Chapter 2- Biological Molecules

Question 1

Explain the facts on carbohydrates

Answer 1

Commonly either hexose(6C), pentose(5C) or triose(3C)

Made up of carbon, hydrogen and oxygen

Monomer of carbohydrates are called monosaccharides(glucose, fructose, galactose, ribose)

Glucose: alpha glucose(OH below at 1C) and beta glucose(OH above at 1C)

Formation of dissacharides via the process condensation(removal of a water molecule) forms a glycosidic bond

Maltose = glucose + glucose
Lactose = glucose + galactose
Sucrose = glucose + fructose

Adding more monosaccahrides to disaccahrides forms polysaccharides

Question 2

3 main types of biological molecules

Answer 2

Carbohydrates: enegy source and storage
Lipids(fats): energy storage
Proteins: enzymes, hormones, carrier proteins, pigments, receptors, antibodies

Question 3

Explain the carbohydrate food test

Answer 3

STARCH
A drop of the food sample is put onto an indent of a spotting tile. A drop of iodine solution is added to the sample. Change of color to blue black indicates presence of starch

REDUCING SUGAR
5 cm3 of food sample is added inside a boiling tube. An excess of Benedict’s solution is added to the boiling tube. The tube is immersed inside a water bath of temperature >80 degree Celcius. A change of color to brick red indicates presence of reducing sugar

NON REDUCING SUGAR
This test should follow from a negative result from the above test. Few drops of hydrocholric acid is added to 5 cm3 of food sample inside a boiling tube and immersed inside a water bath of temperature >80 degree Celcius. After 5 minutes, remove the tube from the water bath and cool it under tap water. Add sodium hydrogen carbonate powder to neutralise it(alkaline condition is also acceptable). Conduct a Benedict’s test upon the neutralised sample. A change of color to brick red indicates presence of reducing sugar

Question 4

Explain the protein food test

Answer 4

5 cm3 of food sample is added to a boiling tube. Excess of biuret solution is added to the food sample. The boiling tube is shaken and left to rest for a few minutes. A change of color from blue to purple lilac indicates presence of protein

Alternatively, biuret solution can be replaced by adding excess potassium hydroxide solution followed by drop by drop of copper(II) sulphate solution until a blue ring forms.

Question 5

Explain the lipid food test

Answer 5

The food sample is to be made into liquid. 5 cm3 of liquified food sample is added to a boiling tube. Excess ethanol is added to it. The tube is shaken and left to rest. A cloudy suspension formed indicates presence of lipid

Question 6

What is the difference between cellulose, starch, amylose, amylopectin and glycogen?

Answer 6

Cellulose is made up of beta glucose and for every other beta glucose molecule, it is rotated 180 degrees to form beta 1-4 glycosidic bond

Starch is a mixture of amylose and amylopectin

Amylose is made from alpha glucose and forms alpha 1-4 glycosidic bond between each molecule. It is shaped as a spiral chain

Amylopectin is made from alpha glucose and forms alpha 1-4 and alpha 1-6 glycosidic bonds. It is a bramched molecule

Glycogen is more branched version of amylopectin. It is also larger than amylopectin and used as a storage molecule