Functions of FoodFunctions of Food

• to maintain life

• to supply energy & give warmth

– carbohydrates

– fats

– proteins

• for growth and repair

– Proteins

• keep healthy & help to fight against disease

• elements : C, H, O

– ratio of H : O

= 2 : 1

• monosaccharides

– C6H12O6

– all are sweet & soluble in water

– all are Reducing Sugars

– include Glucose, Fructose & Galactose

CarbohydratesCarbohydrates

disaccharides (from 2 monosaccharides)

– C12H22O11

– all are sweet & soluble in water

– Maltose ( 2 glucose molecules )

– Sucrose ( glucose + fructose)

(non-reducing sugar)

– Lactose ( glucose + galactose)

CarbohydratesCarbohydrates

surcosesurcose

CarbohydratesCarbohydrates

• polysaccharides (NOT sugar)

• for energy storage

– starch (store in plants)

glycogen (store in animals)

• Hydrolysis:

Polysaccharide +H2O

Disaccharides +H2O

Monosaccharides

main source of energy

cellulose:

- fibrous material of Plant Cell Wall

- dietary fibre: stimulates Peristalsis

excess carbohydrates

- stored as glycogen in liver & muscle

- stored as fats under skin

Functions of CarbohydratesFunctions of Carbohydrates

Test for Reducing Sugars

(Benedict’s Test)

Is there any colour change in

tubes A and B ?

Ans: Only the mixture in tube

A has a colour change.

A B glucose

solution +

Benedict’s

solution

distilled

water +

Benedict’s

solution

water

bath

What is the sequence of

change when there is a colour

change ?

Ans: The blue solution changes

first to green, then to a

yellow coloration and

eventually a brick-red

precipitate is produced.

A B glucose

solution +

Benedict’s

solution

distilled

water +

Benedict’s

solution

water

bath

Test for Starch

(Iodine Test)

What is the final colour in

tube A ?

Ans: The solution in tube A

changes from brown

to blue black.

iodine solution

starch

solution

distilled

water

A B

What is the purpose of

setting up tube B ?

Ans: To act as a control.

iodine solution

starch

solution

distilled

water

A B

ProteinsProteins

• elements: C, H, O, N,

sometimes S, P

• components : amino acids

forms dipeptides & polypeptides

• Condensation:

Amino acids –H2O Dipeptides

–H2O Polypeptide

Proteins

• are compounds of carbon, hydrogen, oxygen,

nitrogen and sometimes sulphur, phosphorus

• amino acids:

– unit of proteins

– about 20 different types

– essential and non-essential types

An amino acid molecules

H2N

amino group

COOH

carboxyl group

C

R

H

The condensation and hydrolysis of a

polypeptide

1 4

1 2 3 4 5

hydrolysis condensation

Condensation of two amino acid to form a

dipeptide

H2N C

R1

H

C

O

OH H

N

H

C

H

R2

COOH

H2N C

R1

H

O

H

C

H

R2

COOH C---N

H2O

cannot be stored

- excess proteins are deaminated by liver

~ to Urea which will be excreted away by

Kidney

~ to Carbohydrates (Glycogen) which will

be stored in Liver

ProteinsProteins

Functions of ProteinsFunctions of Proteins • for growth and repair of body cells

(as structural components (cell

membrane and cytoplasm) of cells)

• to produce hormones and

enzymes and antibodies

• to give energy

• for making Haemoglobin in blood

• for making Antibodies

Deficiency disease of proteins

Kwashiorkor

Test for Proteins

(Biuret Test)

Test for proteins / Biuret test egg white solution

1 cm3 NaOH solution

Put CuSO2 solution drop by

drop, and shake the mixture

after addition of each drop

Positive result: purple colour

What colour changes in

tubes A and B ?

Ans: Mixture in tube A

changes from blue

to purple while

mixture in tube B

remains blue

without any change.

copper sulphate solution

egg white +

sodium

hydroxide

solution

water +

sodium

hydroxide

solution A B

Lipids ( fats & oils )Lipids ( fats & oils )

• elements : C, H, O

• components of 1 lipid molecule:

1 glycerol + 3 fatty acids

• insoluble in water

• soluble in organic solvent

Functions of LipidsFunctions of Lipids

give energy

component of cell membrane

form fatty tissues under skin

> to store energy

> acts as insulator to keep warm

to transport fat-soluble vitamins (A, D, E, K)

Spot Test for Fat

Which substance, oil or water,

leaves a permanent

translucent spot on the filter

paper ?

Ans: Oil.

Emulsion Test for Fat

Which test tube has an emulsion formed ?

Ans: Test tube A.

2 drops of

cooking oil

2 cm3 of

alcohol

2 cm3 of

distilled

water

after shaking to

form a clear

solution

shake and then

allow to stand emulsion

oil

water

shake and then

allow to stand

shaking

2 drops of

cooking oil 2 cm3 of

distilled

water

2 cm3 of

distilled

water

A

B

What happens to the other tube ?

Ans: The mixture separates into two layers because

fats do not dissolve in water.

2 drops of

cooking oil

2 cm3 of

alcohol

2 cm3 of

distilled

water

after shaking to

form a clear

solution

shake and then

allow to stand emulsion

oil

water

shake and then

allow to stand

shaking

2 drops of

cooking oil 2 cm3 of

distilled

water

2 cm3 of

distilled

water

A

B

VitaminsVitamins

• no energy value

• essential for small amount to

maintain good health

• water soluble vitamins

( B, C )

• fat soluble vitamins

( A, D, E, K ) excessive of some vitamins may

be harmful

Vitamin AVitamin A

formed in the body from Carrotene

(a yellow pigment in carrots)

destroyed at high temperature

essential for forming visual purple

(maintain dim light vision)

Vitamin CVitamin C

Destroyed after prolonged cooking

Necessary for wounds-healing

Vitamin DVitamin D

Formed in Skin from Ultraviolet Light

Help to regulate Ca & P metabolism

Detection of Vitamin C in Lemon

Juice by using DCPIP

What colour change has

occurred ?

Ans: The blue DCPIP

decolourizes.

syringe

lemon juice

DCPIP solution

What conclusion can you draw ?

Ans: Lemon juice contains

vitamin C which

decolourizes blue DCPIP.

syringe

lemon juice

DCPIP solution

Vitamin

Sources

Deficiency

Disease

A

Egg yolk, milk,

cheese, carrot,

green

vegetables

Night blindness

C

Fresh fruits &

green

vegetables

Scurvy

D

Cod liver oil

& egg yolk

Rickets

Food TestsFood Tests

Substances

Test

Original

Colour

Positive Result

Reducing

Sugar

Benedict’s

Blue

Orange ppt

Starch

Iodine

Brown

Blue-black

Protein

Biuret

Blue

Violet

Fats/Oils

Spot

---

Translucent

spot

Fats/Oils

Emulsion

Clear

Milky emulsion

Vitamin C

DCPIP

Blue

Colourless

(decolourize)

Mineral SaltsMineral Salts

regulate body metabolism

essential for healthy growth

necessary for construction of certain

tissues

needed in small amount

include Ca, S, K, Na, Mg, Fe, I

Mineral(s)

Sources

Functions

Deficiency

Disease

Calcium &

Phosphorus

Cheese, milk,

vegetables

Making bones &

teeth

Important for

blood clotting &

muscle

contraction

Rickets

Iron

Liver, eggs,

beef, green

leafy

vegetables

Structural

component of

Haemoglobin

Anaemia

Dietary FibreDietary Fibre

mainly cellulose

indigestible material for human

give bulk to food & stimulate peristalsis

prevent Constipation

lack of dietary fibre: Large Intestine Cancer

sources: wholemeal cereals, unpolished

rice, fresh vegetables & fruits

Balanced DietBalanced Diet • have enough food to supply enough

– energy

• carbohydrates, fats, proteins

– body building materials

• proteins

– substances to maintain health

• vitamins, minerals, water & dietary fibres

• malnutrition : not having balanced diets for long time

Energy Contents in FoodEnergy Contents in Food

• Calorimeter is used to measure the

amount of energy contained in a

particular type of food

• Carbohydrate (17kJ/g)

• Protein (18kJ/g)

• Fat (39kJ/g)

Factors affecting energy Factors affecting energy

requirementrequirement • Sex

• Age

• Occupation

• Physical Activities

• Stage of individual

(pregnancy, breast-feeding)

Measure the Energy

Value of Food

boiling tube

water

burning

peanut

thermometer

Explain why the energy value

of the peanut is lower than

those from standard tables.

Ans: Because there are a

number of inaccuracies

associated with this

method due to

incomplete combustion

and heat loss.

Basal Metabolic Rate (BMR)Basal Metabolic Rate (BMR) minimum amount of energy needed by an

individual lying awake in bed to maintain

breathing, body temperature & heartbeat

varies from one individual to another

daily energy requirement > basal

metabolic rate

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