Fundamental Unit of Life

Class 9th
Introduction to Cell
● Cells are the structural, functional, and biological
units of all living beings.
● The entire functioning of living organisms begins from
basic unit called cell, that's why it's called
fundamental unit of life.
● A cell can replicate itself independently. Hence, they
are known as the building blocks of life.
Discoveries related to Cell:
● The cell was first discovered and named by ROBERT
HOOKE in 1665.
Cell Theory:
● All the living organisms are composed of cells.
● The living organisms could be made up of one or more
cells.
● The new cells arise from pre- existing cells through
the process of cell division.
Cell - different cell number:
● • If an organism is made up of a single cell, it is
called a unicellular organism (uni: one; cellular:
cell). e.g. bacteria, protozoa, etc.
● • The organisms which are made up of more than one
cell are called multicellular organisms (multi: many;
cellular: cell). e.g. humans, animals, etc.
Cell - different shapes:
Cell - different size:
Types of organisms:
● (i) Unicellular Organism: These organisms are single
celled which perform all the functions. Example:
Amoeba, paramecium, bacteria.
● (ii) Multicellular Organism: Many cells grouped
together to perform different function in the body
and also form various body parts. Example: fungi,
plants, animals.
● The shape and size of cell are different according to
the kind of function they perform. There is division
of labour in cells.
● Each cell has certain kind of cell organelles to
perform different type of function like mitochondria
for respiration.
Diffusion:
● The spontaneous movement of a substance from a region
of high concentration to the region of low
concentration is called diffusion.
● Some substances like carbon dioxide or oxygen can move
across the cell membrane by a process called
diffusion. Cell also obtains nutrition from the
environment.
Osmosis:
● The movement of water molecules through selectively
permeable membrane along the concentration gradient is
called osmosis.
● Plant cell tend to obtain water through osmosis.
Plasma membrane or Cell membrane:
● This is the outermost covering of the cell that
separates the contents of the cell from its external
environment.
● The plasma membrane allows or permits the entry and
exit of some materials in and out of the cell.
● It is made up of lipid and protein.
● It also prevents movement of some other materials. The
cell membrane is called selectively permeable
membrane.
● It is flexible (made up of organic molecules called
lipids and proteins).
● Its flexibility enables cell to engulf in food and
other from the external environment. This process is
called endocytosis. Amoeba acquire food through this
process. → Functions of Plasma membrane.
Cell Wall:
● Cell wall is another rigid outer covering in addition
to the plasma membrane found in plant cell. The cell
wall lies outside the plasma membrane.
● The plant cell wall is mainly composed of cellulose.
Cellulose is a complex substance which provides
structural strength to plants. → Function of Cell Wall
● In such media the cells tend to take up water by
osmosis. The cell swells, building up pressure against
the cell wall.
Plasmolysis:
● When a living plant cell loses water through osmosis
there is shrinkage or contraction of the contents of
the cell away from the cell wall. This phenomenon is
known as plasmolysis.
Nucleus:
● It is called the brain of the cell as it controls all
the activities of cell.
● The nucleus has a double layered covering called
nuclear membrane.
● The nuclear membrane has pores which allow the
transfer of material from inside the nucleus to the
cytoplasm.
● The nucleus contains chromosomes, which are visible as
rod-shaped structures only when the cell is about to
divide.
Functions of chromosomes:
● Chromosomes contain information for inheritance of
features from parents to next generation in the form
of DNA (Deoxyribonucleic Acid) molecules. Chromosomes
are composed of DNA and protein.
● DNA molecules contain the information necessary for
constructing and organising cells.
● Functional segments of DNA are called genes.
● In non-dividing cell, this DNA is present as part of
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 and perform cell division.
Functions of Nucleus:
● The nucleus plays a central role in cellular
reproduction. It is the process by which a single cell
divides and forms two new cells.
● It also plays a crucial part, along with the
environment, in determining the way the cell will
develop and what form it will exhibit at maturity, by
directing the chemical activities of the cell.
Cytoplasm :
● The cytoplasm is the fluid
content inside the plasma
membrane.
● It also contains many
specialised cell organelles.
Each of these organelles
performs a specific function for
the cell.
endoplasmic reticulum (ER):
● The endoplasmic reticulum (ER) is a large network of
membrane-bound tubes and sheets.
● It looks like long tubules or round or oblong bags
(vesicles).
● The ER membrane is similar in structure to the plasma
membrane. It is also made up of lipid and proteins.
Golgi Apparatus:
● The Golgi apparatus consists of a system of
membrane-bound vesicles arranged approximately
parallel to each other in stacks called cisterns.
● These membranes often have connections with the
membranes of ER and therefore constitute another
portion of a complex cellular membrane system.
Lysosomes:
● Lysosomes are a kind of waste disposal system of the
cell.
● It helps to keep the cell clean by digesting any
foreign material as well as worn-out cell organelles.
● Lysosomes have membrane-bounded structure whose sacs
are filled with digestive enzymes.
Mitochondria:
● Mitochondria are known as the powerhouses of the cell.
● The energy required for various chemical activities
needed for life is released by mitochondria in the
form of ATP (Adenosine triphosphate) molecules.
● ATP is known as the energy currency of the cell. The
body uses energy stored in ATP for making new chemical
compounds and for mechanical work.
Plastids:
● Plastids are present only in plant cells. • There are
three types of plastids:
○ Chromoplasts (coloured plastids).
○ Leucoplasts (white or colourless plastids).
○ Chloroplasts (contains the pigment chlorophyll).
Vacuoles:
● Vacuoles are storage sacs for solid or liquid
contents.
● They are small sized in animal cells while plant cells
have very large vacuoles.
● The central vacuole of some plant cells may occupy
50-90% of the cell volume.
● In plant cells vacuoles provide turgidity and rigidity
to the cell.