Chapter 3Biological molecules

Prepared by Ms Lam and Mr Alex Lim

Learning outcomes

1) State the roles of water in living organisms2) List the chemical elements which make up

• Carbohydrates• Fats• Proteins

3) Describe and carry out tests for • Starch (iodine)• Reducing sugars (Benedict’s solution)• Protein (Biuret test)• Fats (ethanol emulsion)

4) State that large molecules are synthesised from smaller basic units• Glycogen from glucose• Polypeptides and proteins from amino acids• Lipids such as fats from glycerol and fatty acids

Prepared by Ms Lam and Mr Alex Lim

Prepared by Ms Lam and Mr Alex Lim

Importance of Water

1) Vital constituent of protoplasm (70%)

2) Best solvent for both organic and inorganic substances

Functions of water

1) Medium in which various chemical reactions occur

2) Transporting agent– Digested food substances from intestines to

other parts of body– Excretory products from tissues to excretory

organs – Hormones from glands to regions of body

required

Functions of water

3) Essential component of– Digestive juices– Blood– Lubricants in joints

4) Reactant in photosynthetic process in green plants5) Helps to regulate body temperature when

sweat evaporates from skin surface (maintenance of body temperature at 37°C)

Prepared by Ms Lam and Mr Alex Lim

Summary

Water

• Can be obtained by drinking it and also by drinking milk, juices and soup

• Is lost during urination, perspiration and even breathing

• If not replaced, will lead to dehydration, causing muscle cramps and weakness

Sources

What is biological molecule?

Any molecule that is produced by a living organism

Carbohydrates

Organic compound composed of carbon,

hydrogen and oxygen in the ratio of 1:2:1

monosaccharides (simple sugars) or disaccharides (complex sugars)

Many monosaccharides and disaccharides can be linked together

to form polysaccharidesE.g. Starch (C6H10O5)n

Generalized formCnH2nOn

Carbohydrates

Functions of Carbohydrates

1) Source of energy2) Form supporting structures3) Converted to other organic compounds– Amino acids and fats

4) Formation of nucleic acids– Deoxyribonucleic acid

Test for reducing sugars

• Benedict’s test– To 2 cm3 of glucose solution (or sample to be tested)

in a test tube, add 2cm3 of Benedict’s solution– Shake the test tube and immerse it in a beaker of

boiling water for 2 minutes– At the end of 2 minutes, observe any change in

colour• Negative: Remains blue• Positive: Brick-red precipitate . Sometimes, green, yellow

or orange ppt may appear

Test for reducing sugarsTest for reducing sugars

This sugar test is performed by adding a small amount of Benedict’s solution to the sample and bringing the mixture to a boil.

If Benedict’s solution remains clear blue, and no precipitate is formed, sugar is absent.

The presence of sugar is indicated by the formation of a precipitate from green, through to yellow and orange, to brick red.

Is there any colour change in tubes A and B ?Ans: Only the mixture

in tube A has a colour change.

A Bglucose 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 Bglucose solution

+ Benedict

’s solution

distilled water +

Benedict’s solution

water bath

Benedict’s test

Glycogen

• Storage form of carbohydrate in animals and in fungi

• Formed by many glucose molecules• Highly branched chains of glucose units

Starch

• Storage form of carbohydrate in plants• Form by many glucose molecules• Can be hydrolysed by acid – Bonds broken– Glucose units released

Conversion of glucose to starch (in Plants)

Why?

Conversion of glucose to glycogen (in

Human) Why?

Iodine test for starch

This starch test is performed by adding a few drops iodine solution to the sample.

Starch present

Starch absent

What is the final colour in tube A ?

Ans: The solution in tube A changes from brown to blue black.

iodine solution

starch solutio

n

distilled water

A B

What is the purpose of setting up tube B ?Ans: To act as a

control.

iodine solution

starch solutio

n

distilled water

A B

Cellulose

• Main component of cell walls of plants• Inert only few organisms can digest it• Man cannot digest it– Become bulk of undigested matter – Ease the movement of food and undigested waste

along digestive tract

Fats• Organic compounds– Carbon, hydrogen, oxygen

• Unlike carbohydrates• Contain much less oxygen in proportion to hydrogen

Functions of fats1) An efficient source and storage of energy2) An insulating material to reduce heat loss3) Constituent for protoplasm4) Solvent for fat-soluble vitamins (A,D,E,K)5) Restrict water loss from surface of skin

Structure of Fats

Fats are compounds that contain carbon, hydrogen and oxygen.

A molecule of fat consists of glycerol with chains of fatty acids attached to it.

Different kinds of fats have different fatty acids attached to glycerol.

Structure of fats

Type of fats• Saturated fats– Occurring in animal bodies– Cholesterol found together with saturated fats can

get deposited on inside of arteries and lead to heart attacks

• Unsaturated fats– Vegetable fats– Do not cause heart diseases– Healthier alternative

Fats

Oils are fats which are liquid at room temperature.Waxes are fats which are solid at room temperature.

Fats

• Fats that are solid at room temperature tend to be high in saturated fats.

• Saturated fats increase the level of cholesterol in the blood.

• Diets rich in cholesterol and saturated fats increases the risk of heart disease.

Healthy artery

Damaged artery

Which substance, oil or water, leaves a permanent translucent spot on the filter paper ?

Ans: Oil.

Ethanol Emulsion test

• Ethanol Emulsion test– On liquids• To 2cm3 of an unknown liquid in a test tube, add 2cm3

of ethanol• Shake the mixture thoroughly• Add 2cm3 of water to the mixture and observe for any

changes– Negative: Remain clear and colourless– Positive: Form cloudy white suspension

Ethanol emulsion testThe presence of fats can be identified with the emulsion test. The test is done by dissolving the sample liquid in ethanol. Water is then added to the mixture.

water

2cm3 of ethanol

one drop of sample

Emulsion formed.Fat is present.

• Ethanol Emulsion test– On solids• Cut a solid sample into small pieces• To a few pieces of solid sample in a test tube, add 2cm3

of ethanol• Allow the solid particles to settle and decant the

ethanol into another test tube containing 2cm3 of water• Observe for any changes to ethanol solution

– Negative: Remain clear and colourless– Positive: Form cloudy white suspension

Ethanol Emulsion test

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

oilwater

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

oilwater

shake and then allow to

stand

shaking

2 drops of

cooking oil

2 cm3 of distilled water

2 cm3 of distilled water

A

B

Proteins

Proteins are compounds that contain carbon, hydrogen, oxygen and nitrogen.

Proteins

Proteins are nutrients which are made up of basic units called amino acids. Essential amino acids are necessary to keep the body healthy. There are about 20 different amino acids. 9 of them are essential.

different types of amino acids

peptide links

An amino acid molecules

Functions of proteins

• Essential for tissue repair– synthesis of cytoplasm, cell membranes and

chromosomes• Synthesis of enzymes and some hormones• Formation of antibodies to combat diseases

Effects of Protein Deficiency

• In children– Lead to a disease called Kwashiorkor• Swollen abdomen• Scaly skin• Permanent growth defects

Test for Proteins

• Biuret test– To 1 cm3 of an unknown liquid in a test tube, add 1

cm3 of sodium hydroxide solution– Shake thoroughly– Add 1% copper(II) sulphate solution, drop by drop

into the mixture, shaking after every drop• Observe for any colour change

– Turn Violet: Proteins are present– Turn Pink: Peptones are present– Remain Blue: Proteins are absent

Biuret test

The presence of proteins can be identified with the Biuret test. The test is done by dissolving the sample liquid in ethanol. Water is then added to the mixture.

2cm3 of sodium hydroxide

2cm3 of protein sample

1% copper sulphate solution added drop by drop

Purple solution formed Protein is present

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

Prepared by Ms Lam and Mr Alex Lim

Biuret Test

Balanced 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

Factors affecting energy requirement

• Sex• Age• Occupation• Physical Activities• Stage of individual

(pregnancy, breast-feeding)

GOOD LUCK FOR EXAMINATIONS!!!!!

(don’t depend too much on luck though… work smart and hard too!)