Biology Notes on Classification with Assertion-Reason Questions for AIIMS

Classification of Living Organisms


Nature is formed by nonliving and living organisms. One who has life and performs biological processes and manifests to environment is called living organism.

Living organism possess certain characters like-reproduction, growth, development, reaction with environment adaptation and death. Besides these it has characteristics like metabolism, entropy, efficiency to maintain heredity. It produces new generation through reproduction. Energy is required during metabolism. Growth is a output of metabolism. Quantity increases due to growth. Tissues and organs are formed due to differentiation and organogenesis. occurs during development living organism shows efficiency of manifestations of feeling towards environment . It obtains adaptations to sustain in an environment and creates variations for adaptation. New species is created due to variation, hence biodiversity forms. It has efficiency to maintain heredity before its death. There is an aggregation of different layers in living organisms. Membranes are formed of large molecules and molecules are formed from atoms. Cell is formed by membranous organelles. Tissue is constituted by group of cells which are present in organ and organ system. Body is composed by such organ systems. Such living organism is known as species. Population is structured by group of species. Such combined population of a same habitat form a biotic community. By interaction between biotic community and environment is constituted an ecosystem. By composition of ecosystems biosphere is constituted. The study of living organisms can be done by nomenclature and identification characters. Living organisms are classified in to groups.

Meaningfully called classification which has species, genus, order, family, class, phylum and kingdom. There are certain rules and regulations. Many scientist’s have contributed in this field. There are various sources for study.
A number of taxonomical aids have been developing for identification naming and classification of organisms. Actual specimens are collected from the field and preserved in the form of herbaria and museum. Line specimen of plants and animals are found in botanical garden and Zoological park respectively. In museum there are specific methods to store plant specimens.

Plant Kingdom

Five kingdom classification system was given by Whittaker on the basis of following four criteria :
(i) Cell structure
(ii) Body Structure
(iii) Mode of nutrition – Autotrophic and Heterotrophic
(iv) Major ecological role.

The five kingdoms are Monera, Protista, Fungi, Plantae and Animalia.
The three-domain system is closely based on five-kingdom system.
The Eukarya are then divided into 4 kingdoms :
Protistsa, Fungi, plantae and anamalia.

Viroids were discovered by Diener from infectious agents which are even smaller than viruses. It consists of a very simple structure and short RNA strand viroids lack protective protein coat known as capsid. Viruses are self reproducing and obligate parasite in living cells : They Viroids
remain inactive and behave as non-living things. When they enter inside of the living cells, they are active and behave as living organisms Because of this they are intermediate between living and non-living things. They are also called as living chemical.

Algae, Fungi and Lichens are included under thallophyta. The gametophytic plant body is thalloid, without differentiation in to true root, stem and leaves. Zygote does not develop into embryo. Algae have Chlorophylls and they synthesize their one own food so it is Autotrophs, while Fungi are non chlorophylls and they do not Synthesize their own food so it own is called heterotrophs. The lichens, show symbiotic relationship between algae and fungi components.

Bryophytes are non vascular plants. After fertilization Zygote undergoes divisions to form embryo.
The life cycle of bryophytes has two distinct phases.
(1) Gametophytic phases – Haploid, main, autotrophic and gametes formative
(2) Sporophytic phase – Diploid, Subsidiary, heterotrophic, spores formative.

Pteridophytes have vascular tissues, and develop the embryo. The life cycle of pteridophytes shows alternation of generation. Gemetophytic phase is haploid, subsidiary, short lived and Gametes producing while sporophytic phase is diploid, main long lived and spores producing.

Gymnosperm is sporophytic. It is differntiated into root, stem and leaves. Ovules are naked and not enclosed by the ovary therefore recognized as a gymnosperm. The sporophytic and gametophytic phases alternate with each other to complete the life cycle. Gametophytic phase is haploid,
subsidiary, short lived and under ground and sporophytic phase is diploid, Main, long and as a whole plant.Endosperm develops before fertilization, ovules are orthotropous and true fruits are lacking because, of the absence of ovary.

In angiosperms, sporophytic plant body is in the form of herbs, shrubs, trees, climbers or lianas. Ovules are enclosed in the ovary therefore, recognized as angiosperms. Endosperm is developed after fertilization. Members of this group show double fertilization. After fertilization ovules are trans formed into seeds and ovary into fruit. The plant life cycle shows alternation of generation.

Bentham and Hooker classified the angiosperms into two classes.
(1) Dicotyledon and (2) Monocotyledon

Animal Kingdom

The basic fundamental features such as level of organisation, symmetry, cell organisation, coelom, segmentation, notochord, etc., have enabled us to broadly classify the animal kingdom. Besides the fundamental features, there are many other distinctive characters which are specific for each phyla or

Porifera includes multicellular animals which exhibit cellular level of organisation and have characteristic flagellated choanocytes. The coelenterates have tentacles and bear cnidoblasts. They are mostly aquatic, sessile or free-floating. The ctenophores are marine animals with comb plates.
The platyhelminths have flat body and exhibit bilateral symmetry. The parasitic forms show distinct suckers and hooks. Aschelrninthes are pseudocoelomates and include parasitic as well as non-parasitic roundworms.

Annelids are metamerically segmented animals with a true coelom. The arthropods are the most abundant group of animals characterised by the presence of jointed appendages. The molluscs have a soft body surrounded by an external calcareous shell. The body is covered with external skeleton made of
chitin. The echinoderms possess a spiny skin. Their most distinctive feature is the presence of water vascular system.
The hemichordates are a small group of worm-like marine animals. They have a cylind rical body with proboscis, collar and trunk.

Phylum Chordata includes animals which possess a notochord either throughout or during early embryonic life, Other common features observed in the chordates are the dorsal, hollow nerve cord and paired pharyngeal gill slits. Some of the vertebrates do not possess jaws (Agnatha) whereas most of
them possess jaws (Gnathostomata) Agnatha is represented by the class, Cyclostomata. They are the most primitive chordates and are ectoparasites on fishes. Gnathostomata has two super classes, Pisces and Tetrapoda.

Classes Chondrichthyes and Osteichthyes bear fins for locomotion and are grouped under Pisces. The Chondrichthyes are fishes with cartilaginous endoskeleton and are marine. Classes, Amphibia, Reptilia, Aves and Mammalia have two pairs of limbs and are thus grouped under Tetrapoda.

The amphibians have adapted to live both on land and water. Reptiles are characterised by the presence of dry and cornified skin. Limbs are absent in snakes. Fishes, amphibians and reptiles are pofkilothermous(cold­blooded), Aves are warm­blooded animals with feathers on their bodies and forelimbs modified into wings for flying. Hind limbs are adapted for walking, swimming, perching or clasping.
The unique features of mammals are the presence of mammary glands and hairs on the skin. They commonly exhibit viviparity.

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