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Biology Paper 3,Nov/Dec 2008  
Questions:   1 2 3   Main
General Comments

Question 1

An experiment was performed as shown in the diagram below.  A glass funnel was placed over some water weeds in a beaker of water.  A test tube filled with water was carefully inverted over the funnel stem, which was raised above the bottom of the beaker by plasticine.

A small amount of sodium bicarbonate was then added to the water in the set-up, and the apparatus was exposed to sunlight for some hours.

Use this information and the diagrams below to answer questions 1(a) to 1 (c)

Beginning of experiment                                    After some hours







(a)        What do X and Y represent in diagram B?                                            [1 mark]
(b)        State the aim of the experiment.                                                  [1 mark]

(c)        Give one reason in each case for the following procedures:
(i)    The funnel was placed on blocks of plasticine.                           [1 mark]
           (ii)    Sodium bicarbonate was added to the water.                                [2 marks]
           (iii)       The set-up was placed in sunlight.                                            [2 marks]
           (iv)      A glass funnel was used.                                                 [1 mark]

        (d)            (i)         Briefly describe what happened in  the experiment during the time
                                     interval between diagrams A and B.                                        [4 marks]
                        (ii)        Describe a simple test that would be carried out to identify X in the
diagram.                                                                                  [4 marks]

                        (iii)      Write a balanced chemical equation for the process involved in the          experiment.                                                                            [4 marks]
       (e)  State five ways in which the process in the experiment is important.         [5 marks]


The performance of candidates on this question was fair.  Many candidates could identify X and Y as Oxygen and gas bubbles respectively and could state the aim of the experiment as an experiment to show that oxygen is released during the process of photosynthesis.   Some candidates however said it was an experiment to demonstrate the process of transpiration and so lost marks on many of the sub items. 
Most candidates could say why a glass funnel was used for the experiment and could describe a simple test to identify X /oxygen but many candidates wrote insert a glowing splinter instead of a glowing splint.
Some candidates however could not state why the funnel was placed on plasticine, why sodium bicarbonate was added to the water and why the setup was placed in sunlight.

Expected answer include

-           Funnel was raised to allow free movement of water.

-           Sodium bicarbonate was added to provide (sufficient) carbon-dioxide; for the water plant to photosynthesize. 

-           The set-up was placed in the sunlight to provide sunlight/light/solar energy; which is used for photosynthesis/the light stage of photosynthesis.
Many candidates could not describe what happened in the time interval between diagrams A and B, neither could they write a balanced chemical equation for the process nor state five ways in which the process is important.  Expected answers include
Description of what happened between diagrams A and B.

  1. Sunlight energy is absorbed by the plant;
  2. Carbon dioxide produced by the bicarbonate and water are used for photosynthesis;
  3. Bubbles of gas/oxygen are released into the funnel;
  4. And accumulate in the test tube;
  5. Which displaces water/water level falls in the test tube.



Balanced Chemical equation

Chemical Equation

            6H2O       +    6CO2      light                C6H12O6    +    6O2

Many candidates wrote Co2  instead of CO2.

Importance of the process

  1. The Oxygen is released into the environment during the process;
  2. It reduces CO2 concentration in the atmosphere/it reduces the greenhouse effect;
  3. Conversion of solar energy to chemical energy;
  4. Production of food/carbohydrates/fats protein for herbivores/carnivores/animals.
  5. Source of energy for all life processes.


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