Biology is the science of living organisms. Like other branches of science, biology also evolved with man’s curiosity about his surroundings. He observed growth, decay and death in the living world. His knowledge of different forms of life helped him to use different plants and animals for his food, shelter and clothing. Today, the knowledge of biology is used in the study of agriculture, medicine, sericulture, pisciculture and selective breeding of animals. This kind of human interest in nature has existed since the earliest days of recorded history. The writings of the Indian Ayurvedic physicians like Charaka and Sushrut of ancient India tell us many things about plants and animals.

Biology, as a branch of human knowledge started with many observations made by the ancient Greek Philosopher Aristotle (384-322 BC). He is known as the father of biology. The term ‘Biology’ was however first used by the French naturalist Jean Lamarck and Treviranus. Andreas Vesalius was the first to challenge the teaching of Aristotle.

Biological evolution

As a result of chemical evolution from non-living compounds followed by development of a cell membrane and protoplasm, single celled organisms like bacteria and amoeba evolved first. The first forms of life on earth appeared 750 million years ago and later multicellular plants and animals were derived.

In animal kingdom as well as plant kingdom evolutionary development lead to increasing complexity of structure and adaptation involved development of a vascular system for efficient transport of water and origin of the seed. Simultaneously there was a gradual evolution of a motile sperm in all seed plants except in the ‘living fossils’.

Theories of biological evolution

(a)   Lamarck’s theory of the inheritance of acquired characters: Lamarck’s theory was based on the following four points:

(1)   All organisms tend to increase in size to a predetermined limit.

(2)   New organs are produced due to a new need.

(3)   Development or degeneration of organs are due to constant use or disease of them.

(4)   The modifications acquired by an organ of an individual in one generation are transmitted to the next generation.    The theory of Lamarck was criticized. To test Lamarck’s theory Weismann reared several generations of mice and in each generation cut off their tails but tails appeared in all generations.

(b)   Darwin’s theory of Natural selection: Darwin published his view of evolution in a book called the “Origin of Species”. His contemporary Russell Wallace also had independently formulated the same ideas on evolution. Darwin’s theory of evolution by natural selection was based on the following points.

1.    Over production

2.    Variation

3.    Struggle for Existence

4.    Natural selection

But Darwin was unable to explain how continuous and discontinuous variations arose. Now we know that all sexually reproducing organisms produce continuous variations due to genetic segregation and recommendation. Discontinuous variation results from natural gene mutations or chromosomal changes.

Present state of Evolution Theory

The current synthetic theory of evolution recognizes the basic types of processes-Gene mutation change in chromosome number and structure, genetic recombination, natural selection and reproductive genetic variability with which change cannot take place natural selection and reproductive isolation guide populations of organisms into adaptive channels. In addition, three accessory processes affect the working of these 5 basic processes migration of individuals from one population to another, as well as hybridization between races or closely related species which both increase the amount of genetic variability available to a population.

The English scientist Robert Hooke was the first to assign the name cells. In the year 1838 the cell theory was enunciated by two German scientists. Schleiden and Schwann. It simply stated that all plants and animals are composed of cells and all cells come from the pre-existing cells.

Plant and Animals Cells

The Animal cell is bound by a very delicate and elastic membrane forming its outer surface known as the cell membrane or plasma membrane, which is selectively permeable. Besides protecting the internal structure, it also helps in maintaining internal environment of the cell in a desired state. Other differences include:-

1.    Plant Cells have a rigid nonliving cell wall made up of cellulose outside the plasma membranes. The cell wall of plant cell is protective and it largely determines the shape of the cell.

2.    The cytoplasm of cell has one or more vacuoles, which seem to be empty but really are filled with watery liquid. Vacuoles are usually small and few in animal cells but are large and numerous in plant cell.

3.    Just outside the nucleus is a globular centrosome with one or two dot like centrioles and numerous radiating astral rays. The centrosome is not found in plant cells.

4.    In the cytoplasm there are small groups of fine sacs known as golgi-apparatus (in animal cells) or the dictyosome (in plant cells).

5.    A group of small granular organelles are found exclusively in plant cells. They are known as plastids.

Structure and function

The eukaryotic cells (Eu=true, karyotic=nucleated) are essentially two envelope systems and they are larger than prokarytoic cells. The eukaryotic cells are true cells, which occur in the plants and the animals. Though the eukarytic cell have different shape, size and physiology, all the cells are typically composed of plasma membrane, cytoplasm and its organelles viz. mitochondria, endoplasmic reticulum, ribosomes, golgi-complex etc. and a true nucleus. The nuclear contents such as DNA; RNA nucleo-protein remain separated from the cytoplasm by the thin, perforated nuclear membrane. By and large, an eukarytic cell consists of the following components:

Cell Wall and Plasma membrane

(i)    Cell wall: The protoplasm of plant cells is separated from the external world by the cell wall, which is semi rigid, laminated, external and non-living covering of the cell. The cell wall is secreted by the cell itself and provides protection and support to the plasma membrane and the cytoplasm, both of which lie beneath it. Canal-like cockpit-like minute apertures by which the cells remain connected with adjacent cells are known as plasmodesmata.

(ii)   Plasma-membrane: It is a living, ultra thin, elastre, porous, semi permeable membranous covering of cell. Primarily, it provides mechanical support and external form to the protoplasm and it also delimits the protoplasm from the exterior, checks the entry or exit of undesirable substances and due to its semi permeability it transmits necessary materials to and from the cell. It consists of an outer and inner layer of proteins and a middle layer of lipids.

Cytoplasm

The plasma membrane is followed by the cytoplasm, which is distinguished, into following:

(a)   Cytoplasmic Matrix: The space between the plasma-membrane and the nucleus is filled by homogeneous colloidal liquid known as cyto-matrix. It consists of various inorganic molecules such as water, salt of Na, K and other metals and various organic compounds such as carbohydrates, lipids, proteins and a variety of enzymes.

(b)   Cytoplasmic structures: In the cytoplasmic matrix certain nonliving and living structures remain suspended. The non-living structures are called Paraplasm or uetoplasm, while the living structures are membrane bounded and are called organoids.

Cell-division

The growth and the development of every organism depend exclusively on the multiplication and enlargement of its cells. The development of a multicellular organism from the unicellular zygote is achieved by the cell division, growth and differentiation.

The division of the nucleated cells is achieved by two integral activities such as division of the nucleus (called Karyokinesis) and the division of the cytoplasm (that is called cytokinesis). Usually the Karyokinesis is followed by cytokinesis, but sometimes is does not follow and results in multinucleated cells.

In animals and plants following three types of cell division have been observed:

1. Amitosis           2. Mitosis                             3. Meiosis

Amitosis

It is the means of asexual reproduction in unicellular organisms like bacteria and protozoan. It also occurs in embryonic membranes of vertebrates. In Amitosis, the splitting of nucleus is followed by cytoplasmic constriction. In Amitosis, two daughter cells are formed without the occurrence of any nuclear event. It is also known as direct nuclear division.

Mitosis

An organism’s body has two kinds of cells, the somatic cells and germ cells. Germ cells occur only in the gonads and are concerned with reproduction whereas Somatic cells in the remaining bulk of the body. Mitosis occurs in the somatic cells whereas meiosis occurs in germ cells.

A cell, from the time it makes its appearance as a daughter cell to the time when it starts dividing is called an Interphase cell or Resting phase.

The process of mitosis can be distinguished into 4 stages.

There is no sharp line of demarcation between the successive phases. Prophase is the longest phase of mitosis. First of all the two sets of paired centrioles move apart and migrate towards the opposite poles of a cell. Side by side, the five-chromatin threads shorten and thicken by spiral coiling. These threads are now known as chromosomes. The chromosomes further shorten and thicken and each splits into two identical chromatids. The two chromatids, remain attached to the centromere and each possesses a complete set of DNA molecule. Simultaneously, the nucleolus and nuclear membrane gradually disappear and the nuclear material mixes with the cytoplasm. In metaphase, the chromosomes with their chromatins still attached, move towards the equatorial plane of the cell i.e. the plane midway between the two poles. In anaphase each centromere divides to separate the two sister chromatids. The sister chromatids, now called chromosomes, repel each other and migrate towards the opposite poles. After the arrival of the chromosomes at the poles the spindle fibers disappear. This is followed by Telephase which may be called as reconstruction or reorganization phase. At this stage the chromosomes uncoil, become thinner and longer and finally form the chromatin. A nuclear membrane reappears around each set of chromosomes leaving the centroles outside it. A nucleolus appears in each daughter nucleus. The division of the nucleus i.e. Karyokinesis, is followed by division of the cytoplasm i.e. cytokinesis. It begins as a constriction or cleavage furrow that appears in the cytoplasm in the equatorial region during the late anaphase. The furrow gradually deepens and finally divides the cell into two daughter cells.

Meiosis

Meiosis occurs in the cells of gonads (testis and ovaries), resulting in the formation of gametes (sperms and ova). Unlike mitosis, meiosis involves two cell divisions. The first meiotic division is the reduction division resulting in two haploid daughter cells, while the second is the equational division dividing each haploid daughter cell into two similar haploid cells. Thus four haploid daughter cells are produced by a diploid parent cell.

First meiotic division, like mitosis, can be distinguished into four phases viz. Prophase I Metaphase I, Anaphase I, and Telophase I. Out of all these four, prophase I is the largest phase of meiosis. It has been distinguished into 5 sub stages. These are leptotene, zygotene, pachytene, diplotene, and diakinesis. In leptotene, (meaning slender ribbon) stage chromosomes appear longitudinally single and are characteristically beaded to the presence of dense granules called Chromomeres. In zygotene stage and chromosomes shorten and thicken and the homologous Chromosomes approach each other and pair up. The pairing of homologous chromosomes is known as synapsis. The two chromosomes forming a synapsis are referred to as a bivalent or diad. The chromosomes of the diad continue to shorten and thicken, hence the name pachytene meaning thick ribbon. Each chromosome splits up longitudinally into two chromatids attached at the centromere. Each diad now appears to have four strands and is called a quadrivalent or tetrad. At the point of contact of two sister chromatids of tetrad, chaisma is formed. The phenomena of Crossing over (the breakage, accompanied by reunion of two sister chromatis or tetrad) which is significant in heredity, is completed by diplotene stage which is followed by diakinesis, in which the homologous chromosomes move further away from each other and the nuclear membrane and nucleolus disappear. The two sets of centrioles reach the opposite poles. Anaphase I followed by Telophase I. At this stage, at each pole a nucleus is reconstructed as in mitosis but these daughter nuclei are haploid. Each containing of chromosome from each homologous pair. Nuclear division is usually followed by cytokinesis, resulting in two haploid daughter cells. After a short interphase, these cells enter the second meiotic division, in which each haploid daughter cell, resulting from the first meiotic division, now divides mitotically to give rise to two haploid daughter cells. This process resembles normal mitosis very closely and consists of four phases i.e. Prophase II, metaphase II, Anaphase II, and Telophase II.

Four types of tissues have been identified in the animal kingdom, they are:

1.    Epithelial tissues

2.    Connective tissues

3.    Muscular tissues

4.    Nervous tissues

Epithelial Tissues

This tissue covers, organs both externally and internally. Cells are closely placed together without any intercellular spaces. The shape and size of the cells constituting epithelial vary from organ to organ. Epithelial tissues are of two kinds:

(a) Simple epithelial tissues: Cells are arranged to form single layer. It is of the following types:

1.    Squamous type                          

2.    Coboidal type                             

3.    Columnar type                           

4.    Ciliated type

5.    Clandular type                            

6.    Sensory type                               

7.    Germinalt ype                            

8.    Pseudo-stratified

1.    Squmous epithelium: The cells of this tissue are extremely thin and flat and placed compactly with their edge fused completely without any space and fluid. This tissue is found in the alveoli of the lungs. Bowman’s capsule of the kidney and also form lining of the blood vessels etc.

2.    Cuboidal epithelium: The cells are cube like, on the surface view they loop polygonal hexagonal in shape, this is found in kidney tubule salivary glands etc.

3.    Columnar epithelium: Cells are longer than broad and placed side by side like columns. Nuclei are found near the base of each cell. This is found in mucosa layer of food duct.

4.    Ciliated epithelium: The free surface of the cells bear extremely thin, short motile hair like structures called cilia. The cilia beat in the characteristic fashion so as to push the substance in definite direction. This tissue is found in trachea, renal tubule, oviduct etc.

5.    Glandular epithelium: This is modification of columnar epithelium. The epithelium lines the stomach, intestine and other organs performing the function of secretion. It may be of two kinds (i) unicellular (ii) multicellular.

6.    Sensory epithelium: They are similar to columnar epithelium, here, the free end of the cell is provided with fine hair like structure called sensory hair and from the other end of the cell arises sensory nerve. They perceived stimuli both external and internal.

7.    Germinal epithelium: They are found only in the gonads et. Testes and ovary. Cuboidal in shape, these cells divide for certain duration of produce gametes.

8.    Pseudostratified epithelium: It is composed of columnar cells of unequal size so the nuclei are placed at different level. They are found in certain glands and trachea.

(b)   Compound epithelium tissue: These epithelia are made of more than one layer. It is of two kinds:

i)      Stratified epithelium

ii)      Transitional epithelin

In stratified epithelium, the innermost layer rests on basement membrane and is called germinative layer. These cells divide and new cells so produced are pushed outward gradually. It is found in skin.

Transitional epithelium found in urinary bladder, ureter, oesophagus, and tongue.

Connective Tissue

It connects one tissue or organ with other tissues/organ, hence called connective tissue. The cells are found scattered in the ground substance and the matrix is an inert fluid. By and large, three kinds of connective tissues have been recognized:

a.            Connective tissue proper

b.            Skeleton tissue

c.             Fluid connective tissue

Connective tissue proper

(a)     Connective tissue proper: It has been further divided into four types i.e.

(i)    Areolar connective tissue          (ii)           Tendon                 

(iii)  Ligament                                    (iv)          Adipose

(i)    Areolar tissue: Areolar tissue lies just below epidermis of the skin found distributed in the whole body filling all spaces and gaps found between the organs. It has viscous matrix in which are embedded to types of fibers i.e. white fibers and yellow fibers. In addition to these fibers different types of cells like fibroblasts, histiocytes, and lymphoid cells, keep floating and moving in the matrix. White fibers are unbranched wavy and thick filamentous structures made of collagen protein. Sometimes they are called as collagen fibers. This fiber is made from the secretion of fibroblast cell, which lie, either close or embedded in the fibers. yellow fibers are branched, thin and straight made of elastin proteins.

(ii)   Tendon: Tendon is white fibers that run parallel and form cord like structure. They are strong and resistant to pull. They connect muscles with bones and help in the movement of limbs.

(iii)  Ligament: They are exclusively made of elastin fiber protein; thread run parallel to form cord and join bone to bone.

(iv)  Adipose or Fat Tissue: The matrix contains large number of cells that store fat drops or oil globule. This tissue is either found in fat bodies or in deeper layer of dermis. It has got two functions in body system i.e. as a stored reserve food and act as heat insulator cum shock absorber.

Skeleton tissue

It is supporting connective tissue and gives definite shape to the organism. It also protects internal delicate organs like brain, heart, and lungs etc. It is of two kinds: (1) Cartilage (2) Bone.

Cartilage

It is relatively soft, flexible supporting tissue. It has dense matrix with cartilage cells, chondrocytes, scattered in the matrix. Smaller chondrocytes are more numerous found near the margin and larger but few cells in the centre, cells are found individually or in groups of two or four and are surrounded by smooth and clear wall called lacunae. Surface of cartilage is formed by tough connective tissue fiber called perichondrium. There are no blood vessels hence nutrition is simply by diffusion. It is of three types: (i) Hyaline (ii) Elastic, (iii) Fibrous

(i)    Hyaline Cartilage: It is homogenous semitransparent in appearance, and elastic. It is found in cartilage of nose, larynx, trachea, etc. It also forms the articular cartilage of joints that prevents friction.

(ii)   Elastic Cartilage: It constitutes pinna of the ear and epiglottis which is composed of branched elastic fibers and show elasticity.

(iii)  Fibrous: It consists of parallel running bundles of collagen fibers. Embedded within the fibers are cartilage cells. It is found between the vertebrae as intervertebral discs and other articular surface.

Bone

Bones are relatively harder than cartilage because the matrix consists of elastin fibers, which are saturated with phosphates and carbonates of calcium and magnesium. Bones of the whole body articulate to form skeleton that provide definite shape to the body. Almost all long bones of the vertebrate bodies are hollow. Around this cavity are osteocytes that are the bone cells in concentric rings. The outer surface is composed of dead lamellae enclosed by periosteum. The osteocytes are embedded in the matrix. Short filamentous structures arise from every osteocyte and with which all osteocytes are inter connected with each other. These are called canaliculi. The central cavity is filled with bone marrow, which is of two types:

(i)    Red marrow: which produce Red Blood Cells (RBC).

(ii)   Yellow marrow: primarily stores fat but sometimes produces RBC also.

Haversian canals: These are found in mammalian bones which is system of branched tubular structures that traverse through the thick solid portion of the bones and supply nourishment to the bones.

Fluid connective tissue

This is found in fluid state and circulates through the body. There are two types of fluid connective tissue, that is:

(i) Blood                                                 (ii) Lymph

(i) Blood: Blood performs several vital functions of the body. It distributes nutrients, oxygen etc. and maintains a uniform body temperature. It is made up of fluid matrix called plasma and several types of cells termed formed elements. The plasma form 54% of the blood volume and the cells 45%. The formed elements consist of red blood cells (RBC) or erythrocytes. White Blood Cells (WBC) or leucocytes and Platelets or thromobocytes. Blood plasma is the straw-coloured fluid obtained when all formed elements are removed from blood. The ratio of plasma to formed element is held relatively constant by the homeostatic control of the body.

(ii) Lymph: Lymph is an extracellular fluid drained by lymph vessels. Lymph is yellow coloured fluid when is formed by the selective absorption of the interstitial fluid in the lymphatic capillaries. Lymph constitutes about 25% of the total weight of all mammals, the lymphatics are present nearly in all parts of the body. The movement of the lymph is brought about by breathing movements and contraction and relaxation of skeletal muscles of the body. At many points in the body where the smaller lymph vessels join the larger ones, masses of cells aggregate to form lymph nodes, which produce lymphocytes and antibodies, and filter out bacteria and foreign particles. Lymph plays a major role in body’s Defence against disease by the production of lymphocytes and antibodies, and by filtering action of the lymph nodes.

Facts about bone
1.	Bones are made up mostly of the mineral, calcium and phosphorous. There is protein in bones too, to give them some flexibility.
2.	Bones have spaces inside them filled with soft marrow. Yellow marrow which is mostly fat is found inside the middle part of long bones. There is red marrow in the ends of long bones and in ribs and same other flat bones. The red marrow is the place where the red and most white blood cells are made
3.	There are 206 bones in the body. The tiniest one is inside the ear names stapes
4.	The skull is made of 22 bones. The ceranium acts as a helmet to the brain
5.	The longest bone in the body is femur

Muscular Tissue

Muscles that are made of long muscles fibers constitute muscular tissue. It is mainly concerned with the movements of different kinds. The muscular fibers are placed tightly to form bundles. Broadly speaking there are 3 kinds of muscles in mammals:

(i)    Fast moving muscles or striated muscles (e.g. voluntary muscles striped muscles or skeletal muscles etc.)

(ii)   Slow muscles or non-straited muscles including visernal minusedes, smooth muscles, involuntary muscle etc.)

(iii)  Medium fast muscles or cardiac muscles (e.g. Heart muscles).

(i)    Striated or fast moving muscles: This muscle is chiefly found around bones and concerns with the movement of limbs, so it is also called as skeleton muscle. Each muscle consists of many elongated fine fibers that are called myofibrils, with their blunt tips. In between successive myofibril there is thin layer of cytoplasm called Sarcoplasm, there are many nuclei located near the periphery. All the myofibrils have dark and light bands alternately and are enveloped by thin membrane called sarcolemma.

(ii)   Non-striated or slow muscles: This muscle forms the visceral organs. Each muscle fiber the visceral organs. Each muscle fiber is long spindle shaped. The nucleus is situated in the centre of each fiber; these are without light and dark bands and myofibrils. The fibers are held together to form sheet of organs.

(iii)  Cardiac or medium fast muscles: This muscle is exclusively found in the heart. Vertebrate cardiac muscles are composed of branched cells. The cardiac muscle fibers are like striated one but are branched and never get fatigued. Along the length of fibers are found transverse band, that is called he Intercalated disc which is cell like structure each with single nucleus. Muscles are surrounded by plasma membrane instead of sarcolemma.

Nervous Tissue

In mammals all body activities are either directly or indirectly under the control of the nervous system. The stimuli, which the nervous system receives, are of two kinds: stimuli or sensations. Like pressure, pain, heat etc. which are caused due to changes in the external conditions, are called exteroceptive sensations. Sensations, which convey the physical state of the body, are called proprioceptive sensations. Both extero and proprioceptive sensations are brought to the brain and spinal cord through sensory nerves. Orders, which are carried out in response to these sensations, are conveyed to the appropriate organs through motor nerves.