PCG 202 (HISTOLOGICAL AND ANATOMICAL FEATURES OF VEGETABLE DRUGS (1).pdf

COURSE OUTLINE: 1. General histological and anatomical features of drugs of vegetable origin. 2. Macroscopy, microscopy, basic histology, micro- morphology and anatomy of woods from Nigeria flora. 3. Herbarium Preparation 4. Introduction of products of plant primary metabolism...

COURSE OUTLINE: 1. General histological and anatomical features of drugs of vegetable origin. 2. Macroscopy, microscopy, basic histology, micro- morphology and anatomy of woods from Nigeria flora. 3. Herbarium Preparation 4. Introduction of products of plant primary metabolism (Mrs. K.O.Olufolabo) Histology is the microscopic study of plant cells and tissues through sectioning, staining and examining them under the microscope. Histological studies are used to investigate tissue characteristics and microscopic structures of the cells in forensic evaluation, diagnosis, authentication and in education. Histological examination is a series of processes which involves fixation, processing, embedding, sectioning and staining undertaken in the preparation of sample tissues using histological stains to aid in the microscopic study. Anatomical features or internal structures of crude drug are very useful in assigning the morphological groups of powdered, sectioned, broken crude drugs. In anatomy, the study of cell, tissue and their organization in specific organs like root, stem, leaf, flower, fruit, seed in plants are considered. The embryo develops three fundamental (primary) tissues: protoderm, ground, and provascular. The protoderm develops into the epidermis of all three organs (root, stem, leaf). Anatomical study of crude drugs offer important diagnostic characters for the identification of both entire and powdered crude drugs and detection of adulterants in them. The stem is an axial organ of shoot, the vegetative body of the plant that bears leaves and flowers, and they are the continuation of the vascular system that started in the roots. It has functions of support, transportation, photosynthesis, and storage. Stem has radial structure and it grows continuously. The Stem has the nodes (places where leaves are attached) and internodes, which may be long or short. Stems are different by the type of phyllotaxis. The phyllotaxis refers to the arrangement of leaves. If there is one leaf per node, it is a spiral (alternate) arrangement. Two leaves per node means opposite arrangement.  Histological and anatomical study entails the use of microscope with its accessories for drawing and measuring minute details and for examination of the crude drugs. This provide useful information in regard to correct identity and could help to differentiate one species from closely related other species. Reliable anatomical descriptions of the crude drugs provide a basis for identification and purity of medicinal drugs. The importance of histological and anatomical features of plant parts in their identification and detection of adulterants was first recognized in 1857 when Schleiden distinguished various types of arsaparilla roots on the basis of their endodermal cell characteristics. Anatomical perspective of drug plants is an integral component of Pharmacognosy, especially while proposing diagnostic protocols for establishing the botanical identity and ascertaining the quality control of crude drugs. Leaves are flattened lateral outgrowths of the stem, generally green in color and form the foliage of a plant. A typical leaf consists of two main parts, an expanded blade or lamina and a stalk or petiole. Leaves of monocot and dicot plants differ in shape, size, margin, base, apex,venation and orientation. Unlike stems or roots, leaves are determinant in growth activities; they grow and achieve a specific pattern and shape, then stop. Figure 1: (a) A typical Dicot Leaf (b) Monocot and Dicot leaves with respective plants 1. Axil: The leaf axil is where the petiole of the leaf attaches to the stem. 2. Leaf blade: The blade is the broad expanded region of the leaf. The broader the leaf, the greater the surface area exposed to sunlight which leads to a higher rate of photosynthesis. 3. Petiole: The leaf petiole is a thin short stalk that attaches the leaf blade to the stem. 4. Stipule: A leaf stipule is a leafy outgrowth on either side of the leafstalk or petiole. It exhibits no specific function. 5. Vein: The veins are the vascular tissue, made up of xylem and phloem, and are located in the spongy layer of the mesophyll. The pattern of ramification of the veins is called venation. Leaf venation may be reticulate (net-like ramification as in case of the dicot leaf) or parallel (veins run parallel from end to end through the lamina as in case of monocot leaf). 6. Midrib: The main strand of vascular tissue that runs centrally through the lamina from base to apex is called the midrib. 7. Lamina: The lamina is the flat part of the leaves, which constitute the major portion in the leaf which could be a simple or compound leaf. The lamina comprises: the leaf apex, base, margin and shape of lamina which vary from plant to plant. These variations constitute important morphological characters for identification and differentiation of leaves of different drug plants.  The leaf apex or leaf tip is the terminal point of the leaf, it is also the tip of the lamina. An apex or tip of different leaves vary greatly from plant to plant, it may be acuminate, acute, obtuse, caudate, or acuminate, cuspidate, retuse; emerginate, mucronate, cirrhose, etc. Figure 2: Types of Leaf Apex (a) Acuminate (b) Aristate (c) Acute (d) Obtuse (e) Caudate (f) Emerginate (g) Cuspidate( h) Mucronate (i) Apiculate ( j) Obcordate ( k) Retuse ( l) Truncate ( m) Cirrhose  The base of a leaf is the lower part of the lamina, where it is attached to the petiole or stem. The base of different plant vary from plant to plant. Figure 3: Type of Leaf Base (a) Attenuate (b) Auriculate (c) Clasping (d) Cordate (e) Cuneate (f) Hastate (g) Oblique (h) Peltate (i) Perfoliate (j) Round (k) Sagittate (l) Truncate (m) Rounded (n) Acute(o) Equilateral (p) Sheathing The leaf margin is the edge of the leaf lamina, it is the boundary area extending along the edge of the leaf. There are lots of different types of leaf margins that are important for plant identification and provide important diagnostic character for the identification of a leaf drug. In addition to these types, the leaf margin may vary greatly under various conditions of growth. Figure 4: Types of Leaf Margin (a) Simple Leaf is a leaf containing single lamina and auxiliary bud at its axis. Lamina may be entire or if dissected from the margin but notch is incomplete e.g., Mango (b) Compound Leaf: In a compound leaf, lamina is divided into independent leaflets, and notch is completed and reaches to the tip of the petiole e.g. , Neem. Below is the diagram of the simple and compound leaves. The leaf shape is the shape of the leaf blade or lamina. Figure 5: Different types of leaf shape  The pattern of arrangement of these lateral organs along the plant stem is called phyllotaxy or phyllotaxis, means ‘leaf arrangement’ in Greek. Figure 6: Leaf Arrangement (i) Opposite (ii) Alternate (iii) Opposite decussate (iv) whorled The surfaces of leaves provide many characteristics that are used in identification. Surfaces of a leaf may be smooth or may have some epidermal outgrowths. a) Glabrous: the leaf surface is smooth or free from hairs. (b) Pubescent: the leaf surface is hairy. (c) Glaucous: the surface is covered with a whitish powder or waxy coating. (d) Scabrous: the surface is rough or harsh to touch. (e) Puberulent: the surface is covered with very fine, down like hairs. Texture or feel of the leaf to touch varies from leaf to leaf and is sometimes very important in its identification. The texture may be: (a) Succulent: the leaf texture is fleshy, thick, soft, and juicy (b) Hyaline: the texture is thin and almost wholly transparent. (c) Chartaceous: if papery, thin, pliable, and opaque but thin. (d) Scarious: the texture is thin and dry, appearing shriveled. (e) Leathery or coriaceous :if tough, thickish, and leathery and leather like. The arrangement of the veins and veinlets and their mode of distribution in the leaf blade are called the venation. The veins consist of vascular tissues (xylem and phloem) of the leaf and are located in the spongy layer of the mesophyll. The veins connect the blade to the petiole and lead from the petiole to the stem. Venation is characteristic of the type of a leaf. Figure 7: Leaf venation (a) Pinnate (b) Parallel (c) Dichotomous The anatomy of the leaf reveals the basic structural features which allow the identification of the genus and ultimately of the species of a vegetable drug. It also allows the detection of a leaf in a powder form. A typical foliage leaf has a flattened form with two obvious surfaces (bifacial). The upper surface is called adaxial and lower surfaces is called abaxial layer of the leaf. Figure: A typical foliage leaf of a rose plant (Rosa sp.) showing (a) the adaxial surface, (b) the abaxial surface with ridges formed by the major veins Anatomically, the leaves of monocot and dicot plants consist of a protective epidermis with stomata, a parenchymatous mesophyll and vascular bundles (or veins). The epidermis which contain a upper and lower epidermis includes typical epidermal cells, stomata surrounded with guard cells (also optionally with subsidiary cells), and trichomes. Almost all epidermal cells are covered with waterproof cuticle, that is rich in lignin and waxes. The cuticle prevents water from escaping, it check transpiration. The cuticle on the upper epidermis is thicker than that of lower epidermis in dorsiventral dicot leaf and undifferentiable in isobilateral monocot leaf. The stomata (singular: stoma) are used for gas exchange, cooling and water transpiration. The stomata can be open or close, the opening and closing are controlled by two guard cells paired together on each side of the stoma. These guard cells are kidney beans shaped and have a thicker cell wall in the middle. Stomata are present in both the epidermis of monocot leaf but more often in the lower epidermis of dicot leaf except in some special cases. In grass family of monocot, a few cells in the upper epidermis are enlarged to form motor cells called bulliform cells, which help in rolling and unrolling and thereby regulate moisture loss under water stress. The mesophyll (parenchyma) consists of the palisade and spongy cells. The mesophyll is undifferentiated in case of monocot leaf, but in dicot leaf, it is distinguished into palisade mesophyll (on the adaxial) and spongy mesophyll (on the abaxial). Palisade mesophyll is located in the upper layer and is responsible for most of the photosynthesis in the leaf. The palisade mesophyll consists of elongated, cylindrical, tightly arranged cells with chloroplasts mostly along the sides. The spongy mesophyll cells are roughly rounded, loosely packed, and have multiple chloroplasts.  The spongy mesophyll encloses lots of air spaces that hold raw materials, and products of photosynthesis. The mesophyll, although typically parenchymatous may contain collenchyma or sclerenchyma, secretion ducts or latex tissue, oil or mucilage cells, or hydathodes (water pores). Cells may contain inclusions such as crystals or calcium oxalate. The vascular bundles creates the skeleton of the leaf, and they are termed as veins. The veins supply minerals and water to the photosynthetic tissue. The vascular system of leaves fall into two main classes: the reticulate venation (dicot) and parallel venation (monocot). The midrib bundle may be poorly or markedly differentiated. In leaves with a well differentiated midrib, the palisade tissue is usually interrupted in the mid rib region and collenchyma frequently occurs above and below the midrib bundle. Vascular bundles in dicot leaves are open and usually collateral, less commonly bicollateral. They are composed of xylem and phloem. Xylem is present toward the upper epidermis, while the phloem toward the lower epidermis. Vascular bundles are surrounded by a compact layer of parenchymatous cells called bundle sheath or border parenchyma. Xylem consists of vessels and xylem parenchyma. Tracheids and xylem fibers are absent.. Phloem consists of sieve tubes, companion cells, and phloem parenchyma. Vascular bundles in monocot leaves are closed bundle, the bundle sheath of the midrib vein is connected to the upper and lower epidermal layers by sclerenchyma cells representing bundle sheath extensions or hypodermal sclerenchyma. In most Xeromorphic plant (plants adapted to dry habitat), thick cuticle with dense covering of trichomes is present. It also has a large, thin walled bulliform cells occur in the adaxial epidermis. These cells preferentially lose water and so contribute to the rolling of the leaf. The stomata often occur in grooves, or pits. Stomata are usually absent from the epidermis, but overlies the hypodermal sclerenchyma which often occurs in extensive tracts at the margins of the leaves or associated with large veins. Stomata and subsidiary cells occasionally occur in clusters. Aquatic plants have hydromorphic leaves. Their leaves may be floating or submerged. In aquatic plants with floating leaves, stomata is usually confined to the air-exposed adaxial surface, a palisade parenchyma underlies the adaxial epidermis while large, air-filled aerenchyma tissue occupies the bottom portion of the leaf. Submerged leaves has thin cuticle, and the stomata are generally absent. The morphological and anatomical features of the leaf assist in the physiological functions of it. Anatomical structures of a isobilateral (monocot, e.g., maize) leaf and b dorsiventral (dicot, e.g., sunflower) leaf ANATOMICAL DIFFERENCE BETWEEN THE MONOCOT AND DICOT LEAF INTERNAL STRUCTURE DICOT MONOCOT 1. Embryo Presence of two Presence of one cotyledons cotyledons in the embryo in the embryo 2. Leaf Venation Leaf vein are reticulate Leaf vein are parallel 3. Types of Leaves Dorsiventral Isolateral 4. Stomata in leaves Hypostomatous (that is, Amphostomatous (that they have stomata only is, the leaves have on one surface of their stomata on both the leaves) upper and lower surface 5. Bulliform cells/motor Do not have bulliform Have bulliform cells on cells cells their leaves 6. Flowers Petals in multiples of Petals in multiples of four or five three The stem is an axial organ of shoot, the vegetative body of the plant that bears leaves and flowers, and they are the continuation of the vascular system that started in the roots. It has functions of support, transportation, photosynthesis, and storage. Stem has radial structure and it grows continuously. The Stem has the nodes (places where leaves are attached) and internodes, which may be long or short. Stems are different by the type of phyllotaxis. The phyllotaxis refers to the arrangement of leaves. If there is one leaf per node, it is a spiral (alternate) arrangement. Two leaves per node means opposite arrangement. The stem is composed of three tissues: dermal, ground, and vascular based on their developmental origins and their final function. Dermal tissues originate at the shoot apex, they are derived from the protoderm, and function largely in protection. Protoderm cells differentiate into epidermal cells. The ground meristem produces the ground tissues of cortex, pith, and conjunctive tissues that provide bulk to the stem as well as possible organic storage. The procambium forms the vascular bundles of the xylem and phloem. It occur between the cortex and the pith. The middle layer can be entirely spent or will make cambium for the secondary thickening. At times, the layers of the outside of the procambium can form a pericycle. Sometimes the innermost layer of the cortex can form an endodermis (endoderm) and outermost layer makes the exodermis (exoderm). Another frequent variant is the development of collenchyma in the cortex adjacent to epidermis.  Monocot and dicot stems differ slightly in their arrangement of dermal tissue, ground tissue, and vascular tissue. Both have an outer epidermis and inner vascular bundles.  The circular arrangement of the dicot vascular bundles divides the ground tissue into two zones. The region between the epidermis and the vasculature is called the cortex While the ground tissue in the center of the stem is termed pith (or medulla).  The vascular system consists of separate vascular bundles that typically form a peripheral cylinder in dicot and gymnosperms but are scattered in monocot. In dicot, a parenchymatous pith is usually present centrally. In the majority of monocots, the bundle occur throughout the ground tissue. Sclerenchyma fibres are often present in the ground tissue. The parenchyma may become lignified. Collenchyma frequently occur just beneath the epidemis. Vascular bundle in stem are commonly collateral with phloem lying nearest to the epidermis and the xylem situated internally and on the same axis. Bicollateral bundle, in which the phloem lies both externally and internally to the xylem. This occur in Family Curcubitaceae, Solanaceae In many monocot stem e.g., Aloe spp, the bundles are amphivasal with a central strand of phloem surrounded by the xylem. Amphicribal bundle, in which xylem is surrounded by phloem, this occur in ferns and few angiosperms. In great majority of dicot stem, a cambial layer is located between the xylem and phloem but in monocot, this is absent. In dicot, the metaxylem vessels are frequently arranged in a radial files separated by parenchyma or sclenrenchyma. Pericycle fibres often develop in the in the outer procambium. In monocot, the relatively few vessels are usually wider and parenchyma or sclerenchyma occurs between them. Anatomy of the primary stem Anatomy of the secondary stem Stem structure of dicotyledons (transverse section). A, primary structure showing seven vascular bundles; B, development of a complete cambial ring by formation of the interfascicular cambium; C, beginning of a secondary growth; D, stem after a number of seasons of growth, outer cork now present. E–H, types of vascular bundle: E, collateral; F, bicollateral; G, amphivasal; H, amphicribal. c, Cambium; c1, fascicular cambium; c2, interfascicular cambium; ck, cork; ct, cortex; en, endodermis; ep, epidermis; g.r, growth ring; pd, phelloderm; p.f, pericyclic fibres; pg, phellogen; pg1, developing phellogen; pi, pith; r, rays; r1, primary medullary ray; sc, sclerenchyma; xy, xylem; xy1, primary xylem; 1, phloem; 1a, protophloem; 2, fascicular cambium; 3, xylem; 3a, protoxylem. Secondary growth takes place in woody perennial plants (trees and shrubs) and normally result in the formation of secondary xylem called wood, and of an external cork layer known as bark. Wood is the multiyear accumulation of xylem growth, it functions to provide support to the plant and acts as a conduit for the transport of water from the soil to the leaves. Wood represents the central harder part of all shrubs and trees inside the secondary cambium ring. Wood consists of mainly the secondary xylem tissues - tracheids, vessels, wood fibers, wood parenchyma, and medullary ray parenchyma.  Tracheids form the great bulk of the woods of the Conifers, e.g., Pinus andEphedra. Vessels are characteristic of the Angiosperm woods and are absent in Conifers. Tracheids and vessels occur in various sizes with characteristic secondary thickenings of walls in various groups of wood. Wood parenchyma occurs in association with the vessels in most woods, whereas the medullary ray parenchyma transverses the wood tissue forming the medullary rays in a radiating pattern.  Fibers constitute the major bulk of the wood and determine the texture of the wood. In transverse section wood usually show annual growth ring, each of which represent the seasonal growth. In some tropical species, the annual rings are not well marked, owing to the seasonal interruption in growth due to variation in temperature or rainfall. Therefore, tropical hardwood species such as Eucalyptus and Mahogany have less prominent annual growth rings. There are Quassia species which has irregular false annual rings e.g rings formed by alternating zones of wood parenchyma and fibres. In Rhubarb species, the width and height of medullary rays are of diagnostic importance. The grain of wood primarily results from the arrangement of the annual rings and medullary rays, but is modified by wavy course of the wood elements which causes the wood to split irregularly. Irregular splitting is largely dependent on the number of lateral branches which cause knots in the wood. It should be evident that not all woods are the same. Of course, the taxonomic nature of the species has much to do with the overall internal structure of wood, but other factors such as environmental conditions (e.g., temperature, humidity, and elevation) along with age also play key roles. Tissue component of wood show specific features which are of great significance in wood identification. Some of the characteristics are: Anatomy of a tree trunk showing plane of sections and tissue arrangements in wood and bark in transverse, radial, and tangential surfaces Morphology and Macroscopy The wood is moderately hard, light coloured, diffuse porous. It contains tannin cells and resin canal, also the sapwood and heartwood is clearly demarcated. Anatomy and Microscopy (a) Transverse Section of Stem 1. The cork, cork cambium and secondary cortex form the periderm. One side of the cork cells consists of sclerotic walls. 2. Tannin cells, crystals, stone cells and resin canals are present in the cortex as seen in figure A,B. 3. The peripheral side of the each resin canal remains enclosed by an arc-shaped strand of fibres which represent pericycle. 4. Tannin sacs and fibres containing crystals are present in the region of secondary phloem 5. A continuous cylinder of secondary xylem is traversed by medullary rays. 6. Medullary rays are narrow, closely spaced, and unseriate or sometimes biseriate. 7. Vessels are angular, large, thin-walled, numerous and uniformly distributed throughout the wood. They make the wood diffuse porous. 8. Xylem parenchyma is large and paratracheal, i.e., found round the vessels. 9. Parenchymatous pith contains secretory ducts covered with epithelial cells. 10. Growth rings are clearly observed, and usually they are limited by xylem parenchyma. (b) R.L.S. Wood: 1. Large and broad vessels with simple or sometimes scalariform perforation plates are present. Living cells of parenchyma remain always associated with vessels as seen in figure C. 2. Medullary rays consist of two types of cells, and are seen at right angle to the long axis of xylem elements in R.L.S. 3. Fibres contain simple pits and septa. 4. Tyloses are inconspicuous and scanty. (c) T.L.S. Wood: 1.Large and distinctly broad vessels with simple or scalariform perforation plates are also seen. Parenchyma cells also remain associated with the vessels as seen in Figure D. 2. The medullary rays are cut transversely. They are mostly uniseriate and only rarely biseriate. 3. Fibres contain simple pits and septa. 4. Tyloses are inconspicuous and scanty  Wood of Sagwan, a high-quality, timber-yielding plant, is ring-porous. It emits a leather-like odour, and its all the medullary rays are of one type. It is used for wrapping and preservation of food items. (a) Transverse Section of Stem: 1. The stem is circular is outline (Fig. 142A, B) and remains surrounded by a few-layered zone of cork followed by cork cambium and secondary cortex. The endodermis and pericycle are not clearly demarcated. 2. During secondary growth, vascular cambium cuts secondary phloem towards outer side and secondary xylem towards inner side. 3. Wood or secondary xylem is formed by tracheids, vessels, xylem fibres and xylem parenchyma. 4. Due to the presence of vessels, the wood is said to be porous. Vessels present in the early-wood are very large and form conspicuous rings in this wood while those present in the latewood are small or only moderately large. The wood with these characteristics is called ring porous. 5. Distinct annual rings or growth rings (Fig. 142A.B) are seen in the wood. A gradual transition from early-wood to latewood is also visible. 6. Around the vessels is present a clear and thin sheath of wood parenchyma cells. 7. Medullary rays are 1-3 cells wide and are usually not very prominent. 8. Patches of primary xylem are present near the centrally located parenchymatous pith. (b) R.L.S. Wood: 1. Vessels are quite broad and long (Fig. 142C) and their end walls are simple and perforated. 2. Parenchyma cells remain associated with vessels. 3. Usually tyloses and sometimes yellowish deposits are clearly seen in the vessels. 4. Medullary rays are broad, scanty and hetero- cellular, and are seen at right angle to the long axis of xylem elements in R.L.S. 5. Xylem fibres are septate and inter-vascular pittings are fine and alternate. (c) T.L.S. Wood: 1. Tracheids, vessels, fibres and parenchyma are cut longitudinally in T.L.S. (Fig. 142 D). 2. Vessels are broad, connected end to end with perforated end walls, and the perforations are simple. 3. Tyloses and sometimes yellowish deposits are seen in the vessels. 4. Medullary rays are cut across in T.L.S. They are 1-3 cells wide and several cells long. Wood of Eucalyptus (Fig. 140) is diffuse porous with solitary vessels and biseriate medullary rays and vasicentric tracheids. It exhibits following details: (a) Transverse Section of Stem: 1. It is circular in outline and remains surrounded by outer few layers of cork. Usually, two types of cells constitute the cork. Some are thick-walled, radially flattened and filled with resinuous substances, while other types of cork cells are thin-walled and hollow lacking resinuous substances. 2. Inner to the cork is present cork cambium followed by secondary cortex. Some scattered stone cells are present in the secondary cortex which is parenchymatous. 3. Some sclerenchyma cells represent the discontinuous ring of pericycle. 4. Narrow and distinct medullary rays (Fig. 140 A, B) traverse through the continuous cylinders of secondary phloem and secondary xylem. Medullary rays connect the phloem to the pith. 5. Tracheids, vessels, xylem parenchyma and fibers constitute the secondary xylem. 6. Vessels are distributed uniformly in the wood, and the wood is thus diffuse porous. Sometimes, the vessels show oblique or radial pattern. 7. Vasicentric tracheids (short, thin and irregular- shaped) are also seen around the vessels. 8. Xylem parenchyma is present close to the vessels, i.e., paratracheal 9. Usually tyloses but sonietimes solid gummy deposits are seen in the vessels. 10. Clear annual rings are also present. (b) R.L.S. Wood: 1. In radial longitudinal section (Fig. 140C), the vessels, tracheids, medullary rays and parenchyma cells are visible. 2. Vessels are very long (sometimes 2-5 metres), broad and usually solitary, and their end walls are simple and perforated. Simple pits and reticulate thickenings are also seen in the vessels. Herbarium is a storehouse of plant specimens which are collected, dried and mounted on handmade paper sheets. They are arranged in plant families and kept in pigeon holes of steel or wooden cupboards and maintained carefully for current and future studies. Herbarium is a reference material for naming, identification and classification of plants. The herbaria are indexed with unique codes. Each herbarium is assigned an official code that is used as a standard reference for citation. The Voucher herbarium specimen is pressed plant sample deposited for future reference and it will be verify the identity of the specific plant used in a study. The vouchers are crucial in authenticating the taxonomy of an organism, as a tool for identifying localities of the taxon, and for additional taxonomic, genetic, ecological, and/or environmental research  Metabolism can be defined as the sum of all the biochemical reactions carried out by an organism. Plant metabolism is the set of life-sustaining chemical transformations within the plant cell. Metabolites are the intermediates and products of metabolism and are usually restricted to small molecules. Metabolite are also chemical compounds which take part in the process of metabolism. A plant cell produces two types of metabolites: (1) Primary metabolites 2) Secondary metabolites  Figure 1: Primary metabolites Primary metabolism comprises all metabolic pathways that are essential to the plant's survival, generating compounds (metabolites) that are directly involved in the growth and development of the organism. The role of primary plant metabolites in basic life functions are cell division and growth, respiration, storage and reproduction. They include the components of processes such as glycolysis, the Krebs or citric acid cycle, photosynthesis and associated pathways. Primary metabolites include small molecules such as sugars, amino acids, tricarboxylic acids, or Krebs cycle intermediates, proteins, nucleic acids and polysaccharides which are essential for plant growth, development, reproduction, stress adaptation, and defense. Figure 2: Examples of Primary metabolites Biological Significance of Carbohydrate a) Saccharides play a role as structural elements and cell wall polysaccharides, e.g. cellulose b) In energy storage, e.g. starch c) As constituents of various metabolites, e.g. nucleic acid d) In protecting tissues against dehydration Biological Significance of Proteins a) Proteins can be part of a structural element of a cell b) Enzymes and many receptors are also proteins Biological Significance of Lipids a) The main biological function of lipids is energy storage b) As structural components of cell membranes c) As important signaling molecules Biological Significance of Organic Acids Some of the organic acids play a role in the kreb cycle. These are citric, malic, succinicc, fumaric, oxaloacetic, ketoglutaric, and pyruvic acid. This cycle plays a role in synthesis within the plant cells, while it is mainly an energy source in animals. Figure 3: Roles of some plant primary metabolites  Secondary metabolism produces a large number of organic compounds that are not essential to the functioning of the plant (growth, development and eproduction) but are required for the plant to survive in its environment. Secondary metabolites are compounds biosynthetically derived from primary metabolites.  Secondary plant metabolites are useful for defence purposes (against insects of fungi), they give plants characteristics colour,they also attract pollinators. Secondary metabolites have shown to possess various biological and pharmacological effects (antibiotic, antifungal, antiviral, anti-inflammatory, anti-cancr etc.) which provide the scientific base for the use of plants in the traditional medicine. Some primary metabolites are precursors of secondary metabolites. Secondary metabolites have shown to possess various biological and pharmacological effects(antibiotic, antifungal, antiviral, anti-inflammatory, anti-cancr etc.) which provide the scientific base for the use of plants in the traditional medicine. Secondary metabolites are used commercially as biologically active compounds as pharmaceuticals, flavors, fragrances, and pesticides. Examples of secondary metabolites are alkaloids, phenolics, tannins, flavonoid, anthraquinones, essential oils, steroids etc. There are four different pathway used by plants to produce secondary metabolites and each is responsible for the synthesis of different classes of compounds. 1.) Shikimic acid pathway: simple phenolics glycosides, lignans, coumarins and napthoquinones 2.) Acetate-malonate pathway: anthraquinone, flavonoids, tannins 3.) Mevalonate pathway: tannins 4.) Amino acid pathway: alkaloids