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Biology Paper 2, Nov/Dec 2010  
Questions:   1 2 3 4 5 6 7 8   Main
General Comments

Question 3

(a) Explain the following terms giving two examples of each:
      (i)         decomposers;
      (ii)        scavengers.                                                         [9 marks]
(b)        (i)         Name the major source of energy in an ecosystem.               [1 mark]
           (ii)        Which group of organisms receives the most amount of energy in an ecosystem?
           (iii) Give reasons for your answer in 3(b)(ii) above.                                                [6 marks]
(c)        Construct a detritus food chain consisting of four organisms.                                     [ 4 marks]





Many candidates could properly define decomposers and scavengers and give appropriate examples but in defining decomposers a lot of candidates omitted to say that they secrete enzymes into the substrate. Most candidates could correctly state the source of energy in an ecosystems as well as the organisms that receive the most amount of energy namely sunlight/sun/sola/energy and producers/green plants but could not state the reasons. In stating the organisms which receive the most amount of energy some candidates merely wrote plants rather than green plants and so lost the score.

Reasons for saying that producers receive the most amount of energy in the ecosystem include
- Producers absorb sunlight energy; and convert it into chemical energy through the process of photosynthesis'; not all the energy which the plant gets from sunlight is converted to chemical energy; some is lost through respiration/other metabolic activities;
- All consumers depend directly or indirectly on plants for food;
- Not all parts of the plant are eaten by consumer;
- Only a fraction of the energy gotten by plants is passed to consumers.
Only a few candidates could construct a detritus food chain. Most candidates made chains that begun with a producer and many candidates drew line linking the organisms which did not have arrow heads and so could not score. Expected answers include:

Detritus food chain
Begin with detritus/correctly named detritus
e.g plantlanimallitter/humus
→ Detritivore (earth worm, fly, maggots, woodlice)
→ Chichken → snake → hawk.

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