The living world consists of a vast diversity of life forms that are interconnected through various aspects like classification, external appearance, and internal physiology.

Biology (bios–life; logos–study) is the branch of science that deals with the study of life and living organisms. Aristotle is known as the father of biology. Life is a characteristic quality that differentiates animate (living forms) from inanimate (non-living objects).

Attributes of Living Forms

1. Cellular organization: Refers to the composition and arrangement of cells in the body.
2. Growth: All living organisms grow by an increase in mass and in the number of individual cells. Multicellular organisms grow by cell division.
3. Metabolism: A series of chemical processes catalyzed by enzymes, occurring within the body of living beings.
4. Homeostasis: The process of regulating body temperature and maintaining internal environment to continue metabolic processes (e.g., thermoregulation).
5. Reproduction: The process of producing young ones by living beings, which can be sexual or asexual.
6. Adaptation: Refers to the genetic mechanisms of an organism to survive, thrive, and reproduce by constantly enhancing itself.
7. Consciousness and irritability: The ability of an organism to sense the environment (consciousness) and respond to external stimuli (irritability).

Classification of Living World

Each living being, whether plant, animal, or microorganism, is unique. This uniqueness forms the basis of diversity among living organisms.

• Biodiversity: Refers to the diverse or varied forms of living beings that differ from one another.
• Taxonomy or Systematics: The science of classification that projects the relationship between the diversity of organisms.
• Taxon: A group of any rank that is sufficiently distinct to be worthy of being assigned a definite category.

Bi nomial Nomenclature and Classification

This system was proposed by Carolus Linnaeus in 1753 in his book ‘Species Plantarum’. The name of any organism consists of two parts or epithets:

Five-Kingdom System of Classification

Proposed by Robert H. Whittaker in 1959, the Five-Kingdom System divides all organisms into the following kingdoms:

1. Kingdom Monera: Consists of prokaryotic organisms, including Archaebacteria and Eubacteria.
2. Kingdom Protista: Comprises unicellular eukaryotic organisms, including protozoans and algae.
3. Kingdom Fungi: Simple non-green plants, heterotrophic and eukaryotic organisms, including decomposers.
4. Kingdom Plantae: Multicellular eukaryotic organisms with membrane-bound cell organelles.
5. Kingdom Animalia: Eukaryotic cells with heterotrophic nutrition and a variety of organizational levels.

Classification of Kingdom Plantae

Plants, classified in Kingdom Plantae, are multicellular, photoautotrophic, and embryo-forming organisms. They play a crucial role in ecosystems by fixing solar energy through photosynthesis. Kingdom Plantae is further divided into divisions:

1. Division Algae (alga-Sea weed): Consists of non-differentiated plant bodies, typically found in water, and containing various pigments for photosynthesis.
2. Division Bryophyta (bryon-a moss, a liverwort): Small multicellular green land plants, including liverworts and mosses, with no true vascular system.
3. Division Pteridophyta (pteruis-idos-ferns): Found in shady and damp places, with a well-defined vascular system and sporophylls.
4. Division Gymnosperms (gymno-naked; sperma-seed): Primitive seed plants with naked seeds and aggregated sporophylls forming cones.
5. Division Angiosperms (angeion-case; sperma-seed): Highly evolved plants producing seeds enclosed within fruits, with reproductive organs aggregated in flowers.

Major Differences between Dicotyledons and Monocotyledons

Plant Classification Based on External Characters

Plants are classified into three categories based on their external characteristics:

• Herbs: Green and softer stems, smaller in size.
• Shrubs: Branches arise from the base of the stem, hard but not thick stems.
• Trees: Very tall with hard and thick stems (trunks), branches arise from the upper part of the stem.

Parts of the Plant

Plants consist of four main parts:

1. Root: Underground non-green part providing anchorage. Types of root systems:
   • Tap root system: Prominent tap root with smaller lateral roots.
   • Fibrous root system: Extensive network of similar-sized roots, e.g., rice, grasses.
   • Adventitious roots: Similar-sized roots arising from different places apart from the base of the stem.
2. Stem: Ascending part of the plant above the soil, bearing leaves, flowers, and other structures. Types of stem modifications:
   • Underground Modifications: Food storage and thickening, e.g., stem tuber (potato), bulb (onion), corm (gladiolus), rhizome (ginger).
   • Sub-aerial Modifications: Above-ground but not completely formed, e.g., runner (gram), stolon (mint), offset (water hyacinth), sucker (roses).
   • Aerial Modifications: Completely aerial, e.g., stem tendril (grapes), stem thorn (lemon), phylloclade (cactus), bulbils (ruscus).
3. Leaves: Contain photosynthetic pigments and perform photosynthesis. Types of venation:
   • Reticulate venation: Net-like pattern on both sides of the midrib, e.g., most dicots.
   • Parallel venation: Veins parallel to the midrib and to each other, e.g., most monocots.
4. Flowers: Reproductive parts of the plant that maintain generation. Parts of a flower:
   • Sepals: Small leaf-like structures that protect the flower.
   • Petals: Brightly colored to attract insects for pollination.
   • Stamen: Male reproductive part consisting of anther (produces pollen) and filament.
   • Carpel/Pistil: Female reproductive part consisting of stigma (sticky part), style (stalk), and ovary (contains ovules).
5. Fruits: Ripened ovary of a flower, classified into types:
   • Simple fruits: Develop from a single ovary, e.g., mango, apple.
   • Aggregate fruits: Develop from multicarpellary or polycarpellary apocarpous ovaries, e.g., raspberry, blackberry.
   • Composite fruits: Develop from a complete inflorescence, e.g., pineapple, fig.
6. Seeds: Ripened ovule containing the embryo. Types of seeds:
   • Non-endospermic (exalbuminous) seeds: Embryo consumes all endosperm, e.g., most dicots.
   • Endospermic (albuminous) seeds: Endosperm persists in the mature seed, e.g., most monocots and some dicots.

Nutrition in Plants

Nutrition refers to the mode of taking in food and utilizing it by the body. Plants have various modes of nutrition:

• Autotrophic Nutrition: Plants synthesize food from simple inorganic substances like carbon dioxide and water in the presence of sunlight through photosynthesis.
• Heterotrophic Nutrition: Some plants depend on other plants or organisms for food. Types include:
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  • Parasitic Nutrition: Plants derive nutrients from other plants (hosts), e.g., Cuscuta.
  • Insectivorous Nutrition: Plants consume insects to obtain nitrogen, e.g., Sundew, Bladderwort, Nepenthes.
  • Saprotrophic Nutrition: Plants absorb nutrients from dead and decaying matter, e.g., mushrooms, bread mold.
  • Symbiotic Nutrition: Plants form mutually beneficial relationships, e.g., lichens (fungus and alga).
  • Rhizobium: Bacteria that fix atmospheric nitrogen into soluble compounds in legume root nodules.

Mineral Nutrition in Plants

Plants absorb minerals from the soil in the form of ions and incorporate them into vital biomolecules. Essential elements for plant growth can be classified as:

• Macroelements: Required in large amounts, e.g., C, H, O, N, S, P, K, Mg, Ca.
• Microelements: Required in smaller amounts, e.g., Zn, Cu, Mn, Fe, B, Cl, Mo.

Respiration in Plants

Respiration is the process by which plants break down food materials in cells to release energy. There are two types of respiration:

• Aerobic Cell Respiration: Occurs in the presence of oxygen, where glucose is completely broken down into CO2, H2O, and energy.
• Anaerobic Cell Respiration: Occurs in the absence of oxygen, resulting in the incomplete breakdown of glucose into C2H5OH, CO2, and energy.

The basic reaction of cellular respiration is:

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

This energy is utilized by the plants to synthesize other molecules required by the cell.

Transportation in Plants

Plants transport water and minerals to the leaves through vascular bundles, which consist of xylem and phloem:

• Xylem: Transports water from roots to leaves.
• Phloem: Transports food from leaves to all parts of the plant.

Root hairs increase the surface area of roots for better absorption of water and mineral salts. A suction force due to transpiration pull helps in the upward movement of water from the soil.

Transpiration in Plants

Transpiration is the evaporation of water from plants, mainly through stomata in leaves:

• Plants have adaptations to reduce excess transpiration, such as thick waxy cuticles, modified thorn-like leaves, or even no leaves (xerophytes).

Reproduction in Plants

Reproduction in plants involves the production of offspring, which can occur via:

• Asexual Reproduction: Involves vegetative propagation where new plants develop from roots (e.g., ginger), stems (e.g., rose), or leaves (e.g., Bryophyllum).
• Sexual Reproduction: Involves the fusion of male and female gametes, produced by flowers, through processes like pollination and fertilization.

Pollination

Pollination is the transfer of pollen grains from the anther to the stigma:

• Self-pollination: Pollen is transferred to the same flower.
• Cross-pollination: Pollen is transferred to another flower of the same or different plant.
• Agents of pollination include wind, water, insects, birds, and bats.

Fertilisation and Dispersal

Fertilisation leads to the formation of seeds and dispersal ensures wider distribution:

• Fertilisation: Fusion of male and female gametes to form a zygote, which develops into an embryo.
• Dispersal: Scattering of seeds by wind, water, animals, or through explosive mechanisms to avoid competition and inhabit new areas.

Excretion in Plants

Plants eliminate waste products and excess substances in the following ways:

• Oxygen is released as a byproduct of photosynthesis.
• Excess water is released as water vapor through transpiration.
• Plants do not have specific excretory organs; waste gases like oxygen and carbon dioxide are exchanged through small pores on leaf surfaces.
• Excess salts are deposited in various plant parts, sometimes altering taste, e.g., quinine crystals in orange peel.

Classification of the Kingdom Animalia

General Characteristics

Animals are eukaryotic, multicellular, heterotrophic organisms without cell walls. They are divided into two branches: Parazoa (loosely aggregated cells) and Eumetazoa (cells arranged into tissues and organs).

Eumetazoa are further classified based on:

1. Number of germ layers in the embryo
2. Symmetry of the body
3. Origin of the mouth

Germ Layers and Symmetry

• Diploblastic: Two germ layers in the embryo
• Triploblastic: Three germ layers in the embryo
• Bilateral Symmetry: Mirror-image symmetry
• Radial Symmetry: Regular pattern around a central axis

Origin of Mouth

• Protostomia: Mouth arises from or near the blastopore
• Deuterostomia: Mouth arises anteriorly at a distance from the blastopore

Body Cavity (Coelom)

• Acoelomates: No body cavity, filled with parenchymatous tissue
• Pseudocoelomates: Body cavity not derived from mesoderm
• Coelomates: True body cavity derived from mesoderm, lined with epithelial cells

Phylum – Porifera (Sponges)

Sessile, marine animals with cellular level of body organization and radial symmetry. They have a canal system for filter feeding, perforated with numerous pores (ostia) and large-sized outlets (oscula). Skeleton made of calcareous or siliceous spicules. Reproduce asexually by budding and gemmules, and sexually by fertilization.

Phylum – Cnidaria (Coelenterata)

Aquatic, marine animals with tissue level of organization and radial symmetry. They possess nematocysts (sting cells) and show polymorphism with two main forms: polyps (sedentary) and medusae (free-swimming).

Phylum – Ctenophora (Comb Jellies)

Transparent, marine animals with biradial symmetry, triploblastic organization. They have tentacles and ciliary plates for locomotion. Colloblasts (adhesive cells) are present and no polymorphism is observed.

Phylum – Platyhelminthes (Flatworms)

Bilaterally symmetrical, flat animals with organ level of organization. They are triploblastic, acoelomate with a digestive cavity but no anus. Hermaphrodites with excretion via flame cells. Includes free-living and parasitic species.

Phylum – Aschelminthes or Nematoda (Roundworms)

Bilaterally symmetrical, unsegmented animals with pseudocoelomate body plan. They have a complete alimentary canal, lack respiratory and circulatory systems. Excretion occurs through renett cells. Includes many pathogenic species.

Phylum – Annelida (Segmented Worms)

Triploblastic, bilaterally symmetrical, elongated, segmented animals with metamerism. They have a complete alimentary canal, true coelom, and excretion via nephridia. Includes various marine and terrestrial species.

Phylum – Arthropoda (Jointed-legged Animals)

Triploblastic, bilaterally symmetrical, segmented animals with jointed appendages and an exoskeleton. They have a complete alimentary canal, open circulatory system, and excretion via Malpighian tubules. Largest phylum with diverse species including insects, crustaceans, and arachnids.

Phylum – Mollusca (Molluscs)

Triploblastic, bilaterally symmetrical, soft-bodied animals with a hard shell. They have a haemocoel body cavity, open circulatory system, and excretion via metanephridia. Includes species like snails, clams, and squids.

Phylum – Echinodermata (Echinoderms)

Triploblastic, coelomate animals with pentamerous symmetry as adults. They have a unique water vascular system for locomotion and respiration, a complete digestive system, and no excretory organs. Includes sea stars, sea urchins, and sea cucumbers.

Phylum – Chordata (Chordates)

Characterized by dorsal nerve cord, notochord, and pharyngeal gill slits. They have a segmented body with a cartilaginous endoskeleton, complete digestive system, and kidneys for excretion. Includes Urochordata (tunicates), Cephalochordata (amphioxus), and Vertebrata (vertebrates).

Subphylum – Urochordata

Marine animals with a notochord present in the larval tail, and a dorsal tubular nerve cord. Hermaphroditic with a simple body structure.

Subphylum – Cephalochordata

Marine animals with a notochord and nerve cord extending the length of the body, and a large pharynx perforated by gill slits. Sexes are separate and lack exoskeleton, head, jaws, and paired fins.

Subphylum – Vertebrata

Animals with a vertebral column replacing the notochord, and a cranium surrounding the brain. They have an endoskeleton of cartilage or bone, a ventrally situated heart, and kidneys for excretion.

Superclass Pisces (Fishes)

Fishes are exclusively aquatic, cold-blooded animals with backbone, gills, and fins. Their body is divided into head, trunk, and tail. They have a two-chambered heart and are anamniotes, lacking an extra embryonic membrane (amnion).

• Body may or may not be covered by scales.
• Examples include flying fish (Exocoetus), Labeo (rohu), shark, electric ray, and sting ray.

Superclass Tetrapoda

Class Amphibia

Amphibians are cold-blooded animals that are anamniotes. Their skin may be smooth or rough and moist due to glands. They have gills in the larval stage and use lungs, buccopharyngeal cavity, and skin for respiration. Their heart is three-chambered, and they undergo metamorphosis with iodine or salinity being essential for the transformation from tadpole to frog.

• Examples include Rana tigrina (common frog) and Hyla (tree frog).

Class Reptilia (Creeping Vertebrates)

Reptiles are cold-blooded (ectothermal) vertebrates with epidermal scales. They respire through lungs, most having a three-chambered heart, except crocodiles which have a four-chambered heart. Fertilization is internal, and they generally have teeth except for tortoises and turtles.

• Examples include Draco, Chamaleon, Python, and Naja.

Class Aves (Birds)

Birds are warm-blooded tetrapods with various flight adaptations. They have feathers covering most of their body, and forelimbs modified into wings (except in flightless birds like kiwis). Their alimentary canal has a crop and gizzard, and they have a four-chambered heart. Birds have a beak instead of jaws, exhibit sexual dimorphism, and are oviparous.

• Examples include Gallus (chicken), Passer (house sparrow), and Corvus (crow).

Class Mammalia (Mammals)

Mammals are warm-blooded (endothermic) and the most evolved animals in the animal kingdom. They have hairs and a subcutaneous fat layer for insulation, and their body is divided into head, neck, trunk, and tail. Females have milk-producing mammary glands for the nourishment of their young. Mammals have thecodont and heterodont teeth, a dicondylic skull, and two pairs of pentadactyl limbs. They respire through lungs only, excrete urea, have a four-chambered heart, and non-nucleated biconcave red blood cells. Mammals are viviparous, and parental care is highly developed.

• Examples include Macropus (kangaroo), Sorex (shrew), Panthera (lion, tiger, leopard), Rattus (rat), Macaca (monkey), and Homo (human).