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Chemical Composition of the Cell

4.7.2 Enzymes (Structured Question 1 & 2)

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Question 1:
Diagram I shows the organelles involved during the synthesis and secretion of an enzyme in an animal cell.

(a) Name the organelles labelled J and L.

(b)(i) State the function of organelle M.
(b)(ii) Explain the role of organelle K in the synthesis of the enzyme.


(c)
Enzymes are widely used in our daily life and industries.
Explain how enzymes act in:
(i) helping to cook meat.
(ii) extracting agar from seaweeds.


(d) Diagram II shows the structure of an enzyme and three substrates W, X and Y.


Based on Diagram II, complete the schematic diagram below to show the mechanism of enzyme action on a suitable substrate.


Answer:
(a)
J: Golgi apparatus
L: Secretory vesicles

(b)(i)
Produces energy

(b)(ii)
K stores genetic information in the DNA and this information is transferred to the RNA which then carries it out to the cytoplasm.


(c)(i)
Protease enzyme softens the meat.

(c)(ii)
Cellulase breaks down cell walls of seaweed and frees agar contained in it.

(d)



4.7.1 Proteins (Structured Question 1 & 2)

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Question 1:
Diagram I shows two levels of organization in protein structure.





(a) In Diagram I, name the level of organization in the protein structures of P and Q.


(b) Describe the structure of protein P.

(c) Diagram II shows how protein P is broken down to form products.


(d) Explain how the products in Diagram II are formed.


(e) State why animal proteins are first class protein.

Silk is an example of protein. A silk garment crumples when it is washed using hot water at a temperature of 65OC.
(i) Explain the above statement.

(ii) Based on the above statement, suggest two ways to maintain the quality of silk garments.



Answer:
(a)
P: Primary structure
Q: Secondary structure

(b)
The linear sequence of amino acids in a polypeptide chain.

(c)
Protein P is broken down by an enzyme (polypeptidase) in hydrolysis to form dipeptides.

(d)
Because it contains all the 8 essential amino acids needed by humans. 


(e)(i)
Silk is a protein fibre. High temperatures will reduce the strength of the hydrogen bonds and disrupt the structure, thus denaturation of the protein occurs.

(e)(ii)
1.Wash it in cold water
2. Do not wring
3. Do not expose to direct sunlight
4. Never use chlorine bleach on silk
(Choose any 2)

4.2.2 Disaccharides

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  1. Disaccharides are formed when two monosaccharides combine together through a chemical process called the condensation.
  2. Disaccharides are known as complex sugar.
  3. Examples of disaccharides are,
    1. Maltose 
      1. known as malt sugar. 
      2. It is a reducing sugar
      3. It is formed from condensation of two glucose molecules.
        Glucose + glucose → maltose + water 
      4. Found in germinating cereals
      5. Produced during digestion of starch
    2. Sucrose
      1. It is found in sugar cane, beet root and  sweet fruits
      2. It is a non-reducing sugar
      3. It is  formed from glucose and fructose.
        Glucose + fructose → sucrose + water
    3. Lactose
      1. Present in all mammals
      2. Is a reducing sugar
      3. It is formed from glucose and galactose
        Glucose + galactose → lactose + water
  4. Disaccharides are decomposed to units of monosaccharide through hydrolysis (the addition of one molecule of water)

4.2.1 Monosaccharides

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  1. Monosacharides are monomers of carbohydrates. 
  2. Examples of monosacharides are:
    1. Glucose — found in plants and animals (It is the energy source for cellular respiration)
    2. Fructose — found in fruits and honey
    3. Galactose — present in milk
  3. They are reducing sugars, they can remove oxygen from other compounds..
  4. They can be detected by using Fehling's or Benedict's test. 
  5. When monosaccharides are heated with Fehling's or Benedict's solution, they will reduce the blue copper(II) sulphate in the two solutions to a brick red precipitate.

4.2 Carbohydrate

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  1. Carbohydrate consist of the elements carbon, hydrogen and oxygen. 
  2. The H : O ratio in all carbohydrate molecules is 2 : 1.
  3. Examples of carbohydrate include starch, sugar, glycogen and cellulose.
  4. Excess carbohydrates are stored in the form of glycogen in the liver. Some are converted to fat and stored in the adipose tissues below the skin and around organs.
  5. There are three types of carbohydrates:
    1. Monosaccharides
    2. Disaccharides
    3. Polysaccharides

4.1 Chemical Composition of the Cell

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  1. The elements found in the cell include
    1. carbon, 
    2. oxygen, 
    3. hydrogen, 
    4. nitrogen, 
    5. sulphur, 
    6. phosphorus, 
    7. calcium,
    8. sodium,
    9. potassium, 
    10. magnesium,
    11. iron 
    12. chlorine, 
  2. The chemical compounds in the cell can be classified into two groups, 
    1. organic: compounds which contain the element carbon and originate from living things. Example: carbohydrates, lipids, proteins and nucleic acids
    2. inorganic: compounds which do not originate from living things. Example: water and mineral salts.
  3. .Organic compounds are synthesised by the cells themselves whereas inorganic compounds are not synthesised by the cells themselves but are obtained from the external environment.