Biological molecules

Question 1

Why do we need food?

Answer 1

1. To provide energy for vital activities
2. Provide raw materials to make new protoplasm
3. Help us stay healthy

Question 2

What are carbohydrates?

Answer 2

Carbohydrates are organic molecules made up of the elements carbon, hydrgogen and oxygen. The hydrogen and oxygen atoms are present in the ratio 2:1

Question 3

What are the functions of carbohydrates?

Answer 3

1. As a source of energy for cell activities (sugars, starch, glycogen)
2. Structural roles in living things
   * cellulose in plant cell walls
   * chitin in exoskeleton of arthropods and fungal walls
3. To be converted into other organic compounds such as amino acids and fats
4. To form nucleic acids for example DNA
5. To synthesise lubricants for example, mucus, which consists of a carbohydrate and a protein
6. To synthesise nectar in some flowers

Question 4

How are carbohydrates classified?

Question 5

How are monosaccharides formed? And name the monosaccharides

Answer 5

They are the basic unit of carbohydrates and formed by the hydrolysis of disaccharides.

Monosaccharides include :
1. Glucose - found in plants and animals
2. Galactose - present in milk sugar in mammals
3. Fructose - common in plants but rare in mammals

Question 6

How are dissacharides formed? and name the dissacharides

Answer 6

Dissacharides are formed by the condensation of monosaccharides and the hydrolysis of starch (in the case of maltose)

1. Lactose
2. Maltose
3. Sucrose

Question 7

How are polysaccharides formed? And name the polysaccharides

Answer 7

They are formed by the condensation of many monosaccharides.

1. Starch
Role - Storage form of carbohydrates in plants. Can be digested into glucose to provide energy for cell activities

Occurence - Found in storage organs of plants eg potato tubers and tapioca

2. Cellulose
Role - Cellulose cell wall protects plant cells from bursting or damage. Cellulose cannot be digested in our intestines. They serve as dietary fibre that prevents constipation

Occurence - Present in cell walls of plants

3. Glycogen
Role - Storage form of carbohydrates in mammals. Can be digested into glucose to provide energy for cell activities

Occurence - Stored in the liver and muscles of mammals

Question 8

Define hydrolysis, and give chemical equations

Answer 8

Hydrolysis is a reaction in which a water molecule is needed to chemically break down a complex molecule into **smaller molecules **

lactose + water → glucose + galactose (w/ enzyme lactase)
maltose + water → glucose + glucose (w/ enzyme maltase)
sucrose + water → glucose + fructose (w/ enzyme sucrase)
starch + water → maltose (w/ enzyme amylase)

Question 9

Define condensation and give chem equations

Answer 9

Condensation is a reaction in which two simples molecules are chemically joined together to form a larger molecule with the removal of one molecule of water.

glucose + galactose → lactose + water
glucose + glucose → maltose + water
glucose + fructose → sucrose + water

Question 10

What is the food test for reducing sugars? And give the steps to it

Answer 10

Benedict’s test.

1. Place 2cm³ of food sample into a test tube
2. Add 2cm³ of Benedict’s solution to the food sample
3. Shake the mixture and place the tube in a boiling water bath for 2-3 minutes

Positive test :
Observation: Brick-red precipitate is observed.
Conclusion: Reducing sugar is present (large amount)

Observation: Benedict’s solution turned from blue to yellow or orange
Conclusion: Moderate amounts of reducing sugar are present

Observation: Benedict’s solution turned from blue to green
Conclusion: Traces of reducing sugar is present

Negative test
Observation: Benedict’s solution remains blue
Conclusion: Reducing sugar is absent