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Biology Paper 2,May/June 2009.  
Questions: 1 2 3 4 5 6 7 8 Main
General Comments

Question 8
(a.)Explain the following modes of nutrition which occur in plants:
    1. Autotrophic;
    2. Chemosynthetic;
    3. Carnivorous.                                                                          [6 marks]

      (b.)Give one example each of a plant which undergoes the mode of nutrition listed in 8(a) above  [3 marks]
      (c)      (i)     Describe three ways by which nitrogen is added to the soil for plant use.                                 [9 marks]
      (ii)        State two ways by which nitrogen is lost from the soil.    [2 marks]





This was the more popular question in this section and candidates who attempted it performed fairly.  Most candidates could explain autotrophic mode of nutrition in plants and state examples but only a few could explain chemosynthetic and carnivorous modes and state appropriate examples. Some candidates wrote on carnivorous mode of nutrition in animals. Appropriate answers include:

Chemosynthetic -  These plants/bacteria manufacture their food; using atmospheric carbon dioxide; and energy from the breakdown of inorganic compounds.

Carnivorous      -   These plants in addition to photosynthesizing have special devices for trapping and digesting insects and other small   animals; to balance a deficiency in nitrogen.

Nitrosomonas, Nitrobacter, Fungi, Nitrococcus.

Pitcher Plants, Bladderwort, Sundew, Venus flytrap, Butterwort.

Candidates responded poorly to the aspect that dealt with ways by which nitrogen is added and lost from the soil. Most candidates could only state the actions of bacteria in nodules of leguminous plants as well as the action of thunderstorms respectively. Expected answers include.

Addition of nitrogen to soil
Decomposition of dead organic matter         
Dead organic matter; are decomposed by putrefying bacteria; into ammonium compounds, ammonium compounds are converted to nitrites by nitrifying bacteria, in the soil; nitrites are converted into nitrates by nitrifying bacteria in the soil.

Nitrogen fixation in root nodules
Nitrogen fixing bacteria’ living in the root nodules of leguminous plants;
Convert gaseous nitrogen in the soil/atmosphere/air-spaces; into nitrates;

During thunder storms; the energy from electrical discharge/action of lightning; enables atmospheric nitrogen to react with oxygen to form oxides of nitrogen/nitric oxides; which dissolve in rain water to form weak nitric acid; which is washed in to the soil and converted into nitrites and nitrates.

Loss of nitrogen from the soil

  • Absorption by plant roots
  • Action of denitrifying bacteria
  • By leaching/erosion.
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