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Excretion
is the elimination of wastes that are formed during the metabolism of various
substances in the body.
The
main substances excreted are nitrogenous substances like Ammonia, Urea and Uric
acid. Nitrogenous wastes are formed in the metabolism of amino acids and
nucleic acids.
Along
with these excess water, certain salts and organic substances are also excreted.
The main purpose of excretion is to maintain ionic balance in the body.
If
the metabolic wastes are not discharged from the body, they become toxic and
harm the body.
All the animals do not excrete the type of nitrogenous
wastes.
Some
animals, like crabs excrete ammonia, while frogs and man excrete urea, insects,
reptiles and birds excrete uric acid.
The
type of the product excreted may change during different stages of development
and availability of water.
Aquatic
animals prefer to excrete ammonia while animals like reptiles and birds, to
whom water is very scarce, excrete uric acid.
Connective
tissue helps in binding the other tissues and organs together and provides a
frame work and support to various organs in the body.
This
tissue also plays a major role in the transport of material from one tissue to
another.
It also helps in the body defense, body repair and in the storage of
fat.
There
are different types of connective tissues each performing a different function.
Areolartissue is one type of connective tissue which joins different tissues. It helps
in packing these tissues and thus help to keep the organs in pace.
These Cells called fibroblasts are the major
components in this type of connective tissue. These cells secrete fibrous
material which holds the other tissues in position. These cells also help in
repair of the tissues when they are injured.
Cartilage
is a type of connective tissue found in the joints of bones, tip of ribs, of
the nose, external ear and in trachea. Embryos of several vertebrates do not
have bone but have cartilage. The entire skeleton of fishes like sharks is made
of cartilage. Cartilage is hard but not as hard as bone.
Boneis another type of connective tissue. It is a major component of the skeletal
system of several vertebrates. Bone is
made calcium, phosphate and calcium carbonate.
These
salts are secreted by cells called osteocytes. These are present in the central
hollow portion of the bone called marrow.
Ligament
is yet another types of connective tissue that connects bones at the joints and
holds them in position. It is made us. These fibres are made of a protein
called collagen.
Tendon
is a type of connective tissue which is also made of fibres. The tendon joins
the muscles to the bone. It is also made of collagen.
Adipose
or fat tissue is a type of connective tissue which store fat. This tissue is
present beneath the skin, around the kidneys and in bone marrow. This tissue is
made of large numbers of cells called adipocytes or fat cells.
Cytoplasm
of these cells is filled with fat is used for the production of energy required
by the body during starvation. The layer of fat, especially, below the skin,
prevent heat loss from the body.
Blood
is called fluid connective tissue. It differs from other from types of
connective tissues. There are several different types of cells in blood each
one has a different function.
All
the cells in the blood float freely in the plasma. Extracellular space is filled
with a fluid called plasma. There are no fibres in blood. A normal adult humans
being has about 5 liters of blood. Chief component of blood is water.
Besides water, it also has several nutrients
such as glucose, amino acids, proteins, vitamins, and hormones etc. required
for the body and excretory products such as lactic acid, urea, salts etc.
plasma
also contain factors responsible for blood clotting. Cells present in blood are
called corpuscles and they are of three types- red blood cells, white blood
cells and blood platelets.
Red
blood cells, also known as erythrocytes, are red in color . In mammals, red
blood cells do not have mitochondria, nucleus, endoplasmic reticulum, lysosomes
and ribosomes.
In
fishes, amphibians and reptiles, they have a nucleus. These cells are concave
on both the sides.
They
have red colored protein called hemoglobin, which helps in the transport of
oxygen and carbon dioxide .One ml of human blood has about 500 crores of red
blood cells. In adults humans, red blood cells are formed in bone marrow and
live for 120 days I blood.
The
second type of cell present in blood are white blood cells. These cells do not
have hemoglobin, hence they are colorless. These cells are less in number.
There
are two kinds of white blood cells, they are granulocytes and a granulocytes.
There
are three types of granulocytes, neutrophils, basophils and eosinophils. These
three kinds of cells attack and destroy the micro organisms that enter the
blood . all these types of cells have granules in their cytoplasm. Hence they
are called granulocytes.
There
are two types of a granulocytes- they are lymphocytes and monocytes.
Lymphocytes secrete antibodies towards foreign material that enter into blood .
monocytes move like amoeba and along with granulocytes they attack the foreign
materials and engulf them. The foreign material are destroyed inside these
cells.
Blood
platelets are a separates group of cells which do not have a nucleus. They are
disk like bodies. Whenever a blood
vessel is injured, platelets accumulate at the site of injury and help in the
formation of a blood clot.
The
clot seals the wound in the blood vessel and prevent blood loss. Several
chemicals present in the blood required for clot formation.
On the basis of their function, animal tissues are
of four major types. They are epithelial tissue, connective tissue, muscle
tissue and nervous tissue.
Let us examine the structure the structure and
functions of these four kinds of tissues in animals ( including man).
Epithelial tissue: epithelial tissue forms a
covering over the surface of the body as well as on the outside and inside of
several internal organs example skin is an epithelial tissue on the outer side
of the body. Lining of the digestive tract is a covering of epithelial cells on
the internal organ.
Epithelial cells perform several functions- they
offer protection, produce structure like hair, nails, feather and horns, in the
form of gland cells, they secrete a variety of chemical and may involve in
receiving external stimuli.
Cells in the epithelial are very close to each other
and are joined together by a cementing substance. Epithelial cells occur in
various shapes, flat, cube like, long and cylindrical.
Epithelialcells lining the digestive and respiratory system of several animals have hair
like projections called cilia. They help in moving the food particles or mucus.
Epithelial cells may be present in a single layer or
may occur in multiple layer stratified epithelium.
There are simple and complex tissues in plants.
Parenchyma, collenchyma, sclerenchyma, are examples of simple tissues . PlantTissues fall into two large categories: Meristematic, or Permanent
(Non-Meristematic)
Xylem and phloem are example for complex tissues.
Simple tissues have only one type of cell while complex tissues have several
different types of cells.
Parenchyma:parenchyma is the fundamental and simple tissue in plants. Almost all the
primary structure like leaves, flowers and young stem made up of parenchyma.
Cells in parenchyma are living cells. They can be
either round, suare or polyhedral. There are different types of parenchymatous
cells.
Chlorechyma:
cells in this type of parenchyma have chloroplasts and are commonly seen in
leaves.
Aerenchyma:
air spaces are present between cells in this type of parenchyma.
This type of
parenchyma is seen in plants which float on water such plants are called
hydrophytes.
Storage tissue:
cells in this type of parenchyma store food materials. This type of parenchyma
is seen in tubers and rhizomes.
Water storage:
cells in this type of parenchyma store water – such cells are seen in desert
plants.
Collenchyma :
collenchymas is a simple living tissue.
This tissue is present in the stems of herbs and shrubs. It gives mechanical
strength and support. Cells may have chloroplasts. Cells in collenchymas are
short and square or long and fibre like with pointed ends. Cell wall is made up
of cellulose and pectin which gives. Collenchyma gives flexibility and tensile strength to the
plants.
Sclerenchyma :
sclerenchyma is a tissue with dead cells and is designed to give mechanical
strength to the plant. It also protects the parenchyma from damage due to
stretching, bending and pressure lignin is the major component in cell walls of
the cells present in sclerenchyma.
Xylem:xylem is a conductive tissue which has both
living and non-living cells. It conducts water and minerals from roots to other
parts of the plant. A part from this
xylem gives mechanical strength to plant. Xylem tissue is also helpful in
identifying the plant species.
There are three kinds of non-living cells in xylem-
they are fibers, tracheas and vessels. Xylem parenchyma is the living component
in xylem. Cell walls of the xylem cells are thick because of lignin deposition.
It is also commercially very useful.
Phloem:
phloem is also called bast or leptome. Unlike xylem, phloem is a living
tissues. Cell walls have lignin deposition. Phloem is composed of five types of
cells. They are sieve cells, sieve tubes, companion cells, phloem fibres and
phloem parenchyma.
Sieve cells and sieve tubes are long with tapering
ends. Sieve plate with one to many sieve pores is present at the ends of sieve
tubes. The conductuion of food material from leaves to other parts takes place
through phloem tissues. Besides, phloem has commercial value- bast fibres
present in phloem are used for making ropes.
