3. Biological Molecules

Question 1

state the roles of water in living organisms

• animal (4)
• plant (4)

Answer 1

Animals
• required for chemical reaction such as
hydrolysis of food molecules
• key component for tissue and bodily fluid
• regulation of body temperature through
sweat
• allow blood to transport substances

Plants
• reactant for photosynthesis
• provides physical support to the plant in the
form of turgor pressure.
• allow dissolved mineral salts to be
transported from roots to other part of plants
• allow sugars to be transported from leaves to
other parts of the plant

Question 2

describe and carry out tests for starch (3)

Answer 2

1. Place food substance on a white tile. Solid foods may need to be
   chopped up to smaller pieces.
2. Add 2-3 drops of dilute iodine solution to substance to be tested.
3. Iodine solution is yellowish brown, if it changes to blue black, starch
   is present. If it remains yellowish brown, starch is absent

Question 3

describe and carry out tests for reducing sugars (5)

Answer 3

1. Add 2cm3 of Benedict’s solution to 2cm3 of solution to be tested.
2. Shake the mixture
3. Heat the test tube in boiling water bath for 5 minutes.
4. Observe precipitate formation and colour changes.
5. Benedict solution is blue. Remain Blue (reducing sugar is absent) →
   Green (little amount) → Yellow (moderate amount) → Orange →
   Brick-red (most amount)

Question 4

describe and carry out tests for protein (4)

Answer 4

1. Add 2cm3 of sodium hydroxide solution to 2cm3 food solution.
2. Shake thoroughly.
3. Add 1% copper (II) sulfate solution, drop by drop, shaking after
   every drop. Allow the mixture to stand for 5 minutes.
4. Copper (II) sulfate is blue, remains blue —> protein is absent.
   Solution changes from blue to violet —> protein is present

Question 5

describe and carry out tests for fats (4)

Answer 5

1. Add 2cm3 of ethanol to the substance in a dry test tube.
2. Shake the mixture thoroughly.
3. Add 2cm3 of water to mixture.
4. If fats are present, a white emulsion will be observed

Question 6

State the smaller basic unit of glycogen

Answer 6

glucose

Question 7

State the smaller basic unit of polypeptides and protein

Answer 7

amino acids

Question 8

State the smaller basic unit of lipids

Answer 8

glycerol and fatty acids

Question 9

explain enzyme action in terms of the ‘lock and key’ hypothesis (6)

Answer 9

‘LOCK AND KEY’ HYPOTHESIS
1. The substrate is they “key”, while the enzyme is the “lock”
2. Only substrate that is complementary in shape to the active site of
the enzyme can fit into the active site.
3. The substrate binds to the active site of the enzyme, forming an
enzyme-substrate complex.
4. The formation of enzyme-substrate complex lowers the activation
energy of the chemical reaction as enzyme molecule holds the
substrate molecule(s) in an arrangement that forces them together in
the correct orientation.
5. The enzyme then catalyse the reaction at its active sites to convert
the substrate into product molecules
6. The product(s) dissociate from the enzyme, and the unchanged
enzyme is free to catalyse another reaction

Question 10

investigate and explain the effects of temperature on the rate of enzyme-catalysed reactions (5)

Answer 10

1. At low temperatures, enzymes are inactive and the rate of reaction is low.
   Substrate and enzyme molecules have little kinetic energy, hence the
   frequency of collision is low. In addition, most substrate molecules do not
   contain suficient energy to overcome the activation energy required to
   start a reaction.
2. As temperature increases, enzyme and substrate gain kinetic energy and they
   collide more often, increasing the formation of enzyme substrate complex
   thus increase rate of reaction. Rate of reaction doubles with every 10°C rise in
   temperature.
3. Reaction rate is its maximum at enzyme optimum temperature.
4. As temperature increases beyond optimum temperature, enzyme is
   denatured. The enzyme loses its 3D shape and active site is unable to bind to
   the substrate. Rate of reaction thus decreases.
5. Denaturation is irreversible even when temperature is lowered. At extremely
   high temperatures, the enzyme is completely denatured and the rate of
   reaction drops to zero

Question 11

investigate and explain the effects of pH on the rate of enzyme-catalysed reactions (3)

Answer 11

1. Enzyme activity is the highest at the optimum pH of the enzyme.
2. As the pH deviates from the optimum, enzyme activity sharply
   decreases. This is because the hydrogen bonds and ionic bonds
   that hold the 3-dimensional structure are disrupted. The
   enzyme is denatured and the shape of the active site is altered
3. At extreme pH levels, the enzyme is completely denatured and
   the rate of reaction drops to zero. The optimum pH for each
   enzyme differs