Taxonomy is the branch of biology dealing with the identification, nomenclature and classification of organisms based on their characteristics. A taxon is a unit or group of organisms of any rank in classification (for example, mammals, insects, dogs). Taxonomic hierarchy is the arrangement of taxonomic categories in a definite descending order, from kingdom down to species.

Binomial nomenclature is the system of naming organisms with two words, given by Carolus Linnaeus, where the first word is the genus and the second is the species (for example, Mangifera indica for mango). Rules: (1) the generic name begins with a capital letter and the specific name with a small letter; (2) the biological name is printed in italics (or underlined separately when handwritten) and is usually in Latin.

Kingdom Monera includes bacteria, which are prokaryotic (they lack a true membrane-bound nucleus and membrane-bound organelles). They are microscopic, unicellular organisms with a cell wall (except mycoplasma). Their nutrition may be autotrophic (photosynthetic or chemosynthetic) or heterotrophic (saprophytic or parasitic). They are grouped as Archaebacteria (live in extreme habitats) and Eubacteria (true bacteria).

Kingdom Protista includes single-celled eukaryotes. They have a well-defined nucleus and membrane-bound organelles. Some have flagella or cilia for locomotion, and they may be autotrophic or heterotrophic. This kingdom forms a link between plants, animals and fungi. Examples: Chrysophytes (diatoms), dinoflagellates, Euglenoids (Euglena), slime moulds and protozoans (Amoeba, Paramecium).

Viruses are non-cellular infectious agents that are inert outside the host but reproduce inside a living host cell. A virus has a protein coat called a capsid enclosing genetic material of either DNA or RNA (never both). Viroids are the smallest infectious agents, made of free naked RNA without a protein coat (e.g. potato spindle tuber disease). Lichens are symbiotic associations between an alga (phycobiont) and a fungus (mycobiont) and are good indicators of air pollution.

Bryophytes are called the amphibians of the plant kingdom because they can live in soil but need water for sexual reproduction, as the male gametes (antherozoids) require a film of water to swim to the female sex organ. They lack true vascular tissue and true roots (they have rhizoids). Examples: Funaria (moss), Marchantia and Riccia (liverworts).

Pteridophytes are the first terrestrial plants to have true vascular tissues (xylem and phloem). The plant body is a sporophyte differentiated into true root, stem and leaves. They reproduce by spores produced in sporangia, and water is required for fertilisation. Examples: Selaginella, Equisetum and ferns such as Pteris and Dryopteris.

Diploblastic animals have cells arranged in two embryonic germ layers, an outer ectoderm and an inner endoderm (e.g. coelenterates). Triploblastic animals have three germ layers: ectoderm, mesoderm and endoderm (e.g. from flatworms upward). A coelomate animal is one that has a true body cavity (coelom) lined by mesoderm, for example annelids, arthropods and chordates.

Porifera (sponges): (1) they are primitive multicellular animals with a cellular level of organisation and body with pores (ostia) and a canal system; (2) the body is supported by spicules and they show intracellular digestion (e.g. Sycon, Spongilla). Coelenterata (Cnidaria): (1) they are diploblastic with tissue-level organisation and radial symmetry; (2) they possess stinging cells called cnidoblasts and show two body forms, polyp and medusa (e.g. Hydra, jellyfish).

A tap root system has one main primary root growing vertically downward from which lateral roots (secondary and tertiary) arise; it develops from the radicle and is characteristic of dicots (e.g. mustard, mango). A fibrous root system has a cluster of thin, thread-like roots of similar size arising from the base of the stem; the primary root is short-lived and it is characteristic of monocots (e.g. wheat, grass).

Modifications of the stem include: (1) underground stems for storage such as the tuber of potato and the rhizome of ginger; (2) stem tendrils for climbing, as in gourds and grapevine; (3) thorns for protection, as in Citrus and Bougainvillea; and (4) runners or stolons for vegetative propagation, as in grass and strawberry. These modifications help in storage, support, protection or propagation.

A typical flower has four whorls: (1) Calyx (sepals) - the outermost green whorl that protects the flower in the bud stage; (2) Corolla (petals) - usually coloured, it attracts pollinators; (3) Androecium (stamens) - the male reproductive whorl producing pollen; and (4) Gynoecium (carpels/pistil) - the female reproductive whorl containing ovules that develop into seeds after fertilisation.

Venation is the arrangement of veins and veinlets in the lamina of a leaf. In reticulate venation the veinlets form a network, typical of dicot leaves (e.g. mango); in parallel venation the veins run parallel to one another, typical of monocot leaves (e.g. grass). Phyllotaxy is the pattern of arrangement of leaves on the stem or branch; it may be alternate, opposite or whorled.

Meristematic tissue consists of actively dividing cells that are responsible for the growth of the plant. The cells are small, thin-walled, with dense cytoplasm and prominent nuclei, and little or no vacuole. Based on position the three types are: apical meristem (at the tips of root and shoot, causing increase in length), intercalary meristem (at the base of internodes or leaves) and lateral meristem (along the sides, e.g. cambium, causing increase in girth).

(1) In a dicot root the number of xylem bundles (vascular bundles) is few, usually two to four (tetrarch or less), whereas in a monocot root they are many (polyarch, often more than six). (2) A dicot root has secondary growth due to the presence of cambium, whereas a monocot root usually lacks secondary growth as cambium is absent. Also, the pith is small or absent in dicot roots but large and well developed in monocot roots.

Stomata are tiny pores present mainly in the epidermis of leaves. Each stoma is bounded by two bean-shaped (kidney-shaped) guard cells that contain chloroplasts; in monocots the guard cells are dumb-bell shaped. Functions: stomata allow the exchange of gases (carbon dioxide and oxygen) between the plant and the atmosphere and control transpiration (loss of water vapour). The opening and closing of the stoma is regulated by the turgidity of the guard cells.

The three types of muscular tissue are: (1) striated (skeletal) muscle - cylindrical, unbranched, multinucleate and voluntary, attached to bones for movement; (2) smooth (unstriated) muscle - spindle-shaped, uninucleate and involuntary, found in the walls of internal organs such as the stomach, intestine and blood vessels; and (3) cardiac muscle - branched, striated and involuntary, found only in the wall of the heart.

A neuron is the structural and functional unit of nervous tissue. It has three main parts: (1) the cell body (cyton), which contains the nucleus and cytoplasm with Nissl granules; (2) dendrites, which are short branched processes arising from the cell body that receive impulses and carry them towards the cell body; and (3) the axon, a single long process that carries impulses away from the cell body to the next neuron or effector. The axon may be covered by an insulating myelin sheath.

The cell theory was proposed by M. Schleiden (a botanist) and T. Schwann (a zoologist) in 1838-39. Its postulates are: (1) all living organisms are composed of cells and products of cells; (2) the cell is the basic structural and functional unit of life. Later Rudolf Virchow (1855) added that all cells arise from pre-existing cells (omnis cellula-e-cellula).

The endoplasmic reticulum (ER) is a network of membranous tubules and sacs in the cytoplasm. Rough ER (RER) has ribosomes attached to its surface and is mainly involved in the synthesis and transport of proteins. Smooth ER (SER) lacks ribosomes and is involved in the synthesis of lipids and steroids and in detoxification. Both help in intracellular transport of materials.