NUTRITION

All living organisms need matter to build up the body and energy to operate the metabolic reactions that sustain life. The materials which provide these two primary requirements of life are called nutrients or food. The sum of the processes by which the living organisms obtain matter and energy is termed nutrition. All the processes involved in the taking in and utilization of food substances by which growth, repair and maintenance of activities in the body as a whole or in any of its parts are accomplished is included in nutrition. Nutrients in the food provide the necessary chemicals for growth, maintenance and energy production. Overall, the nutritional requirements of an animal are inversely related to its ability to synthesize molecules essential for life. The fewer such biosynthetic abilities n animals has, the more kinds of nutrients it must obtain from its environment. Green plants and photo synthetic protists have the fewest such nutritional requirements because they can synthesize all their own complex molecules from simpler inorganic substances; they are called autotrophs. Animals, fungi and bacteria that are called heterotrophs, cannot synthesize many of their own organic molecules and must obtain them by consuming other organisms of their products. Animals, such as rabbits that consist entirely on plant material are called herbivores. Carnivores, such as hawks, are animals that eat only meat. Omnivores, such as humans, eat both plant and animal matter. Modes of nutrition: two types

A. Autotrophic or Holophytic Nutrition

All green plants and certain protists (Euglena Viridis) have evolved a mechanism to directly use the energy of sunlight for preparing organic food in their own body from simple inorganic materials. This process of making food is called photosynthesis and the organisms capable of it are termed phototrophs. Some bacteria have developed a technique to capture energy released during oxidation of inorganic chemical substance and prepare organic food with its help. They are known as chemotrophs and the process as chemosynthesis. Nitrifying bacteria, Nitrosomonas and Nitrobacter are chemotrophs. Both phototrophs and chemotrophs do not take organic molecules predicted by other organisms, their modes of feeding are together reffered to s autotrophic nutrition. Since, it is characteristics of plants, it is also called holophytic nutrition.

B. Heterotrophic Nutrition : Animals, fungi, some protists (Amoeba) and many bacteria cannot utilize sun energy. They use chemical bond energy of organic molecules synthesized by other organisms in building their own organic molecules. Such a mode of feeding is termed heterotrophic nutrition and the organisms having it are called heterotrophs.

Heterotrophic Nutrition are of following types:

Saprophytic Nutrition: Many organisms absorb fluid food through the body surface. This is called saprophytic nutrition. Bacteria and fungi flourish on dead, decaying organic matter of both plant and animal origin. They secrete digestive enzymes onto this matter.The enzymes hydrolyze he organic matter into simple soluble products that are then absorbed. This method of taking up organic food is known as saprophytic nutrition. Some parasitic protists, such as Trypnosoma and a few invertebrates, such as tapeworms, live in a medium that contains simple organic compounds ready for absorption. This mode of taking up organic compounds is termed saprozoic nutrition.

Holotrophic Nutrition : Majority of invertebrates and all vertebrates take plant, animals or their products through the mouth and break up the large organic molecules into smaller ones in their own body with the help of digestive enzymes. The simple molecules are then absorbed into the cells and utilized.This mode of taking organic food is called holotrophic nutrition. Since, it is characteristic of animals, it is also called holozoic nutrition.

Mixotrophic Nutrition: Euglena carries an autotrophic and saprotrophic nutrition at the sme time. This is called mixotrophic nutrition.

The Metabolic fates of nutrition in Heterotrophs :

The nutrients ingested by heterotroph can be divided into macronutrients and micronutrients.Macronutrients re needed in lrge quantities and include the carbohydrates, lipids and proteins.The micronutrients are needed in small amount and include organic vitmins and inorganic minerals. These nutrients make up the animl’s dietry requirements. Besides these nutrients, animals require water.

Calories and energy:

The energy value of food is measured in terms of calories. A calorie is the amount of energy required to raise the temperature of 1 gm of water 1^o degree celcius. A calorie, with small ‘c’, is also called gram calorie. A kilocalorie, also known as calorie or kilogram calorie (kcal), equal to 1000 calories {kilojules(kj=4.1855*kcal)}. A food’s calorie content is determined by burning it in a bomb calorimeter, chamber surrounded by water.When burning food is placed in the chamber, the energy released raises the water temperture and the energy is measured in kilocalories. Bomb calorimetry studies have shown that 1 gram of carbohydrate yields 4.1 kilocalories, 1 gram protein yields 4.3 kilocalories, 1 gram of fat yields 9.3 kilocalories. These value explain why a fatty diet may cause weight gain; fats supply more energy than most people can use.

The Average Caloric Values of macronutrients

Macronutrients	Calories per gram
Carbohydrates	4.1
Lipids	9.3
Proteins	4.4

Macronutrients: With a few notable exceptions, heterotrophs require organic molecules, such as carbohydrates, lipids and proteins, in their diets. When these molecules are broken down by enzymes into their components, they can be used for energy production or as sources for the ‘building blocks’ of life. The major dietary source of energy for heterotrophs is complex carbohydrates. Most carbohydrates originally come from plants sources. This dietary need can be met by various polysaccharides, disaccharides or any variety of simple sugars (monosaccharides). Carbohydrates also serve as major carbon sources for incorporation into important orgnic compound. Many plants also supply cellulose, an indigestible polysaccharides, for humans and other animals (with the exception of hervivores). Cellulose is sometomes called dietary fibre. Neutral lipids (fats) or triacylglycerols are contained in fats and oils, meat and dairy products, nuts and avocados. Lipids are the most concentrated sorce of food energy. They produce twice the energy available from an equal mass of carbohydrate or protein. Most heterotrophs have an absolute dietary requirement for lipids. Sometimes for specific types. For example, unsaturated fatty acids (e.g. linoleic acid, linolenic acid and arachidonic acid) are required by a variety of animals. Their most obvious function is to act as precursor molecules for the synthesis of sterols, the most common of which is cholesterol. The sterols are required for the synthesis of steroid hormones and incriporated into cell membranes. Other lipids insulate the body of some vertebrates and help maintain a constant temperature. The animal source of protein include : eggs, meats of animals and milk. The plant source include: beans, peas and nuts. Proteins are needed for their amino acids, which heterotrophs use to build their own body proteins.

Micronutrients : Micronutrients are usually small ions, organic vitamins, inorganic minerals and molecules that are used over and over enzymatic reactions or as parts of certain proteins (e.g. copper in haemocyanin and iron on haemoglobin). Even though they are needed in small amounts, animals cannot synthesize them rapidly, thus they must be obtained from the diet.

Food : Major contents of food are carbohydrates, fats, proteins, water, minerals salt and vitamins. According to their utility in body, the various nutrients of food can be placed into the foollowing three categories-

Energy producers : Oxidative combustion of three substances (mainly carbohydrates and fats) yield bioenergy required for perforance of all biological activities.

Body builders : These (mainly proteins) are the major structural components of body and, hence, required for growth and repair.

Metabolic regulators : These substances (vitamins, water and mineral salts) control and regulate the internal environment of body and metabolism.

Carbohydrates:

These are carbon ‘hydrates’ (polyhydroxy aldehydes and polyhydroxy ketones), e.g, compound of carbon, hydrogen and oxygen(1:2:1 ratio) with the ratio of hydrogen and oxygen being the same as in water (H₂O). Obviously their emperical formula is (CH₂O)_n. These occur in food as soluble sugars and insoluble starches. Chemicallly, these are three main categories, viz, monosaccharides, oligosaccharides and polysaccharides.

Monosaccharides: These are simplest, colourless, soluble and sweet carbohydrates. Their molecules may have three to sesven carbon atoms. Monosaccharides having five or six (pentose or hexoses) carbon atom in their glucose, fructose, galactose and mannose. Glucose is the most common and most important hexose sugar. Animals mainly use it for energy production (main fuel substances). Of the pentose sugar, most important are ribose and deoxyribose, because these participate in the composistion of DNA and RNA.

Oligosaccharides and Disaccharides: When two molecules of the same or different monosaccharides link by a ‘glycosidic bond’, a disaccharide molecules is formed. Disaccharides are also sweet and soluble sugars. These are maltose (malt sugar) and sucrose(cane sugar) of plants and lactose (milk sugar) of animals. Maltose is formed from glucose monomers (α, 1-4 linkage) sucrose from combination of glucose and fructose (α, 1-2 linkage) and lactose from combination of glucose and galactose (β, 1-4 linkage). Amount of lactose is highest in the milk of human mothers. Linkages of a few (upto 10) monosaccharides are generally called oligosaccharides.

Polysaccharides: When several (more than 10) monosaccharides molecules link by glycosidic bonds, insoluble carbohydrate molecules, called polysaccharides are formed. These are the polymers (C₆H₁₀O₅)_n of monosaccharides units or monomers. Starch and insulin are common polysaccharides found in plants but cellulose of their cell wall is the most abundant structural polysaccharides of nature. Glycogen is the common polysaccharides found in animals. Chitin of arthopoda exoskeleton is nitrogenous polysaccharide.

Use of Carbohydrates: The carbohydrate of food eaten, after being processed in the alimentary canal and liver, are supplied to the tissues as glucose, often called blood sugar. The carbohydrates serve a variety of function.

As fuel: Carbohydrates from the major fuel in the cells to provide energy for life process. One gram of carbohydrates on complete combustion in a bomb calorimeter yields 4.1 kilocalories of energy. This is called caloric value of carbohydrates. One gram of food carbohydrate on oxidation in the cells produces 4 kcal of energy. This is known as the physiological fuel value of carbohydrates. Carbohydrate form a better fuel than proteins and fats because their molecules have relatively more oxygen and, therefore need less molecular oxygen for oxidation that those of proteins and fats.

Reserve food materials: Carbohydrates form storage products. If in excess, glucose is converted into glycogen and stored in liver cells and muscle cells. This conversion is called glycogenesis. It may be changed ito fat and stored in liver. Adipose tissue and mesenteries. This changes is termed lipogenesis. In case the food provides inadequate glucose, reserve glycogen is converted into glucose for energy producton work. This conversion is known as glycogenesis.

Components of cellular compounds and organelles

Pentose sugar ribose is a component of ribonucleic acids (RNA) energy carrirs, such as adenosine triphosphate (ATP) and certain coenzymes, such as Ncotinamide Adenine Dinucleate(NAD). Another pentose sugar deoxyribose is a component of deoxyribonucleic acid(DNA). The RNA and DNA are in turn components of ribosome’s and chromosomes respectively.

Formation of Amino Acids- Amino acids may be formed from intermediates of carbohydrate Ketabolism.

Heteropolysaccharides- These consist of modified monosaccharides units. They form the following important substances.

Anticoagulant heparin; blood group substances, such as A, B and Rh antigens of erythrocytes. Lubricant hyaluronic acid present in the synovial fluid of the joints. Cerebrospinal fluid and vitreous humour. Protective coats, such as glycocalyx, that covers the intestinal epithelium and mucous which covers all mucous membranes. Luteinizing hormone that causes ovulation, formation of corpus lutenum and secretion of female sex hormone. Cells can absorb only monosaccharides from tissue fluid. Therefore, all disaccharides and polysaccharides of food are broken down into their digestion. Since their synthesis is condensation(=dehyhdration) process, their digestion is ‘hydrolysis process’.

Lipids

Three categories of lipids occur in animal food, i.e., simple, compound and derived.

Simple lipids:

These are neutral or true fats and compounds of carbon, hydrogen and oxygen but the ratio of H₂ and O₂ is never 2:1 unlike water. A molecules of fat is formed by linking a molecule of glycerol with three molecules of fatty acids (aliphatic carboxylic acid) by an ester-bond. These fats are therefore, also called triglycerides. This linkage is also a dehydration-condesation reaction, yielding three molecules of water. Ghee, oils, cuard, butter etc. are common neutral fats. Waxes (such as beewax) are also simple lipids. Most animal fats are saturated and hence, solidify at low temperatures. Most vegetables oils are unsaturated and, hence remain fluid. Oxidative breakdown of fats yields more than double the amount of energy yielded by glucose, because of their poor oxygen contents. These can be stored in an almost pure unhydrated form in large amounts in lesser space. Hence, fats serve as the best storage of spare energy amounts in lesser space. Fats also serve as the best storage of spare energy in the forms of ‘reserve stored food’. These are stored in adipose tissue, which also serve for heat insulation.

Compound or Conjugated lipoids:

These lipids contain traces of nitrogen, phosphoric acid, or carbohydrates. Phosphoric acid containing phospholipids are components of membrane system of cells. Of these lecithin commonly found in liver, nervous tissue, yolk and muscles. Carbohydrat containing lipids are called glycolipoids, occur in cellmembranes of brain cells.

Derived fats

These are formed when neutral and conjugated fats are hydrolysed. Hence, these are fat-like alcohol’s, usually called lipids or steroids. The most common steroids are sterols. Cholesterol is the main sterol found in blood plasma and cell membranes. Bile acids, sex hormones, vitamin D, ergosterol, hormones of adrenal cortex are examples of sterols.

Use of fats- Fats are used as ‘fuel substances’. Their caloric value is 9.4 kcal and physiological fuel is 9 kcal. There are important food – reserves and produce more energy on oxidation than glycogen. Fat deposited in layers provides thermo-insulation and protection against pressure. Conjugated lipids are components of membrane systems of cells. Connective tissues and myelin of nerve fibres. Man can synthesize most of the fatty acids in his body from the food taken. A few fatty acids are not synthesized in the body and must be present in the diet. These are called essential fatty acids. They include linoleic acid and arachidonic acids. They are present in unsaturated vegetables oils, such as groundnut oil, sunflower oil etc.

Protein

Proteins account for about 14% part of living and 75% part of dead and dried animal body. These are the major components of the body and more important for anabolism (architecture, growth and repair of body), than for catabolism (energy production). There are components of C, H₂ and O₂ but in addition, these essentially contains about 16% nitrogen and may also contain traces of sulphur, phosphorus, iodine, iron etc. protein is polymer of very large molecular mass, composed of one or more polypeptide chins and whose monomers are amino acids, joined together (in condensation reactions) by peptide bonds. In addition, some have covalent ‘sulphur bonds’ formed by oxidation between two cysteine radicals, in the polypeptide. Biological polypeptides are often several hundred amino acids long. So few of the possible polypeptides actually occur in organisms, linking by peptide (=amide), amino acid molecules from dipeptide, xipeptide, oligopeptides and polypeptide. The latter then link with each other. Forming first the peptones, the proteoses. Various proteins of biological system can be classified into three categories.

Simple proteins-these contain only amino acid monomers.

Globular proteins : In the molecules of these proteins, the polypeptide chains are folded into compact globular or spherical shapes. Hence, the length to9 breadth ratio of molecules is usually 1:3 or 4(never more than 1:10). That is why, these proteins are non contractile and soluble in aqueous systems. Forming colloidal solutions and easily diffusible. All enzymes, many hormones (insulin, thyroxin, ACTH), the antibodies, albumins and globulins of blood plasma, globin of haemoglobin, myoglobin of muscles, histones of blood nucleoproteins, glutelins of cerals, prolamines of pulses are example of globular proteins.

Fibrous proteins

In the molecules of these proteins the length to breadth ratio is always more than 1:10. Hence, these are insoluble structural proteins that make the body architecture. The collagen, elastin and reticulin of connective tissues, tendons, ligaments, cartilages and bones; the kertin of skin, horns, nails, feathers, hairs; the fibroin of silk; the actin and myosin of muscles, fibrinogen of blood plasma, tubuilin of microtubules are examples of fibrous proteins.

Conjugated proteins: