20 April 2015

#81 Question 4

The diagrams below show five molecules found in living organisms.

(a) Give the letter of one molecule that fits each of these descriptions.

You can use each letter once, more than once or not at all.

   (i) the form in which carbohydrates are transported through phloem tissue in plants
(1 mark)
   (ii) the form in which carbohydrates are stored in animals
(1 mark)
   (iii) a molecule that is insoluble in water
(1 mark)
   (iv) a molecule that links together with others to form a polypeptide
(1 mark)
   (v) a molecule that contains ester bonds
(1 mark)

(b) Explain how the structure of water molecules makes water a good solvent.
(3 marks)
(Total 8 marks)

Candidate A

(a) (i) A û
    * A is a glucose molecule, but plants transport sucrose. Even if you did not know what a sucrose molecule looks like, you should know that it is a disaccharide.

   (ii) Cü
   * Correct. 1/1

   * Amino acids are soluble. Either C or D would be correct.

    *  Correct. 1/1

   *  Correct. 1/1

(b) Water has dipoles and hydrogen bonds,ü which help it to dissolve other substances.

* There are no wrong statements in this answer, but it does not really give an explanation of why water is a good solvent - it just states two facts about water molecules. 1/3

Candidate B

(a) (i) Bü
    *  Correct. 1/1

    (ii) Cü
    *  Correct. 1/1

   (iii) D or Cü
   *  Correct. However, the candidate took an unnecessary risk with (iii),by giving two answers. If the second one had been wrong, it could have negated the first correct one. If you are asked for one answer, it is best to give only one, 1/1

   (iv) ü
   *  Correct. 1/1

  (v) D ü
   *  Correct. 1/1

(b) In a water molecule, the hydrogen atoms have a tiny positive electrical charge and the oxygen atom has a similar negative charge. üOther atoms or ions with electrical chargesü are attracted ü to these charges on the water molecules. This makes them spread about ü among the water molecules.

* This is a good answer. It really does explain how a substance dissolves in water and relates this clearly to the structure of a water molecule. The candidate has actually earned four possible marking points, but there is a maximum of three marks available in total. 3/3

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