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UNIT 1 NUTRITION, FEEDING, DIGESTION Structure Introduction Objectives Nutrition Proteins Carbohydrates Lipids Vitamins

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UNIT 1 NUTRITION, FEEDING, DIGESTION Structure Introduction Objectives Nutrition Proteins Carbohydrates Lipids Vitamins UNIT 1 NUTRITION, FEEDING, DIGESTION Structure Introduction Objectives Nutrition Proteins Carbohydrates Lipids Vitamins. Minerals and Trace Elements Water Feeding Mechanisms Feeding on Small Particles Fteding on Food Masses Feeding on Liquids Digestion lntrllccllular Digestion Digestive Tract Dibestive Enzymes Maintenance of Gut Lining Coordination of Digestion Absorption Energy Metab-olism Summary Terminal Questions Answers 1.1 INTRODUCTION All organisms require a fairly steady supply of nutrient materials from the environment to obtain energy in order to stay alive. You would recall from FST-1, Unit 14 that animals are heterotrophs because they depend on already synfhesised organic compounds from plants and other animals to obtain their food. Unlike autotrophs (plants and chemosynthetic bacteria) animals have only limited synthesising abilitiks. In LSE-01, you have read that cellular metabolism provides energy for various processes in organisms, like locomotion., excretion, osmoregulation, synthesis of new materials for growth and maintenance and reproduction. To provide energy for these processes raw material or nutrients are required which are supplied by food. In addition animals require amino acids, vitamins and minerals which they cannot synthesise. The study of nutrition involves both the need for food to provide energy and the need for specific food components. The process by which animajs acquire and ingest their food is referred to as feeding. Diverse types of feeding mechanisms have been evolved by different groups of animals. Virtually all foofwhether of plant or of animal origin has to be broken down to simple compounds by the process of digestion. Digestion and absorption of food constitute the essential link between nutrition and metabolism. In this unit we shall first discuss the nature and components of food and the specialised feeding mechanisms. There exists a relati~nshiphetween the nature of ingested food and type of feeding mechanism used in acquiring the food. Then we will consider the digestion and absorption of nutrients. Towards the end of the unit we shall be discussing the energy metabolism in animals. i Objectives After studying this unit you should be able to : distinguish between essential and non-essential nutrients and explain why animals exhibit differences in their essential food requirements describe the various feeding strategies evolved by the animals in relation to the available food distinguish between intracellular and extracellular digestion of proteins, carbohydrates and fats and explain the role of gastrointestinal hormones summarise the process of absorption of food from the alimentary canal explain energy metabolism in animals relating it to oxygen consumption. 1.2 NUTRITION As we have said earlier all animals are heterotrophs and require food from the I environment. What is this food made up of? If the food of a number of different animals is broken down we find that it consists of proteins, carbohydrates, fats, water, minerals and vitamins. All animals require the above-mentioned broad categories of nutrients although in different amounts. Some of these nutrients are ,used mainly as fuel (carbohydrates and fats), while others'are required principally as structural and funational components (proteins, minerals and vitamins). However, proteins, carbohydrates and fats can all serve as fuel for the body's energy needs, but no animal can survive on fuels alone. Therefore, a balanced diet is needed to meet all the requirements of the body for energy, growth, maintenance, reproduction and physiological regulation. Now.let us discuss the importance of these different classes of food in relation to animal nutrition. 1.2.1 Proteins Proteins are continually synthesised in the cells as they are the principal component required for growth. Proteins are composed of amino acids which are derived largely from the diet and partly from the breakdown of protein available in. the body. The terms essential and non- Generally all proteins are made from about 20 different amino acids in various mntial ami* acids are not very combinations. However, it is not necessary to supply.al1 the 20 amino acids. Some significant because the can be formed in the body, using other amino acids but others have to be supplied non-essential amino acids are jbt through diet because they are not formed in the body. The amino acids that are , as important for the body. May hi so important that the body cannot synthesised in the body are called nonsssential amino acids while those that have to leave them to be supplied be supplied through diet are known as essential amino acids. externally and so has mechanisms to synthesise them. The requirement of 'essential amino acids differs from organism to arganism. Some bacteria require only one amino acid in sufficient quantities in the growth medium to be able to synthesise the rest. In contrast mammals certainly cannot fulfil their protein requirements by only one amino acid. How can one determine wbich amino acid is essential and which is non-essential? The nutritional requirements are determined by deletion experiments i.e. by removing a single nutrient from the diet and then observing the growth and health of the animal. By this method it was found out that 10 amino acids are essential for the growth and well-being of rats (see Table 1.1). Table 1.1 : Amino acids clssdCied according to dietary needs d hum- and rats Essential Non-essentid Rats Humans Rats Humans Lysine Phenylalanine I Glycine Glycine Tryptophan Lysine I Alanine Alanine Histidine Isoleucine I Serine Serine Phenylalanine Leucine I Cysteine Ty rosine hucine Valine I Tyrosine Aspartate Isbleucine Methionine -1 Aspartate Glutamate Theronine Cystine Glutamate Proline Methionine Tryptophan Proline Hydroxyproline ' ,, Valine Theronine Hydroxyproline Citrulline . Arginine Citrulline Histidine Arginine Absence of anjlone of these except arginine produces nutritional deficiency and even death. Rats are able to synthesise arglnine but at such a slow rate that it does not meet the demands of normal growth. To what extent the animal requires a particular amino acid in diet depends on the synthetic ability of the body cells. Organisms with. marked synthetic ability, for example, bacteria (mentioned earlier) require a few ' essential amino acids. Organisms like mammals, that require many essential amino acids have a marked synthetic disability. 1.2.2 Carbohydrates Fifty five to seventy per cent of the required energy in animals is derived from carbohydrates. However, fats and proteins can also be broken down and used for supplyin4 energy. In most animals this happens only when the dietary intake of carbohydrates is low. In contrast, Drosophila uses only carbohydrates as a source of energy for its flight muscles and when the supply is exhausted the insect cannot fly even though it uses stored fat for other metabolic processes. Whereas, locusts are known to use only lipids for their long migratory flights. Most animals, however, use a variety of hexose sugars like glucose, fructose, mannose, and galactose as interchangeable sources of energy. In this way no particular carbohydrate is really considered essential in a way similar to amino acids. But even if no carbohydrate is considered essential, growth of certain animals will be better on one type of sugar than on another. This can be explained better by the results of the following experiment. Young locusts showed that when dietary sugar was maltose growth was maximum or optimum and growth was minimum when no carbohydrate was given. Other sugars supported sub-optimal growth. What could be the reason for this difference? One of the main causes is the difference in the rate of movement of sugars across the gut wall into the blood. From the above experiment we can conclude that certain insects have a preference for a certain carbohydrate which can be called an essential or preferred nutrient. In the above experiment with locusts, maltose was the preferred nutrient. 1.2.3 Lipids All animal tissues contain lipids or fats as essential components of cell membrane. It is also stored in certain tissues. Lipids are body's chief source of energy and are essential for diverse functions such as insulation, padding, synthesis of steroid hormones and carriers of fat soluble vitamins. Many animals can live on little or no dietary fat because it can be formed from proteins as well as carbohydrates. But the synthetic ability of many animals is limitec in respect to certain unsaturated fatty acids and cholesterol. For instance, vertebrates can synthesise cholesterol readily: In humans cholesterol is considered harmful in diet because it is a major factor in the development of atherosclerosis or hardening of arteries. On the other hand, insects cannot synthesise cholesterol from their precursors. Therefore, it must be supplied in their diet. Studies on rats show that three fatty acids - linoleic, Liolenic and archidonic acids are not synthesised. Therefore, they are considered essential fatty acids. Many insects, birds and some mammals also reveal such a dietary requirement of fatty acids. It seems that animals in general have a better synthetic ability for lipids than for amino acids. 1.2.4 Vitamins Animals cannot sustain a healthy life if they are fed on a diet having only carbohydrates, fats and proteins. They also require vitamins in small quantities in the The physiological role of vitamin K range of milligrams or micrograms. These function as coenzymes in metabolic was discovered in birds fed a reactions. You might find it useful to reread Unit 21 in FST-01 for the list of vitamins, cholesterol free diet for the putpose of studying cholesterol their main functions and their dietary sources. Table 1.2 gives a more detailed list of synthesis. The birds developed vitamins important in animal and human nutrition along with their diverse functions. severe bleedings which were The synthetic ability for vitamins also varies among different animal species and those traced to a vitamin deficiency. essential vitamins that the animal cannot synthesise must come from its dietary Dnly after this it was discovered sources.
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