Group of similar cells performing the same function
is called a tissue. If all the functions are performed by
one cell then it is called unicellular or acellular eg amoeba,
In the beginning unicellular organism stuck together to
form colonies but they were independent in their own respect.
They had a colonial life. Colony of chlamydomonas is called
volvox. It is a colonial organism but they were independent.
By living in a colony they could protect each other. They
live in a jelly-like material.After having a colonial life
there was a division of labour like in a honeybee colony.
The colony performs the total living function, that developed
a formation of single organism into a tissue. When many
cells join together to form a single organism it is called
Tissues are of two types - plant tissue
and animal tissue.
In plants there are two types of tissues - permanent
tissue and meristematic tissue.
Meristematic tissues have the capacity to divide. Depending
upon the position meristematic tissues are of three types
- apical , lateral , intercalary.
Apical meristematic is the tissue present
on the tips of root and shoot. By division of the apical
meristem the length of the plant increases.
Lateral meristem is present on the lateral
side of the plant, all around the plant. By division of
the lateral meristem the width of the plant increases.
Intercalary meristem is present on the
internodes. The distance between adjacent nodes is called
internode. Its function is to increase the length of the
internode. This meristem is short lived and it does not
All the permanent tissues are made up of meristematic tissues.
There are two kinds of plants dicots and monocots. In dicot
the apical and lateral meristem keep on dividing. In monocots
only apical is present as it lacks lateral and intercalary
. Hence the width of the plant remains the same.
Permanent tissues are of two types - simple tissue and complex
tissue. Simple tissues are divided into three types - parenchyma,
collenchyma, and sclerencyma.
Parenchyma is made up of only one kind
of cells. These cells are thin walled loosely arranged with
plenty of intercellular space. Sometimes if chloroplasts
are present then they are called chlorenchyma. Its function
then is photosynthesis. If they are loosely arranged with
plenty of intercellular space then they are called aerenchyma.
They are present on the lower side of the leaf. Aerenchyma
can also contain chloroplasts. Their main function is the
supply of carbon dioxide aerenchyma and chlorenchyma are
present to perform additional functions.
In collenchyma the intercellular space
is filled with hardening material. Hence collenchyma are
more stronger. Hardening material is peptin and pectin.
Both are mixed together. The corners being thick it therefore
provides mechanical support. It is found in few layers below
the epidermis of dicotyledonous stems. It is generally absent
in monocot stem root and leaves.
Sclerenchyma cells have thicker walls than
collenchyma. Most of the sclerenchyma cells are dead cells.
It cannot take any material from outside and cannot give
out any material. They cannot grow and only provide mechanical
Complex tissues are of two types - xylem
Xylem is composed of three types of cells.
Xylemparenchyma is formed when xylem and parenchyma combine.
Parenchyma is compelled to do the work of xylem.Tracheids
are elongated cells and hollow for the transport of water.
The adjacent tracheids are joined in a slanting way. These
are dead cells.Vessels are elongated cylindrical cells they
are hollow and dead cells. Their function is transportation
of materials. Xylemparenchyma are present outer to the tracheids
and vessels to support them. They are living cells.
Phloem is composed of three types of
cells - sieve tubes, companion cells and phloem parenchyma.Sieve
tubes are the main types of cells present in phloem through
which prepared food is transported. They are thin-walled
elongated cells placed one above the other and joining walls
are perforated (porous). Sieve tubes have hollow structures
for the transportation of material.Companion cells are small
cells arranged together with sieve tubes. They contain dense
cytoplasm. Cytoplasm is confined to one region. Companion
cells help the sieve tubes.
Phloem parenchyma also helps the sieve tubes for the transport
of material. These are simple parenchyma cells associated
with phloem. Some regions on the sieve tubes are connected
to companion cells while some with phloem parenchyma.
Epithelial tissues are divided into four types depending
upon the shape size and function. Epithelial means covering
body organs as well as internal organs and cavity of organs.
Squamous epithelium cells are of hexagonal
disc shaped because the outer region is thin and central
region is broad to accommodate a bigger nucleus. It function
is to protect the underlying parts of the body from mechanical
injury, entry of germs, chemicals and drying. It is located
in the lining of cavities of ducts and blood vessels and
covering of the skin.
The length of the columnar epithelium
cells much more than the width of the cell that is why it
is called columnar epithelium. The surface is provided with
hair like structures called cilia. It is present in larynx,
trachea, salivary gland and liver. It is present in trachea
to prevent the passage of duct particles.
Connective tissue connects adjacent tissues
or organs. It contains different types of cells. Connective
tissue is of four types.
Ligaments and tendons
come under areolar connective tissue. Ligament
connect bone to bones and tendons connect muscle to bones.
Different types of cells are present in areolar connective
tissue like shapeless cells called fibroblasts are present.
Fibroblasts are living cells. Its function is to connect
different organs. It produces two types of proteins collagen
fiber and elastin fiber. Depending upon the amount of collagen
and elastin the fibers are divided into two -
More collagen than elastin - white collagen fiber
More elastin than collagen - yellow elastin fiber
Adipose connective tissue is present below
the skin so it acts as heat or thermal regulator by storing
fat. Fat molecules do not allow body heat to go outside
or outside heat to come in. only some heat is released.
Sweat formation is a part of heat regulation. Blood releases
some extra water as sweat and sweat when evaporates produces
a cooling effect. Then the blood vessels also help as they
become large to carry more blood. Adipose tissue is an insulator
of organs. Fat storage cells are present. It also acts as
a shock absorber. It connects skin to internal organs. Here
collagen and elastin fibers are absent. In it matrix also
fat is deposited. Matrix present inside the cell is called
tissue fluid and matrix present outside the cell is called
Skeletal connective tissue is the framework
of the body. Supporting material like cartilage and bones
are present. The outer membranous covering of the cell is
called perichondrium. Matrix is present inside which several
cavities are present called lacunae. They are fluid filled
spaces. Cartilage cells present in lacunae are called chrondriocytes.
Osteoblasts produces osteocytes. All RBCs ,WBCs, and platelets
are formed in the bone marrow.
Fluid connective tissue - blood
has two parts plasma and formed elements. The main component
of plasma is water and dissolved material, organic or inorganic.
Formed elements are of three types RBC, WBC and platelets.
RBCs are biconcave because they are enucleated i.e. without
nucleus. WBC are having nucleus but they are irregular in
shape. The main function of RBC is to carry oxygen to all
parts of the body. Function of WBC is to destroy foreign
objects. Platelets are fragmented cells i.e. they are cut
into smaller pieces. Their shape is irregular but not amoeboid
i.e. they cannot change their shape themselves. It does
not contain nucleus it is only a small part of the protoplasm.
Function of platelets is to help in the clotting of blood.
Lymph is also a part of the blood. It is the clear fluid
that has filtered out of the blood. It contains WBC and
lacks RBC. Lymph contains nitrogenous waste and no oxygen.
In tissue fluid oxygen and RBC are present. Its function
is to bring back tissue fluid from the blood vessel otherwise
we will suffer from elephantitis.
Muscular tissue - there are three types
of muscular tissues striated, unstriated and cardiac. In
sraited muscles light and dark bands are present because
of this we call it striated as strait means marks. These
are voluntary muscles. Muscle cell is covered with a membrane
called sarcolemma. The cytoplasm of the muscle cell is called
In unstriated muscles the markings are absent. They are
also called involuntary muscles, as they are not controlled
by will. It is controlled by medulla oblongata (part of
brain). It is present in internal organs.
Cardiac muscles show characteristics of both striated and
unstriated muscles. Straits are present but they are not
present in a proper manner. They are not controlled by the
will of the person. It performs non-stop rhythmic contraction.
It consists of branches which form a network. A covering
is present similar to sarcolemma it is like plasma membrane.
Cardiac muscles are uninucleated. Their function is contraction
and relaxation of heart. It then contracts the heart chambers.
By this the blood is pumped into the body.
Nervous tissue- the schwann nucleus is
present for protection. Nervous tissue is formed of neurons.
Nephrons is a collection of cells while neuron is one single
cell. The region where nucleus and cytoplasm are present
is called cyton. From this cyton there are several cytoplasmic
projections. These projections are called dendrites. Even
dendrites have cytoplasm the biggest dendron is called axon.
Dendron is singular while dendrite is plural.
In most cases the axon is covered by myelin sheath. If covered
then they are called myelinated neuron while if uncovered
then non-myelinated neuron. Myelin sheath increases the
speed of conduction of nerve impulse. It will also prevent
mixing of impulses because it is a protective covering.
Myelin sheath is a part of another cell. It is not continuous
and is interrupted in between called Node of Ranvier.
Synapse is the juncture between axon
terminal and dendrite of adjacent neurons. Message is transmitted
in only one direction not from dendrite to dendrite because
the chemical used for transmission of message is present
only in axon terminal.
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