMS A
SPECIAL FUNCTION.
 ALL CELLS ARE FOUND TO HAVE THE SAME
ORGANELLES,IRRESPECTIVE OF THEIR FUNCTION AND
WHICH ORGANISM THEY ARE FOUND IN.

Why are cells of different shapes and sizes
found in the human body?
> The shape and size of the cells depend upon
the function they perform. In a human body,
each organ performs a specific function,
hence the cells of different organs have
different shapes and sizes.

Who discovered cells, and how?
> Cells were first discovered by Robert Hooke,
in1665. He observed a thin slice of cork
under his self designed microscope.This
slice of cork resembled the structure of the
honeycomb consisting of small
compartments. He named these small
compartments as cells.

Why is the cell called the structural
and functional unit of life?
> Cell is called the structural and functional unit
of life because
a) It gives structure to the living organism.
b) It helps the living organism to perform
various life functions.

Cell membrane or Plasma membrane
 It is the outermost covering of the
cell that separates the contents of
the cell from the external
environment.
 Allows the entry and exit of
materials in and out of the cell.It
also prevents the movement of
some other material and is
therefore called a selectively
permeable membrane.
 The plasma membrane is flexible
and is made up of organic
molecules like proteins and lipids.

DIFFUSION
 GASEOUS EXCHANGE TAKES PLACE FROM A
REGION OF HIGHER CONCENTRATION TO A
REGION OF LOW CONCENTRATION.

OSMOSIS
The movement of water from a region of high
water concentration through a semi- permeable
membrane to a region of low water
concentration.

How do substances like CO2 and water move
in and out the cell? Discuss.
As the cell functions the concentration of CO2 in
the cell increases whereas the concentration of
CO2 in the external environment is low. CO2
moves out of the cell from a region of higher
concentration to the external environment which
is the region of lower concentration.
Water moves in and out the cell from a region of
higher concentration through a semi- permeable
membrane to a region of lower concentration by
the process of osmosis.

Difference between
Diffusion Osmosis
1)It can occur in any
medium.
2)It does not require
a semi-permeable
membrane.
1)It occurs only in a
liquid medium.
2)It requires a semi-
permeable membrane.

What is a hypotonic solution?
Hypotonic solution is the solution
surrounding the cell which has higher water
concentration than that in the cell.
Water molecules are free to pass across the
cell membrane in both directions, but more
water will come into the cell than will leave.
The net result is that the water enters the
cell and the cell is likely to swell up.

When is a solution said to be isotonic?
When the medium surrounding a cell has
the same concentration of water as that in
the cell it is said to be isotonic.
Water crosses the cell membrane in both
the directions, but the amount going in is
the same as the amount going out, so
there is no overall movement of water.
The cell will stay the same size.

What is a hypertonic solution?
When the medium surrounding a cell has
lower concentration of water than that in
the cell it is said to be hypertonic.
Water crosses the membrane in both the
directions, but this time more water
leaves the cell than enters it. Therefore
the cell will shrink.

What is Plasmolysis?
Plasmolysis is the phenomenon by which
a plant cell shrinks away from the cell wall
when it loses water by osmosis.

What is endocytosis?
Endocytosis is the process by which a cell
is able to engulf food and other material
from its external environment due to the
flexibility of the plasma membrane.
Example : Amoeba acquires its food
through such processes.

Plasma membrane is called a selectively
permeable membrane. Why?
> Plasma membrane is called a selectively
permeable membrane because it allows
some materials to enter and leave the cell.

Cell wall
>Hard outer covering of the cell.
>Present outside the cell
membrane.
>Cell wall is composed of
cellulose.
>Cellulose is a complex
substance and provides structural
strength to plants.
>Cell wall is present in the plant
cell and helps the plant to
withstand the environmental
changes.

Nucleus
>Has a double layered covering called
nuclear membrane.
 Nuclear membrane has pores which
allow the transfer of material from inside
the nucleus to its outside (cytoplasm).
 Contains chromosomes which are
visible as rod-shaped structures only
when the cell is about to divide.
Chromosomes contain information for
inheritance of features from parents to
the next generation in the form of DNA
molecules. (Deoxyribo Nucleic Acid).

Nucleus
 DNA molecules contain information necessary for constructing
and organising cells.
 Functional segments of DNA are called genes.
 When the cell is not dividing, DNA is present as a part of the
chromatin material. Chromatin material is visible as entangled
mass of thread like structures. Whenever the cell is about to
divide , the chromatin material gets organised into
chromosomes.
 Plays an important role in cellular reproduction.
 It helps in determining the way the cell will develop and what
form it will exhibit on maturity by directing the chemical activites
of the cell.

Haploid Cell
 The cell having one set of chromosome (n
number of chromosome).
 Gametes or sex cells (Sperm and Ova) are
the example.
 They are formed as a result of meiosis.
 They fertilize to form to form offsprings
(which are diploid)
 eg. Human Sperm cell/ Ova cell (23 number
of chromosome: n)

Diploid Cell
 The cell having two sets of chromosome (2n
Chromosome)
 Somatic cells have diploid set of
chromosome.
 Diploid cell reproduce by mitosis.
 eg. Human Muscle cell or blood cell (46
number of chromosome: 2n (2 x 23 = 46))

Significance of membranes
 Example : Viruses
 Viruses lack any membranes and hence
do not show any characteristics of life
until they enter a living body and use its
cell machinery to multiply.

Cytoplasm
• It is the fluid content inside the plasma
membrane.
• Contains specialised cell organelles.

Cell organelles
1) Endoplasmic reticulum
2) Golgi apparatus
3) Lysosomes
4) Mitochondria
5) Plastids
6) Vacuoles

Endoplasmic Reticulum
 ER is a large network of membrane bound tubes and
sheets.
 Extends throughout cytoplasm
 Two types - Rough Endoplasmic Reticulum (RER)
Smooth Endoplasmic Reticulum (SER)
 Serves as channels for the transport of materials
between various regions of the cytoplasm or between
the cytoplasm and the nucleus.
 Functions as a cytoplasmic framework providing a
surface for biochemical activities.

RER
 Arranged into flattened sacs
 Ribosomes on surface give it a rough
appearance
 Ribosomes are the sites of protein synthesis.
 Cells that specialize in secreting proteins
have lots of rough ER

SER
 A series of
interconnected tubules .
 No ribosomes on surface.
 Helps in the manufacture
of fat molecules or lipids.
 In liver cells of
vertebrates, SER
detoxifies many poisons
and drugs.

Membrane Biogenesis
 It is the process of formation of the cell
membrane from proteins and lipids.

GOLGI APPARATUS
First discovered by Camillo Golgi.
 It consist of a system of membrane-
bound vesicles arranged parallel to each
other in stacks called cisterns.
 The material synthesised near the ER is
packaged and dispatched to various
targets inside and outside the cell through
the Golgi apparatus.
 Its functions include storage ,
modification and packaging of products in
vesicles.
Involved in the formation of the lysosome.

LYSOSOMES
>Waste disposal system of the
cell.
Keep the cell clean by digesting
any foreign material as well as
worn out cell organelles.
 Contain powerful digestive
enzymes capable of breaking
down all organic material.
When the cell gets damaged,
lysosomes may break open and
the enzymes digest their own cell.
Thus known as suicide bags of a
cell.

Mitochondria
Known as the powerhouse of the cell.
 The energy required for various chemical activities is
released by the mitochondria in the form of ATP
molecules.( Adenosine triphosphate).
ATP is known as the energy currency of the cell.
 Mitochondria have two membrane coverings.
 Outer membrane is very porous.
Inner membrane is deeply folded.
Mitochondria are able to make their own proteins as
they have their own DNA and ribosomes .

Plastids
 Present in plant cells.
 Two types- 1) Chromoplasts (coloured)
2) Leucoplasts (colourless)
> Plastids containing green pigment chlorophyll are called
Chloroplasts.
 Leucoplasts store materials as starch, oils and protein
granules.
 Plastids contain membrane layers embedded in a material
called stroma.
 Has a similar structure like the mitochondria.
 Have their own DNA and ribosomes.

Vacuoles
 Storage sac for solid or liquid contents.
 Small size vacuoles are present in animals.
 Large in plants.
 In plant cells, vacuoles are full of cell sap and provide
turgidity and rigidity to the cell.
 Vacuoles store amino acids, sugars, various organic acids
and some proteins.
 In Amoeba, the food vacuole contains the food items that
the Amoeba has consumed.
 Vacuoles play an important role in expelling excess water
and some wastes from the cell.

Ribosomes
 Are dense, spherical and granular particles.
 Present freely in matrix (Cytosol) or remain
attached to the RER.
 Major constituents is RNA and Proteins.
 Not bounded by membrane.
 Present in both prokaryotic and eukaryotic
cell except mammalian RBC.
 Function- Important role in Protein synthesis.

Peroxisomes
 Are small and spherical organelles
containing powerful oxidative enzymes.
 Mostly found in kidney and Liver cells.
 Present in both plants and animal cell.
 Functions: Specialized for carrying out some
oxidative reaction such as detoxifications or
removal of toxic substance from the cell.

Centrosome
 Found only in animal cell.
 Consists of two granule, like – Centriole.
 Centrioles are hollow and cylindrical structures which
are made up of microtubules.
 In plant cell, Polar caps perform the function of
centrioles.
 Function:- Centrosome helps on cell division in animal
cell. During cell division centrioles migrate to the poles
of animal cell and are involved in spindle formation.
 In plant cell, cell division involves polar caps for the
spindle formation.

Plasmodesmata
 Are microscopic channel which traverse the
cell wall of plant cell and some algal cell,
enabling transport and communication
between them.
 channel through the cell wall that allows
molecules and substances to move back and
forth as needed.
 Found only in Plant cell.

Cell Division
 Process by which cell
divides into two or
more daughter cells.
 By three process:
Binary Fission, Mitosis
and Meiosis

Active Transport
 movement of dissolved molecules
into or out of a cell through the cell
membrane, from a region of lower
concentration to a region of higher
concentration.
 The particles move against
the concentration gradient, using
energy released during respiration.
 Sometimes dissolved molecules are at a
higher concentration inside the cell than
outside, but, because the organism needs
these molecules, they still have to be
absorbed.
 Carrier proteins pick up specific molecules
and take them through the cell membrane
against the concentration gradient.

Types of Active transport
 There are two types of active transport namely – Primary active
transport and secondary active transport.
 Primary active transport
 In this process of transportation, the energy is utilized by the breakdown
of the ATP – Adenosine triphosphate to transport molecules across the
membrane against a concentration gradient. Therefore, all the groups of
ATP powered pumps contain one or more binding sites for the ATP
molecules, which are present on the cytosolic face of the membrane.
Basically, the primary active transport uses external chemical energy
such as the ATP.
 Sodium-potassium pump, the most important pump in the animal cell is
considered as an example of primary active transport. In this process
of transportation, the sodium ions are moved to the outside of the cell
and potassium ions are moved to the inside of the cell.

 Secondary active transport
 Secondary active transport is a kind of active transport that
uses electrochemical energy. It takes place across a biological
membrane where a transporter protein couples the movement
of an electrochemical ion (typically Na+ or H+) down its
electrochemical gradient to the upward movement of another
molecule or an ion against a concentration or electrochemical
gradient.

Examples of active transport include:
 Phagocytosis of bacteria by Macrophages.
 Movement of Ca2+ ions out of cardiac muscle cells.
 Transportation of amino acids across the intestinal lining in the
human gut.
 Secretion of proteins like enzymes, peptide hormones, and
antibodies from different cells.
 Functioning of the White Blood Cells by protecting our body by
attacking diseases causing microbes and other foreign
invaders.
 uptake of glucose by epithelial cells in the villi of the small
intestine

Diffusion Osmosis Active transport
Down a
concentration
gradient
✓ ✓ ✗
Against a
concentration
gradient
✗ ✗ ✓
Energy needed ✗ ✗ ✓
Substance moved Dissolved solutes Water Dissolved solutes
Notes
Gases and
dissolved gases
also diffuse
Partially permeable
membrane needed
Carrier protein
needed
Active transport vs diffusion and osmosis

Active Transport in Plants
 Like humans and animals, plants also require transport systems
which are mainly involved in the transport of materials, such as
water, minerals, and necessary nutrients to all parts of the plant
for its survival.
 Active transport is a mode of transportation in plants, which
uses stored energy to move the particles against the
concentration gradient. In a plant cell, it takes place in the root
cells by absorbing water and minerals. Active transport always
leads to accumulation of molecules are ions towards one side
of the membrane. This mode of transportation in plants is
carried out by membrane proteins and transports the substance
from the lower concentration to higher concentration.

Passive Transport
 It is the biological process of movements of
the biochemical across the cell membranes
and tissues.
 natural phenomenon, which does not
require any external energy.

Types of Passive Transport
1. Simple Diffusion
2. Facilitated Diffusion
3. Filtration
4. Osmosis

1. Simple Diffusion
 movement of substances from a region of higher concentration
to lower concentration.
 The difference in the concentration of the two areas is termed
as concentration gradient and the process of diffusion
continues until this gradient neutralizes.
 Diffusion occurs in liquid and gases because their particles
move randomly from one place to another.
 It is an important process in living things required for
different life processes. The substances move in and out of
the cells by simple diffusion.

2. Facilitated Diffusion
 Passive transportation of ions or molecules across the cell
membrane through specific transmembrane integral proteins.
 The molecules, which are large and insoluble require a carrier
substance for their transportation through the plasma membrane.
 This process does not require any cellular or external energy.
 Glucose transporter, ion channels and aquaporins are some of
the examples of facilitated diffusion.
 The cell membrane is permeable only to a few molecules that
are smaller in size and non-polar.
 Therefore, facilitated diffusion with the help of transmembrane
proteins is important.

3. Filtration
 Process of separating solids from liquids and gases.
 The selective absorption of nutrients in the body is an example
of filtration.
 This process does not require any energy and takes place
along the concentration gradient.
 In the process of filtration, the cell membrane permits only
those substances which are soluble and could easily pass
through its pores.
 The kidneys are an example of a biological filter. The blood is
filtered by the glomerulus and the necessary molecules are
reabsorbed.

4. Osmosis
 water and other molecules pass through a selectively
permeable membrane in order to balance the concentration of
other substances.
 Osmosis is affected by the concentration gradient and
temperature. The greater the concentration gradient, the faster
is the rate of osmosis. Also, the rate of osmosis increases with
the increase in temperature.

Examples Of Passive Transport
 Ethanol enters our body and hits the bloodstream. This
happens because the ethanol molecules undergo simple
diffusion and pass through the cell membrane without any
external energy.
 Reabsorption of nutrients by the intestines by separating them
from the solid waste and transporting the nutrients through the
intestinal membrane into the bloodstream.
 When a raisin is soaked in water the water moves inside the
raisin by the process of osmosis and it swells.

Active Transport Passive Transport
Requires cellular energy. Do not require cellular energy.
It circulates from an area of lower concentration to a region of
higher concentration
It circulates from a region of higher concentration to a region
of lower concentration
Required in the transportation of all the molecules such as
proteins, large cells, complex sugars, ions, etc.
Required in the transportation all the soluble molecules which
include oxygen, water, carbon dioxide, lipids, sex hormones,
monosaccharides, etc.
It transports various molecules in the cell.
It is involved in the maintenance of the equilibrium level inside
the cell.
Active transport is a dynamic process. Passive Transport is a physical process.
It is highly selective. It is partly non-selective
Active transport is a rapid process. Passive transport is a comparatively slow process.
Transpires in one direction. Transpires bidirectionally.
Active transportation is influenced by temperature. Passive transportation is not influenced by temperature.
In Active transport carrier proteins are required In passive transport carrier proteins are not required
This process reduces or halts as the oxygen content level is
reduced.
This process is not affected by the level of oxygen content.
Metabolic inhibitors can influence and stop the active
transport.
Passive transportation is not influenced by metabolic
inhibitors.
Different types of Active Transport are –
Exocytosis, endocytosis, sodium-potassium pump
Different types of Passive Transport are – Osmosis, diffusion,
and facilitated diffusion

Can you name the two organelles
we have studied that contain their
own genetic material?
Ans : Mitochondria and plastids are the two
organelles that contain their own genetic
material. Both these organelles have their
own DNA and ribosomes.

Make a comparison and write
down ways in which plant cells are
different from animal cells.
Animal cell Plant cell
Animal cells are generally small
in size.
Plants cells are usually larger
than animal cells.
Cell wall is absent. Cell wall is present.
Except the protozoan Euglena,
no animal cell possesses
plastids.
Plastids (chromoplasts and
leucoplasts) are present.
Vacuoles are smaller in size. Vacuoles are larger in size.

What would happen to the life of a
cell if there was no Golgi
apparatus?
 (i) Membranes of the Golgi apparatus are often connected
to ER membranes. It collects simpler molecules and
combines them to make more complex molecules. These
are then packaged in small vesicles and are either stored
in the cell or sent out as per the requirement. Thus, if the
Golgi apparatus is absent in the cell, then the above
process of storage, modification, and packaging of
products will not be possible.
 (ii) The formation of complex sugars from simple sugars
will not be possible as this takes place with the help of
enzymes present in Golgi bodies.
 (iii) The Golgi apparatus is involved in the formation of
lysosomes. Thus, if the Golgi body is absent in a cell, the
synthesis of lysosomes will not be possible in the cell.

Which organelle is known as the
powerhouse of the cell? Why?
 Mitochondria are known as the powerhouse of cells.
Mitochondria create energy for the cell, and this
process of creating energy for the cell is known as
cellular respiration. Most chemical reactions involved
in cellular respiration occur in the mitochondria. The
energy required for various chemical activities
needed for life is released by the mitochondria in the
form of ATP (Adenosine triphosphate) molecules. For
this reason, mitochondria are known as the
powerhouse of cells.

Where do the lipids and proteins
constituting the cell membrane get
synthesized?
Lipids and proteins constituting the cell
membrane are synthesized in the
endoplasmic reticulum.
1)SER (Smooth endoplasmic reticulum)
helps in the manufacturing of lipids.
2)RER (Rough endoplasmic reticulum) has
particles attached to its surface, called
ribosomes. These ribosomes are the site
for protein synthesis.

How does an Amoeba obtain its
food?
 Amoeba obtains its food through the process
of endocytosis. The flexibility of the cell
membrane enables the cell to engulf the solid
particles of food and other materials from its
external environment.

What is osmosis?
 The movement of water molecules from a
region of high concentration to a region of
low concentration through a selectively
permeable membrane is called osmosis.

"The Fundamental Unit Of Life" Class 9

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