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Biology III

Being Alive-What does it mean

     Living organisms, like any non-living matter, are composed of some basic elements. The same physical laws such as gravitation, magnetism, thermodynamics etc., also bind living organisms. Yet, living organisms differ from non-living in fundamental characteristics such as cellular organisation, nutrition, metabolism, growth and reproduction. It is these characteristics that make 'life'.

Nature and Scope of Biology

     Biology is a branch of science, which deals with the study of life. It is concerned with the study of living organisms, which are immeasurably diverse and complex than the non-living matter. Hence, biology is also described as life science or natural science.

The Realm of Science

     Science is the hallmark of progress in the realm of naturally occurring matter and energy. It is the way one has to get at the truth or at least, as close to the truth as possible.

Origin of Life Index

      Philosophers and scientists from time immemorial have attempted to explain how and where life originated on this planet earth. Today, earth is the only planet where life exists. However, when the earth was formed about 4.8 billion years ago, there was absolutely no sign of life on earth. The first form of life is said to have appeared about 3.8 billion years ago.

Organic Evolution

     Chemical evolution resulted in the formation of life on earth about 3.8 billion years ago. The primitive cells that resulted from this process gradually gave rise to primitive forms of organisms such as bacteria, algae, fungi and protozoan. These organisms in turn, gave rise to a variety of forms of life that exist today on Earth.

Classification of Living Organisms

     More than a million species of living organisms have been discovered and described so far and a large number of them are yet to be discovered. Scientists involved in this task, called taxonomists, estimate that there may be around 30 million species of living organisms of which the known number of species forms a very small percentage.

Modern Taxonomy

     The modern classification is simply an extension of the system of classification proposed by Carolus Linnaeus. Linnaeus introduced four categories namely class, order, genus and species in each of the two kingdoms proposed by him - the plant kingdom and the animal kingdom.

Kingdoms of the Living World

     The present system of classification, recognises the living organisms under five kingdoms.

Angiosperm Classification

     There have been several attempts to classify angiosperms, the flowering plants. One of the earliest attempts in this direction was that of Carolus Linnaeus. He attempted a classification of angiosperms based on the characteristics related to flowers. Subsequent taxonomists also have found that floral characteristics provide the main basis for angiosperm classification.

Detailed study of Animal Kingdom

     The animal kingdom includes eukaryotic multicellular organisms which exhibit heterotrophic nutrition. They are commonly known as animals and differ characteristically from the other major groups of plants, in their capacity to be able to move.

Plant and Animal Study

     Plants and animals occupy diverse habitats. It becomes practically impossible for a biologist to study each and every plant or animal species in its natural habitat. Hence, over the years attempts have been made to preserve the plant species or animal species in a simulated natural condition, by maintaining the plant or animal live or by preserving them in some form. Such attempts have helped biologists in making a detailed study of the given species. Moreover, such attempts also help in educating students and the general public in developing awareness about plant and animal life and their importance.

Cell As A Unit Of Life

     All living organisms are composed of structural and functional units called cells. The body of a living organism may be composed of a single cell - as in unicellular organisms or many cells - as in multicellular organisms. The number of cells in the body of multicellular organisms is correlated with the size and complexity of their body.

Micro Molecules of Cell Unit

     At the molecular level all the different types of cells in living organisms are composed of certain chemical substances. The structure and function of the cell and its components are dependent on the molecular structure, arrangement and properties of these chemical constituents. The molecules found in the cells are called biological molecules or biomolecules. The collection of various types of molecules in a cell is known as cellular pool. It consists of water, inorganic substances and organic substances.

Macro Molecules of Cell Unit

     The living cells contain specific macromolecules with a high molecular weight and a poor solubility. Generally in a cell, macromolecules are represented by polysaccharides, proteins, nucleic acids and enzymes. These compounds are formed by polymerisation of micromolecules such as sugars, amino acids and nucleotides.


     Every cell, whether it is prokaryotic or eukaryotic, is enclosed in a thin covering called cell membrane. It is also called as plasma membrane or plasma lemma. The cell membrane can only be seen through the electron microscope since its thickness is well below the resolving power of human eye.

Cell Reproduction

     One of the main ideas in the modified cell principle is that new cells arise from pre-existing cells (Omnis cellula e cellula). Every new cell formed must inherit not only the nucleus which contains the genetic information, but also should inherit the cell organelles that are required to express this genetic information. This process occurs by division of the pre-existing cell and is called cell reproduction or cell division. In spite of the enormous diversity in organisms, the mode of cell division is essentially similar indicating the Unity of Life.


     Genetic engineering can be described as an in vitro manipulation of genes. It refers to artificial synthesis, modification, removal, addition and repair of the genetic material (DNA) to alter the genotype at will. It has evoked great interest because it may enable the geneticists in the near future to correct the disease-causing defective genes for the improvement of human race, and may be to even create life.


     Chromosomes (chroma = colour, soma = body) are tiny thread-like structures found in the nucleus of a cell. In a non-dividing cell they appear as a chromatin network while during cell division they become condensed to form short and thick chromosomes. Chromosomes are unique cell structures which are capable of replication. They store and transmit the coded information which is responsible for all the life processes of an organism. Hence, chromosomes are commonly described as carriers of heredity. The term "chromosome" was coined by Waldeyer in 1888 since these structures easily take up dye stains.

Developoment and Differentiation

     Differentiation is the process by which the unspecialised embryonic cells change in structure and function during development and growth of an organism to form specialised cell types, tissues, and organs, distinct from one another. Differentiation makes cells of the same genotype to assume different phenotypes.

Gene Expression

     The important characteristic of the gene is to store and express the genetic information that will contribute towards the phenotype, and will be passed on to successive generation.

Gene Regulation

     Genes do not operate to produce enzymes all the time. Each gene is normally more or less repressed (inactive); but, when a particular enzyme is needed, the relevant gene becomes active to bring about the production of that enzyme. A totally active gene can produce a very large amount of enzyme. If the synthesis of all the enzymes is fully accelerated in a cell, it would enter into a metabolic chaos.

Heredity and Variation

     All living organisms reproduce. Reproduction results in the formation of offspring of the same kind. A pea plant produces only pea plants each time it reproduces. A rat produces only rats. Humans produce only humans.

Human Genetics

     The branch of genetics, which deals with the inheritance of characters in human beings, is called human genetics. Eugenics is a branch of human genetics which is concerned with the use of knowledge of genetics for the welfare of the human race.

Nucleic Acids Index

     Nucleic acids are giant macromolecules having a complex structure and very high molecular weight. They are polymers of nucleotides and hence, described as polynucleotides.

Angiosperm Families

     In the description of Angiosperms, family is the most useful taxonomic unit. Ranks higher than family are not of much practical significance and are used mostly in classification systems and academic discussions. The taxonomic category genus also does not get much importance as plants are scientifically described in binomial nomenclature. Use of only the generic name isolated from the specific name may lead to confusion. The genus Solanum includes several species of which Solanum tuberosum alone represents the potato plant.

Angiosperm Morphology

     Angiosperms are a group of flowering plants that represent a major community in the plant kingdom. Angiosperms occur in all typical habits and constitute a very useful group of plants for animals and human beings. They include about 2,50,000 species distributed all over the world. They are the most highly evolved group of plants and appeared on Earth about 130 million years ago.

Animal Histology

     A tissue is a group of cells having a common origin, similar structure and performing a common function. Tissues are characteristic of advanced multicellular animals, commonly called as eumetazoans. They are true multicellular animals where the body cells aggregate to form tissues, organs and even organ systems.

Animal Morphology

     Animals can be broadly distinguished into two major groups the nonchordates (or invertebrates) and the chordates.Invertebrates account for more than 95% of the animal species that have been identified. These are animals which do not have a vertebral column or a back bone.

Plant Histology

     A tissue is an aggregation of cells that have a common origin and structure, and perform similar functions. Tissues are meant for meeting the physical and physiological needs of the plant body.

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