UNIT 1 NUTRITION, FEEDING, DIGESTION

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. T o 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~nship hetween 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.

Objectives i

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 I

The terms essential and non- m n t i a l ami* acids are not very significant because the

non-essential amino acids are j b t as important for the body. May hi so important that the body cannot leave them to be supplied externally and so has mechanisms to synthesise them.

As we have said earlier all animals are heterotrophs and require food from the 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.

Generally all proteins are made from about 20 different amino acids in various combinations. However, it is not necessary to supply.al1 the 20 amino acids. Some can be formed in the body, using other amino acids but others have to be supplied through diet because they are not formed in the body. The amino acids that are ^, synthesised in the body are called nonsssential amino acids while those that have to be supplied through diet are known as essential amino acids.

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

Lysine Phenylalanine

I

^{Glycine Glycine}

Essential

Rats Humans

Tryptophan Lysine

I

^{Alanine Alanine}

Non-essentid

Rats Humans

Histidine Isoleucine

I

^{Serine Serine}

Phenylalanine Leucine

I

^{Cysteine Ty rosine}

hucine Valine

I

^{Tyrosine Aspartate}

Isbleucine Methionine

-1

^{Aspartate Glutamate}

Theronine Cystine

Methionine Tryptophan

Valine Theronine

Arginine

Glutamate Proline

Proline Hydroxyproline ^' ,,

Hydroxyproline Citrulline .

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 range of milligrams o r micrograms. These function as coenzymes in metabolic reactions. You might find it useful t o reread Unit 21 in FST-01 for the list of vitamins, their main functions and their dietary sources. Table 1.2 gives a more detailed list of vitamins important in animal and human nutrition along with their diverse functions.

The synthetic ability for vitamins also varies among different animal species and those essential vitamins that the animal cannot synthesise must come from its dietary sources. For instance, most animals can synthesise ascorbic acid but humans cannot.

We also depend on intestinal bacteria to synthesise vitamin K and BI2. Fat sol 'rle vitamins like A , D, E and K can be stored in the fat deposits of the body but vit -ts

The physiological role of vitamin K was discovered in birds fed a cholesterol free diet for the putpose of studying cholesterol synthesis. The birds developed severe bleedings which were traced to a vitamin deficiency.

Dnly after this it was discovered that vitamin K is necessary for mammals.

L Table 1.2 : The Vitamine and their chPRfterWcs

R ~ o t ~ o q a ~

la hunmb, except M aotcd

~ipM-so~ubIe viernlns:

A (CZoH,O) anti- xerophthalmic

Plants form (carotene, C&%) in green leaves, carrots, etc; is changed in liver to animal form (C&iWO), present in fish-liver oil (shark); both forms in egg yolk, butter, milk

Maintains integrity of epithelial tissues, especially mucous membrane; needed as part of visual purple in retina of eye

Xerophthalmia (dry cornea, no tear secretion), phrynoderma (toad skin) night blindness, growth retardation, nutritional croup (hoarseness) in birds

Fish-liver oils, especially tuna, less in cod; beef fat; also exposure of skin to ultraviolet radiation

Regulates metabolism of calcium and posphorus;

promotes absorption of calcium*in intestine;

needed for normal growth

& mineralisation of bones

Rickets in young (tqnes soft, yielding, often deformed);

osteomalacia (soft bones), ^, especially in women of Asia D (C,H,O),

antirachitic

E or tocopherol (C2UH~f12), antlstenl~ty

Green leaves, wheat- germ oil and other vege- table fats, meat, milk

Antioxidative; maintains integrity of membranes

Sterility in male fowls and rats, degeneration of testes with failure of sperma- togenesis, embryonic growth disturbances, suckling para- lysis and muscular dystrophy in young animals

Green leaves, also certain Essential production of bacteria, such as those of prothrombin in liver;

intestihal flora necessary for blood clotting

Blood fails to clot (C31%b02)r

antihemorrhagic

Water-soluble vitamins:

B complex Thiamine (B,) ( C I Z H I ~ O N ~ S ) . antineuritic

Yeast, germ of cereals (especially wheat, peanuts, other legumi- nous seed), roots, egg yolk, liver, lean meat

Needed for carbohydrate metabolism; thiamine pyrophosphate an essential coenzyme in pyruvate metabolism (stimulates root growth in plants) ^'

On diet high in polished rice, beriberi (nerve inflammation);

loss of appetite, with loss of tone and reduced motility in digestive tract; cessation of growth; polyneuritis (nerve inflammation) in birds ^, Riboflavin (B2)

(CI~HZOOC.N~)

Green leaves, milk, eggs, liver, yeast

Essential for giowth;

forms prosthetic group of FAD enzymes concerned with intermediate meta- bolism of food and elec- tron-transport system

Cheilosis (inflammation and cracking at corners of mouth), digestive disturbances.

"yellow liver" of dogs, curled- toe paralysis of chicks, cataract

Nicotinic acid, or niacin (C6HS02N), antipellagric

Green leaves, wheat germ, egg yolk, meat, liver, yeast

Forms active group of nicotina- mide adenine dinucleotide, which functions in dehydrogenation reactions

Pellagra in humans and monkeys, swine pellagra in pigs, blacktongue in dogs, perosis in birds

Eolic acid (C19H1906N7)

Green leaves, liver, soyabeans, yeast, egg yolk

Essential for growth and forma- tion of blood cells; coenzyme in- volved in transfer of single- carbon units in metabolism

Anaeniia, haemorrhage from kidneys, and s ~ r u e (defective intestinal abson, tion)'in humans; nutritional cytopen$

(reduction in cellular.elements of blood) in nionkeys; slow growth and anaemia in chicks and rats

Pyridoxine (B6) (C,H,zOzN)

Yeast, cereal grains, meat, eggs, milk, liver

Present in tissues as pyridoxal phosphate which serve as coenzyme in transamination and decarboxylation of amino acids

Anaemia in dogs and pigs; dermatitis in rats; paralysis (and death) in pig, rats and chicks; growth

retardation Pentothenic acid

(C9H1703N)

Yeast, cane molasses, peanuts, egg yolks, milk, liver

Forms coenzyme A, which cata- lyzes transfer of various carbo- xylated groups and functions in carbohydrate and lipid metabolism

Dermatitis in chicks rnd rats, graying of fur in black rats, "goosestepping"

and nerve degeneration in pigs Essential for growth; functions

in CO, fixation and fatty acid oxidation and synthesis Biotin (vitamin H)

(CIOHI~Q~N~S)

Yeast, cereal grains, cane molasses, egg yolk, livkr, vegetables, fresh fruits

Dermatitis with thickening of skin in rats and chicks, perosis in birds

Liver, fish, meat, milk, egg yolk, uysters, bacteria and fer-

. mentations of Srreptomyces;

synthesised only by bacteria

Formation of blood cells, growth;

coenzyme involved in transfer of methyl groups and in nucleic acid metabolism

Pernicious anaemia, slow growth in young animals; wasting disease

in ruminants .

.

Cyanocobalamin (BIZ) - . ( G H P O N I ~ O I ~ P ~ )

Maintains integrity of capillary walls; involved in formation of

"intercellular cement"

Scurvy (bleeding in mucous membranes, under skin, pnd into joints) in humans and gulnea pigs C, or ascorbic acid

(cdi806)

Citrus fruits, tomatoes, vege- tables;.also produced by animals (except primates and

guinea pigs)

- - -

.L. Usinger, R.C. Stebbins, and J.W. Nybakken, General Zoology, 6th ed., Mc

7 .

:Graw-Hill, ~ e w York, 1979

that are water soluble like B or C need to be supplied continually as they are lost 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 muscle contraction 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~l r o l e 4 important minerals

Elements S o d i i ~ ~ n (Na)

Potassium (K)

Calcium

, (Ca) Phosphorus (P) 'Magnesium (Mg) Chlorine ( a )

Main extracellular positive ion:

Regulates plasma volume. acid- hase halance: nerve and muwlc Function

Major intracellular positive ion:

nervc and muscle function:

acid hase balance

Component of bones, teeth;

regulation of nerve, muscle function; blood clotting 9onG formation, part of DNA.

RNA, ATP, etc.; energy metabolism

Bone and teeth: carbohydrate metabolism

Major extracellular negative ion; osmotic and acidibase balance; stomach acid

Deficiency Disease Unknown on normal diet.

Secondary in illness or injury

Secondary t o illness.

injury or diuretic therapy; paralysis, mental confusion muscular weakness Children-rickets Adults - ostmalacia

Children-rick$

Adults - osiomalacia

Secondary to mal- ^' ahsorption o r diarrhoea,

;~icoholism In infants fed on salt free formula; secondary to vomiting. diuretic therapy, renal disease.

Table salt, salt added to prepared food

Dairy products, beans, leafy vegetables Phosphate f w d additives

Leafy green vegetables

Table salt

Mineral requirements are met by a varied intake of adequate amounts of w h o l ~ grain cereals, legumes, leafy green vegetables, meat and dairy products.

Nutrition. Feeding, Digestion

The first conclusive evidence that cobalt is an essential trace element came from Australia ^' where a serious disease of cattle and sheep developed. Addition of small amounts of cobalt, prevented this disease. Each sheep is made to swallow a ceramic coated cobal ball that remains in the mmer and slowly releases the cobalt o\ifr several years.

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 B12 and 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

(Fe) haemoglobins, cytochromes

Copper Needed to make haemoglobin, Anaemia; secpndary to ( c u ) bone, part of cytochrome malnutrition, Menke's

syndrome Iodine Component of thyroid

(1) hormone

Children: cretinism Adults: goitre, hyper- thyroidism, myxedema Manganese Needed in urea formation, Unknown in humans (Mn) protein metabolism, glycolysis,

citric acid cycle

Cobalt Constituent of vitamin B I Z , B I Z deficiency

(Co) RBC formation

Zinc Essential constituent of many Hypogonadism, growth ( z n ) 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

(F) teeth

Selenium Needed in fat

(se) metabolism

Dental caries

Marginal deficiency where salt content is low, secondary to parenteral nutrition

Iron cookware ^'

lodised salt, seafood

Foods of animal origin

Drinking water

Chromium Needed in glucose Impaired glucose

(Cr) metabolism tolerance

Trace element requirements are met by a varied intake of whole grain cereal, legumes, leafy green vegetables, meat and dairy products

Some of the trace elements are essential for complex organisms while others may not be so essential. Flourine, for example, is an essential element for normal growth of 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~tions in 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 (annelids, 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 vacuoles Use of cilia Mucous traps Tentacles Filter feeding

Amoeba, Radiolarians Ciliates, Sponges, Bivalves, Tadpoles Gastropods, Tunicates Sea cucumbers

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

solution solution

Aquatic invertebrates

Symbiotic supply of Intracellular symbiotic Paramecium, Sponges, Flatworms,

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

r

oduce 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.

Water currents

v

ngocoel (body cavity)

move water) . Amebocyte

Choanocyte mucus

Amebocyte receiving food vacuole

Fig. 1.1 : Sponges obtain their fdod by filtering seawater. Food particles pass down the collar cells or choanocytes.and enter 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.

Fig. 1.2 : Some filter feeders and I

their feeding mechanisms Many animals use methods for chewing and sctaping to obt'ainlfood. Their food is often of ~ l a n t origin. Numerous insects and other invertebrates as well as herbivorous vertebraies use Gese methods of feeding. A rahp-like structurs known i s radula is used by gastropods to scrape algae from roqks of; t9 rasp through vegeta.tiio (Fig. 1.3).

R a d a retractor Radula protractor

-

(b') ( 4

(a)

Flg.l.3:a) S s s i t E . l & ~ b a d + ~ p W s b o w h g t ~ d u L P w W ~ ~ t o ~ 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 pharynx 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 v h , an e m ~ t pdy-,

a) antdw end with everted Jaw to apturn Prcr

b ) e x t m u l ~ ,

15

Spiders provide an interesting example of fluid feeding. Their preybare usually larger in size and covebed by a hard chitinous covering. Spiders, therefore, pierce the covering by their hollow jaws and pump digestive juices into the prey's body. These liquify the tissues and then the 'kpider'sucks the prey empty.

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

M

^premolars

^A

^~c~~

Fig. 1.7;,Mammaliandentitlon - teeth of (a) generalised mammal, (b) squirrel, (c) AMcan Uon, (d) OX

1.3.3 Feeding on Liquids

Animals feeding on liquids are generally highly specialised for their feeding habits.

Certain protozoa, endoparasites and aquatic invertebrates take up nutrient molecules 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 2) Cilia

a) Blaod, plant sap 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

-

intracellular digestion 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

<v'"

. I

Lysosome 3:

.'..",.. .- *""; . .

. ..._ ..:

.

⁴

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 nematodes,

,"

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 *

I

I

:

^Storage

\

,'

(Some spccies)

I

^(Add)

I

^Digestion

Jl

(Alkaline) Absorption and

1

Assimilation

J

Storage of waste Defecation

+

FLa.L.9: Ccncrdbed dlgwtlvt trod. Om way pslrs~ge

of food allows sequtntlal stag- In dlgcstlon.

. Duhcd Ilna represent crop Lo m m . . n l d s .

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 chordates 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 month. The herbivore animal spends hours masticating the food it takes in hurridly and stores it in its stomachsfor further use.

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 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 of the body. In vertebrates this function is carried out in the large.intestine.

In higher vertebrates, each area of the gut is specialised for a certain activity,

1

digestive enzymes are produced in glands as well as in the wall of the gut. Absorption occurs in the intestine predominantly.

1.4.3 Digestive Enzymes

Now let us consider the general principles of digestion that are applicable t6all animals. We will start with the digestive enzymes that breakdown the large food molecules into smaller soluble component units. This breakdown involves the uptake of water and is called hydrolysis. Before reading the following sub-sections, you 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:

a) Digestive enzymes are not as narrowly specific as other enzymes rather they show group specificity. For example, enzymes that digest carbohydrates can digest polysaccharides of both animal and plant origin,

b) Even though enzymes performing similar functions in different animals are given same names, they are not identical chemically. For example, trypsin (an enzyme, 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 vertebrate 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 peptide 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.

Inactive form

a - Active form Preferred Peptide

Link Attacked Endopepticlaw

Pepsinogen ^HC1

,

^pepsin'

pepsin

entemkinase ' trypsin Trypsinogen

trypsin

'

GYrn-in~gen

w *

chymotrypsin

Carboxpeptidase (H+) (trypsin)

Link to amino group of aromatic amino acid (tyrosine and phenylalanine)

Link to carbxyl end of arginine or lpine

Link to czubxyl group of aromatic amino acid (tryptophan, tyrosine,

' phenylalanine) and also bonds adjacent to methionine and leucine when'they are present

Link to terminal anjno acid with h e amino group

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 ,

-

HN-CH-CO ^I HN-CH-CO _I

-

^{NH-CHz-CO NH-CH-CC I}

^-

^NH-

Clutamic acid ^{b' v 6}

Clycine Lysine

Tyrosine

H3CVCH 3

CH

I 4$

^{.. /}

CHa Am~nopeptldase Carboxypeptidase

N Termlnal

I

NH,-CH-CO H N -CHt-CO

;

*

HN-CH-COOH

1"'

1

Leucine Clycine

I (b) . ^Tyroslne

I

I mg. 1.10 : Rotdn d i g d o g enzymm:

a) a ~ s p c d l l e p e p t i d e b o m b i o a p r o t e ~ ~ t

T w i n is 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.-Other exopeptidases 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 peptides 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 disaccharides such as sucrose o r lactose and polysaccharides such as starch and glycogen have to be broken down to monosaccharides 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 ~ ~ Glyccwidaws ~ ~ h r ~Monosaccharide i d ~ ~ .

(c6111005)x ( C ~ ~ H 2 2 0 ~ ~ ) (CnHZnOn)

Between 2-18 per cent of caucasians loose the capacity to produce lactase and between 95-1W per cent oriental and native ~ f r i c a n races loose the

.

ability td produce lactase as they grow older. They can no longer digest milk which ferments in their gut and produces diarrhoea and related problems. Interestingly yoghurt and cheese do not create any problems as these contain less than 2 per cent lactose due to .

action of bacteria.

Glycogen Amylases Maltose

(animals)

Amylases

Starch Maliose

(Plants)

Cellulases

Cellulose

--

Cellobiose

(Plants & animals)

Trehalose (insects and some plants) Lactose

M ~ I ~ ~ S C

,

^{, .}

. . 'Maltase * ^{' .} Glucose

Cellobioses

r Glucose Trehalase

r Glucose

Lactase

r Galactose Glucose lnvcrtase

Sucrose r Fructose

Glucose

Carbohydrate digestion in vertebrates and invertebrates is very similar. All the enzymes shown in Table 1.8 are not required by all animals. The enzymes present are related to the food habits of the animal. However, amylase and maltase are of universal occurrence. Amylase is secreted in the saliva of man and in larger amounts by the pancreas. Enzyme production in some animals is also influenced by genetic characteristics and enzyme induction. For example, production of maltase and sucrase by the intestinal villi depends on the amount of ingested sugar. If a high maltose or sucrose diet is taken it induces the villi to produce more maltase and 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 protistans which live symbiotically in many herbivores and insects. Cellulose digestion is carried on by the help of these symbiotic microorganisms. The microorganisms live . . in the s t o h c h of the ruminants (i.e. cow, sheep, etc.) and breakdown the cellulose.

The breakdown pioducts are then utilised by the host. In some invertebrates like silver fish (Ctenolepisma lineata) true cellulases have been reported but the insect cannot survive on an only cellulose diet. Some other invertebrates also have some cellulases that partly digest cellulose but none show conclusive evidence of a complete breakdown of cellulose into glucose without the help of symbionts.

Lipid Digestion ^{. . .. . .}

Digestion of fats is also similar inboth invertebrates and vertebrates. Lipases are the

-

enzymes that hydrolyse fats. A single l\pise can catalyse many steps in the break down 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 epithelium without hydrolysis.

1.4.4 ~aintenance of 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~elease of 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 ~ o i m o n e s

I The thiee Atlain mammalian gastrointestinal hormones are secretin, gastrin and

i

chdecy&kinh (CCK). There are several other hormones, all peptides. The physiology of only. thrce'major hormones is listed i i Table 1.9.

Symbiotic flagaates from termites are obligate anaerobic organisms. Because of this sensitivity these flagellates can be removed from termite gut by exposing termites to oxygen at 3.5 atm pressure. The protozoa are selectively killed within half an hour and the termites survive.

Such treated termites do not survive when fed on wood though they stiU have bacteria in the gut.

This shows that anaerobic p-rotozops rather than bacteria, are responsible for cellulqse digestion in temiites.

.I"- a.r

.

,..uulururu -uu",-uuu . M - - 9

- ..-

m.""".""", I O Y " . Y U . . V Y ,

+++

^{.hormdne nuwe} important than other two

Gastrin Seeretin

Secreted by Stimulus for release

Effect on : Gastric Motility Gastric HCI Secretion

Pancreatic secretion bicarbonates enzymes

Stomach Peptides Parasympathetic Nerves

Duodenum Duodenum

Acid (HCI) Amino acids

fatty acids

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 o r 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 t o 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