Constituents of food – the major nutrients, Biology

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Composition of Plant and Animal Tissues

Food supplies nutrients popularly known as major and minor nutrients based on the quantities in which they are required for the living being and the amounts in which they are present in the living body. The major nutrients are carbohydrates, lipids and proteins. Water is also an essential nutrient required for structural and metabolic functions in addition to its role as a solvent and a medium in which most of biological reactions take place. The minor nutrients are vitamins and mineral elements. Plants build up nutrients in their system during their growth and maturity. The animals utilize these nutrients and transform them by various metabolic processes. Thus, the plant and animal tissues are basically made up of those constituents, namely nutrients which make up the food material. Plants display much greater variation than do animals in their chemical make up. As the plant matures, lignin is deposited over the structural and protective parts of the plant and is hence found in close association with the main structural compound, the cellulose. Cellulose and lignin make up most of the crude fibre material of the plant. Hemicellulose and pentosans are other notable constituents of crude fibre. Carbohydrate also occurs as reserve food material in plants. In seeds such as cereal grains it occurs mainly as starch. Large reserves of starch are also noticed in root stems and tubers such as potatoes, sugar beets, turnips, tapioca etc.

Lipids are found highest in seeds. Oilseeds like soybean, linseed, cottonseed, groundnut, mustard, sesame, coconut etc. are rich in oil content. Lipids are also found in leaves, but to a much less extent and to a further less extent in stems. In these parts, the lipid portion includes a considerable proportion of plant pigments. 

Protein is the basic structural material of protoplasm. Protein is found highest in actively growing parts of the plant. At the time of maturity and germination, seeds contain more protein than other parts of the plant. Oilseeds contain higher amount of protein than cereal grains. Leguminous plants are more proteinecious than non-legumes. 

The mineral content also varies widely among different species and different parts of the same plant. Legumes are particularly rich in calcium. Leaves in general contain more calcium than stems. Seeds are poor in calcium and rich in phosphorus. Seed coats and embryo contain most of the mineral matter of the seed while endosperm is poor in mineral content. Soil conditions greatly influence the mineral status of plants.

As in plants, water forms the principal constituent of the animal body. As the growth proceeds, there is reduction in water content until the stage of maturity. The presence of water is not uniform in the animal body. It is over 90% in the blood and only about 5% in the enamel of teeth. Fat is the most variable component of the animal body.

There is an increase in fat content with age. Fat is the form in which surplus energy is stored in the animal body. In contrast to plants, the animal body contains a rather low glycogen. Glycogen is the storage form of carbohydrate occurring in liver and muscles.

The total carbohydrate in the animal body is around 0.5% of the body weight and it never exceeds 1.0% at any time. Protein is present as the basic structural material in all the cells and tissues of the animal body. It is an important constituent of the body fluids also. Its level is quite constant and any variation observed in its proportion is largely due to the variations in fat and water contents. Fat-free animal body contains about 73% water, 21.5% protein and 5.5% ash. The moisture-free, fat-free body contains 80% protein and 20% ash.


Water is the mot important nutrient for animals in view of their multifarious functions, largest quantitative requirements and highest turnover in the animal body. Water is available to the livestock not only as free drinkable water, but also as a constituent of feedstuffs in variable amounts. It is present to an extent of 70 to 90% in succulent fodders and about 10% in concentrate feed ingredients. There is an internal source of water known as ‘metabolic water’ which is derived during the metabolic processes in the body system. Water is an excellent solvent and a good dispersing medium. It has unique qualities of high surface tension, high dielectric constant and highest latent heat of vaporization, i.e., the ability to absorb heat with out rise in temperature. Its ionizing and hydrating qualities are most essential for body reactions. Apart from being an integral part of cell structure and body fluids, it is the medium in which all biochemical processes in the body take place. As a solvent it forms a medium of transport of nutrients, metabolites and waste products. It regulates body temperature through its latent heat quality. Water balance has a direct influence on electrolyte balance and acid-base equilibrium. It is a shock absorbing cushion to protect the tissues from physical pressure by being a constituent of tissues and fluids. In fact, water is an inseparable component of the body system and no body function can be imagined without water being involved directly or indirectly.


They are primarily energy yielding to the animal body. In fact the chief function of any nutrient is to provide energy or to aid in the release of energy from other nutrients. The animal body system uses energy chiefly through the metabolism of carbohydrates and stores energy in the form of fat. The simple sugars or monosaccharides contain single basic structural units. Based on the number of carbon atoms in their basic units, they are classified into trioses, tetroses, pentoses, hexoses and so on. Some of the important monosaccharides are glyceraldehyde (triose), erythrose, threose, erythrulose (tetroses), xylose, ribose, arabinose, xylulose, ribulose (pentoses), glucose, galactose, mannose, fructose, sorbose (hexoses). Monosaccharides are highly soluble in water and are sweet in taste. Disaccharides contain two molecules of monosugars as basic units. The common disaccharides include maltose (containing two glucose units), sucrose (containing fructose and glucose), lactose (containing glucose and galactose) and cellubiose (containing two glucose units). Maltose occurs as an intermediary product when starch is digested. Sucrose is the sugar present in sugarcane, ripe fruits and sugar beet etc. Lactose is the milk sugar. Lactose fermenting organisms ferment lactose to cause souring of milk. Cellubiose is an intermediary product of cellulose digestion. Raffinose is a trisaccharide containing glucose, galactose and fructose molecules. It is available in cottonseed and sugar beet. Sugars containing 2 to 10 units of monosugars are known as oligosaccharides.

Polysaccharides occupy an important position in the nutrition of farm animals. They are polymers of simple sugars. The polysaccharides of nutritional importance are starch, dextrins, inulin, glycogen, cellulose, pentosans and their intermediate compounds of digestion. Starch is the reserve food material found in most plants. It is abundantly present in grains, fruits and tubers. The starch granules vary in size and shape and help in the identification of starches. The breakdown products of starch are dextrins, achroodextrins, maltose and finally glucose. Dextrins are products of partial hydrolysis of starch and are abundantly available in germinating seeds. Inulin is a polysaccharide consisting of fructose units. It replaces starch in certain plants like artichoke and dahlia. Inulin is mostly not digested in the animal system. Glycogen or animal starch is the reserve carbohydrate in the animal body. It is found in liver, muscles, kidneys and other tissues. On breakdown, glycogen yields maltose and finally glucose. Cellulose is the most important structural carbohydrate in plants, but plays a significant role in the nutrition of ruminants. Cellulose also possesses glucose as its building units. In aging plants, cellulose occurs in close association with lignin. Cellulose digestion in the complex stomach of ruminants yields volatile fatty acids like acetic acid, propionic acid and butyric acid which are utilized by the animal system upon absorption. The animal system does not possess any enzyme to digest cellulose, hence requires the symbiotic assistance of microbes occupying the complex stomach for its digestion.

Pentosans are made up of pentose units. Xylan is a pentose containing xylose units. It is present in corn cobs, oat hulls, hay and woody plants. Araban is a pentose containing arabinose units. It is found in gum Arabic. Hemicelluloses are made up of pentosans in combination with some hexosans. Hemicelluloses make up over 20% of the crude fibre material found in the plant cell wall. They also occur in close association with cellulose. Lignin is a polymer of some complex alcoholic compounds. It is indigestible in the animal system. Its gives strength and rigidity to the cell wall in plants and serves as a protective cover to the plant cells. It limits the digestibility of crude fibre and thus requires to be considered for effective digestion of crude fibre through right time of harvesting of fodders to have less levels of lignin or through appropriate processing to bring about delignification of crude fibre.


Lipids are esters of fatty acids or such compounds which can form these esters. Lipids are generally grouped into simple lipids, compound lipids and derived lipids. Fatty acids are the essential components of lipids. Fatty acids possess high levels of Naturally occurring fatty acids contain even number of carbon atoms. Fatty acids containing ten or less number of carbon atoms are liquid at room temperature and are generally volatile. Those with higher number of carbon atoms are solid and non-volatile. The fatty acids may be saturated or unsaturated, depending upon the absence or presence of double bonds in their structure. With the presence of double bonds, the fatty acids become highly unstable and reactive. Animal fat contains oleic, palmitic and stearic acids. Butter fat is rich in lower steam volatile fatty acids. Essential fatty acids which are not synthesized in the animal tissues such as linoleic, linoleinic and arachidonic acids are present abundantly in vegetable oils. Animal body cannot synthesize fatty acids containing more than one double bond, though some inter-conversion may take place to meet body requirements.


Proteins form the major part of protoplasmic structure of plant and animal tissues. These are polymers of amino acids, which form the building blocks of protein molecules. Amino acids are derivatives of lower fatty acids. These are water soluble compounds and possess an amino- group at one end and a carboxyl group at the other end. Amino and carboxyl groups successively link together to form a chain of amino acids constituting the protein molecule.

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