This question was the least popular in the paper and the candidates performed poorly. They had little or no knowledge of the nitrogen cycle. Most candidates could not state the organisms in the labelled stages or state what happened in the various stages. It was only in 4(a)(ii) that candidates performed well as most could state Leguminous plants associated with the cycle. Expected answers include.
Organism found in Stages
I - Rhizobium;
II - Azotobacter, Clostridium, Nostoc
V - Nitrosomonas
VI - Nitrobacter
What happens to stages III and IV
III - Some denitrifying bacteria/Psuedomonas/Psuedomonas
- breakdown soil nitrates;
- and release free nitrogen to the atmosphere
IV - Lightning discharge/thunderstorm;
- Causes nitrogen in the air to combine with oxygen
- to form oxides of nitrogen/nitrous oxide;
- which mixes with rain water to form acids/nitric acid;
- in the soil they react with minerals;
- to form nitrates.
In answering question 4(b), most candidates could properly identify the specimens as Rice/maize weevil, Spider and Earthworm, however some candidates lost marks due to spelling errors. Some candidates wrote earthworm as two separate words.
Most candidates could not classify the organisms into their classes with reasons or state the structural differences between the spider and weevils or state the importance of the earth worm to the farmer.
Expected answers include.
Classification with reason
Organism Class Reason
Weevil Insects Three body divisions/three pairs
Spider Arachnida Possession of four pairs of legs;
Two body divisions/Prosoma and
Earthworm Oligochaetae. No distinct head region; Presence of Clitellum.
Structural differences between C & D
- Three body divisions/head - Two body divisions/cephalo
thorax and abdomen; thorax and abdomen;
- Has three pairs of legs; - Has four pairs of legs;
- Has antennae; - No antennae;
- Has wings; - No wings;
- Smooth body; - Hairy body;
Points must correspond to score.
Importance of E/earthworm
- By its burrowing action it aerates the soil;
- Enhances decomposition of organic matter/improves humus content of soil;
- The worm casts produced improves soil fertility;
- Tunnels made in the soil allow easy percolation of water/rain water.
Most candidates however could state the similarities between the weevil and the spider namely possession of exoskeleton, and jointed appendages.
Tolerance – Ability of organisms to live/withstand/survive/endure; prevailing/adverse
conditions/biotic/abiotic conditions; within certain minimum and maximum
limit in the habitat.
Emigration – movement of organisms out of the population/community/habitat; to other
places; due to adverse/biological conditions.