Chapter 3: Biological molecules

Question 1

Iodine test (starch)

Answer 1

1. Add 2/3 drops of iodine solution to food sample
   Positive results: Iodine solution turns blue-black
   Negative Results: Iodine solution remains yellow-ish brown

Question 2

Dealing with solid food samples

Answer 2

1. Cut small section of sample
2. Put sample into mortar and grind with pestle
3. Put crushed sample into test-tube
4. Add water and stir
5. Let crushed sample settle at the bottom
6. Decant the clear liquid and conduct the test on liquid

Question 3

Benedict’s Test (reducing sugar)

Answer 3

1. To a liquid sample, add equal volume of Benedict’s solution
2. Place test tube in boiling water bath for a few minutes
   - Red-brick ppt formed ➡️ large amounts of reducing sugar
   - Turned from blue to yellow/orange ➡️ moderate amounts of sugar
   - Turned from blue to green ➡️ traces of reducing sugar
   - Solution remained blue ➡️ no tracing sugar

Question 4

Reducing Sugars

Answer 4

• Glucose, fructose & maltose are reducing sugars
• sucrose is not a reducing sugar

Question 5

Ethanol Emulsion Test (Fats)

Answer 5

1. Add 1cm3 of ethanol to liquid sample and shake
2. Add 1cm3 of water and shake
   Positive result: Cloudy white emulsion observed
   Negative results: Solution remained clear

Question 6

Biuret test (Protein)

Answer 6

1. To liquid sample, add equal volume of sodium hydroxide solution
2. Add 1-2 drops of copper sulfate solution
   Positive results: Solution turned purple
   Negative results: Solution turned blue

Question 7

Function of carbohydrates

Answer 7

1. as a substrate for respiration, provides energy for cell activities
2. form supporting structures like cell walls
3. converted into other organic compounds
4. formation of nucleic acid like DNA
5. synthesises lubricants like mucus
6. produces nectar

Question 8

Composition of carbohydrates

Answer 8

• made out of carbon, hydrogen and oxygen
• hydrogen to oxygen ratio is 2:1
• glucose: C6H12O6
• sucrose: C12H22O11

Question 9

Simple Sugars

Answer 9

• basic unit of carbohydrate
• pass through the cell membrane and absorbed into cells
• glucose, fructose and galactose

Question 10

Double sugars

Answer 10

• formed when 2 simple sugars are joined tgt
• maltose ➡️ glucose + glucose
• lactose ➡️ galactose + glucose
• sucrose ➡️ fructose + glucose

Question 11

Complex carbohydrates

Answer 11

• made out of many similar molecules of single sugars joined to form a large molecule
• starch, glycogen and cellulose

Question 12

Starch

Answer 12

• made out of several thousand glucose molecules joined tgt
• storage form of carbohydrates in plants
• it is digested to glucose to provide energy for cell activities

Question 13

Cellulose

Answer 13

• made out of many glucose molecules joined tgt, but the bonds between them is diff from starch
• forms cell wall
• cannot be digested, so it serves as dietary fibre that prevents constipation

Question 14

Glycogen

Answer 14

• a branched molecule that is made up of many joined glucose molecules
• storage form of carbohydrates in mammals
• is digested to glucose to provide energy for cell activities
• stored in liver and muscles in animals

Question 15

Why glucose and starch are suitable as storage materials in cells

Answer 15

• they are insoluble in water ➡️ do not change water potential
• cannot diffuse through cell membranes since they are large ➡️ will not be lost from cell
• can be easily broken down to glucose when needed
• have compact shapes ➡️ occupy less space than individual glucose molecules

Question 16

Functions of fats

Answer 16

1. Source and long-term storage of energy
2. insulating material that prevents excessive heat loss
3. a solvent for fat-soluble vitamins and other vital substances such as hormones
4. essential part of cells, especially in cell membranes
5. a way to reduce water loss from skin surface

Question 17

Composition of fats

Answer 17

• type of lipids
• made out of carbon, hydrogen and oxygen
• contains much less oxygen atoms in proportion to hydrogen atoms

Question 18

Sources of fats

Answer 18

• foods rich in fats: dairy products, fatty meats, olives, many nuts, peas, beans, oil
• some fishes have little fats, but some have a lot

Question 19

Breaking down of fats

Answer 19

• break down with lipase to form 1 glycerol molecule and 3 fatty acids molecules

Question 20

Functions of proteins

Answer 20

1. Synthesises new cytoplasm for growth and repair worn-out cells
2. synthesises hormones and enzymes
3. source of energy in absence of carbohydrates and fats
4. formation of antibodies

Question 21

Composition of proteins

Answer 21

• made out of carbon, hydrogen, oxygen, nitrogen, sulfur and phosphate
• contains amino (NH2) and acidic (COOH) group

Question 22

Sources of proteins

Answer 22

• found in animals & plant foods
• milk, egg, seafood & meat
• soya beans, nuts, grains & vegetables

Question 23

Amino acids

Answer 23

• building blocks of proteins
• 1 amino acid is made out of amino group (-NH2), acidic group (-COOH), and side chain
• amnio acid join tgt to form polypeptides
• polypeptides join tgt to form proteins