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Unit 5: Animal Nutrition 5.1-NUTRIENTS, 5.2 –DIET, 5.3-WORLD FOOD SUPPLIES, 5.4-HUMAN ALIMENTARY CANAL,

5.5-CHEMICAL DIGESTION, 5.6-ABSORPTION AND ASSIMILATION

SUFEATIN SURHAN BIOLOGY MSPSBS 2012

SYLLABUS CHECKLIST:

Candidates should be able to: (a) list the chemical elements that make up:

carbohydrates; fats; proteins;

(b) describe tests for: starch (iodine in potassium iodide); reducing sugars (Benedict’s solution); protein (Biuret test); fats (ethanol emulsion test);

(c) list the principal sources of, and describe the dietary importance of carbohydrates, fats, proteins, vitamins (C and D only), mineral salts (calcium and iron only), fibre (roughage) and water;

(d) describe deficiency symptoms of vitamins C and D and mineral salts calcium and iron; (e) understand the concept of a balanced diet; (f) explain why diet, especially energy intake, should be related to age, sex and activity of an individual; (g) state the effects of malnutrition in relation to starvation, heart disease, constipation and obesity; (h) discuss the problems that contribute to famine (unequal distribution of food, drought and flooding,

increasing population); (i) identify the main regions of the alimentary canal and the associated organs: mouth, salivary glands,

oesophagus, stomach, duodenum, pancreas, gall bladder, liver, ileum, colon, rectum and anus; (j) describe the main functions of these parts in relation to ingestion, digestion, absorption, assimilation and

egestion of food, as appropriate; (k) identify the different types of human teeth and describe their structure and functions; (l) state the causes of dental decay and describe the proper care of teeth; (m) describe peristalsis; (n) explain why most foods must be digested; (o) describe:

digestion in the alimentary canal; the functions of a typical amylase, protease and lipase, listing the substrates and end-products;

(p) describe the structure of a villus, including the roles of capillaries and lacteals; (q) describe the significance of villi in increasing the internal surface area; (r) state the function of the hepatic portal vein as the route taken by most of the food absorbed from the small

intestine; (s) state:

that large molecules are synthesised from smaller basic units: glycogen from glucose; proteins from amino acids; lipids (fats and oils) from glycerol and fatty acids;

the role of the liver in the metabolism of glucose and amino acids; the role of fat as a storage substance; that the formation of urea and the breakdown of alcohol occur in the liver.

SUF/BIO/MSPSBS/2012 2 OF 15

NUTRIENTS

Nutrients are chemical substances in food that

provide energy and materials needed by the body.

Food is a store of raw materials and potential

energy. An organism needs food:

as a source of energy for the chemical

reactions that take place in its body to keep it

alive and active.

for growth: to provide the substances needed

for making new cells and tissues, to repair and

replace worn and damaged tissues, and for

reproduction.

for making important chemicals in the body,

e.g. haemoglobin, enzymes, hormones,

antibodies, etc.

for maintaining health.

Food is made up of:

nutrients: carbohydrates, fats and oils,

proteins, vitamins, minerals and water;

fibre (roughage).

Of theses, only minerals and water are inorganic,

the rest are organic, i.e. carbon compounds.

Carbohydrates, fats and oils, and proteins are

required in large quantities as they supply energy

and materials to the living organism.

CARBOHYDRATES

Carbohydrates are made up of the elements carbon,

hydrogen and oxygen.

The hydrogen and oxygen atoms are present in the

ratio 2:1 in each carbohydrate molecule (the same

ratio in which they are present in water).

Carbohydrates may be classified into:

1. monosaccharides,

2. disaccharides and

3. polysaccharides.

Monosaccharides (Mono:One, Saccharides:Sugar)

Monosaccharides are simple sugars with the formula

C6H12O6 but their atoms are arranged differently

within the molecules. These different arrangements

give the sugars different chemical and biochemical

properties.

They are soluble in water.

Examples of monosaccharides are:

1. Glucose; it is the energy source of both plants

and animals cells respiration.

2. Fructose; it is found in honey, tree fruits,

berries, melons and some root vegetables.

3. Galactose; it is found in dairy products and

sugar beets.

They produce a brick-red precipitate of

copper(I)oxide when boiled with Benedict’s

solution. Sugars with this property are called

reducing sugars.

Disaccharides (Di:Two, Saccharides:Sugar)

Disaccharides are complex sugars with the formula

C12H22O11.

They are soluble in water.

They are formed when two monosaccharides are

combined by a process of removing water called

condensation.

Examples are:

1. Maltose (malt sugar; found in germinating

grains) made up of condensing two

molecules of glucose.

2. Sucrose (cane sugar) made up of condensing

a molecule of glucose and fructose.

3. Lactose (milk sugar) made up of condensing

a molecule of glucose and galactose.

Maltose and lactose are reducing sugars while

sucrose is not.

Polysaccharides (Poly:Many, Saccharides:Sugar)

Polysaccharides are macromolecules which are

made up of thousands of simple sugar units.

They are insoluble in water.

Polysaccharides are produced by the condensation

of many glucose units to form a long molecule but in

each, the glucose units are linked in a different

way.

Examples of polysaccharides are:

1. Starch; the storage form of glucose in green

plants. Cereals, potatoes, yam and tapioca are

rich sources of starch.

2. Glycogen; the storage form of glucose in

animals.

3. Cellulose; forms the cell walls in plants.

Mammals cannot digest cellulose but herbivores

have bacteria in their intestines to digest it. In

other mammals, cellulose forms fibre in the

diet.


They can be broken down to smaller molecules by

hydrolysis using acid or enzymes.

Glycogen and starch are suitable as storage

materials because:

They are insoluble in water and so do not

change the osmotic pressure in the cells.

They are large molecules which are unable to

diffuse through the cell membranes.

They are easily hydrolysed to glucose when

needed and so is ideal as a stand-by source of

energy.

The molecules have compact structures so they

occupy less space.

They are inactive.

Dietary Importance of carbohydrates

As a substrate for respiration, to provide energy for

cell activities (1g yields 17kJ of energy);

To form supporting structures, eg. cell walls in

plants;

To be converted into other organic compounds such

as amino acids and fats;

For the formation of nucleic acids, eg. DNA;

To synthesise lubricants, eg. mucus, which consists

of a carbohydrate and a protein; and

To synthesise the nectar in some flowers. Nectar is

a sweet liquid that plants produce to attract

insects.

LIPIDS

Fats and oils are made up of the elements carbon,

hydrogen and oxygen but they contain very little

oxygen unlike carbohydrates.

Fats are made up of fatty acids and glycerol joined

up as shown:

They are all insoluble in water.

There are two types of fats:

1. Saturated fats (Animal fats).

Contain saturated fatty acids.

Solids at room temperature.

Examples are butter and lard.

Bad for health.

2. Unsaturated fats (Vegetable fats).

Contain unsaturated fatty acids.

Liquids at room temperature.

Examples are corn oil and palm oil.

Good for health.

Dietary sources of fats

Animal fats: meat, milk, lard, butter and egg yolk.

Plant fats: found as oils in nuts and seeds.

Dietary importance of fats

As a source of energy (1g yields 39kJ of energy);

To form part of the cell surface membrane;

As solvents for fat-soluble vitamins (A, D, E and K)

and some hormones;

As an insulation to prevent excessive heat loss from

the skin of mammals;

As shock absorber to protect internal organs from

physical damage;

To prevent water loss from the skin surface (in

human)

Secretion of sebum (oily substance from

sebaceous glands) over the skin surface provides

a waterproof layer

Rate of water loss from evaporation decreases

For buoyancy in aquatic animals (blubber in

whales).

PROTEINS

Proteins are made up of the elements carbon,

hydrogen, oxygen and nitrogen. Often, sulphur

and phosphorus are also present.

They are made up of basic units called amino acids

linked together by condensation to form polypeptide

chains.

These polypeptide chains are folded and twisted to

form large, complex structures.

There are twenty-two types of naturally occurring

amino acids.

In each protein, some or all the amino acids are

present in different numbers and order so that each

protein is unique.

The essential amino acids are amino acids that

cannot be made in our body and so must be present

in our dietary protein foods.

Dietary sources of proteins

Animal proteins: lean meat, fish, eggs, milk and

cheese.

Plant proteins: beans and cereals.


Dietary importance of proteins

For synthesis of new protoplasm, i.e. for growth and

repair of worn-out body parts e.g. hair, nails, red

blood cells, muscles, ligaments, tendons.

As a source of energy (1g yields 17kJ of energy);

only used when carbohydrates (glucose or glycogen)

and fats are all used up e.g. during starvation.

For synthesis of enzymes, hormones, antibodies,

haemoglobin, etc.

Note: Lack of proteins in daily diet will result in a

deficiency disease known as Kwashiorkor.

Kwashiorkor sufferers show signs of thinning hair,

edema, inadequate growth, and weight loss.

FOOD TEST

1. Test samples can either be a:

o Solid sample; the sample must be crushed and a

solvent (water or alcohol) will be used to

extract out the molecules.

o Liquid sample; unless stated, the volume of test

sample used would be 2cm3.

2. Keep the foods completely separate from each

other. Always use clean test-tubes, spatulas, white

tiles, Petri dishes and knives for each kind of food

to avoid contamination.

3. Use the same amount of reagent for each test.

(A) Test for simple sugars or reducing sugars.

Reducing sugars (glucose, fructose, galactose,

maltose and lactose) react with copper(II) ions in

Benedict’s solution and causes these ions to be

reduced to copper(I) ions which is red and insoluble.

This is the only food test which requires heating.

If the sample given is a solid, then begin with:

1. Crush the solid food specimen on a white tile

provided.

2. Place some crushed specimen in a test tube.

3. Add 2cm3 water (optional) and shake thoroughly

to mix the sample.

4. Add 2cm3 Benedict’s solution. Shake well.

5. Heat the mixture in a hot boiling water bath for

5 minutes.

If the sample given is a liquid:

1. Add equal volume of Benedict’s solution to the

liquid food specimen in a test-tube (2cm3).

2. Shake the mixture and heat it in a hot water

bath for 5 minutes.

Depending on the concentration of reducing sugar

present, the possible colour changes are from blue

to green to yellow to orange to red precipitate:

Green colour: very low concentration of

reducing sugar present.

Yellow colour: low concentration of reducing

sugar present.

Orange colour: high concentration of reducing

sugar present.

Red colour: very high concentration of reducing

sugar present.

Blue colour remains: reducing sugar absent.

(B) Test for starch

If the sample given is a solid:

1. Cut and crush the solid food specimen on a

clean glazed white tile.

2. Place some specimen in a clean test tube.

3. Add 2cm3 water to dissolve it (optional).

4. Add 1cm3 iodine solution. Shake well. Iodine

solution is 1% iodine in potassium iodide

solution. Iodine solution is brown in colour.

If the sample given in a liquid form:

1. Place 2cm3 of the specimen into a test-tube.

2. Add 1cm3 iodine solution. Shake well.

If the colour of iodine changes from brown to blue-

black, starch is present.

If the colour remains brown, starch is absent.

(C) Test for fats – The Ethanol Emulsion test

Fats will dissolve in alcohol but not in water.



clean DRY glazed white tile. Place some crushed

specimen in a clean dry test-tube. DO NOT ADD

WATER!

2. Add 2cm3 of 100% ethanol.

3. Shake well and leave to stand for a few

minutes.

If the sample given is in a liquid form:

1. Place 2cm3 of the specimen into a clean DRY

test-tube.

2. Add 2cm3 of 100% ethanol.

3. Shake well and leave to stand for a few

minutes.

After step 3 for both samples either:

1. Decant the top portion of the liquid into another

clean test-tube containing 2cm3 of water; OR

2. Add water drop by drop to the mixture in the

test-tube.

Observation:

If a cloudy white emulsion is formed followed

with an increase in temperature, fat is present.

If a cloudy white emulsion is not formed, fat is

absent.


(C) Test for proteins – Biuret Test



clean glazed white tile.

2. Place some crushed specimen in a clean test

tube.

3. Add 2cm3 water (option). Shake well.

4. Add 2cm3 of Biuret reagent. If Biuret reagent is

not available or provided, add 1 cm3 of sodium

or potassium hydroxide followed by 1% copper

sulphate solution, drop by drop, shaking after

every drop.

If the sample given is a liquid:

1. Place 2cm3 of the liquid sample into a clean

test-tube.

2. Add 2cm3 of Biuret reagent. If Biuret reagent is

not available or provided, add 1 cm3 of sodium

or potassium hydroxide followed by 1% copper

sulphate solution, drop by drop, shaking after

every drop.

Observation:

If the colour changes from blue to purple,

protein.

If the colour remains blue, protein is absent.

MINERALS

Minerals are inorganic substances required only in

small amounts in the daily diet.

Calcium Iron

Sourc

es Flour

Milk

Cheese

Liver

Red-meat

Spinach

Functi

ons

For healthy bones and teeth (combines with phosphates to form calcium phosphates, one of the main components of teeth and bones)

For muscular contraction

For blood clotting

For formation of haemoglobin, oxygen carrying pigments in red blood cells

Defi

cie

ncy d

isease

and

sym

pto

ms

Rickets. Osteoporosis (porous bones);

Brittle bones which fractures easily.

Stunted growth.

Muscle spasm.

Dental decay.

Anaemia;

Dizziness.

Pale appearance.

Breathlessness.

Easily fatigued and weak.

VITAMINS

Vitamins are organic substances required only in

small amounts in the daily diet. They are needed for

normal health and development.

There are two types of vitamins, water soluble

(Vitamins B and C) and fat soluble (Vitamins A, D,

E and K) vitamins.

Water soluble vitamins cannot be stored in the body

and are removed from the body in the urine.

Therefore, they have to be supplied from the daily

diet.

Fat soluble vitamins can be stored in the body i.e.

in the body fats e.g. liver and adipose tissue.

Therefore, these do not have to be consumed daily.

Vitamin C

(Ascorbic acid) Vitamin D

(Calciferol)

Sourc

es

Citrus fruits (fresh)

Raw vegetables (fresh)

Fish liver

Egg yolk

Dairy products

The action of sun on the skin

Import

ance

For healthy gums

For aiding absorption of iron in the small intestine

For healthy capillaries and epithelial tissues

For skin repair especially in healing of wound

For aiding absorption of calcium in the small intestine

For formation of strong teeth and bones

Defi

cie

ncy

Scurvy: swollen bleeding gums, loosening of the teeth, painful swelling of joints, internal bleeding of muscles and skin.

Rickets: soft weak bones that bend and break under pressure, poor teeth and bone formation, bowed legs and knock-knees develop.

DIETARY FIBRE (ROUGHAGE)

Dietary fibre is indigestible cellulose cell walls of

plants.

Humans cannot digest cellulose or lignin due to the

absence of the enzyme cellulase.

Dietary sources of roughage

Fruits and vegetables.

Outer husks of cereal grains (bran) such as oats,

wheat and barley.

Whole meal bread, brown or unpolished rice.


Dietary importance of roughage

Prevents constipation:

Provides bulk to feaces in the large

intestine/colon aiding the process of peristalsis.

Acts as a sponge to absorb water softening the

feaces.

Reduces the amount of fat absorption.

Reduces the risk of colon cancer.

WATER

In mammals, about 70% of the body weight is water

and in most conditions, humans can survive longer

without food than without water.

Dietary importance of water

Water is the medium or substance in which chemical

reactions occur in an organism such as hydrolysis

(breakdown of large, complex food molecules into

smaller simpler molecules).

Water helps to transport dissolved substances

around the body, such as:

digested products from the small intestine to

other parts of the body;

excretory products or waste products from the

tissue cells to the excretory organs for removal

from the body and

hormones from the glands to parts of the body

which require them.

Water is a key component of:

protoplasm;

lubricants found in joints;

the digestive juices;

blood and

tissue fluid.

Water helps to control the body temperature.

Water is a component of sweat and the evaporation

of sweat removes excess sweat from the body

preventing the body from overheating.

DIET

The food an animal eats everyday is called its diet.

A balanced diet is a diet that contains adequate

amounts of all the necessary nutrients required for

healthy growth and activity.

The body needs sufficient energy to maintain the

basal metabolic rate (BMR) and to sustain additional

activities of the individual.

Basal metabolic rate (BMR) is defined as the energy

needed for vital body functions (such as heartbeat,

circulation, brain function and essential chemical

reactions in liver and other organs) to keep alive

when the body is at rest.

The basal metabolic rate depends on the life-style,

climate, occupation, body size, gender, activities

and age; therefore, a balanced diet for one person

may not be balanced for another.

Climate

A person living in a cold country tends to lose more

heat to the surrounding and will require more food

to keep warm. This person will therefore have a

higher basal metabolic rate than a person living in

the tropics.

Body size

A person with a bigger build will require more

energy for their basal metabolism than a person

with a smaller build.

Age

Growing children has a higher basal metabolic rate

than adults as they need more energy for growth.

A child requires extra amounts of carbohydrates,

proteins and calcium in his/her balanced diet:

Carbohydrates: for energy.

Proteins: for growth.

Calcium: for formation of healthy bones and

teeth.

Gender

The basal metabolic rate for males is higher than

that of females. This is because women usually

have a greater amount of fatty tissue in their

bodies. This causes women to be more efficient in

preventing heat loss.

A menstruating female requires extra iron in her

balanced diet. Iron is important for the formation

of extra red blood cells to replace those lost during

menstruation.

A pregnant women requires extra amounts of

proteins, iron and calcium in her balanced diet:

Proteins: for healthy growth of embryo into

foetus (formation of new cells).

Iron: for formation of extra red blood cells to

accommodate foetal requirement for oxygen.

Calcium (+phosphate): for formation of foetus’s

bones.

Vitamins: for healthy development.

A lactating mother requires more intakes of

proteins, vitamins and calcium to produce milk of

adequate quality and quantity.


Activities

An active adult or individuals who have an

occupation that involves heavy work will need more

energy than a person who is moderately active.

These active adults and manual workers need extra

amounts of carbohydrates, fats, proteins and water

in their balanced diet.

PROBLEMS CAUSED BY UNBALANCED DIET - MALNUTRITION

Malnutrition is the result of unbalanced diet of

either lack of food (undernutrition) or excess of

food (overnutrition).

Undernutrition

This is a lack of one or all types of food.

Starvation is the massive lack of all kinds of

food. It restricts growth, development of

muscles and deficiency diseases which

eventually causes death.

Famine is the lack of adequate amount of food

to support the population.

Starvation and famine may result from one or many

of the following factors:

Unequal distribution of food

The world produces enough food to sustain the

current population. However, some areas

overproduce while others do not produce enough.

Poverty

The cost of transporting food to where it is required

is often too high to those who need it, as they tend

to be those in the poor regions of the world.

Overpopulation

Some regions have more population than what their

food can adequately support. This may be

controlled through education and availability of

birth control methods. Some countries even

imposed laws to control population growth, such as

restricting the number of children per family.

Natural disasters

Quantity of products from farming can be severely

reduced by natural events such as flooding and

draughts. Sudan, Ethiopia and Chad have been

experiencing decreased rainfall for the last 200

years.

Crop failure due to diseases

Biological research is developing disease-resistant

crops and improved methods of disease control.

Poor farming techniques

Education on improved farming practices and

understanding on the effects of deforestation and

land clearing may ease the problems.

War or political instability

War or political instability within a region affects

the distribution of food in that region, where some

parts may not receive any food at all for long period

of time.

Overnutrition

This is the excessive intake of energy food everyday,

more than what the body requires. Excess energy

food will be stored up as either glycogen in the liver

or as fats in the body. This often leads to obesity.

Obesity means being very, very fat. It is a condition

that is very dangerous to health.

Physiologically, obesity has been related to many

other health problems such as hypertension,

diabetes and coronary heart disease.

Psychologically, it causes emotional stress and the

feeling of being socially rejected as a result of poor

self image.

Cardiovascular disease

The excess weight stresses the joints and also puts

extra strain on the heart as it has to work extra

hard to pump blood to the skeletal muscles.

The excess animal fats and cholesterol in blood

forms deposits called atheroma on the walls of

arteries. This causes the walls to harden.

Atheroma decreases the diameter of the lumen of

the arteries (atherosclerosis).

Both the extra volume of blood as well as the

formation of atheroma results in hypertension,

which increases the risk of heart disease.

If atheroma occurs on the walls of the coronary

artery (the blood vessel that carries blood to the

cardiac muscles of the heart), it will restrict the

blood flow to the heart muscles, decreasing their

oxygen supply.

In severe cases, the artery may become blocked

either by the atheroma itself or by a blood clot,

leading to cardiac arrest (heart attack). If this

occurs in the brain, this may lead to stroke.


NUTRITION

The intake of food, with the changes that result in

the conversion of food substances into living matter,

is known as nutrition.

Nutrition may be divided into two types:

1. Autotrophic Nutrition.

2. Heterotrophic Nutrition.

Plants feed by autotrophic nutrition. “Auto” means

self and “trophic” means feeding. So autotrophic

nutrition means that plants feed itself and does not

rely on other living things to make its food for it.

Animals cannot make their own food. They feed on

organic substances which have been originally made

by plants. This is called heterotrophic nutrition.

“Hetero” means other and so heterotrophic

nutrition means that an animal feeds on substances

made by other organisms.

Heterotrophic nutrition in animals

Human (heterotrophic) nutrition involves five basic

stages:

(1) Ingestion (eating/feeding)

Taking food into the alimentary canal via mouth.

(2) Digestion

Mechanical digestion: Chopping and grinding of

food (with teeth) and muscular churning of food (in

the stomach).

Chemical digestion: Breaking large, insoluble

molecules into small soluble ones via the action of

enzymes in the alimentary canal (in the presence of

water = hydrolysis)

(3) Absorption

Transport of soluble food products into the

bloodstream via villi in the ileum.

(4) Assimilation

Using the absorbed food in the metabolic processes

and conversion into new protoplasm.

(5) Egestion / Defecation

Removal of any undigested and unabsorbed food

from the alimentary canal via anus.

The need for chemical digestion

Most foods contain insoluble starch, proteins and

fats, as these are large and complex molecules.

These large insoluble molecules cannot be absorbed

into the bloodstream so they need to be broken

down into small, soluble and absorbable / diffusible

ones.

Glucose, amino acids, fatty acids and glycerol are

the smallest forms of their ‘parent’ molecules.

HUMAN DENTITION

The function of teeth is to chew food and cut it into

tiny pieces.

Chewing increases the surface area of food and

helps to speed up the action of digestive enzymes

allowing digestion to be more rapid and efficient.

It also helps to dissolve the soluble parts of the

food.

Chewing is only a mechanical digestion of food

where the food remains chemically unchanged.


Types of teeth, dentition, tooth structure and function

Every human being has two sets of teeth in his life-

time.

The first set of teeth is called milk teeth or

deciduous teeth. Later this milk set is replaced by

a permanent set of 32 teeth.

If the permanent teeth are lost any reason, they will

not grow again.

The number, the type and the arrangement of the

teeth in a mammal is called dentition. This can be

expressed using the dental formula.

In the dental formula, the letters express the

different types of teeth in one half of the upper

and lower jaws, and the numbers expresses the

number of each type of teeth.

Dental formula of milk teeth:

i 2

2 : c

1

1 : pm

0

0 : m

2

2

Dental formula of permanent teeth:

i 2

2 : c

1

1 : pm

2

2 : m

3

3

There are four different types of human teeth:

Types of teeth Function

Incisors

Sharp-edge, chisel-shaped teeth at the front jaw.

Single-rooted.

2 in each quarter jaw (milk and permanent dentition).

For biting and cutting food.

Canines

Sharp, cone-shaped and pointed.

Single-rooted.


For biting and tearing food.

Premolars

Surface of each tooth have two projections or cusps.

Single-rooted.

2 in each quarter jaw (permanent dentition only).

For cutting, crushing and grinding food.

Molars

Tooth surface is square with 4 cusps.

Double-rooted.


For crushing and grinding food.


Parts of a tooth

Composition Function(s)

Enamel Hard, non-living layer of calcium salts.

Forms a hard, biting surface; Protects dentine.

Dentine Softer, bone-like layer.

Acts as a shock absorber.

Pulp cavity

Contains blood capillaries; Tooth cells and nerves present.

Supply food and oxygen to cells; Tooth cells divide to form dentine.

Cement A thin layer of bone-like material

Covers dentine of root and holds root in socket in jaw

Fibres Embedded in the cement.

Allow slight movement of root in socket.

Jawbone and

socket

Bone structure made of calcium phosphate.

Tooth fits into sockets of jawbone.

Root opening

Entrance for blood capillaries and nerves

For blood supply, growth and replacement of tooth; Nerves carry messages to and from brain.

DENTAL DECAY

If teeth are not properly cared for, they may suffer

from dental caries or decay.

The process of dental decay:

1. Small amounts of food, particularly those rich in

sugar, are left on and between the teeth.

2. There are large numbers of harmless bacteria

living in the mouth. Some of these bacteria

stick to the surfaces of the teeth forming a thin

film called dental plaque.

3. Bacteria feed on the sugary deposits using them

for their own metabolism and grow within the

plaque.

4. Bacteria secrete acids as a result of their

metabolism. These acids dissolve the outer non-

living enamel covering of the tooth.

5. A cavity develops, in which more sugary deposits

collect. More bacteria settle secreting more acids.

The size of the cavity increases.

6. Eventually, the cavity reaches the living parts of

the tooth. The first part affected is the dentine,

where there are nerve endings causing toothache.

Then the decay reaches the pulp cavity leading to

an abscess i.e. painful swelling of the gums filled

with puss.

Oral hygiene

Do not eat sweet or starchy foods before going to

bed.

Reduce daily intake of sugary food.

Ensure daily diet contains sufficient calcium,

phosphorus and Vitamin D for the formation and

maintenance of strong teeth.

Brush teeth regularly particularly after eating, first

thing in the morning and before going to sleep.

Use dental floss regularly to remove fragments of

food from between the teeth.

Use a toothpaste which contains:

Fluoride: to strengthen tooth enamel.

Bacteriacide / Antiseptic: to kill bacteria.

Alkaline: to neutralise acids released by the

bacteria

Visit a dentist regularly for an examination and have

treatment if required.

DIGESTIVE SYSTEM: HUMAN ALIMENTARY CANAL


The alimentary canal consists of the following parts:

The whole of the alimentary canal consists of layers

of muscles and cells which secrete a slimy liquid

called mucus.

The slimy mucus makes it easier for the food to

move along.

The layers of muscles bring about wave-like

contractions called peristalsis which push food

along the alimentary canal.

The Mouth

Food enters the body through the mouth, which

leads into the buccal cavity (pharynx).

In the mouth there are:

1. Teeth: Chewing action of the teeth

(mastication) breaks down large pieces of food

into smaller pieces. This increases the surface

area to volume ratio of the food so enzymes

can act it more efficiently.

2. Salivary glands: These three pairs of glands

which produce saliva; a slightly alkaline

digestive juice, contain an enzyme called

salivary amylase (ptyalin) which digests starch.

3. Tongue: It mixes the food with saliva and then

rolls it into small, slippery balls called bolus

(plural: boli). These balls of food are then

swallowed. Mucin, which is present in the

saliva helps to soften food pass it smoothly from

the mouth to the oesophagus.

The Oesophagus

The oesophagus or gullet is a narrow, muscular tube

leading from the mouth to the stomach.

The wall of the oesophagus is made up of two layers

of muscles. These muscles are present along the

gut from the oesophagus to the rectum.

The two layers of muscles are:

1. Longitudinal muscles on the outside of the gut.

2. Circular muscles on the inside of the gut.

These two muscles are called antagonistic muscles

which means that if one muscle contracts the other

relaxes. The movement of these two muscles

produces long, slow contractions which move food

along the gut.

During peristalsis:

Behind the food bolus: Circular muscle contracts

(small lumen), longitudinal muscle relaxes.

Aim: To decrease the diameter of the lumen

pushing the food bolus forward.

Region of food bolus: Circular muscle relaxes

(wide lumen), longitudinal muscle contracts

(shorter distance).

Aim: To allow the lumen to widen to

accommodate the food bolus, and to shorten

the distance across which the food bolus has to

move.

For smooth movement of food throughout the

alimentary canal, the food passage is lubricated

with mucus (to reduce friction).

The Stomach

The stomach is a distensible (can be stretched or

expanded) muscular bag, with thick and well-

developed muscular walls.

When the stomach is fully distended, it sends signals

to the brain that it is full or sated.


The stomach has two rings of muscles called

sphincters, one at its entrance and one at its exit.

These sphincters control food going into and out of

the stomach.

Gastric glands in the wall of the stomach secrete an

acidic digestive juice called gastric juice. Gastric

juice is a dilute solution of hydrochloric acid (about

pH 2) and two enzymes, pepsin and rennin.

The dilute hydrochloric acid:

Stops the action of salivary amylase by

denaturing it.

Provides an optimum pH for pepsin and

rennin. Pepsin belongs to a type of enzyme

known as protease which digest proteins by

breaking them into amino acids.

Rennin curdles or coagulates milk by changing

soluble caseinogens to insoluble casein.

Kills certain potentially harmful

microorganisms in food.

Food normally remains in the stomach for about

three to four hours. The partly digested food

becomes liquefied, forming chyme.

Chyme passes in small amounts into the duodenum

when the pyloric sphincter relaxes and opens.

The Small Intestines

The small intestine is a long narrow tube of about 5

– 6.4 metres long in a normal adult human and is

divided into the duodenum and ileum.

The Duodenum

The duodenum is the first part of the small

intestine.

It is a U-shaped tube of about 30 cm long.

It receives three types of digestive juices:

1. Bile juice from gall bladder in the liver.

Liver is the largest internal organ and it

produces bile; a green-yellow coloured

fluid, which is stored temporarily in the gall

bladder. Bile is transported into the

duodenum via the bile duct.

Bile is the product of breakdown of old red

blood cells in the liver. Bile juice contains:

a) Bile salts: emulsifies fats i.e. converts

large fat droplets into small droplets.

Emulsification increases the surface

area for the enzyme lipase to act on

therefore, faster enzyme action.

b) Bile pigments: gives the colour of bile

juice, removed with faeces (gives colour

to faeces)

Bile juice has an alkaline pH and is

important for neutralising the acidic chyme

from the stomach.

2. Pancreatic juice from the pancreas.

Pancreas lies between the stomach and the

duodenum.

Pancreatic juice contains:

1. Lipase: digests fats into fatty acids and

glycerol.

2. Amylase: digests any remaining starch

into maltose.

3. Trypsin: digests proteins into

polypeptides.

4. Sodium Hydrogen Carbonate (NaHCO3):

To provide a slightly alkaline pH for

the enzymes to work in.

To neutralise the acidic chyme from

the stomach.

The pancreas is also an endocrine gland

where it produces the hormones insulin and

glucagon for controlling the concentration

of glucose in the blood and carbohydrate

metabolism.

3. Intestinal juice produced by duodenal walls.

The juice contains:

1. Lipase: digests fats into fatty acids and

glycerol.

2. Erepsin: digests polypeptides into amino

acids.

3. Maltase: digests maltose into glucose.

4. Sucrase / Invertase: digests sucrose into

glucose and fructose.

5. Lactase: digests lactose into glucose and

galactose (This enzyme is absent in lactose

intolerant individuals)


SUMMARY OF DIGESTION

REGION OF DIGESTION

SECRETION SOURCE ENZYME ACTION

MOUTH Saliva Salivary Glands

Salivary Amylase

Starch maltose

STOMACH Gastric juice

Gastric glands

Pepsin Proteins polypeptides

Rennin Soluble caseinogens insoluble casein

DUODENUM

Bile Liver - Bile salts emulsify fats.

Pancreatic juice

Pancreas

Amylase Starch maltose

Trypsin Proteins polypeptides

Lipase Fats fatty acids and glycerol

Intestinal juice

Intestinal glands

Sucrase Sucrose glucose + fructose

Maltase Maltose glucose

Erepsin Polypeptides amino acids

Lactase Lactose glucose + galactose

Lipase Fats fatty acids and glycerol

OPTIMUM pH

pH 2.0 pH 6.5 – 6.8 pH 8.5

Pepsin and Rennin Salivary Amylase

or Ptyalin Trypsin and other intestinal juice

END PRODUCTS OF DIGESTION

SUBSTRATE END PRODUCTS

Carbohydrates Glucose, Fructose and

Galactose

Proteins Amino acids

Fats Fatty acids and glycerol

ABSORPTION

Products of digestion such as simple sugars, amino

acids, fatty acids and glycerol are absorbed

throughout the small intestines especially the ileum.

The absorbed nutrients pass from the small intestine

into the blood stream.

Ileum

The ileum is the last part of

the small intestine. This is the

region where absorption of

digested food takes place.

The structure of the ileum is

highly adapted in order to

carry out its function. The

main aim of the adaptations is

to increase the surface area

for absorption as much as

possible.

THE PROCESS OF ABSORPTION

Simple sugars and amino acids

diffuse through the walls of

the villi into the blood

capillaries.

They can also be absorbed by

active transport against the

concentration gradient in

times of starvation, diets,

anorexia, etc.

The blood capillaries are

connected to the hepatic

portal vein where they are

transported to the liver.

Minerals salts and vitamins

also diffuse into the blood

capillaries of the villi.

Glycerol is soluble in water

and it diffuses into the

epithelium.


Fatty acids react with bile salts to form soluble

soaps, and then diffuse into the epithelium.

In the epithelium, the soaps and glycerol recombine

to form minute fat globules, which are absorbed by

the lacteals.

FEATURE OF SMALL INTESTINE (ILEUM)

HOW THIS HELPS ABSORPTION TO TAKE PLACE

1. It is very long, about 6m in adult.

This gives plenty time for digestion to be completed, and for digested food to be absorbed as it passes through.

2. It has villi. Each villus is covered with cells which have even smaller projections on them called microvilli.

This increases the surface area of the small intestine for rapid and efficient absorption of digested food.

3. Villi contain blood capillaries.

Digested food diffuse into the blood and is transported to the liver by the hepatic portal vein.

4. Villi contain lacteals, which are part of the lymphatic system.

Fats are absorbed into the lacteals.

5. Villi have walls only one cell thick.

The digested food can easily cross the wall to reach the blood capillaries and lacteals.

The large intestine (Colon), Rectum and Anus

The large intestine or colon is about 1.5m long.

The colon absorbs excess water, salts and vitamins.

For more effective absorption, its walls are folded

transversely to increase its surface area.

The indigestible fibre provides the faeces bulk

against which the muscles of the colon can push.

The large intestine also secretes mucus, which acts

as a lubricant to facilitate the peristalsis of faeces

through the rectum and anus.

The rectum is a muscular storage chamber where

undigested food (faeces) is held temporarily and

moulded before being pushed out through the anus

during egestion.

The anus is the exit to the alimentary canal. It is

closed by a ring of muscle called the anal sphincter

which is relaxed during egestion.

ASSIMILATION

After the digested food has diffused into the blood

capillaries of the villi, the food is taken to the liver

via the hepatic portal vein. Here, the small food

substances, absorbed as small soluble molecules,

must now be built up into the larger molecules

needed by the body.

Sugars

Glucose and any other simple sugars absorbed by

the villi may be used as it is; as a substrate for

respiration to release energy.

However, after a meal, there is more glucose

available than is needed immediately and so, these

excess glucose molecules need to stored.

Insulin, a hormone secreted by the pancreas,

regulates blood glucose concentration.

Insulin converts glucose into large insoluble

molecule, glycogen, which is stored in the liver and

muscles.

Glycogen can be converted back into glucose when

there is insufficient glucose supply in the diet, using

the hormone glucagon (also from pancreas).

Amino acids

Amino acids are transported around the body for

growth and repair of worn out cells.

They are also used for the formation of enzymes and

hormones.

Any excess amino acids are broken down in the liver

by a process called deamination.

This is the removal of their amino group (-NH3)

leaving a carbohydrate backbone.

Two separate molecules are produced as a result of

deamination:

1. A carbohydrate which can be changed into

glucose. Excess glucose will be converted into

glycogen and stored.

2. Urea, a nitrogenous waste product which passes

in the blood from the liver to the kidneys for

excretion in urine.

Fats

Once in the blood, fatty acids and glycerol

recombine to form tiny fat droplets.

Lipids are stored in special storage cells in the skin

known as adipose tissue and around the body

organs such as the kidneys.


Other functions of the liver

The liver is the largest gland in the body and weighs

about 2kg.

It is situated just below the abdomen.

It is dark red in colour because it contains a lot of

blood.

Apart from the production of bile, the regulation

of blood glucose and the deamination of amino

acids, other functions of the liver include:

1. Iron storage: vitamin B12 is necessary for the

maturation of red-blood cells and this is stored

in the liver until required. The liver also stores

iron for the manufacture of haemoglobin.

2. Detoxification: the removal and breakdown of

poisons (toxins) from the blood such as alcohol.

Liver can remove small quantities of alcohol on

a regular basis. However, high levels of alcohol

in the blood can eventually lead to liver disease

or ‘cirrhosis’.

3. Heat production: heat is formed as a result of

numerous chemical activities occurring in the

liver and is distributed by the blood to other

parts of the body, thus helping to maintain the

body temperature.

4. Prothrombin and fibrinogen production: these

two substances are essential for the clotting of

blood and are formed in the liver.

5. Fat metabolism: fat is mainly stored in the

saturated form. When it is required by the

tissues to provide energy, the liver performs a

chemical change by removing the hydrogen and

converting the fat into the unsaturated form for

use.

6. Storage of vitamins: the liver stores vitamin A

and D.

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