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Biology: Introduction, Living things, Cell,Tissue.

Biology is said to be the study of life.It has a varying number of branches. Biology has been divided into Botany and Zoology on the basis ...

Biology is said to be the study of life.It has a varying number of branches.
Biology has been divided into Botany and Zoology on the basis of nature of
living beings. Inspite of this, now-a-days extensive research has been done on
Microorganism. They influence human life in various ways. So for discussion
about them a new branch is made. This is known as Microbiology.
Considering the subjects of living bodies on which discussion is made the
whole range of Biology is divided into the following main branches.
1. Morphology: This branch deals with both the external and internal
structures of organism. The subject concerning internal . structures is also
known as Anatomy.
2. Cytology: Each living organism consists of one or more cells. Structures
and functions of cells are treated in this branch of Biology.
3. Histology: In this branch discussion is made on structure, location and
function of different tissues.
4. Physiology: This branch includes all the activities of living things e.g
growth respiration, excretion, photosynthesis and other biological activities.
5. Taxonomy: In this branch discussion is made on identification
nomenclature, and classification of plants and animals into groups and
subgroups.
6. Genetics: How different characters are inherited from parents to offsprings,
and how the processes can be controlled and improved etc. are brought under
study and research in this branch.
7. Ecology: This branch considers the effects of environment on living
organisms or living communities and also interaction between them.
8. Evolution: This branch deals with the origin and successive transformations
of living organisms.
The groups discussed above are the basic branches of Biology. Scientists
gradually started to utilize the knowledge of these branches for human welfare,
and as a result applied branches of Biology were created. Agriculture,
Medical science, Breeding etc. are some of the important applied branches of
Biology. Applied Biology also includes Forestry and Horticulture, Fishery,
Pest Control. Animal Husbandry etc.
There are numerous varieties of plants and animals on earth. Generally similar
plants or animals are arranged in particular groups. Some special branches of
Biology have been created on the basis of different types of living things are
under discussion and research: for example. Phycology includes only members
of algae; fungi are treated in Mycology; Virology deals with viruses only;
bacteria are considered in Bacteriology; Helminthology is based on study of
worms only; insects are discussed in Entomology.
Each of the above mentioned divisions has been divided into subdivisions or
branches. It has been mentioned earlier that research in Biological fields has
greatly extended now-a-days.
Biology, as it appears today, was not the same the past 50 years ago. It was
believed that sweat-soaked warm clothing of men and some wheat, if kept
together in a box for few days, rats would be produced. This wrong concept
about the origin of life prevailed for a long time. Subsequently ideas about
origin of life have changed by the research works of various scientists.
Since biology is the study of life,life which entails plants and animals, we need to know the characteristics of Plants and Animal.
CHARACTERISTICS OF PLANTS AND ANIMALS
Taking a few main characteristics from many we shall now compare plants
with animals.
1. Size and shape: Plants have no definite size and shape while animals have
 definite size and shape.
2. Life Time: Life time is not limited for plants and under suitable conditions
 they live for a long period. But animals live up to a certain length of time
 under suitable conditions.
3. Growth: In suitable environment growth of many plants continue for a long
 period. Growth in animals takes place up to a certain stage of life and growth
 ceases after that.
4. Movement: Most land plants remain fixed in soil with the help of roots as a
 result they cannot move. But some organs of plants may show some
 movement. A few animals (Such as Sponge) are not capable of movement.
 Most of the animals can move freely.
5. Nature of Food: Plants cannot take solid food Animals can take solid and
 liquid type of food.
6. Nutrition: Plants are autotrophic, it can prepare food through
 photosynthesis. Animals cannot prepare their own food. For foods they are
 to depend on plants and other animals.
7. Structure of Cell: A Plant cell has dead cell wall. Most of the mature cells
 have a large vacuole and plastids. The animal cell has no cell wall and it
 contains small vacuoles and centrioles.
8. Reproduction: Plants reproduce by vegetative, asexual and sexual
methods. Reproduction in animals mainly takes place by sexual
method.Vegetative and asexual reproduction may be found in some animals.
9. There are different systems in animal body. But plants have no body system.

ROLE OF BIOLOGY IN SOCIAL, ECONOMIC AND
ENVIRONMENTAL DEVELOPMENT
The two main branches of Biology are Botany and Zoology. In addition to
these, there are other branches and sub-branches of Biology. You have come
across some of those in the preceding parts of this chapter. Now let us discuss
the importance of Biology on the basis of knowledge you have already
attained.
Agricultural science has many branches of which Agronomy, Horticulture,
Plant breeding, plant pathology, Fishery, Animal husbandry, Poultry,
Honeybee culture (Apiculture), Sericulture etc. are important. With the
application of knowledge of these branches economic development is possible.
Both plants and animals are the essential components of natural environment. It
is known to you that plants and animals are interdependent in many ways.
Ecology makes us alert about the importance of plants and animals and also the
necessity of their presence in nature, Educated and conscious citizens can play
important role in conservation and development of environment. Economic
development sometimes produces adverse effects on natural environment.
Environmental degradation and hazards can be encountered through proper
application of our knowledge in ecology, Economic solvency and good
environment help social development.
NATURE AND ORIGIN OF LIFE
You know what nature is. We live in nature. We have soil, water, air different
plants, animals, insects and worms etc. all around us. All these together
constitute nature. Millions of years ago nature was not as we see it today. Nor
will it remain the same after millions of years. This indicates that nature is
always changing. Nature undergoes changes due to various natural
forces such as river-currents, earthquakes, volcanic eruptions etc. Different human activities, such as destruction of forests, construction of dams etc. also bring
about changes in nature.
Scientists have been thinking about the origin of nature. There are different
theories about the origin of the Earth. Among these the commonly acceptable
theory explains that a part of the Sun or a similar star got detached as a result of
the attraction of another star. The planets including the Earth and then satellites
have been created from the detached part of the sun.
According to this theory at the beginning the Earth was in a hot, gaseous state
Subsequently the Earth gradually cooled and condensed to form the Earth as it
is now. The core of the Earth is still in an extremely hot liquid and gaseous
state. Nature of the Earth at the primary stage was not at all suitable for
inhabitation of life. Gradually it became favourable for the existence of living
forms. Now a question may arise in your mind as to how life originates in
nature. Did plants and animals exist in nature from the very beginning?
There are different theories regarding the origin of life on the Earth. In the
beginning of this chapter we have described an old concept of the origin of life
But the theory presented by Oparin, a Russian scientist, about the origin of life
is still accepted.
According to the theory of Oparin nature was quite hot when life was created
Different gases such as ammonia (NH3), hydrogen (H2), methane (CH4)
hydrogen cyanine (HCN) etc. were present in the atmosphere. During that
period there was heavy rainfall and thunderstorm. Under such environmental
condition amino acids were formed by the interaction of various gases and
these amino acids are considered by scientists to be the first molecule of life
The first cell or life came into being through incorporation of amino acids. It is
also believed that life was created first in the primitive aquatic environment.

Nature,Structure and classification of cell

All living things are made up of small, basic and functional units referred to as the cell.Hence cell in terms of biology is the basic and functional unit of life.
Those who have life are living beings. Microorganisms, Plants and Animals
have life, so they all are living beings. Living world is composed of
microorganisms plants and animals. One or more cells make every living
being. Cell is the structural and functional unit of living body. All sorts of
reaction and anti- reaction in living body are cell centered. So to know about
any living beings one should know about the living cell at first.
All living cells are not alike. As they are, different in structure, they are
different in size, shape and functions. Different types of cell are described
below in brief.
All the cells of Plants and Animals are eukaryotic but they have differences as
well. Difference between Plant cell and Animal cell are shown in a tabular
form at the end of this chapter. However the main difference between them is
the Plant cells have a non-living cell wall, which is absent in the Animal cells.

Classification of cells

A. On the basis of Nuclear Structure

Prokaryotie cell or Proto cell : Nucleus is not well organized in these cells;
nuclear membrane and nucleolus are absent here. In these cells Chromosome
contains only DNA. Generally no other organelles are present except
Ribosome. Cell division happens here through Amitosis process.
Eukaryotic cell or Eucell : There are well-organised nucleus in these cells
with membrane and nucleolus. Chromosome contains DNA, proteins
Chloroplasts, Mitochondria and other organelles. Cell division happens through
mitosis process.
B. On the basis of Function
Somatic cell : These cells construct the living body but do not take part in
reproduction. In the lower group of living objects they are haploid i.e. only one
set of chromosomes are present, but in higher groups they are diploid i.e. there
are two sets of chromosome in the nucleus.
Reproductive cell: These cells take part in sexual reproduction but do not
construct the main body. These cells are also haploid. Sperm and ovum are
examples of reproductive cell.
Size, shape and Structure of Living cell :
It is normal to have some diversity in size, shape and structure of cells. The
size, shape and structure of a eukaryotic cell (eucell) are described below in
brief.
Size : A cell may be 0.1 ยต (micron) to 55cm (centimeter) or even more.
[1 Millimeter = 1000 Micron]
Shape: Cells are of different shapes. Mainly they are spherical, oval,
rectangular, barrel shaped or polygonal.
Structure of typical cell :
Structurally cells are of various types. For that in a certain cell all the
organelles or structural ingredients may not be present. For that reason,
considering the presence of all the organelles in a certain cell, it may be termed
as a Generalized Cell. A generalized cell has mainly two parts- Cell wall and
protoplasm.

The Structure to Nature of Cell
Cell Wall:
Cell wall is the unique characteristics of a plant cell. The non-living and hard
wall, which constructs the plant cell, is called the cell wall. At first Robert
Hook observed it in 1665 AD. There is no cell wall in Animal cells.
Chemical composition of cell wall:
The cell wall is mainly composed of cellulose, hemicellulose, pectose, lignin,
suberine etc. Those are made by different carbohydrate compound. The cell
wall of fungi is made of a carbohydrate called chitin. The cell wall of bacteria
made of protein, lipid and polymer.
Function of cell wall:
The functions of cell wall are to give a definite shape to the cell. It protects the
Cell from external injury and gives necessary rigidity and to regulate the flow
fluid between external and internal side of the cell.
Protoplasm:
Protoplasm is made by the mixture of different complex compounds. It is jelly
like, translucent, viscous, colourless semi solid-living substance. All the pro
perties of life are present in it. Protoplasm is divided into three parts: Plasma
membrane, Cytoplasm and Nucleus.
1. Plasma membrane or Cell membrane:
Just beneath the cell wall there is a soft living membrane surrounding the
whole protoplasm. This is called cytoplasmic membrane or cell membrane.
Stucture:
Plasma membrane is bi-layered. Under electron microscope there revealed
a light layer between two dark layers. In the cell membrane of some epithelial
cell there found some finger like out growth-these are called Microvilli
(Singular : Microvillus). They increase the absorption surface of the cell. The
membrane between two adjacent cells modified in various forms to make the Connection between two cells rigid. They also make the movement of different
substances between the cells easier. In many cells spaces in the cell membrane
may be broader.
Functions of Cell membrane :
(a) Transportaion of different materials inside and outside the cell,
(b) Absorption of different substances (mainly nutrients) from outside the cell,
(c) protection of the cell body and giving the cell a definite shape.
2. Cytoplasm : Outside the nucleus, the part of protoplasm, which is surrounded by the cell membrane, is known as cytoplasm. It is composed of different organic and inorganic compounds like water. different nucleic acid and enzymes. The outer area of cytoplasm is more concentrate. less granular and hard, which is called
Ectoplasm, and the central area, Which is less concentrated, called Endoplasm.
Different organelles like plastids, mitochondria, endoplasmic reticulum,
ribosome, golgi bodies, lysosome, centrosome and different nonliving
substances are present in the cytoplasm.
Function of Cytoplasm:
(a) to hold different organelles and
(b) to perform some organic functions.
Description of organelles present in cytoplasm is given below :
Plastid :
Plastids are largest in size among the organelles present in cytoplasm. They can
be seen clearly under microscope. Plastids are not present in Fungi, Bacteria
and Animal cells.
Plastids are mainly of two types: Leucoplastid or Leucoplast and
Chromoplastid or Chromoplast. Leucoplastids are colourless and
Chromoplastids are coloured. Chromoplastid again is of two types
Chromoplast and Chloroplast.
Leucoplast: They are colourless, as they have no pigment. Leucoplast may be
converted into Chromoplast or mainly chloroplast in contact with sunlight. In
absence of light the case may be reversed.
Position: Leucoplasts are present in the cells of organs like root. underground
stem etc. which do not come in contact with sunlight.
Shape: Leucoplast may be semicircular or tubular in shape
Function: Their function is to store food.
Chromatoplast: Chromatoplasts are green or other colour; when green, it is
called chloroplast and when they are other colour it is called Chromoplast.
Chromoplast: They are variously coloured, other than green, mostly yellow
and red in colour. They are also variable in shape. Chromoplast occurred in
coloured part of plants. e.g. petals, coloured fruit and seeds, roots of carrot etc.
Function: Flowers are coloured and beautiful due to their presence.
Chloroplast: They possesses green pigments named chlorophyll in excess as a
result they are green. Other pigments are also present to some extent. Every
cell may contain one or more chloroplast.
In higher plants the shapes of chloroplast is lenceolate. In Algal cells their
shapes are of various types, e.g. cup shaped, spiral, reticulate, star shaped.
semicircular etc.
Structure of chloroplast: The following parts make Chloroplast :
1. A bi-layered semipermiable membrane surrounds the whole chloroplast. It
is composed of protein and lipid. This is called lipoprotein.
2. A hygroscopic matrix surrounded by a membrane is there. This matrix is
called stroma.
3. In the stroma, 40-80 well-arranged barrel shaped grana (sing: granum)
are present. In one granum there are 5-25 granum disc. Inside the granum disc, there are spaces. Probably chlorophyll and other
photosynthetic materials are present in this chamber.
4. Some granum discs of two adjacent grana are connected by minute tubular
bodies, which are called stroma lamelli.
5. Many crystalline bodies are arranged in the membrane of grana disc. These
are called quantosome. In the dark phase of photosynthesis, carbon-di-oxide is
assimilated mainly in the quantosome.
Function: Its function is to prepare carbohydrate food by the process
photosynthesis.
Mitochondria: Krebs cycle, fatty acid cycle, electron transport system etc.
take place in mitochondria (sing: mitochondrion). All energy producing
process occurs in mitochondria for which mitochondria are compared to the
powerhouse of the cell. Number of mitochondria may vary according to the
species. Normally each cell contains average 300-400 mitochondria. Their
shape may be globular, rod, thread, star-shaped or ring-shaped. The outside of
mitochondria is surrounded by a bi-layered membrane. The membrane is made
by lipoprotein, which is enriched with lipid and protein. The outer membrane is
smooth but the inner one has a series of enfolding into the inner cavity of
mitochondria. These enfolding are called cristae. Small stalked granular bodies
are arranged on the cristae and they are called auxisome.
Endoplasmic reticulum: ER
In a mature cell a network is found in the cytoplasm. This is called endoplasmic reticulum. Endoplasmic reticulum is of two types - smooth and rough. ER having ribosome on its body is called rough endoplasmic reticulum.
Structure: They are bounded by double-layered semipermiable membrane.
Normally they are branched but may occur in parallel. They are continuous
with the nuclear membrane and the cell membrane. Chemically the membrane
is made by lipoprotein. Small granular bodies may be present on it.
Function: They form the skeleton of protoplasm. Protein synthesis occurs in
rough endoplasmic reticulum. Lipid, in alternative opinion, different hormones,
glycogens etc. are synthesized in the smooth endoplasmic reticulum. They play
the role of internal carrier of lipid and protein.
Ribosome:
Ribosomes are small spherical /granular
organelle. They may occur freely in
the cytoplasm and may be attached on the outer wall
of endoplasmic reticulum.
Structure: These are spherical and
bounded by double layered membrane.
Mostly they are made by protein.
Function: In ribosome various amino acids are combined to synthesize protein.
Golgi bodies/Golgi apparatus:
Golgi bodies may be flat, spherical or elongated. Normally they are present near
the nucleus. It was first observed by a scientist named Golgi in 1898 AD in the
nerve cells of owl and cat. This organelle is afterward named Golgi apparatus
after his name. In plant cells their number is small for which it is not always
visible under microscope.
Structure: Golgi apparatus is tubular, small vesicle, vacuolar, elongated vessel
like or lamillar bodies. They are vacuolar space bounded by double-layered
membrane.
Function: Function of Golgi bodies are- synthesis of lysosome and non-protein
substances, releasing some enzymes, expelling cell water and attaching
substances to its membrane produced by endoplasmic reticulum.
Lysosome:
Lysosome is formed by various enzyme bounded by a membrane. Normally
they are spherical. Their membrane is bi-layered.
Function: Their functions are phagocytosis i.e. to eat the invading enemy in
the cell, to protect different organelles in the cell by dissolving enzymes and to
help in digestion.
Centrosome:
In animal cell there is a spherical body outside the nucleus, which is called
centrosome. The fluid by which it is composed is called centrosphere. At the
center of centrosphere there are two cylindrical objects called centriole. At the
time of cell division the pair of centriole is separated and moves to two
opposite pole (of the cell).
Function: At the time of cell division centrosome directs the poles of spindle
apparatus and help in cell division.
Vacuole:
The open spaces that found in the cytoplasm is the vacuole. In immature cell
their number are many and small in size. But in a mature cell all the vacuoles
combined together to form a large vacuole. The thin membrane that covers the
vacuole is called tonoplast. The internal fluid of the vacuole is called cell sap.
Different kind of inorganic salts, organic acid, carbohydrate, protein, fat,
various complex substances and various colour are present in the cell sap.
Nucleus:
Denser and clearer organ found in the protoplasm is Nucleus. Robert Brown
discovered and named nucleus in 1831 AD in the cell of orchid leaf. Normally
each cell contains one nucleus. Some eukaryotic cell like sieve tube, mature red
blood cells of mammal do not have nucleus. Usually nucleus is spherical and
present at the centre of the cell. They may be present by the side of a large
vacuole. Nucleus may smaller or larger in size and shape.
Function : Nucleus controls the total activity of the cell.
Structure: Chemically they are made of nucleic acid and protein. It contains
some protein, trace of DNA (Deoxi-ribo nucleic acid) and RNA (Ribo Nucleic
Acid), little amount of Co-enzyme and other materials. Physically they are
made by the following parts: - Nuclear membrane, Nucleoplasm, Nucleolus
and Chromosome
Nuclear membrane: The double
layered transparent membrane,
which make the outer covering of
the nucleus is the nuclear
membrane
The outer membrane porous but
the inner one is not. Chemically the
membrane is made of protein and
lipid.
Function: The main function of
this membrane.is to keep the
nucleoplasm, chromosome and the
nucleus distinct from cytoplasm.
Transportation and communication
between internal materials and
cytoplasm is also done through this
membrane.
Nucleoplasm: It is a transparent
and dense fluid bounded by nuclear
membrane. Nucleolus and
chromosomes are present in it.
Function: It holds chromosomes and
performs various organic functions.
Nucleolus: The denser, small and round body found in the nucleus is the
nucleolus. Every nucleus normally contains a single nucleolus. Nucleolus is
usually attached to a certain area of a Particular chromosome. The region of
chromosome where it remain attached is called `secondary constriction'.
Chemical composition: The main compositions of the nucleolus are protein,
RNA and a trace of DNA.
Physical structure: Nucleolus is usually divided into three parts namely
fibrous, granular and matrix.
Function: To synthesise various types of DNA and protein and preserve them.
Chromosome: In every nucleus there is a definite number of chromosomes
according to the characteristic of definite species. It can be seen under
microscope after proper staining, only in a dividing cell. Every chromosome
contains one or more centromere, one chromonema or more chromonemata and
some chromosome may have satellite. Chromosome bears a number of gene
and genes are responsible for expressing characteristics of different species.
Chemical composition of chromosome: Chemically each chromosome is
composed of DNA, RNA, Histon and non-histon protein. Besides, some
calcium and magnesium are also present here.
Function: Chromosome is the bearer and carrier of hereditary properties of the
organism.
4. Ergastic substances :
In a mature cell, in addition to the above-mentioned substances. there are
various types of non-living objects. They are present in the cytoplasm. Non￾living objets may be classified into three types, namely reserve food, secretory materials and excretory materials.
Note that the parts of  cell are what is called the cell organelle

Cell Division

Every living body is composed of cell. Some living bodies are made of only 
one cell. They are called unicellular organism, e.g. Bacteria, Amoeba, 
Plasmodium, some Fungi and some Algae. Some living bodies are made of 
more than one cell. These are called multicellular organism. There are many 
living bodies, which are made of millions of cells. Living bodies like human. 
beings or mango tree etc. are composed of millions of cells. Unicellular 
organisms increase their number (multiply) by cell division. In this process one 
cell divides into two, two to four and so on. In multicellular organisms, a large 
body consisting of millions of cells develops by cell division from the embryo, 
which also develops from a single fertilized egg. A young seedling developed 
to a large tree by cell division. Again new generation is created from male and 
female gametes formed by cell division. But all these divisions are, not alike. 
The processes of division are of different types and the results are also 
different.
Types of cell division:
Cell division is of three types: (i) Amitosis (ii) Mitosis and (4i) Meiosis.
1. Amitosis: This type of cell division occurs in unicellular prokaryotic 
organisms like Bacteria, Yeast etc. In this division, at first the nuclear 
materials are directly splitted into two portions and then the cell divides into 
two from the middle region. As a result, from one cell there' develops two.
2. Mitosis: Mitosis is a type of cell division by which a eukaryotic cell divides 
into two by a special method. In this process, the nucleus and chromosome 
are divided once and the number, structure and properties of chromosome in 
the newly formed cell remain just alike the mother.
Mitosis is also termed as equational division. Normally this 
division occurs in somatic cell. As a result of this division, the plant 
and animal increases in length and breadth. Mitosis occurs in all 
meristamatic cells of plants.
3 Meiosis: Meiosis is a type of cell division by which a Eukaryotic cell 
divides into four cells by a special method. In this process the nucleus 
divides twice but the chromosome divides once. The number of 
chromosome in the newly formed daughter cell reduces to the half the 
number of mother cell. As the number of chromosome reduces to half 
this process is also termed as reduction division. In Greek `Meiosis' 
means `to reduce' and from this the term Meiosis comes in use. This 
division occurs in reproductive mother cells of diploid organisms. As 
a result haploid gametes develop. In haploid organisms, this. division 
occurs in the zygote, as a result of which the organism becomes 
haploid again.
Stages of Mitosis : 
Mitosis is a continuous 
process. The process is 
completed by a complex 
method. According to the 
sequence and stages, this 
continuous process is divided 
into five stages. The stages are: 
(1)Prophase, 
(2) Pro-Metaphase, 
(3) Metaphase,
(4) Anaphase 
and 
(5) Telophase.
1. Prophase: At this stage, the 
nucleus swells up. 
Chromosomes begin to be de￾hydrated. As a result, the chromosomes gradually become shorter and thicker.
Then they are visible under microscopes. At the end of this stage nucleolus and nuclear
membrane become disappear.
2. Pro-Metaphase : At the beginning of
this stage. the fibrous protein converse to
form a bi-polar spindle apparatus. Each
chromosome is then become attached to a
fibre of the spindle apparatus by its
centromere. Each fibre of the spindle
apparatus is called spindle fibre. The fibre
to which the chromosomes are attached is
called traction fibre. As they are attached
with the chromosomes so they also called
chromosomal fibre. In animal cell aster rays
are radiated from centrioles present at two
poles.
3. Metaphase : Chromosomes are arranged
at the equatorial plane of the spindle. The
centromere of each chromosome remains at
the equatorial plane and the two arms are
placed towards two poles. At this stage, the
chromosomes become maximum thick and
short. Two chromatids of a chromosome
become maximum thick and short. Two
chromatids of a chromosome become
clearly visible and the centromere is divided
in to two parts.
4. Anaphase : Two separate chromatids of
a chromosome move towards the opposite
pole of the spindle apparatus. Centromere
goes ahead at the movement of the
chromatids towards the pole and the arms
follow them. when the daughter
chromosomes reaches near the poles the
anaphase stage ends.


5. Telophase: Daughter chromosomes take position at two opposite poles.
As a result, the mother cell divides into two daughter cells. In case of
animal cells, instead of formation of cell wall the cell membrane is
constricted inwardly and the cell divides into two.
Significance of Mitosis:
The significance of mitosis in the living world is unlimited. Some of them are
described here:
1. Growth of the body: Growth of the body of a living being takes place by
mitotic division. A unicellular zygote is transformed to a human body
consisting of millions of cells. A small zygote forms a large Banyan tree.
2. Maintaining equality of Chromosome number: By this division the
number and properties of a chromosome in each cell of a multicellular body
remains constant.
3. Keeping the size and shape constant: By this division the definite shape
and size of the cell remains constant.
4. Healing of injuries: By producing new cells this process repairs the various
types of damage of multicellular organisms.
5. Formation of sex organs: By this process sex organs are formed. As a
result, continuity of reproductive sequence is maintained.
6. Qualitative stability: Qualitative stability is maintained by mitosis.
Chromosomes gradually take water and
become elongated, thin and long. Nuclear
membrane develops encircling the
chromosomes. Nucleolus reappears at the
secondary constriction of the Sat
chromosome. Spindle apparatus disappears.
At the end of this stage, gradually a cell
wall develops at the equatorial region of the cell.

Abnormal cell division :
 We are all acquainted with the terms, 'Tumor', 'Cancer', etc. These
are the result of abnormal cell division. In mitosis one cell divides into
two, two to four and in this way the number of cell increases But there is
a regularity in this process. If by any circumstances this regularity is
lost, cell divides abnormally. As a result, a tumor is formed.
 Cancer cell is also a product of uncontrolled abnormal cell division.
It has been found in experiment that various type of Papilloma virus
help to produce cancer cell. Two genes named E6 and E7 of this virus
(Papilloma) produce some chemicals, which displace two protein molecules
responsible for controlling the cell division. As a result, control over cell
division is lost and thus a tumor is produced. Sometimes these two genes may
be united with-the genes of host cell and stop the action of protein to control the
cell division and thus there develop cancer cell or Cancer.
 Cancer is a fatal disease. Every year five to six hundred thousand of patients
die only because of various forms of cancer. Cancer generates in liver, lunge,
brain, breast and skin i.e. almost all the organs of the body. No effective
medicine has yet been discovered to prevent this disease.
Male-Female determining chromosome:
 Every living being has a definite number of chromosomes. In human beings
there are 23 pairs of chromosome. Among them 22. pairs are similar in both
male and female, these are Autosomes, AA. Members of the remaining pair of
chromosome are different in male and female. This pair of chromosome
determines the sex of human beings. As they determine the sex, so they are
called X and y sex chromosome or sex determining chromosome. One of the
sex chromosomes is X chromosome and the other is Y chromosome. If the sex
chromosome pair is XY, the child will be male (son), and if it is XX, the
child will be female (daughter). In Drosophylla (a fly) also XX indicate female and XY male. So in case of determining the sex, the role of X and Y
chromosome is vital.
Whether the child will be a son or daughter:
Now we will discuss whether the father or mother is responsible for the
birth of a son or daughter. Both male and female have 44 autosomes, A
and a pair of sex chromosome. Sex chromosome of a female is XX and
that of the male is XY. The male gamete (sperm) bears X and Y
chromosome. But the female gamete (egg) bears X and X. If the X
bearing egg of the female is united (fertilize) with Y bearing sperm of
the male, the child will have XY. A child having such chromosomes
will be a male child. It is seen that if there is no Y chromosome, the
child will never be a son. There is no Y chromosome in a female; Y
chromosome is present only in male. So if a couple do not have any
male child, the husband is responsible not the wife.

Division Of Labour In Multicellular Plants :
Tissue and Tissue System

Tissue :
 In unicellular organisms a single cell does all the functions. There
is no chance of division of labour on the basis of cell. In contrast, this
kind of division is found in multicellular living body. In this case, it is
observed that a group of cells performs the duty of manufacturing food;
another group stores it, while the other group conducts the transport of
food materials. There is further a group of cells that gives mechanical
strenght to some organs. In many cases it is observed that a group of
cells originating from a single origin remain closely attached in a place
and collectively perform a similar type of function. This type of cells in
a group is called tissue. In multicellular organisms division of labour is
the main cause of formation of tissue.
Types of Tissue :
 Cells of all tissues do not have the power of cell division but some
of them have. Therefore tissues are of two types in terms of the power
of cell division :
 1. Meristamatic tissue &
 2. Permanent tissue
1. Meristamatic tissue : Meristamatic
cells compose the Meristamatic tissue.
The cells of this tissue divide
repeatedly. These tissues are found in
the tress. Due to their presence plants
increase in size very rapidly. Other
permanent tissues are originated from
Meristamatic tissue.

Characrteristics of Meristamatic tissues :
 1. Cells possess the power of cell division.
 2. Usually the cells are rectangular of oval.
 3. Cell wall is made of cellulose and is thin.
 4. Nucleus is large and the cytoplasm is denser.
 5. Usually no vacuole is seen in the cell.
 6. Usually there is no intercellular space in Meristamatic tissue, so the
 cells are arranged compactly.
Meristamatic tissues are present at the apex of roots and stems. According to
their position they are Apical meristems and accroding to their origin they are
Primary meristems, because they are originated from the embryonic stage.
Stem and root increase in length by the division of cells of these tissues.
In the roots and stems of gymnosperms and dicotyledonous plants there
develops a new Meristamatic tissue, they are called 'Secondary meristems.
According to their position they are Secondary meristems. Due to the cell
division of these tissues the breath of the root and stem increase i.e. the root and
stem gradually become broader in breath
2. Permanent tissue:
 Cells of permanent tissues are not capable of cell division, for which they are
called Permanent tissue. They are originated from Meristamatic tissues. All
tissues other than the Meristamatic ones are permanent tissues.
Characteristics of Permanent tissue:
1. Cells of these tissues have no power of
 division.
2. Cells are well developed and properly
 shaped.
3. Cell wall is comparatively thick.
4. Nucleus of the cells are bigger and
 cytoplasm is dense.
5. Usually there are vacuoles in the cell.
6. There may have intercellular spaces in
 between cells.

Types of permanent tissue : Permanent tissues are of three types,
namely -
(a) Simple tissue, (b) Complex tissue and (c) Secretory tissue .
(a) Simple tissue: Tissues of same kinds compose simple tissue. Simple tissues
are of three types, namely
Parenchyma: Characteristics of parenchyma tissue are as follows:
* Cells are almost uniform in length, breath and depth.
* Cells are round, oval or polygonal in shape.
* Cell wallls are evenly thick.
* Cells are living and contain sufficient
protoplasm.
* There may have intercellular spaces
between adjacent cells.
Permanent - tissues that Thin cell wall
contain chlorophyll are called
Chlorenchyma. The chlorenchyma of
leaf IS called Messophyll.
Parenchymatous tissues with large air
spaces In aquatic plants are
Aerenchyma.
Functions:
￾ Chlorenchyma manufactures food materials.
￾ They store reserve food.
￾ They help ,in transporting the food materials.
Parenchyma in the epidermis act as a defensive organ.
Position: Usually pith, pith rays, epidermis and most of the cortex are made of
this tissue.
Collenchyma: Characteristics of collenchyma tissues are as follows:
￾ Cells are to some extent elongated. '
￾ Cells are living with protoplasm.
￾ Cell wall is unevenly thick, thickness is greater at eorners of the cells.
￾ There may have intercellular spaces between adjacent cells

Functions:
￾ Cells with chlorophyll manufacture
food.
￾ It gives mechanical strength ' to the
growmg organ.
Position: It is found under the epidermis,
in the petioles and veins of leaves and in
the flower stalks.
Sclerenchyma: Characteristics of
Sclerenchyma are as follows:
￾ Cells elongateq and the ends are
pointed.
￾ Cell walls being lignified become
thick and the thickness is uniform.
Mature cells are dead and without
nucleus and protoplasm.
￾ In transverse section they are
polygonal.
Functions:
￾ To give mechanical strength to
different organs of the plant is the main
function.
￾ Some dead cell may store excretory
substances of plants.
￾ Sometimes it forms hard outer wall to
protect the inner soft portion, e.g. seeds
of coconut and date-palm
Position : They are present in cortex,
phloem and pericycle.
(b) Complex tissue: Complex tissue is composed of more than one kind of
cells. They are two types, namely Xylem tissue and Phloem tissue.
Xylem tissue: Xylem tissue consists of four types of cells, namely:
Tracheids, Vessels or Trachaea, Xylem fibre and Xylem Parenchyma.

￾ Tracheids: Cells are dead, long with
transverse ends, containing large vacuole.
Cell walls are hard, strong. and lignified.
Their main functions are to gIve
mechanical strength and to supply water
and dissolved minerals from root to the
leaves.
￾ Vessels or Trachaea: Cells are broad
and short, placed end to end to form a
continuous hollow tube. Water and
waterdissolved minerals are conducted
from root to leaf by these cells.
￾Xylem fibre: These are sclerenchymatous
cells. Their main function is to give
mechanical strength to the plant.
Xylem parenchyma: These are
parenchymatous .
cells. Functions of these cells are storage
and conduction of food materials.
Functions of xylem tissue: Their
functions are to give mechanical strength to
the plant body, conduction of water,
minerals and food materials and storage of
food.
Phloem tissue: Phloem tissue consists of
four types of cells, namely: sieve tubes,
companion cells, phloem fibres and phloem
parenchyma.
Sieve tube: These are elongate hollow
cells placed end-to-end forming a long tube. The partition walls between two adjacent sieve cells are perforated
and known as sieve plate. In m4;iture sieve tube there is. no nucleus in the cell.
Conduction of food, prepared in the leaves is its main function.
￾ Companion cells: These are parenchymatous, narrow, elongated cells, and
are closely associated with the sieve tube. They have dense cytoplasm and a
large nucleus. Conduction with the sieve tube is done through the pores present
on the walls of these cells. They help the sieve tubes in conduction of food
materials.
Phloem fibre: These are Sclerenchymatous cells. They are also known as bast
fibre. Its function is to give mechanical strength.
￾ Phloem parenchyma: These are parenchymatous cells. They help in storage
and conduction of food materials.
Function of Phloem tissue:
￾ There main function is to conduct food materials, prepared in the leaves, to
different parts of the plant.
￾ They give mechanical strength to plant organ.
￾ In case of necessity they store food materials.
Importance of Complex tissues:
In respect of physiological and economic aspects, complex tissues are of
great importance.
Physiological importance:
Complex tissues perform the duty of conduction of raw food materials to
leaves and prepared food materials to all the living cells.
￾ Beside preparation of food, water is necessary for various reactions. This
water is conducted from root to stem through the complex tissue named
xylem.
￾ Through xylem tissue dissolved minerals along with water also passes
upwardly.
￾ Water used in transpiration also moves to leaf through xylem tissue.
Economic importance
￾ The most important economic crop in Bangladesh is jute fiber. Jute
fiber is the secondary phloem or bast fiber.
￾ The wood we use, in the construction of houses, in making
furniture, boat or musical instruments or as fuel, is the secondary xylem.
(c) Secretory tissue: Secretoy tissues are those tissues, which secrets
various liquids, Resins, gums, rubber etc. are collected from secretory
tissues. They are of two types: .
Laticiferous tissues and Glandular tissues.
Laticiferous tissue: Latex is a white, yellow or colourless liquid
Carbohydrates, protein, gums, and fats etc. are mixed in Latex, Latex is
found in various plants like;- Musa (KALA) Ficus (BOT),etc

Tissue System

 When one or more kinds of tissues are united and together perform a similar
function, then it is known as tissue system. They are classified into three types
namely: i) Epidermal Tissue System, ii) Ground Tissue System and iii)
Vascular Tissue System.
i) Epidermal Tissue System: - This tissue system makes the outer cover of
plant organs like, roots stems, branches, leaves, fruits. etc. Typically it consists
of a closely arranged single layer of parenchymatous cells. There may have
unicellular or multicellular hairs.
Functions: To give protection to the internal portion of the plant organ.
ii) Ground Tissue System :
Theses are the main bulk of tissues
of roots and stems. The ground
tissue is differentiated into outer
cortical region and inner medulla
or pith region. In dicot stem the
cortical region is ususlly subdivided
into three zones namely:-
(a) Hypodermis, (b) General,
Cortex and (c) Endodermis,
Medulla and Medullary Rays
encircled by, the Pericycle form the
central Pith.
Functions : Functions of this tissue
system are to give mechanical
stregth to the stem and to store food
materials.
iii) Vascular Tissue System:-
Vascular tissue system comprises of
two complex tixssue, namely;
Xylem and phloem, These tissues
are embedded in the ground tissues.
In dicotyledonous stems xylem and
phloem lie side by side on the same
radius and between them there is a
kind of Meristamatic tissue named Cambium. Phloem tissue lies towards the
periphery and xylem toward center of the stem.
Functions
*Conduction of water and dissolved food materials.
*Conduction of prepared food.
*Give mechanical strength to the stem, and
*Storage of food where necessary.

ANIMAL TISSUE, ORGAN AND ORGAN SYSTEM 

There is only one cell in the body of the animals of the phylum Protozoa.
Animals of the Phylum Porifera are simplest multi-cellular animals. Their cells
do not form actual tissue. Though tissues, organs and systems are not formed in
members of the phylum Cnidaria (previously known as Coelenterata), their
body cells are, however, arranged in two layers.
In the animals from phylum Platyhelminthes to Chordata, tissues, organs and
organ systems are seen. Members of those phyla are three layered. From there
three embryonic cell layers (namely ectoderm, mesoderm and endoderm),
tissues, organs and systems are formed.
Tissue: In the body of multi-cellular animals some cells remain together and
performa particular function. These cells are either formed from the ectoderm,
mesoderm or endoderm of the embryo and remain close together. Inter-cellular
materials or matrix are present between these cells. These cells may be similar
or of different types. Arising from the embryonic cell layer when certain types
of cells when remain in a particular place of the animals' body and collectively
perform a common function, those cells and the inter-cellular materials or
matrix secreted by them are collectively called the Tissue. That is to say, cells
of a particular tissue are similar in its, origin, function and structure. The
subject in which the different types of tissue are discussed is called Histology.
Differences between Tissue and Cell:
A. Tissue: Originating from a same place when more than one cell with similar
structure and function performs a common function, then those groups of cells
are called Tissue. For example, blood is a kind 'of fluid connective tissue. In
the liquid matrix or plasma of blood there are red corpuscles, white corpuscles
and platelets. Blood is developed from the mesoderm of the embryo.
B. Cell: Cell is the structural and functional unit of tissue. For example, Red
blood corpuscles (Erythrocytes), White blood corpuscles (Leucocytes) and
Platelets (Thrombocytes) are various types of blood cells. Of these the red courpuscles transport oxygen and carb.on dioxide, white corpuscles take part in the
defense of the body and the platelets take part in blood clotting at the site of injury
and stop unwanted blood loss.
Blood cells and the plasma together form the blood tissue.
Blood generally takes part in the internal transportation of the body.
Types of Tissues :
On the basis of number of cells, characteristic and the presence or absence of
the intercellular materials or matrix secreted by cells, the tissue is mainly
divided into four categories.
 1. The Epitherlial Tissue.
 2. The Connective Tissue.
 3. The Muscular Tissue.
 4. The Nerve Tissue.
Location, Structural characteristics and Function of different types of Tissue
of the Animal Body :
1. Structural Characteristics, Function and Location of Epithelial Tissue:
Structural Characterisics : The cells of the epithelial tissue lie closely or side by
side on a basement membrane. There is no matrix in this tissue. On the basis of
cell size, location in the animal body and nature of work, this tissue is of three
types, sch as;
a. Squamous Epithelial Tissue: Cells of this tissue are flat like scales; rnucleus
is large
Example: Wall of the Bowman's capsule of Kindey.
Function: Mainly filtration and covering. ~
b. Cuboidal Epithelial Tissue: Cells of this tissue are cuboidal, as the length,
breadth and height of the cells are nearly equal
Example: Collecting tubules of the kidney.
Function: Mainly absorption and covering.
Columnar Epithelial Tissue : Cells of this tissue are narrow and elongated like a
column
Example: On the internal wall of intestine.
Function: Mainly secretion, protection and absorption.
i. Simple Epithelial Tissue: On basement membrane the cells are arranged in a
single layer.
Example: Bowman's capsule of ki4ney; renal tubules, intestinal wall
ii. Stratified Epithelial Tissue: Cells are arranged on the basement membrane
in more than one layer
Example: Integument of vertebrate animals.
iii. Pseudo stratified Epithelial Tissue: .Cells, of this tissue are arranged in a
single layer on basement membrane. The cells are not of the same height. So
this tissue appears to be stratified tissue
Example: Trachea.
Besides the cells of epithelial tissue. are transformed variously for different
functions. As:
1. Ciliated Epithelial Tissue: Found in the wall of the respiratory tube of
vertebrate animals.
2. Flagellated Epithelial Tissue: Found in the endoderm of Hydra.
3. Pseudopodia Epithelial Tissue: Found in pseudopodial cells in the
endoderms of Hydra and in the intestine of vertebrate animals.
4. Glandular Epithelial Tissue: This is a kind of epithelial tissue transformed
into gland in the stomach and intestine of vertebrate animals.
5. Germinal Epithelial Tissue: This, is a kind of transformed epithelial tissue.
From this tissue sperms and ovum are formed.
General Functions of Epithelial Tissue:
1. This tissue form, the external and internal- covering of any organ or tube.
2. After transformation this tissue takes part in , protection; secretion,
absorption, diffusion, transportation etc. So, it can be said that epithelial tissue
being transformed into glandular tissue and germinal tissue perform various
important functions.
2. Structural Characteristics, Functions and Location of Connective Tissue .
Structural Characteristics: The. amount of matrix is more in connective
tissue but the number of cells is comparatively less. Matrix may be jelly like, soft or hard and fragile. One or more than one type of fibre and materials like
calcium carbonate may remain present in matrix
Functions: Connective tissue connects muscle with muscle and bone
with bone. Connective tissue may transform into skeletal tissue, fibrous tissue
and. fluid connective tissue.
On the basis of structure and function connective tissue is mainly of
three types. As:
A. Films Connective Tissue: This type of connective tissue lies below the
body-integument and sparsely in muscles. In their matrix numerous fibres are
present.
B. Skeletal Tissue: Internal structural building tissue of the body is
called the skeletal tissue.
Functions:
1. This tissue forms the internal structure of the body, e.g. skeletal system.
2. It gives the body definite shape and firmness.
3. It helps in organ movement and locomotion.
I4. It protects the soft and sensitive organs of the body (as brain. spinal cord,
lungs, heart etc.).
5. It produces various types of blood corpuscles.
6. It forms the surface for the attachment of voluntary muscles.
Depending on the formation, skeletat tissue is of two types.
a. Cartilage: Cartilage is a kind of flexible skeletal tissue. Their matrixes are
solid but they are soft and their cells have large spaces. Cartilage is suited at the
two ends of the humerus, femur, and pinna of the ear and nose of the mammals.
b. Bone: Bone is hard, fragile and unflexible skeletal connective tissue.
Bones become rigid due to deposition of lime in their matrix. Some bones are
solid. For an example, long bones of hind limb of vertebrates. Parts of long
bones near-the bone cavities are sponge like.
c. Fluid Connective Tissue:
Structural Characteristics: Matrix of this tissue is liquid. In he matrix there
are various types of organic materials in the form of colloid.
Function: The main function of vascular tissue is to maintain circulation in the
interior of the body and resistance from disease. This tissue is of two types:
Blood and Lymph.
1. Blood: Blood is a type of alkaline, stightly saline, red coloured, liquid
connective tissue. Flowing through the artery, veill and capillaries, blood takes
part in internal circulation. Blood, blood vessels and heart together form
circulatory system.
Structural Characteristics: Blood is formed of two components:
i. Plasma: It is the liquid part of blood. It is straw coloured. It contains
(91-92) % water and (8-9) % organic and inorganic materials. The
organic substances include various types of blood protein and waste
materials. The inorganic part contains different minerals like sodium,
potassium, iron. calcium, magnesium etc.
ii. Blood Cell or Blood Corpuscles: Blood corpuscles form the major
components of blood. Blood cells are of three types. These are:
Red Blood Corpuscle or Erythrocyte: These blood corpuscles contain
haemoglobin. Due to haemoglobin colour of blood is red. the red blood
corpuscles of the amphibians are biconvex, nucleated and oval. On the contrary, the red corpuscles of the blood of mammals are biconvex, non
nucleated and round. Haemoglobin is combined with oxygen forms a unstable
compound oxyhaemoglobin. It breaks down and releases oxygen in places
where it is needed. '
Functions: To, carry oxygen and some carbon dioxide.
a. White Blood Corpuscle or Leucocyte: These generally lack definite shape
and are nucleated. Cytoplasm of white corpuscles are either granular or non
granular.
Functions: To destroy germs and take part in self defense.
Thrombocyte or Platelets: These are present in the blood of vertebrate
animals. These are usually nucleated and spindle shaped. Nucleus is absent in
the Thrombocytes of mammals. The thrombocyte of mammal is also called
platelet.
Function:. Thrombocytes take part in blood coagulation or blood clotting.
2. Lymph:- The fluid materials stored in the spaces between different tissues
are collected by some small vessels. These small vessels are united together to
form larger vessels. The system formed by these vessels known as lymphatic
system. These vessels are lymph vessels and the' fluids are lymph. The large
lymph vessels enter the vain in the shoulder region of man. There are some cells in the lymph known as lymphocyte. Lymph is a kind of slightly alkaline,
transparent -and yellow coloured fluid.
3. The Muscular Tissue:
The particular type of contracting and expanding tissue originates from
embryonic, mesoderm is called Muscular tissue.
Structural Characteristics: Matrix is nearly absent in muscular tissue. The
muscle cells are elongated and fibre-like. The fibres are spindle shaped. The
fibres are known as myofibril. The cytoplasm of muscle cell may have one or
more nuclei. The cell membrane of the muscle cell is known as sarcolemma.
The myofibrils with transverse striation are known as striated muscle and
without striation are known as smooth muscle.
Function: Muscle cells through contraction or expansion take part in organ
movement, locomotion and internal circulation. On the basis of location, structure and functions, muscle tissue is of three types. Such as:
Voluntary or Striated Muscle: This Type of muscular tissue can be contracted or
expanded at the will of the living beings. The cells of the voluntary muscle tissue are
tubular, not branched and provided with transverse striations. These have generally more than one nucleus. This muscle can contract or expand quickly. This muscle is also, called striated or skeletal muscle. 
Location: Voluntary muscles remain attached with the skeletal system as muscles of hand, and feet of man.
Function: To control the movement and mobilization of different organs by
voluntary movement of various bones.
b. Involuntary or Smooth Muscle:
Structural Characteristics: The contraction and expansion of this muscle tissue is
not at all of the living beings. This muscle tissue is spindle shaped. Transverse
striations are not present here. That is why this muscle is called smooth muscle.
Location: Involuntary muscles are found on the walls of blood vessels, alimentary
canal etc. of the vertebrate animals.
Function: Involuntary muscles mainly take part in the movement of internal organs,
e.g. peristalsis. of intestine.

Cardiac Muscle :
Structural Characteristics: The special type of involuntary muscle that forms
the heart of vertebrate animals is called the cardiac muscle. The cells of this
mucle tissue are tubular (very similar to those of voluntary muscle), branched
and provided with transverse striations. Between the cells of this tissue
intercalated disc are present. The contraction and relaxation of this tissue is not dependent on the will of the living beings. That is, the structure of heart muscles is ,like that of voluntary muscle and the function is like that of involuntary
muscle. The cells of cardiac muscle, attached by branch joined together by
branch. The contraction and expansion of all cardiac muscles take part
combindly.
Function: Through rhythmic contraction and relaxation, the cardiac muscles
control the circulation of blood within the body from a particular stage of the
embryonic condition until death.
4. The Nerve Tissue:
Structural Characteristics: The particular type of tissue, which forms the
nervous system, is called nerve tissue. Receiving stimulus from the
environment, the nerve tissue transmits it within the body and accordingly
appropriate responses are created. The special type of cell which forms the
nerve tissue is called nerve cell or neuron
(Fig-5.9) So a neuron is the structural and
functional unit of nervous system. It is
ectodermal in origin.
Neuron or nerve cells can receive various types
of external and internal stimuli or
nerve-sensation and can transmit those inside the body.
Structure of a Neuron: A mature neuron has three parts
These are:
1. Cell body: The cell body is generally
polygonal and nucleated. The cytoplasm of the cell contains mitochondria, golgibody,
ribosome, endoplasmic reticulum etc. But as there is no active centriole in the cytoplasm of neuron, the neuron cannot divide. 
2. Function: Originating from the cell body, two or more branches transmit stimuli or nerve impulse to the neuron's cell body. Generally they are one or several in numbers and present opposite to the axon.
3. Axon : From the neuron's cell body a long fibre branch carries nerve impulse
towards dendrite of the next neuron. A neuron has only one axon. Between the
with the dendrite of the other. It is called synapse. Through the synapse stimuli
of a nerve is transmitted from one neuron to another.
Location : Nerve tissues are located within the nervous system. There are
innumerable neurons is the nerve tissue of any animal.
Function :
1. To receive stimuli and create proper sensation.
2. To store memory in higher animals.
3. To control the works of different organs of the body and coordinate their
activities.
Organs : An organ is a part of animal body formed by the combination of one
or more than one type of tissues performing a particular function is called an organ. That is to say, in any organ there may be one or more than one type of
tissues and the organ can perform a particular work. The branch of biology
where organs are discussed is called anatomy.
Necessary Organs of Human Body : According to location there are two
types of organs in human body. The branch of Biology, where the morphology of
external organs like eye, ear, nose, hand, foot, head etc. are discussed, is called
Morphology. Again, the branch of Biology where the internal organs of the
organism are discussed in detail is called Internal Anatomy. Eye, ear, nose,
hand, foot, head etc. are external organs and stomach, duodenum, ileum,
rectum, heart, liver, pancreas, spleen, lung, kidney, testis, ovary etc. are
internal organs of human body
Systems: For performing various physiological f~nctions, such. as digestion,
respiration, excretion, reproduction etc. several organs together form the organ
systems in animal body. These are:
1. Digestive System: This system ,is associated with ingestion, digestion,
absorption of food and removal of undigested residues. Digestive system has two main parts as:
a. Digestive Canal: This canal consists of mouth opening, mouth cavity, pharynx,
oesophagus, stomach, duodenum, ileum, rectum and anus.
b. Digestive Glands: In man salivary glands, liver and Panereas work. as digestive
glands.
2. Respiratory System: This system with the help of oxygen taken from the
environment produces energy by the oxidation of food present in the body cells to help the body to do every days work. In man the respiratory system .consists of nasal aperture, pharynx, larynx, trachea, bronchus, bronchiole and a pair of lungs made of alveoli.
3. Nervous System: The function of this system is to receive external and internal
stimuli of the body and to create appropriate sensation. Nerve system is formed of  brain spinal cord and cranial nerves and spinal nerves. Besides, the nerve system has  also a part named Autonomous Nervous system. This part of the nerve system controls the involuntary works of the body.
Excretory System: Due to various physiological metabolic activities some waste materials are produced within the body as by products. These materials are generally toxic to the body, so they need to be removed. The process of removing unnecessary waste materials from the body is called excretion. The system by which the excretion is performed is called excretory system. The excretory system of human
beings consists of a pair of kidney, a pair of ureter, one urinary bladder and one
urethra.
5. Reproductive System: Through this system an animal produces one or more
offsprings of its type to maintain the contiriuation of the species. On the maturity, the child becomes capable to reproduce. Man is unisexual. Both
males and females have individual reproductive system of their own. Besides,
males have a few more important systems, such as: .
a. Integumentary System: The membrane covering the body from outside is
called integument or skin. This system covers the body, protects from external
injury and control the movement of various material from outside and inside the
body.
b. Endocrine System: There are few endocrine glands within the body of the
human beings. Secretions of these glands are known as hormone. There are no
ducts for transportation of hormone. Blood carries the hormone from one place
to another. The endocrine system is formed with the endocrine glands like
Pituitary, Thyroid, Parathyroid, Islets of Langerhans of Panqreas, and Suprarenal etc. 
Co-ordination between Organs and Systems to keep the body fit: 
The body of man and other higher animals is formed in coordination with some 
systems. A particular system is again formed of some related organs. Each of 
these organs has particular structure and functions. There exists coordination 
among the functions of the organs of a system. Each system works separately 
and each has its own specific functions. For example, the function of the 
digestive system is mainly intake of food, digestion of food, absorption of 
digested food, and temporary storage and excretion of undigested food. Of this 
system the digestive glands named salivary glands; liver and pancreas take part 
in digestion by secreting various digestive enzymes. In this system, the pharynx 
and oesophagus take part in food intake, stomach and duodenum in digestion of food, ileum both in digestion and absorption of food and the rectum in storing the undigested food. In any living beings, all these systems are interrelated. So, a system works properly only when each of the component organ of that particular system function properly and systematically. 
Again, through the coordinated work of different systems, living beings do their 
regular activities. In case of all higher animals, nervous system and endocrine system together control and conduct the works of all organs, i.e. all systems to keep the body healthy and fit. The work of the nervou5 system is performed directly and generally quickly. On the other hand~ the functioning of the hormones secreted by the endocrine glands or ductless glands are comparatively slow

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