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Agricultural production systems depend on natural resources viz. land, water, biodiversity, forests, pastures and wildlife. Farm activities can also have major impacts on the quality and availability of these resources well beyond the boundaries of the production system. Although natural resources are critical to agricultural production, farm households also frequently depend on them to meet other needs, such as fuel, construction materials and supplemental foods. Thus rural livelihoods are intricately linked to the condition of natural resources. Over the last 40 years as food production has doubled, agricultural production systems have expanded, with significant impacts on the natural resource base:

There appears a general consensus that agriculture sector in India requires to be revitalized to enable it to respond effectively to the multitude of challenges that it now faces, and include assuring food security and environmental sustainability against the background of declining natural resources, changing climate and needs to become increasingly competitive.

Sustainable agricultural practices cover a whole range of improving sustainability: raising soil fertility, improving water storage capacity, increasing water quality, diversification, raising people’s capability to cope with risks and withstand natural calamities, reducing energy consumption, minimizing risk, and so on.

It is impossible to achieve sustainable development without applying sustainable agriculture on a large scale. The relationship between agricultural production and eradication of extreme poverty and hunger is very strong: agriculture is the very basis for food security. Together with fisheries, it provides practically all of the world’s food.

India is on the move, but in part.  
For, the farm sector is lagging behind.
It may soon become a drag on the economy.
Hence, farm development requires renewed efforts,
Farm youth must get critical attention.
Matter brooks no delay.

Farming is still the only source of human subsistence. No alternative is in sight. This ancient family occupation has survived over the ages through a process of transfer from father to son, generation to generation. But, in recent times, with an unprecedented externalization of farming, and under the pervasive influence of Liberalisation, Privatisation and Globalisation, this process of transfer of occupational knowledge within the community is seriously dislocated. Also, the earlier institutional arrangements for formal education in farming like district agricultural schools, gramsevaks training units and farmers training centres have now disappeared. In the meantime, even as more rural youth are becoming literate, formal education system is seen to have shed all aspects of agricultural education. Thus, the rural drop-outs are missing both formal and informal orientation in farming since a comprehension of ‘modern’ farming is beyond the middle level farmers. As such, farm youth today are neither here nor there, with regard to farming.

The word cell was coined by Robert Hooke with the help of compound microscope. Cell is the basic unit of life. A plant cell has three distinct components a). Cell wall b). Protoplasm and c). Vacuole. Cell wall  and  vacuole are considered as non-living substances. The protoplasm which is living is  composed of 1. Cytoplasm and 2. Nucleus. The cytoplasm contains several organelles such as mitochondria, chloroplast, ribosomes, endoplasmic reticulum, golgi complex, lysosomes, plastids etc.

Before the invention of N fertilizer, mineralization of organic N ( present in dead plants and animals or manure) was the main method used to obtain ammonium and nitrate nitrogen. Mineralization is conducted by soil microbes resulting in ammonium ion. Most Nitrogen is absorbed by plants in the form of Nitrate (NO-₃). Ammonium is converted to Nitrate with the help of  soil bacteria. Nitrogen is a very mobile element and is easily lost from the soil. Ammonium (NH₄+) can be converted to Ammonia (NH₃). The gaseous ammonia can be lost to the atmosphere.  Nitrate can be lost through denitrification. Nitrate nitrogen is reduced to nitrous oxide (N₂O), elemental nitrogen (N₂) and nitric oxide (NO). All are volatile. Denitrification occurs under conditions of high water availability and temperatures higher than 10^ºC.

Plant hormones (phytohormones) are chemicals that regulate plant growth and are termed as ‘plant growth substances’. Plant hormones are signal molecules produced within the plant, and are present in extremely low concentrations. Hormones regulate cellular processes in targeted cells. Hormones also determine the formation of flowers, stems, leaves, the shedding of leaves, and the development and ripening of fruit. Every cell is capable of producing hormones in plants. They shape the plant, affecting seed growth, time of flowering, the sex of flowers, senescence of leaves, and fruits. Plant hormones affect gene expression and transcription levels, cellular division, and growth.They direct which tissues grow upward and which grow downwards. Hormones are vital to plant growth. Hence, they are also known as growth factors or growth regulators.

Vitamins are essential organic nutrients required by an organism in small quantities. They are essential since they cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. The term vitamin does not include other essential nutrients like dietary minerals, essential fatty acids, or essential amino acids which are needed in larger quantities than vitamins. The term vitamin is derived from “vitamine,” coined in 1912 by the Polish biochemist Funk. The name is from vital and amine, meaning amine of life. Thirteen vitamins are generally recognized presently. Vitamins are used in small quantities by an organism to keep it in a healthy condition.

Vitamins are given letters of alphabet (A,B, C, D, etc.) in the order in which they have been discovered. The letters do not bear any relation to their structures or their importance. Vitamins are classified by their biological and chemical activity and not by their structure. Thus, each “vitamin” refers to a number of vitamer compounds that all show the biological activity associated with a particular vitamin. Vitamin A includes the compounds retinal, retinol, and four known carotenoids. Vitamers by definition are convertible to the active form of the vitamin in the body, and are sometimes inter-convertible to one another, as well.

Alkaloids are naturally occurring substances of plant origin that are alkaline in reaction. They are optically active, contain nitrogen as part of a heterocyclic ring and have varying degrees of psychological responses in man and animals. They exhibit significant pharmacological activity. Alkaloids are mostly basic in character but also includes some related compounds with neutral and even weakly acidic properties. Presently more than 2000 alkaloids are known.

Nomenclature of alkaloids
Alkaloids are named by adding the suffix ‘ine’ to various distinguishable features such as sources, physiological action in animals, their properties etc. Thus, 1. The name of the genera of the plants containing them Ex. Atropine (Atropa), nicotine (Nicotiana) etc. 2. The names of the species of the plants producing them Ex. Cocaine (Erythroxolon coca). 3. The common name of the organism producing them Ex. Ergotamine (ergot fungus). 4. The specific physiological activity observed in animals Ex. Emetine (emetic – causing vomiting), narcotine (narcotic-alters mind).

Sterols are derivatives of a complex ring system called cyclopentano perhydro phenanthrene. The term sterol means solid alcohol. Sterols, though alcohols, are solids with a M.P. of 100-120^ºC. The 27 carbon atoms impart hydrocarbon character, namely, solubility in non-polar solvents. Free sterols in plants are mono hydroxy  secondary alcohols varying in the constitution of methyl or ethyl group and in double bonds. Cholesterol, ergosterol, bile acids, sex harmones, cortical harmones and D-vitamins are some of the sterol derivatives.
   
The most widely distributed sterols in higher plants are the C₂₉ sterols, stigmasterol, β sitosterol and g sitosterol. Together, they are called steroids. In plants, steroids are present as glycosides. Cholesterol was first isolated from gall stones. The word cholesterol means “solid alcohol from bile”. Bile is a fluid which is formed and stored in the gall bladder. It aids in the digestion of fats in the digestive tract. Sterols form an important group among the steroids. Unsaturated steroids with most of the skeleton of cholestane containing a 3β-hydroxyl group and an aliphatic side chain of 8 or more carbon atoms attached to position 17 form the group of sterols.

Plant pigments impart colour to the different components like leaves, flowers, fruits etc. They in turn are responsible for the colour of raw food, we eat. Pigments like chlorophyll, anthocyanins, flavonoids, tannins, xanthones etc. are the different pigments that are present in plants. The primary function of pigments in plants is photosynthesis, which uses the green pigment chlorophyll along with several red and yellow pigments that help to capture as much light energy as possible. Other functions of pigments in plants include attracting insects to flowers to encourage pollination.

Phenol is normally a benzene ring containing an –OH group, and hence an aromatic alcohol. The term phenolic refers to similar aromatic compounds with one or more -OH groups. Phenolics constitute one of the most widespread classes of secondary metabolites. Commonly talked about class of compounds like anthocyanins, flavonoids, lignins, tannins, coumarins and chalcones are all phenolic substances. Phenolics are water insoluble substances. However, the water solubility increases with the increase in the number of -OH groups. Phenolics absorb electromagnetic radiation strongly. Many of the phenolics absorb in the visible part of the spectrum (Ex. anthocyanins) and are coloured. Phenolics are produced in plants by 1. Shikimate pathway through phenylalanine (C6.C3)  and 2. Acetate/ malonate pathway.

Primary metabolism in a plant comprises all metabolic pathways that are essential to the plant’s survival. Primary metabolites, namely, carbohydrates, lipids and proteins are compounds that are directly involved in the growth and development of a plant whereas the secondary metabolites are compounds produced in other metabolic pathways that, although important, are not essential to the functioning of the plant. However, secondary plant metabolites are useful in the long term, often for defense purposes, and give plants characteristics such as color.
 
Secondary plant metabolites are used in signalling and regulation of primary metabolic pathways. Plant hormones, which are secondary metabolites, are often used to regulate the metabolic activity within cells. Secondary plant metabolites help the plant maintain an intricate balance with the environment, often adapting to match the environmental needs. Plant metabolites that color the plant are a good example of this, as the coloring of a plant can attract pollinators and also defend against attack by animals. Terpenes, alkaloids, sterols, pigments, growth regulators, and phenolics are the important secondary plant metabolites.

Amino acids serve as the building blocks of proteins and as precursors of many other important biomolecules such as hormones, purines, pyrimidines, porphyrins and some vitamins. They also serve as sources of energy when present in excess amounts and when they are used as fuel, amino acids undergo deamination reactions. The remaining carbon skeleton will either convert itself into glucose or undergoes oxidation to CO₂ via TCA cycle.

Central dogma: The Central dogma of molecular biology, which has been formulated as a simple linear progression of information from DNA to RNA to Protein, is summarized in the illustration. The replication process on the left consists of   passing  information from a parent DNA molecule to daughter molecules. The middle  transcription process copies this information to a mRNA molecule.  Finally, this information is used by the chemical machinery of the ribosome to make polypeptides.

Plants can synthesize all the 20 amino acids, which include essential and non-essential amino acids. They can be degraded by removal of the amino group and conversion of the remaining skeleton to useful metabolic intermediates which participate in the synthesis of glucose and other ketone bodies.
 
Biosynthesis of amino acids
 
The biosyntheses of amino acids have an important feature. Their carbon skeletons come from intermediates from glycolysis, pentose phosphate pathway and citric acid cycle etc. The biosynthetic pathways depend on the intermediates from which they are derived. The primary amino group normally comes from the transamination of glutamate. Glutamine synthetase is a central control point in nitrogen metabolism, since glutamine is the amino group donor in the formation of many biosynthetic products as well as being a storage form of ammonia. Glutamate is the precursor of proline, ornithine, and arginine. Serine, cysteine, and glycine are derived from 3-phosphoglycerate. Glycine is synthesized from serine by removing CH2OH group. Tyrosine is synthesized from phenylalanine. Asparagine and glutamine are amides that are synthesized from aspartate and glutamate.

The importance of lipids have already been elaborated. They play a major role as source of energy and as components of membranes which have implications in plant growth regulation. The complete metabolism of fat leads to oxidation, the final products being CO₂ and water. The burning of one mole of palmitic acid gives 2340 kcal.  The molecular weight of palmitic acid is 256. This works out to approximately 9 kcal/g of palmitic acid. This energy is twice that derived from either sugars or proteins. Fats and oils are thus better and more efficient forms of storage of energy in living beings.

Carbohydrate metabolism in the cell is essentially the metabolism of glucose and other substances related to glucose in their metabolic processes. The chemical reactions involved in carbohydrate metabolism represents complex groups, sequences, and cycles of reactions which integrate at various points with the reactions concerned in the metabolism of lipids and proteins. The major types of chemical processes involved in carbohydrate metabolism may be grouped as follows: a) Anabolism of starch – Photosynthesis, b) Catabolism of starch, c) Catabolism of glucose – 1. Glycolysis and TCA cycle and 2. Oxidative pentose   phosphate pathway.
 
Plant cells “burn “ glucose for energy. Plants also produce sugars through photosynthesis. This is roughly the reverse of the burning process.

Plants are responsible for highly useful molecular reactions. All the enzymatic  reactions  that  occur in the cell are  termed  as metabolic reactions.  They  take place within organisms in an orderly and regulated way.  They  are  the  metabolic pathways and the compounds  resulting from the process, namely,  metabolism are called  the metabolites. Metabolism in Greek means change. It is the set of life sustaining chemical transformations within the cells of living organisms. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to the environments around them.
 
Metabolism is a highly organized,   coordinated and purposeful enzymatic activity inside a cell. Metabolism has four functions: 1. To obtain energy from the environment, 2. To convert molecules into building blocks, 3. To convert the building blocks into proteins, lipids, sugars, nucleic acids and other cell components and 4. To synthesize and degrade organic molecules for specific needs of the cell.

Nucleic acids were first discovered in 1868 by Friedrich Meischer. Nucleic acids are high molecular weight polymers that are present in our cells. They  store and transfer genetic material from generation to generation. Knowledge of how genes are expressed and how they can be manipulated is becoming increasingly important for understanding   of nearly every aspect of biochemistry. These macromolecules pass on information for cellular growth and reproduction.  All the genetic information in the cells is called the genome. Every time a cell divides, the information is copied and passed on to the new cells.
 
Nucleic acids fall into two main classes according to the type of sugar they contain: the Deoxyribonucleic acids (DNA) and Ribonucleic acids (RNA). Some sections  of  the  DNA  called  genes contain the information to make a protein. RNA translates the genetic information in the DNA to the ribosomes, where the synthesis of proteins takes place.

Enzymes are organic catalysts produced by living organisms. They make possible biochemical reactions. Hence they are called biocatalysts. They are mostly a specialized class of proteins. However, there are some non-protein enzymes like ribozymes, which are RNAses. Enzymes speed up reactions at ambient temperatures. This is in contrast to chemical reactions like saponification of lipids which take place when lipids are boiled for a few hours with concentrated alkali.  Enzymes, namely, lipases, hydrolyze lipids at body temperatures in minutes. They are superior to chemical catalysts by a factor of 107 to 1014 .  An enzyme may be a simple protein or a complex protein. They are produced by cells and are mostly globular proteins. Each cell contains an estimated 3000 different enzymes.

The word protein was first coined in 1838 to emphasize the importance of this class of molecules. The word is derived from the Greek word proteios which means “of the first- rank”. Proteins are polymers/macro molecules made up of several amino acids. When there are more than 50 amino acids in a chain, the polypeptide is normally called a protein. Each protein in the cells of living beings has a unique sequence of amino acids that determines its biological function. The proteins are folded into specific defined structures, which are maintained by a large number of relatively weak bonds. The  three  dimensional structure is well defined and suited to it’s specific function.

Very small changes in the structure can modify the function. Hence, it is important to know the structure of proteins to understand their behavior and function. The biological activity of proteins depends on maintenance of folded conformation. Proteins fold into well defined three dimensional shapes and they are able to recognize their corresponding substrates or antigen molecules and bind them tightly. The protein structure has been classified into four different levels based on the folded conformation of the protein.