UNIT 1 NUTRITION, FEEDING, Structure Introduction Objectives Nutrition Proteins 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 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 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 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 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 , fructose, mannose, and galactose as interchangeable sources of energy. In this way no particular 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 . 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, 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. For instance, most animals can synthesise ascorbic acid but humans cannot. that vitamin K is necessary for We also depend on intestinal bacteria to synthesise vitamin K and BI2.Fat sol 'rle mammals. vitamins like A, D, E and K can be stored in the fat deposits of the body but vit -ts L Table 1.2 : The Vitamine and their chPRfterWcs R~ot~oqa~ la hunmb, except M aotcd

~ipM-so~ubIeviernlns: A (CZoH,O) anti- Plants form (carotene, Maintains integrity of epithelial Xerophthalmia (dry cornea, no tear xerophthalmic C&%) in green leaves, tissues, especially mucous secretion), phrynoderma (toad skin) carrots, etc; is changed in liver membrane; needed as part of night blindness, growth retardation, to animal form (C&iWO), visual purple in retina nutritional croup (hoarseness) present in fish-liver oil of eye in birds (shark); both forms in egg yolk, butter, milk D (C,H,O), Fish-liver oils, especially Regulates metabolism of Rickets in young (tqnes soft, antirachitic tuna, less in cod; beef calcium and posphorus; yielding, often deformed); fat; also exposure of skin promotes absorption of osteomalacia (soft bones), , to ultraviolet radiation calcium*in intestine; especially in women of Asia needed for normal growth & mineralisation of bones E or tocopherol Green leaves, wheat- Antioxidative; maintains Sterility in male fowls and (C2UH~f12), germ oil and other vege- integrity of membranes rats, degeneration of testes antlstenl~ty table fats, meat, milk with failure of sperma- togenesis, embryonic growth disturbances, suckling para- lysis and muscular dystrophy in young animals (C31%b02)r Green leaves, also certain Essential production of Blood fails to clot antihemorrhagic bacteria, such as those of prothrombin in liver; intestihal flora necessary for blood clotting Water-soluble vitamins: B complex Yeast, germ of cereals Needed for carbohydrate On diet high in polished rice, Thiamine (B,) (especially wheat, metabolism; thiamine beriberi (nerve inflammation); (CIZHI~ON~S). peanuts, other legumi- pyrophosphate an essential loss of appetite, with loss of antineuritic nous seed), roots, egg coenzyme in pyruvate tone and reduced motility in yolk, liver, lean meat metabolism (stimulates digestive tract; cessation of root growth in plants) ' growth; polyneuritis (nerve inflammation) in birds , Riboflavin (B2) Green leaves, milk, Essential for giowth; Cheilosis (inflammation and (CI~HZOOC.N~) eggs, liver, yeast forms prosthetic group of cracking at corners of mouth), FAD enzymes concerned digestive disturbances. with intermediate meta- "yellow liver" of dogs, curled- bolism of food and elec- toe paralysis of chicks, tron-transport system cataract

Nicotinic acid, or Green leaves, wheat germ, egg Forms active group of nicotina- Pellagra in humans and monkeys, niacin (C6HS02N), yolk, meat, liver, yeast mide adenine dinucleotide, which swine pellagra in pigs, blacktongue antipellagric functions in dehydrogenation in dogs, perosis in birds reactions Eolic acid Green leaves, liver, soyabeans, Essential for growth and forma- Anaeniia, haemorrhage from kidneys, (C19H1906N7) yeast, egg yolk tion of blood cells; coenzyme in- and s~rue(defective intestinal abson, volved in transfer of single- tion)'in humans; nutritional cytopen$ carbon units in metabolism (reduction in cellular.elementsof blood) in nionkeys; slow growth and anaemia in chicks and rats Pyridoxine (B6) Yeast, cereal grains, meat, Present in tissues as pyridoxal Anaemia in dogs and pigs; dermatitis (C,H,zOzN) eggs, milk, liver phosphate which serve as in rats; paralysis (and death) in pig, coenzyme in transamination and rats and chicks; growth decarboxylationof amino acids retardation Pentothenic acid Yeast, cane molasses, peanuts, Forms coenzyme A, which cata- Dermatitis in chicks rnd rats, graying (C9H1703N) egg yolks, milk, liver lyzes transfer of various carbo- of fur in black rats, "goosestepping" xylated groups and functions in and nerve degeneration in pigs carbohydrate and lipid metabolism Biotin (vitamin H) Yeast, cereal grains, cane Essential for growth; functions Dermatitis with thickening of skin (CIOHI~Q~N~S) molasses, egg yolk, livkr, in CO, fixation and in rats and chicks, perosis vegetables, fresh fruits oxidation and synthesis in birds Cyanocobalamin (BIZ) Liver, fish, meat, milk, egg Formation of blood cells, growth; Pernicious anaemia, slow growth - .(GHPONI~OI~P~) yolk, uysters, bacteria and fer- coenzyme involved in transfer in young animals; wasting disease . mentations of Srreptomyces; of methyl groups and in nucleic in ruminants . . synthesised only by bacteria acid metabolism C, or ascorbic acid Citrus fruits, tomatoes, vege- Maintains integrity of capillary Scurvy (bleeding in mucous (cdi806) tables;.also produced by animals walls; involved in formation of membranes, under skin, pnd into (except primates and "intercellular cement" joints) in humans and gulnea pigs guinea pigs) - - - .L. Usinger, R.C. Stebbins, and J.W. Nybakken, General Zoology, 6th ed., Mc :Graw-Hill, ~ewYork, 1979 7. that are water soluble like B or C need to be supplied continually as they are lost Nutrition. Feeding, Digestion through urine. 1.2.5 Minerals and Trace Elements I Oxygen,l,carbon, hydrogen and nitrogen are the most common elements that make up 96%'of the total weight of a mammal. The next most abundant elements are calcium, phosphorus, potassium, sulphur, sodium, chlorine and magnesium. These make up nearly the remaining 4%. Fifteen additional elements are required but their total combined amount in mammalian body is less than 0.01% of the body weight. Of the known 90 naturally occurring elements how many are essential for life? This is nat known for many animals. In humans 26 elements are known to be necessary.

Table 1.3 : Approximate composition of human tissue

Element Per cent body weight Oxygen 65.0 Carbon 18.0 Hydrogen 10,0 Nitrogen Calcium Phosphorus Pot;~ssiuni Sulphur Sodium

Chlorinc ~. Magnesium 0.05

Table 1.3 gives the approximate coinposition of human tissue. We all know that carbon, oxygen, hydrogen are present in water and other organic building blocks of the body which also contain nitrogen, sulphur and phosphorus. Calcium is an important constituent of the skeletal structuves of animals and its role in physiological processes such as will be studied later in Unit 6: If the level of calcium concentration falls below half its normal value it leads to severe or fatal tetanic cramps. Table j.4 gives the role of these important minerals.

able 1.4 : Physiologic~lrole4 important minerals

Elements Deficiency Disease

Sodii~~n Main extracellular positive ion: Unknown on normal diet. Table salt, salt (Na) Regulates plasma volume. acid- Secondary in illness or added to hase halance: nerve and injury prepared food muwlc Function Potassium Major intracellular positive ion: Secondary to illness. (K) nervc and muscle function: injury or diuretic acid hase balance therapy; paralysis, mental confusion muscular weakness Calcium Component of bones, teeth; Children-rickets Dairy products,

,(Ca) regulation of nerve, muscle Adults - ostmalacia beans, leafy function; blood clotting vegetables Phosphorus 9onG formation, part of DNA. Children-rick$ Phosphate fwd (P) RNA, ATP, etc.; energy Adults - osiomalacia additives metabolism

'Magnesium Bone and teeth: carbohydrate Secondary to mal- ' Leafy green (Mg) metabolism ahsorption or diarrhoea, vegetables ;~icoholism Chlorine Major extracellular negative In infants fed on salt Table salt (a) ion; osmotic and acidibase free formula; secondary balance; acid to vomiting. diuretic therapy, renal disease.

Mineral requirements are met by a varied intake of adequate amounts of whol~grain cereals, legumes, leafy green vegetables, meat and dairy products. The additional 15 elements that make up less than 0.01% of the body weight occur in such small amounts that they are called trace elements. Table 1.5 lists some of these trace elements thatare considered essential. Although present inminute amounts the essential trace elements are just as necessary as the essential amino acids. For instance, cobalt is needed in the specific form of B12and its deficiency leads to severe anaemia. In ruminants'vitamin B12 is formed in the rumen'by bacteria provided a sufficient amount of cobalt is present in the diet.

Table 1.5 : Physiological role of essential trace elements

Element Physiological Role Deficiency Disease source'

Iron Component of haem group in Anaemia Iron cookware ' (Fe) haemoglobins, cytochromes Copper Needed to make haemoglobin, Anaemia; secpndary to (cu) bone, part of cytochrome malnutrition, Menke's syndrome Iodine Component of thyroid Children: cretinism lodised salt, (1) hormone Adults: goitre, hyper- seafood thyroidism, myxedema Manganese Needed in urea formation, Unknown in humans (Mn) protein metabolism, glycolysis, citric acid cycle Cobalt Constituent of vitamin BIZ, BIZdeficiency Foods of (Co) RBC formation animal origin Zinc Essential constituent of many Hypogonadism, growth (zn) enzymes, needed for normal failure, impaired wound senses of smell and taste healing, decreased taste and smell Molybdenum Constituent of some enzymes Secondary to parenteral (Mo) nutrition , Flourine Hardness of Dental caries Drinking (F) teeth water Selenium Needed in fat Marginal deficiency where The first conclusive evidence that (se) metabolism salt content is low, cobalt is an essential trace secondary to parenteral element came from Australia ' nutrition where a serious disease of cattle and sheep developed. Addition of Chromium Needed in glucose Impaired glucose small amounts of cobalt, (Cr) metabolism tolerance prevented this disease. Each Trace element requirements are met by a varied intake of whole grain cereal, legumes, leafy green sheep is made to swallow a vegetables, meat and dairy products ceramic coated cobal ball that remains in the mmer and slowly Some of the trace elements are essential for complex organisms while others may not releases the cobalt o\ifr several be so essential. Flourine, for example, is an essential element for normal growth of years. rats but is not strictly essential for humans, though we know that it has a well-defined role in prevention and treatment of dental caries. It is difficult to find out the role of each and every trace element and further research will be needed to increase our knowledge. 1.2.6 Water Water is the most important constitdent of all living tissue. It forms up to 95% of the fresh weight of some animals. We all know that water is lost through sweat, excretion and evaporation from the respiratory surfaces. It must therefore, be replenished by drinking, through food and in small quantities by metabolic processes of the body like synthesis and oxidation of fats, proteins and carbohydrates. SAQ 1 a) Match the words in column A with descri~tionsin column B. Column A I i) Synthetic ability a) Amino acids that are required for growth but have to be supplied through diet. ii) Synthetic disability b) Most bacteria can produce all the required amino acids from only one nutrient present in their growth medium. iii) Essential nutrient c) Humans can synthesise cholesterol but insects cannot, from non-sterol precursors. b) Why don't domestic cats and dogs need fruit in their diet while humans do? ,

1.3 FEEDING MECHANISMS

All animals have evolved successful methods for extracting their required nutrition from the environment. Thus we find a diversity of feeding mechanisms or strategies according to the nature of food that an animal can obtain. Table 1.6 lists the major feeding methods in-animal groups based on the type of food available. It would not be possible to discuss each food gathering device in detail but in a brief discussion we shall consider the basic principles on which the different feeding mechanism operate. From Table 1.6 you will note that taxonomically different animal groups living in the same habitat obtain food in a similar manner. For example, many marine animals (, molluscs, crustaceans) may be filter feeders though the organs concerned with the process of filtration may not be anatomically similar.

Table 1.6 : Feeding methods classified according to type of food

Type of food Method of feeding Anlmab using the method

Small particles Digestive Amoeba, Radiolarians Use of cilia Ciliates, , Bivalves, Tadpoles Mucous traps Gastropods, Tunicates Tentacles Sea cucumbers Filter feeding Small Crustaceans, Herrings, Baleen Whales, Flamingoes, Petrels

Large food masses Ingestion of inactive Detritus feeders, Earthworm masses Scraping, chewing, boring Sea urchins, Snails, Insects, Vertebrates Capture and swallowing Coelentrates, Fishes, Snakes, Bats, of prey Birds Fluid or soft tissue Sucking plant sap, nector Aphids, Bees, Humming-birds Ingestion of blood Leaches, Ticks, Insects Vampire bats Sucking of milk or Young Mammals, Young Birds Similar secretions External digestion Spiders Uptake from body surface Parasites, Tapeworm Dissolved organic Uptake from dilute Aquatic invertebrates solution solution Symbiotic supply of Intracellular symbiotic Paramecium, Sponges, , nutrients algae Corals, Hydras, Clams.

1.3.1 Feeding on Small Particles Microscopic algae and bacteria can be taken in directly into the cell by the digestive vacuoles. But one of the most successful methods of feeding on sma 1 particulate matter is filter feeding or suspension feeding. Particulate matter incl des detritus, living and dead plankton. Most filter feeders use ciliated surfaces to p roduce currents - that draw drifting food particles into the mouth. The animal extracts the suspended food particles by means of structures that act as filters often aided by secretion of , mucous which traps the food particles. In sponges, the flagella of the choanocytes, I Anlaul phrddogy - I the cells that line the body cavity, create internal water currents. The body wall has numerous pores called ostia and the water is drawn in across the flagellated chamber ... (Fig. 1.1) into the body cavity. Food particles are trapped by the flagella and directed to the surface of choanocyte, which ingest the particles by phagocytosis.

vWater currents

ngocoel (body cavity)

Choanocyte mucus

move water) . Amebocyte Amebocyte receiving food

Fig. 1.1 : Sponges obtain their fdod by filtering seawater. Food particles pass down the collar cells or choanocytes.andenter them through pbagocytosis.

Moreelaborate methods of Qlter feeding are seen in tube dwelling'polychaetes which use teiltacles to entangle the food particles. Figure 1.2 shows some of the filter feeders and their feeding mechanisms. Filter feeders include both sessile and free 'living forms. The sessile forms generally accept what they get, though, some can selectively choose their food according to size. For instance in Sabella a tube living polychaete, while large sand particles are-rejected small food particles enter the food groove. Free rakers swimming forms are selective feeders. Examples are, many of the microcrustaceans, fishes such as herring, menhaden and bbking sharks, certain birds such as flamingo, pelican and the largest of all animals the baleen whale.

Among the fishes, herring have gill rakers (Fig. 1.2b) that function as a sieve to catch plankton. The basking sharks feed exclusively on plankton and can filter up to 200 tons of water in 1 hour.

The flamingo also a plankton eater uses its beak to strain small organisms from the water (Fig. 1.2~).However, the baleen whale is specialised for filter feeding. Its filtering apparatus cons,jsts of a series of horny plates attached to the upper jaw (Fig. 1.2d). As the whale swims, water flows between the plates retaining the planktan. +'- 1.3.2 Feeding on Food Masses Unlike filter feeders which feed in water, animals that obtain and eat solid food are (4 not restricted to the aquatic environment, and show a great variety of adaptations related to their feeding habits. Animals like earthworms ingest the medium in which they live and digest the organic material ih it as the material passes through the 1 digestive tract. I Fig. 1.2 : Some filter feeders and their feeding mechanisms Many animals use methods for chewing and sctaping to obt'ainlfood. Their food is often of ~lantorigin. Numerous insects and other invertebrates as well as herbivorous vertebraies use Gese methods of feeding. A rahp-like structurs known is radula is used by gastropods to scrape algae from roqks of;t9 rasp through vegeta.tiio (Fig. 1.3). - Rada retractor Radula protractor (b') (4 (a) Flg.l.3:a) SssitE.l&~bad+~pWsbowhgt~duLPwW~~to~p~ vegetatba. b) RohctkmdradulP c) Retrrtlon d red& (a) However, most carnivorous animals simply seize and swallow their prey whole. Jaw Relatively few invertebrates feed in this way but an interesting example is the carnivorous polychaete Nereis which has a muscular armed with chitinous tentacles jaws that can be everted to capture prey (Fig. 1.4). Once a prey is caught the pharynx is retracted and the prey is swallowed. The teeth of lower vertebrates (fishes, amphibians, reptiles) are mainly used to grip the prey and prevent its escape till it is Rmtomiu swallowed. Snakes are familiar examples that are well adapted to this kind of feeding \ \

- Fig. 1.5 : Snakes cannot tear or chew food. They swallow prey whole. The mouth is extremely flexible because of thearrangement of bones in the head and jaw. Lower jaw is loosely attached to - quadrate bone and bones of the palate are movable. The help to draw is prey into the gaping i mouth.

Fig. 1.4 :Narlr vh,an em~t pdy-, a) antdw end with everted Jaw to apturn Prcr b)extmul~,

15 True mastication i.e. e&of food is found only in mammals. Their teeth are adapted for this specific function. Mammals have basically four types of teeth (Fig. 1.7) each adapted for different type of feeding. Incisors are adapted for biting and cutting and stripping; canines for seizing and piercing; premolars for crushing; and the molars for crushing and grinding. The number and size of these teeth varies according to the type of foad eaten.,

as Mpremolars A ~c~~

Spiders provide an interesting example of fluid feeding. Their preybare usually larger in size and Fig. 1.7;,Mammaliandentitlon - teeth of (a) generalised mammal, (b) squirrel, (c) AMcan Uon, (d) OX covebed by a hard chitinous covering. Spiders, therefore, pierce the covering by their 1.3.3 Feeding on Liquids hollow jaws and pump digestive juices into the prey's body. These Animals feeding on liquids are generally highly specialised for their feeding habits. liquify the tissues and then the Certain protozoa, endoparasites and aquatic invertebrates take up nutrient molecules 'kpider'sucks the prey empty. through their integument from the- medium in which they live. For example, endoparasites, which include parasitic protozoa, tapeworms, flukes, certain molluscs and crustaceans are surrounded by host tissue or alimentary canal fluids which are highly nutritive. These parasites lack a digestive system of their own. All of us are familiar with insects that have well-developed piercing and sucking organs. Mosquitoes, bedbugs and lice and leaches among annelids are some examples. They use anticoagulant to prevent blood from clotting as it leaves the blood vessels ruptured by their piercing or rasping jaws.

I SAQ 2 a) You.must have observed a squirrel, a cow and a dog feeding. What kind of differences would you expect to find in their dentition?

b) Match the type of feeding apparatus in column A with the kind of food in column B. Column A Column B

1)- Radula a) Blaod, plant sap 2) Cilia b) Detritus in mud 3) Mucous Sheets c) Large chunks of food 4) Sucking mouth parts d) Algae on rocks 5) Teeth e) Suspended particles ,1.4 . DIGESTION

i In the earlier sections we considered the nutritional requirements and the various ways used by heterotrophic organisms to obtain nutrition. Whether food is used to give energy or to build the body, the large molecules of food have to be broken down into simpler constituents before they can be used by the body. The process by which the food is broken down into simpler molecules is known as digestion. This breakdown is achieved with the aid of enzymes and can take place inside the cell - or outside the cell -extracellular digestion often in a specialised digestive tract. Let us first consider intracellular digestion and see how it is different from extracellular digestion.

We all know that unicellular organisms do not have a separate alimentary canal system. All the functions of life are carried out inside a single cell. Food is taken in directly into a cell by phagocytosis/endocytosis and then with the help of enzymes digested in a food vacuGle. Fig. 1.8 shows the process of endocytosis in Amoeba.

Nucleus molecules

Fig. 1.8 :Digestion in amoeba . Similar intracellular digestion occurs in sponges, some coelentrates, ctenophores and turbellarians. Although the process is called intracellular digestion, the food material is actually separated from the rest of the cellular material by a membrane which it can cross after digestion. In organisms such as cnidarians and platyhelmintbs, a gut or enteron is present and here along with extracellular digestion where enzymes are secreted into the cavity, intracellular digestion also takes place within the cells that line the ca2ity. However, in annelids and molluscs more extracellular than intracellular digestion takes place. Digestion is entirely extracellular in , insects, echinoderms and vertebrates. ," 1 1.4.2 Digestive l'rict Extracellular digestion takes place in a tubular cavity that extends tiroughout the length of the organism. All animals after flatworms have-a tubular alimentary organisation open at both ends. The. development of extracellular digestion freed many animals from feeding continuously on small particles. They could now quickly 1 ingest a few large chunks of food. The overall tubular organisation of the digestive tract also allows the food to travel in one direction passing through regions of I digestive specialisation (Fig. 1.9). In general the digestive system of metazoans is divided into 4 major functional regions 1 of: reception conduction and storage : I* digestion and absorption ce?duction and. formation of faeces. I* The region for reception is associated with devices for mastication or chewing of food (like teeth); for paralysing the struggling prey (toxic enzymes from saliva); initiating digestion and lubricating the food with mucous. The oesophagus of and some invertebrates serves to conduct the bolus (mass of chewed food) by peristaltic movement from buccal cavity. In some animals this fegion has a crop for storage. The crop ip birds is also used to ferment mildly or digest food. This is later regurgitated by pare'nt birds for their nestlings. The storage region allows the animals to store food and use it when it is not easily available. For example, leaches take in infrequent large meals of blood and digest it slowly over a

I month. The herbivore animal spends hours masticating the food it takes in hurridly I : Storage and stores it in its stomachsfor further use. \ ,' (Some spccies) In the third region or digestive region the enzymes reduce the food to a forin that can be absorbecfby the body of the organi:m. As the food is digested, the absorbable (Add) food is passed to the blood stream and the unabsorbed material is stored briefly in the final section of the alimentary canal where further removal of excess water and , consolidation of undigested material into faeces takes place, before it is expelled out I of the body. In vertebrates this function is carried out in the large.intestine. Digestion In higher vertebrates, each area of the gut is specialised for a certain activity, 1 I digestive enzymes are produced in glands as well as in the wall of the gut. Absorption occurs in the intestine predominantly. (Alkaline)

1.4.3 Digestive Enzymes Absorption and Now let us consider the general principles of digestion that are applicable t6all Jl1 Assimilation animals. We will start with the digestive enzymes that breakdown the large food molecules into smaller soluble component units. This breakdown involves the uptake

J of water and is called . Before reading the following sub-sections, you Storage of waste would find it useful to read Units 9 and 10 of LSE-01 to recapitulate the nature and properties of enzymes in general. However, digestive enzymes differ in the following + ways: Defecation a) Digestive enzymes are not as narrowly specific as other enzymes rather they show FLa.L.9: Ccncrdbed dlgwtlvt group specificity. For example, enzymes that digest carbohydrates can digest trod. Om way pslrs~ge of food allows sequtntlal of both animal and plant origin, stag- In dlgcstlon. . Duhcd Ilna represent b) Even though enzymes performing similar functions in different animals are given crop Lo mm..nlds. same names, they are not identical chemically. For example, trypsin (an , that hydrolyses proteins) in humans is not identical to that fouqd in fish. Temperature and pH for optimum activity is also different. For example, trypsin from pancreas acts best in the pH range of 7-9 but in silkworm the pH range is 6.2-9.

c) Digestive enzymes from pancreas parti'iularly those that digest proteins are secreted in an inactive form.

Thethree major classes of digestive enzymes are: i) Proteases.that hydrolyse bonds in proteins, ii) Carbohydrases that hydrolyse glycosidic bonds in carbohydrate,

iii) Lipases that hydrolyse ester bonds in fats

.. .. . Protein Digestion Enzymes that digest proteins are divided into two groups endopeptidases and exopeptidases according to site of their action in the protein molecule. Endopeptidases confine their attack to the interior of the protein molecule so that the large peptide chain is broken into smaller fragments/. This provides many sites for action of exopeptidases that attack only peptide bonds at the end of a peptide chain releasing amino acids, dipeptides and tripeptides. There are several types of endopeptidased and exopeptidaseg. They are listed-in Table 1.7. Preferred Peptide Inactive form Active form a- Link Attacked

Endopepticlaw HC1 Link to amino group of aromatic Pepsinogen , pepsin' pepsin amino acid (tyrosine and phenylalanine)

entemkinase ' Link to carbxyl end of arginine Trypsinogen trypsin trypsin ' or lpine Link to czubxyl group of aromatic GYrn-in~gen w*chymotrypsin amino acid (tryptophan, tyrosine, ' phenylalanine) and also bonds adjacent to methionine and leucine when'they are present

Link to terminal anjno acid with heamino group Carboxpeptidase (H+)(trypsin) Link to terming amino acid with free carboxyl group Bonds between pairs of amino acids

From the table youcan see that these exopeptidases and endopeptidases attack specific peptide bonds depending on the chemical group near them. Theeinactive forms need activators and autocatalysts to convert them into active forms. For example pepsinogen is secreted by the vertebrate stomach. The stomach also secretes HCL which makes the medium acidic (pH 2). This activates pepsinogen into pepsin. Pepsin specifically hydrolyses peptide bond between a dicarboxylic and an aromatic amino acid (Fig. 1.10a). In this way short fragments of polypeptide chains are formed. Invertebrates seem to lack pepsin arid their main endopeptidase is more like trypsin. Look at Fig. 1.10a again, chymotrypsin also attacks a peptide bond involving' aromatic amino acid but on the carboxyl terminal end of the molecule while pepsin attacks on themnino terminal end.

FHa .pepsin Chymotrypsin CH, (CHz)3 , I I -HN-CH-CO HN-CH-CO -NH-CHz-CO NH-CH-CC - NH- I Clutamic acid b' v 6 Clycine Lysine

Tyrosine

H3CVCH 3 CH I 4$.. / CHa Am~nopeptldase Carboxypeptidase N Termlnal I 1"' NH,-CH-CO HN -CHt-CO ; * HN-CH-COOH 1 Leucine Clycine Tyroslne I (b) .

I mg. 1.10 :Rotdn digdog enzymm: I a) a~spcdllepeptidebombioaprote~~t Twinis secreted by the pancreas in an inactive form trypsinogen. It is activated by enterokinase secreted by the glands in the intestinal wall. As trypsin is formed, it activates more trypsinogen to be converted into trypsin. This is autocatalytid activation. Trypsin acts in an alkaline medium betwcen pH 7-9. It breaks a peptide bond next to basic amino acid like arginine or lysine. The polypeptide fragments are further digested by the exopeptidases. Carboxypeptidase require the presence,of zinc ion and trypsin.-Otherexopeptidases are secreted in active form but need metal ions as cofactors to increase their activity. Fig 1.10(b) illustrates the action of aminopeptidase which removes terminal amino acids having free amino groups and carboxypeptidase which removes terminal amino acids possessing a free carboxyl group. In this w'ay these two enzymes remove from each end until a dipeptide fragment consisting of only two amino acids remains. Bonds between these pairs of amino acids are split by dipeptidases releasing free amino acid. The amino acids now, may be absorbed through the cells of the indstinal wall.

Carbohydrate Digestion Simple sugars like glucose and fructose can be absorbed and metabolised directly but such as sucrose or lactose and polysaccharides such as and glycogen have to be broken down to before they can be used in metabolic pathways. Carbohydrases, that digest carbohydrates can .be grouped into two categories: i) Polysaccharases that split polysaccharides into disaccharides or trisaccharides. ii) Glycosidases that break up the disaccharides or trisaccharides to monosaccharides. The digestion of carbohydrates, like proteins also occurs in steps. These along with the enzymes responsible for digestion are given in Table 1.8.

, Table 1.8 : Digestion of carbohydrates . . .. . ; ~

Poly-harides P"'ys;lcchar- ~i~~~~h~id~~Glyccwidaws r . (c6111005)x (C~~H220~~) (CnHZnOn)

Glycogen Amylases Maltose M~I~~SC, , . (animals) . .

Amylases 'Maltase '. Starch Maliose * Glucose (Plants) Cellulases Cellobioses -- Cellobiose r Glucose (Plants & animals) Trehalase Trehalose r Glucose (insects and some plants) Lactase Lactose r Galactose Glucose Between 2-18 per cent of lnvcrtase Sucrose r Fructose caucasians loose the capacity to Glucose produce lactase and between 95-1W per cent oriental and native ~fricanraces loose the . Carbohydrate digestion in vertebrates and invertebrates is very similar. All the ability td produce lactase as they enzymes shown in Table 1.8 are not required by all animals. The enzymes present grow older. They can no longer are related to the food habits of the animal. However, amylase and maltase are of digest milk which ferments in their gut and produces diarrhoea universal occurrence. Amylase is secreted in the saliva of man and in larger amounts and related problems. Interestingly by the pancreas. Enzyme production in some animals is also influenced by genetic yoghurt and cheese do not create characteristics and enzyme induction. For example, production of maltase and any problems as these contain less. sucrase by the intestinal villi depends on the amount of ingested sugar. If a high than 2 per cent lactose due to maltose or sucrose diet is taken it induces the villi to produce more maltase and action of bacteria. sucrase within 2-5 days. Lactase production declines in humans as gut develops after infamy. It ceases in some individuals so that they can no longer hydrolase this sugar. Now let us consider the digestion of cellulose, the most important structural material of plants and a major component of the diet of herbivores. Very few animals possess the enzyme.cellulases. Then how do animals that feed on plants breakdown this carbohydrate? Cellulases enzymes are synthesised by many bacteria and Symbiotic flagaates from protistans which live symbiotically in many herbivores and insects. Cellulose digestion termites are obligate anaerobic is carried on by the help of these symbiotic microorganisms. The microorganisms live organisms. Because of this sensitivity these flagellates can be . . in the stohch of the ruminants (i.e. cow, sheep, etc.) and breakdown the cellulose. removed from termite gut by The breakdown pioducts are then utilised by the host. In some invertebrates like exposing termites to oxygen at 3.5 silver fish (Ctenolepisma lineata) true cellulases have been reported but the insect atm pressure. The protozoa are cannot survive on an only cellulose diet. Some other invertebrates also have some selectively killed within half an cellulases that partly digest cellulose but none show conclusive evidence of a complete hour and the termites survive. breakdown of cellulose into glucose without the help of symbionts. Such treated termites do not survive when fed on wood though they stiU have bacteria in the gut. This shows that anaerobic Lipid Digestion ...... p-rotozopsrather than bacteria, are Digestion of fats is also similar inboth invertebrates and vertebrates. Lipases are the responsible for cellulqse digestion - enzymes that hydrolyse fats. A single l\pise can catalyse many steps in the break down in temiites. of fat. The vertebratepancreas secrete an enzyme lipase but before it breaks down ' fat, some detergent-like action is needed to emulsify the fat droplets. Bile salts from the liver, lecithin and cholesterol form miscelles and do this job. They reduce the surface tension at the fat-water interphase in a slightly alkaline medium and tiny emulsific-ation droplets of fat are formed. Then the lipase begins to digest the emulsified droplets. The resultant'fatt'y acids and monoglycerates are kept in solution by help of bile salts again and are finally absorbed. Glycerol is water soluble and easilyiabsorbed and metabolised. Fat like butter is absorbed directly through the intestinal without hydrolysis.

1.4.4 ~aintenanceof Gut Lining After studying the digestive enzymes you would'wonder why the gut linings are not digested themselves. This is because animals have several mechanisms that protect their gut lining from autodigestion. The mucous membranes of vertebrates secrete a slightly'alkaline mucous that lubricates the food and protects the lining'cells from

' corrosive secretions. 1n addition, the lateral surfaces of exposedepithelial cells are joined by tight junctions that prevent the secretions from penetrating between them. Careful studies have also revealed that the entire lining of the gut is renewed every third day in rats and every 2-6 days in humans. Similar mechanisms are present in invertebrates also. In insects, the fore-gut and hind gut are lined by cuticle. This lining is known as intimg. Only in the midgut, the epithelial cells are exposed, where most of the digestion occurs. The midgut is lined by a delicate lining the peritrophic membrane in some insects. This correspends to the mucous lining of vcrtebrates.

1.4.5 Coordination of Digestion You have learnt that digestion is a process in which large food mplecules arc bipken down step by step into their constituents. In primitive metazoans that are continuous feeders, the enzyme producing cells secrete continuously. In higher animals more ' precise controls are needed to regulate the~eleaseof food from stomach to intestine. and also the release of digestive enzymes at the proper time. The interplay of nervous and hormonal control is beautifully illustrated when we study the &ordination of digestive activity. t- . In the mammalian mouth, control of salivary gland secretion is entirely nervous; gastric secretions are under hormoaal.and neural control; and intestinal secretions are slower and are primarily under hormonal control.

I Gastrointestinal secretion is largely under the control of gastrointestinal hormones I secreted by endocririe glands of gastric and intestinal mucosa.

I Gastrointestinal ~oimones I The thiee Atlain mammalian gastrointestinal hormones are secretin, gastrin and chdecy&kinh (CCK). There are several other hormones, all peptides. The i physiology of only. thrce'major hormones is listed ii Table 1.9. .I"- a.r . ,..uulururu -uu",-uuu .M--9 - ..- m.""".""", I OY".YU..VY, +++ .hormdne nuwe important than other two

Gastrin Seeretin

Secreted by Stomach Duodenum Duodenum Stimulus for Peptides Acid (HCI) Amino acids release fatty acids Parasympathetic Nerves Effect on : Gastric Motility Gastric HCI Secretion Pancreatic secretion bicarbonates enzymes

Gastrin secretion is responsible for control of HCI volume; the presence of HCI in turn inhibits further gastrin secretion. Secretin is released under acidic conditions (low pH); digested fat or bile initiates production of pancreatic juice low in enzymes but rich in salts important in neutralising the acid chyme. CCK is secreted when partially digested proteins or HC1 (to a lesser extent) are present. It induces the flow of pancreatic juice rich in enzyme. Fig. 1.11 summaries the action of GI hormones.

I Stomach Gallbladder

Fig.l.11 : Action of several gastrointestinai hormones. Gastrin is secreted in response to intragastrk proteins, stomach distention and stimulation by vagus nerve. Gastrin from the lower stomach stimulates HCI seeretion and pepsin from secretory cells. Cholecystdrlnln (CCK) stimulates pancreas to secrete digestive enzymes and bases to nwtrallse and digest chyme. It also Induces contractlon of gall bladder to secrete bile salts. CCK is secreted in response to arrival of emlno acids and fatty acids In deodenum from stomach.

These two hormones inhibit stomach motility. Arrival of fat from the stomach initiates the release of CCK by intestinal mucosa, this ,causes gall bladder to release bile which aids in fat digestion.

SAQ 3 a) What are the main advantages of having a digestive tract with a mouth and anus?

b) Choose the correct answer. Digestion is brought -about by i) acids, ii) enzymes, iii) alkaline solutions, iv) vitamins and minerals c) Fill in the blanks with appropiiate words. In. the process of digestion proteins are converted into ...... carbohydrates to ...... fats to ...... and ...... d) Three hormones stirnulafe the release of digestive materials, ...... stimulates release of gastric juices ...... stimulates release of bicarbonate ion...... stimulates release of bile and pancreatic enzymes.

The monosaccharides, amino acids and other products of digestion must be passed on to other tissues to be useful for the organism. The process by which the digested material from the alimentary canal enters the blood stream is known as absorption. In intracellular digestion the same cells are concerned with digestion and absorption but in higher multicellular animals there are usually separate tissues and areas of gut for enzyme production, digestion and absorption. In this section we wi.1 mainly be concerned with absorption of amino acids, sugars and fats released during extracellular digestion in vertebrates. In all vertebrates most of the absorption is localised in the intestine. As you already know the wall of the vertebrate intestine is folded and ridged to increase the absorptive surface. These ridges or folds are covered by a velvet like pile of minute absorptive villi (Fig.1.12). These are highly specialised absorptive organs with a core containing a network of capillaries derived from blood vessels in the gut wall. Each villus also contains a central lymph capillary known as 4acteal which begins blindly at the tip of the villus and drains into the main lymph channels of the gut wall. Lipids pass mainly into the lacteals while sugars and aminoacids are absorbed directly by the blood capillaries. The villi and intestinal folds contain smooth muscles that contract to bring the villi in contact with the food in the intestine; and also maintaining the circulation in lacteals, lymphatics and small blood vessels.

Fig. 1.12 :Lining of mnmmpli.n smPU intestine a) Villus covered wlth digestive epithelium which consists of absorptlve cell and occasional goblet cells. b) An absorptive ceU. The apical surface bears a brush border of microvilli.

Now let us examine the processes involved in actual absorption of the digested food. You could recall from LSE-Ol Unit 7 the transport processes across membranes. We can summarise them as follows: Diffusion A material passes down its electrochemical gradient; at the end (passive) of the process the concentration inside the cell is equal to that outside. 23 Mediated Transpon Facilitated A specific membrane component is invplved in the transfer of the substrate along the concentration gradient but the concentration inside the cell never exceeds that of outside. Active A specific membrane component is involved in the transfer of a substrate, which can be accumulated inside the cell against the electrochemical .gradient.

All animal groups make some use of these three processes. The transport of sugars and of amino acids takes place through transport molecules that depend on the action of sodium pumps and can be blocked by metabolic inhibitors such as cynide. Some sugars for example, fructose are carried down their concentration gradient by facilitated transport. This process requires no energy other than the one provided by the concentration gradient of the diffusing substance. Other sugars like glucose and galactose as well as amino acids are absorbed through special transporter molecules in the membrane of the cell and depend on the Na' gradient between the lumen of the gut and of the epithelial cells. Let us first consider the absorption of glucose from the'gut lumen. (Refer to LSE-01, Units 7 and 8). The molecule involved in absorption of,glucose is known as Glu T, is one of the 5 gluwse cotransporter because it couples the transport of a glucose molecule with that of a transport molecules known to sodium ion. The energy needed is provided by the movement of sodium ion along its carry gljlwse across cell. membranes. These transporters gradient. The cotransporter enables cells lining the lumen of intestine to absorb even are broadly similar in structure quite small traces of glucose from food even though the epithelial cells may already and function and numbered in have high concentrations of glucose inside them. order of their discovery. These transporters change their Once inside the cell, the sodium ion is pumped out by ATP energised active transport configuration, one shape binds and the glucose molecule is transferred to the blood stream through another glucose on the extracellular side transporter molecule, Glu Tz, along its concentration gradient (Fig. 1.13). Glu Tz and the other shape binds glucose in the intracellular side. This transports glucose in propdrtion to the sugar concentration present in the blood. If . binding and transportation in a more glucose is present in the blood, transport is slowed and if glucose content of flip-flop manner is very rapid. blood is low then transport is accelerated.

Fig. 1.13 :Suggested mechanism for absorption of glucose. Na' and glucose are transpofled together i through carrier molecule or cotransporter located in the membrane. Inside the cell, sodium moves 1 out by ATP pump and glucose is taken by a transporter molecule to the blood.

Experimental evidence shows that atleast 4 transport processes for amino acids occur in the mammalian gut. Two for neutral amino acids, one for basic and one for , acidic amino acid. Another separate transport system exists for dipeptides and tripeptides. Once inside the cell, these breakdown into constituent amino acids bp intracellular peptidases. The sugar and amino acids reach the from where similar mechanisms use the sodium gradient to transport- amino acids and 24 glucose to the various tissues of the body. Absorption of lipids is quite different from the absorption of monosaccharides and amino acids. Fig. 1.14 shows the process. mefree fatty acids, monoglycerides and lysolecithi?~leave the miscelles and pass through the membrane of the microvilli to enter the epithelial cell. The miscelles may also be transported intact into the epithelipl cells. In ~itherevent, these products are used to resynthesise triglycerides anc$Jphc$pholipids within the epithelial cells. Triglycerides, phospholipids and cholesterol combine with protein inside the epithelial cells to form small particles called chylomicrons that are collected by the lacteals in the intestinal villi. Absorbed lipids thus reach the venous blood through the lymphatic system.

duct aod blood

Fig. 1.14 :Fatty acids and monoglycerides from the mlscelles within the small intestlne are absorbed by the epithelial cells and resyntheslsed into triglycerldes. These anthen coated by protein to form chylomicrons which enter the lacteals of the .

SAQ 4 In the figure of the human digestive tract and the associated glands given below: a) indicate the sites of activity. b) indicate the enzymes responsible for digestion of carbohydrates, proteins and fats.

Salivary gland Pharynx Trachea klrlRw8a-I 1.6 ENERGY METABOLISM .. In the p~cedingsections of the unit, you studied how the products of digestion of food, viz: amino acids, sugars and fatty acids are absorbed and transported to the body tissues. pebxidation of these compounds yields virtually all the chemical energy requiraaioy animals and the use of this chemical energy is referred to as their energy metabolism. You also learnt in Section 1.2 that generally carbohydrates and fats are the fuel which'provide energy but other organic compounds are within wide limits interchangeable in energy metabolism. How do we measure the rate of metabolism or the actual amount of energy liberated during oxidative metabolism? One way could be to measure the total heat produced by the organism per unit df time. This is the metabolic rate of the organism. The metabolic rate can be determined by using the formulation: rate of energy intake - rate of energy loss per unit time = metabolic rate . . . Energy intake is the chemical energy content of ingested food over a given period. Energy loss is the chemical energy that remains in faeces and urine produced by the animal over the same period. The energy content of food and wastes is found out by burning them in .abomb calorimeter (Fig. 1.15). The material to be tested is placed and burned with the aid of oxygen in a chamber surrounded by ajacket of water. ':.. ,The heat produced is determined by the rise i" temperature of the surroundingv+ti$' . . 'Table 1.10 provides the caloric value of the common food stuffs estimaied in b@b ;.- . calorimeter and in the body...... ,

. .

. . heat from the hod

&.

These values are essentially average values. During oxidative degradation in animal body, carbohydrates and lipids are fully oxidised to carbon dioxide and water just as in a bomb calorimeter but proteins are not degraded because the major end-product of protein metabolism, is urea which still possess some energy. Accordingly, the value is lower in the body (4.1 kcaUg) as you can see from Table 1.10. Energy derived from one gram of fat is much more than that derived from 1 gram of protein or of carbohydrate.

Tabk 1.10 : Fuel Cootent of Food Materinls

Food Kiloeolorks per gram

In Bomb ,In Body+ I Calorimeter Carbohydrates 4.1 4.0 Lipids 9.4 9.0 Proteins 5.6 4.1

k*. The heat broduced during the metabolic activities of the body helps in maintaining the body temperature. Generally, warm blooded animals like birds and mammals ' have specific regulatory. mchanisms bv which heat production is either increased or * dissipated in relation to the environmental conditions. You will learn more about nutrl*, Fqdlol, -, temperature regulation in Unit 7 of Block 2. The most commonly used measure to determine metabolic rate is to find the oxygen consumption of the animal. Oxygen consumption is a good indicator of metabolic rate because heat produced for each litre of oxygen used in metabolism is the same irrespective of the food stuff oxidised (see Table 1.1 1). The highest figure of 5.Ucal per litre of oxygen is only 10% more than the lowest figuresMitre of oxygen for proteins. On an average we consider metabolic rate to be equal to 4.8 kcalllitre of oxygen used.

Table 1.11 : Hqt pduetlon and respiratory quotient for foodstuff types

Heat produced Kcal Per litre Per litre C02formed of O2 0.f COz RQ = consumed formed . 0, used

Carbohydrates Fats Proteins

Respiratory Quotient Table 1.11 also shows the ratio of thevolume of carbon dioxide evolved to that of the amount of oxygen consumed during oxidation. This is the respiratory' quotient or RQ. It is an important concept in energy metabolism. From the table you can see that RQ is usually between 0.7 and 1.0. However, RQ near 0.7 shows that fat is being metabolised and RQ near 1.0 suggests carbohydrate metabolism. RQ in between 0.7 and 1.0 could indicate either protein or a mixed diet metabolism. Quite often animals cannot utilise the entire food value because not all the food they consume is fully digested. Also some portion is excreted as urea or ammonia. In general, it has been observed that animals have a higher intake of food than what is indicated by their oxygen consumption data so that their body weight is kept steady'. The oxygen consumption per unit weight/per unit time mm30,1g/hr tends to decrease with higher body weight of aaimals. In other words, small sized animals $ke mouse, shrew, etc., have a higher metabolic rate than a large sized animal (an elephant) as evidenced by their oxygen consumption. Accordingly smaller animals have a need to feed constarTtly. This would also mean that an elephant can survive without food for a much longer period of time than a mouse.

Energy Storage As we said above, food intake and energy expenditure for animals is approximately equal. If energy expenditure exceeds food intake, th,en the excess energy is taken up by utilisation of body fat. However, if food intake is excess, then the surplus is stored as fat irrespective of the kind of food eaten. Excess carbohydrates are changed to fats and accordingly RQ exceeds 1. This is because fats contain relatively less oxygen and the excess oxygen of carbohydrates is used in the metabolism. This reduces the oxygen uptake and the respiratory carbon dioxide, oxygen ratio is increased. For this reason fat is ideal storage material for energy. It is much lighter and yields twice as much energy as carbohydrates. Migratory birds that may have to fly more than 1000 km non-stop, carry fat as 40% to 50% of their body weight. Nonetheless, some carbohydrates are important in energy storage. Glycogen a starch-like carbohydrate polymer is stored as granules in the skeletal muscles and liver of vertebrates. During heavy muscular exercise when blood does not deliver sufficient oxygen to meet demands, glycogen provides the energy. It is broken down directly into glucose-6-phosphate, pToviding fuel fbr carbohydrate metabolism more directly than does fat. . , On the other hand, many animals that do not move about, also store glycogen as excess energy source. For example, clams, oysters and many intestinal parasites like Ascaris use glycogen as the storage material. These animds have to face anaerobic conditio,ns and in such situations glycogen breaks down to acetic acid to yield energy.

SAQ 5 A person on a diet does not consume any fat but only rice and sweets. After some time he finds that he is still gaining weight. Why?

1.7 SUMMARY

All hetesotrophic organisms require a balanced diet of nutrients to survive, grow and meet their energy demands. Nutrients that must be supplied to the animals in diet are called essential nutrients and the requirement for these essential nutrients indicates the animal's synthetic ability. Animals also have a quantitative requirement for nutrients and if they don't get the required amount, deficiency syndromes and suboptimal growth results. Food is obtained by animals in different ways, including absorption through body surface, endocytoiis, filter feeding, mucous trapping, sucking, biting and chewing. The feeding strategies hate evolved according to the nature of food required. Digestion is the breakdown of complex food molecules into simpler constituents and two broad categories of chemical digestion are seen - intracellular and extracellular. Intracellular digestion is characteristic of primitive animals and extracellular digestion of the higher forms. Digestion of proteins, carbohydrates and lipids is a step by step process in which larger molecules are broken down by the action of specific enzymes. Lipases are not as specific as carbohydrases and proteases. Secretion of digestive juices as well as the motility of smooth muscles are under neural and hormonal control. All gastrointestinal harmones are peptides. Both direct activation by food in the gut and neural activation stimulates the endocrine cells of the alimentary canal to secrete hormones. The products of digestion are taken up by the absorptive cells of the intestine and transferred to the blood capillarJes and lymphatic system. Transport of some sugars occurs by facilctated diffusion which does not require'metabolic energy. Most sugars and amino acids are transported with Na+ and a carrier molecule requiring- energy. Tkproducts of fat digestion are absorbed by diffusion across cell membranes. The absorbed nutrients provide fuel for body metabolism. The chemical energy used in metabolism can be measured as heat energy. A given class of food molecufes will liberate the same amount of heat and require the same amount of

oxygen when oxidised to water and carbon dioyide. The heat production and ' oxygen consumption of animals gives their metabolic rate and the respiratory quotient is useful in determining the proportion of carbohydrates, fats or proteins metabolised

1) What are essential nutrients? Why is that some nutrients are essential for some animals and non-essential for others? .. 2) Make a table that shows the characteristics of intracellular and extracellular digestion. List out some advantages of extracellular digestion from that information.

......

' , 3) What prevents the walls of the digestive tract from being digested in animals?

...... d ......

4) What ar,e the end-products'of food that can be absorbed by the body? Explain I how absorption of fats diffkrs from absorption of proteins and-sugars.

- --- - 1.9 ANSWERS

Self-assessment Questions 1) a) i) matches b) ii) .matches c) iii) matches a) b) Because these animals can synthesise vitamin C while humans and some fruit eating mammals have lost the ability to synthesise vitamin C. 2) i) Squirrel has chisel-like incisors used especially for gnawing. Cow has molars that are used in a side to side grinding motion for mastication of vegetation. Dog has pointed dagger-like canines for piercing and tearing food. ii) 1) matches d) 2) matches e) 3) matches b) 4) matches a) 5) matches c) 3) a) It permits different parts of gut to specialise for performing different digestive functions. b) ii) C) amino acids, monosaccharides, fatty acids and glycerol. d) gastrin, secretin, cholesystokinin. 4) a) 1) Salivary glands 2) Stomach . 3) Pancreas 4) Gall bladder 5) Liver 6) Small intestine b) Carbohydrates - amylase, sucrase, maltase, lactase Proteins - pepsin, trypsin, chymotrypsin, carboxypeptidase . Lipids - Lipase. 5) Excess carbohydrates that are not used for metabolism are converted to fats and stored in the body. This increases the weight. Terminal Questions 1) Essential nutrients are materials the animals need to sustain life but cannot synthesise in their cells. The nutrients that are essential'differ for each type of animal because the ability to synthesise is genetic and species specific. For example, most vertebrates can synthesise vitamin C but human and some other fruit eating species have lost this synthetic ability. Therefore, vitamin C is essential for us but non-essential for some others.

2) Intracellular Extracellular

Ingestion of small particles Ingestion of large food masses Digestive enzymes enclosed Usually well-developed digestive tract that in a small area. One kind of allows secreted enzymes to act on food at ' enzyme acts at one time in different parts at the same time, digestion digestive vacuole occurs in phases Suited to continuous feeding Suited to discontinuous feeding Two openings, a mouth and anus in complex animals so that food passes in one direction

For more complex freely moving organisms extracellular digestion is more advantageous because they don't have to feed continuously and a divi'sion of labour in the digestive tract occurs so that only a few cells are devoted to the digestive processes. Digestion can occur in different phases at the same time. 3) A lining of mucous secreted by the goblet cells and the fact the most digestive

' enzyme especially protease are present in inactive form. 4) Carbohydrates are hydrolysed to monosaccharides. Proteins are hydrolysed to amino-acids and fats are hydrolysed to the free fatty acids and glycerol. i) Amino acids and glucose enter the intestinal epithelial cells by a carrier or cotransporter protein molecules that depend on the action of a sodium ion pump. Free fatty acids or monoglycerides form miscelles with bile salts before entering the intestinal epithelium through diffusion. ii) Amino acids and glucose are passed on to the blood stream through anothe. transporter molecule while fatty acids and monoglycerides form chylomicrons which enter the lacteals from where they enter the blood stream.